Current Medicinal Chemistry 2007 14 759-773 759

Recent Advances in Antimalarial Compounds and their Patents

Alka Mital

Department of Pharmaceutical Technology National Institute of Pharmaceutical Education and Research Sector67 SAS Nagar Mohali-160062 Punjab India

Abstract Malaria is one of the most severe tropical parasitic disease causing 1-3 million deaths annually Inthe last 25 years very few new antimalarial molecules have been developed and only a limited number of themare currently in various stages of clinical development The presently available antimalarial drugs includeartemisinin analogs quinoline derivatives and antifolates This review summarizes recent advances inantimalarial drug development and world patents published between 2000-2006 claiming new syntheticantimalarial compounds and their activities The most over-represented classes of compounds in malaria patentliterature in order of frequency are artemisinin analogs quinoline derivatives DOXP reductoisomeraseinhibitors antifolates and febrifugine analogues Many of these patents describe the novelty and potential ofthese synthetic derivatives with an attempt to identify the next generation antimalarials that may havepotential commercial advantages

Keywords Malaria antimalarial activity artemisinin quinoline antifolate Plasmodium falciparum

INTRODUCTION potent antimalarial agent in 1971 and the subsequentdevelopment of its analogues as potent antimalarial agentshave been of immense importance in the field of antimalarialchemotherapy [89] The artemisinin class is the most wellrepresented group of compounds in the patent literature[1011]

Malaria is the most widespread tropical parasitic diseaseand continues to be a major cause of mortality andmorbidity especially throughout the developing world It hasbeen estimated that in the last 20 years mortality frommalaria has doubled (currently 1ndash3 million deaths annually)and a major factor responsible for this increase is theresistance of malaria parasites to antimalarial drugs [12]The emergence and spread of drug resistant malariarepresents a considerable challenge to controlling malaria Todate malaria control has relied heavily on a comparativelysmall number of chemically related drugs belonging toeither the quinoline or the antifolate groups [3] Chloroquinewas once the most effective treatment for malaria butresistance has been developed with its treatment [4] Otherantimalarial treatments exist but they are not readilyavailable due to their formidable costs or toxicity

There have been considerable increase in theunderstanding of developments over the last decade in theprocesses occurring within the parasite that are relevant tothe mode of action of current antimalarials and targets fornew antimalarial compounds

This review summarizes current antimalarial drugdevelopments and world patents filed claiming specific invitro or in vivo antimalarial activities Table 1 summarizesthe synthesized and patented compounds from differentclasses of antimalarials

ARTEMISININ DERIVATIVESMost severe malaria is caused by the blood-borne parasiteP falciparum The emergence and spread of drug-resistantmalaria parasites is the major threat to effective malariacontrol The need for new antimalarials has becomeincreasingly urgent due to the widespread resistance of Pfalciparum to chloroquine as well as to other antimalarialdrugs in this challenging situation Current efforts focus onthe development of new compounds with novel mechanismsof action and on measures to prevent or delay resistance tothe malarial parasite In the last 25 years a number ofsignificant advances have been made that have the potentialto make a major contribution to the control of this disease[5-7] The discovery of artemisinin (1) as a remarkably

Artemisinin (qinghaosu 1) a sesquiterpene trioxanelactone was first isolated in 1971 from Artemisia annua L(sweet or annual wormwood) as the active componentresponsible for antimalarial activity [1213] Artemisinin andits derivatives represent a new class of antimalarialscontaining an endoperoxide moiety [14] The therapeuticvalue of artemisinin is limited by its low solubilitybioavailability short half-life and neurotoxicity [15]

Several semi-synthetic derivatives with betterpharmacokinetic parameters which contain substitution atC-10 have been prepared [1617] The replacement ofoxygen at the C-10 position of dihydroartemisinin (DHA)with carbon produces compounds not only with greaterhydrolytic stability but also with longer half-life and lowertoxicity [10] The first generation derivatives derived fromDHA are artemether (2) arteether (3) artelinic acid (4) andsodium artesunate (5) in Fig (1) These compounds aremore potent than artemisinin but have short plasma half-lives Artemisinin and its derivatives kill all stages of themalaria parasite by interacting with heme to produce carbon-

Address correspondence to this author at the Department ofPharmaceutical Technology National Institute of PharmaceuticalEducation and Research Sector 67 SAS Nagar Mohali-160062 PunjabIndia Tel +91 (0)172-2214682-87 Fax +91 (0)172-2214692 E-mailalkamitalgmailcomDivision of Biological Chemistry amp Molecular Microbiology College ofLife Sciences Universty of Dundee Dow Street Sir James Black CentreDundee DD1 5EH UK Tel +44 (0) 1382 384328386227 Fax +44 (0)1382 386373

0929-867307 $5000+00 copy 2007 Bentham Science Publishers Ltd

760 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Table 1 Patented Synthetic Molecules

Patent Nos Title Structure Authors Pub Date

US06094736WO06046949

Synthesis andantimalarial activity of

pyrrolo[32-F]quinazoline-13-diamine derivatives

N

N

N

F3 C

H2 N

NH2

WR227825

Ai JL Jian G QuanZ Donald RS

2006-05-04

US05038024 Novel artemisininederivatives and usesthereof for treating

malaria

O

O

H

HO

O

R3 R1

R4

R2

Begue J-P BonnetDD Crousse B et al

2005-02-17

US066984640EP1655302

Antiparasitic artemisininderivatives

(endoperoxides)

O

O

H

O

O

Y

H

Haynes R LamWW-L Chan H-W

et al

2006-01-10

2006-05-10

US05119232EP1485389

WO03076446

Antiparasitic artemisininderivatives

(endoperoxides)

O

O

H

O

O

N

H

XR1

R2

Z

Haynes RK 2005-06-022004-12-152003-09-18

WO04028476 Orally activeantimalarial anticancer

artemisinin-derivedtrioxane dimers with highselectivity stability andefficacy and methods of

making the same OCH3

O

H

O

OH

OH

O

O

H

O

OH

H

Posner GH ShapiroTA

2004-04-08

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 761

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

WO03095444 Artemisinin-basedperoxide compounds asbroad spectrum anti-

infective agents O

O

H

O

O

H

X

H(A)o (B)e

R1

R2

R3

R4

(G)I

R5

R9

R6( )

2

O

O

H

O

O

X

H H

(Y)q

R16

R14 R15

Avery M AMuraleedharan KM

2003-11-20

US036649647 Trioxane derivatives

Z

O

H

O

O

Y

CH2X

H

H

Haynes R Chan H-W et al

2003-11-18

WO03082852US04053991

Novel substituted 124-trioxanes useful as

antimalarial agents and aprocess for the

preparation thereof

CH2

O

OO

RR2

R1

Singh CTiwari P Puri SK

2003-10-09

2004-03-18

US026500955 An improved one-potsynthesis of [2 8-bis(trifluoromethyl)-4-

quinolinyl]-2-pyridinylmethanone a mefloquine

intermediate

N

H

OH

HN

CF3

CF3

Chawla HPS JoharPS Mital A Meena

RA Negi VS

2002-12-31

US050165054 Process for thepreparation of quinoline

derivatives

N

HNNH2

OCH3

O

CH3O

CF3Bell D Davies BJ

Kincey PM2005-07-28

762 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

US05131058EP051496056

Novel compounds andantimalarials

O O

O O

R1

Z ( )n

( )n Wataya Y Kim H-SNojima M

2005-06-16

WO05037833 4-Amino-QuinolineOuinolizidinyl-andQuinolizidinylalkyl-

derivatives withantimalarial activity

NR

N

(CH2)n

HN

Sparatore ATaramelli D

Basilico N et al

2005-04-28

US04039008 Spiro and Dispiro 1 2 4-Trioxolane antimalarial

O O

O

R1

R2

Vennerstrom JLDong Y Chollet J

et al

2004-02-26

JP05060396 Spiro and Dispiro 1 2 4-Trioxolane antimalarial

O

O

O

R2 R4

R3R1Vennerstrom J LDong Y Chollet J

et al

2005-03-10

US04192724 Process for preparationof ring-substituted 8-

aminoquinoline analogsas antimalarial agents

N

R

H3 CO

R1

HNNHR3

R2

Jain R Jain M SinghPP et al

2004-09-30

EP1606263WO04085402

Ring-substituted 8-aminoquinolinederivatives as

antimalarial agents

N

R

H3CO

R1

HNNHR3

R2

Jain R Jain M SinghPP et al

2005-12-21

US056949569US04038957

Dual moleculescontaining a peroxide

derivative synthesis andtherapeutic applications

thereof

Z1

R1

R4H Z2

R2

R3A-Y3-U-Y2

Meunier B Robert ADechy-Cabaret O

Benoit-Vical F

2004-02-26

US04180913 Antimalarial pyrimidinederivatives and methods

of making and usingthem N

N

NH2

H2N R2

R1

Yongyuth YBongkoch TSumalee K

2004-09-16

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 763

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

US056844350US02193387

Febrifugineisofebrifugine and

method for producing thesame

N

N

O

O HN

OH

N

N

OO

NHHO

Kobayashi SWataya Y Kim H-S

2005-01-18

2002-12-19

WO04000319 Antimalarial activities offebrifugine analogues

N

N

O

O HN

OH Jiang S Hudson HTMilhous KV

2003-12-31

US04053950 Antimalarial activitiesand therapeutic

properties of febrifugineanalogues

N

N

O

O HN

OH Jiang S Hudson HT 2004-03-18

US056906098US04019200

Mixed steroidal 1 2 45-tetraoxane compoundsand methods of making

and using thereof

O

OO

O

CH3OR

XOH

OR

CH3

H

R1R4

R3

R2

CH3

n

Solaja BOpsenica D MPocsfalvi G et al

2005-06-14

2004-01-29

US026426417 Processes andintermediates useful to

make Antifolates N

N NY

R7

R6HN

R3

R4

O Jackson BCEugene DS et al

2002-07-30

US03203908WO0153276

Dihydrofolate reductaseinhibitors

N

N

N

H2N

NH2

R1

R2

X

Y

N

NH

NH

HN

NH2

R1

R2

X

Y

Gordon L 2001-07-26

WO0170237US03144249

Use oforganophosphorous

compounds forproducing a medicament

for treating infections

O

P

R4

R3B

Jomaa H 2001-09-27

764 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

EP1237579WO0139757

Surfactants as MalarialChloroquine resistance

reversal agentsC9H19

(OCH2 CH2)XOH Kain KC Crandall ICharuk J Reithmeier

R

2002-09-11

US05240034 Artemisinin-basedperoxide compounds asbroad spectrum anti-

infective agents

O

O

O

O

H

H

X

H(A)o (B)e

R1

R2 R4

R3

(G)f

R5

R7

R6n

O

O

O

O

H

HH

X

(Y)q

R16

R1 5R1 4

Mitchell AAKannoth MM

2005-10-27

O

O

O

O

H

H

O

O

O

O

O

H

H

OH

DHA

O

O

O

H

H

OR

O

O

O

O

O

H

O

HHO

O

O

O

H

O

HH

O

O

O

O

O

H

O

HH

F3 C

O

O

O

O

H

O

HH

N

NPhH2 C

O

O

O

O

H

HH

O

O

O

O

O

H

HH

O

O

O

O

H

O

HH

NO O

ONH

OCH3

CH3

NH2

2R = CH33R = C2H54R = CH3C6H4 CO2 H-p5R = C(O)(CH2)2COONa

67

8

9

10 11

Artemisinin (1)

12

16

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 765

Fig (1) contd

O

O

O

O

H

HH

S

O

HN

O

O

H

HH

O

O

O

O

H

O

R

O

O

O

O

H

H

R

O

O

O

O

H

H

R

O

O

O

O

H

H

X

F3C CF3

O

O O

O

O

O

O

H

HR

O O

O

H

HN

NH

N

Cl

CH3

13

14 15a R = F15b R = COOH

19 R=Alkyl

20 R= halogen OMe Alkyl

21 X= O NMe

Arteflene (17)22 R= Alkyl arylalkyl OH

18

O

O

O

O

H

H

N N

CF3 O

O

O

O

H

H

S

O

O

O

O

H

HH H

O O

O

O

O

O

H

HH

O P

OCH3

O

O

O

O

O

O

H

HH O

O

O

O

H

H

H

COOH

O

O

O

O

H

HH

23 TDR 4029224

25 26

Fig (1) Structures of artemisinin derivatives and other semi synthetic peroxides

centered free radicals that alkylate protein and damage themicroorganelles and membranes of the parasites [18] Mostimportantly there is no evidence of drug resistance to anymember of the artemisinin family of drugs [19]

(6) have been reported from Hong Kong University ofScience and Technology [20] This series of compoundsshowed IC50 values in the range of 025-250 ngml againstchloroquine-resistant P falciparum strain W2 Otherderivatives of artemisinin as specified by 10β-dihydroartemisinyl benzoate (7) have IC50 values in therange of 007-119 ngml (7) being most active against bothW2 and chloroquine sensitive strain D6 [21]

Several derivatives of artemisinin (1) were disclosedbetween 2000 and 2006 as world patents many of these areacetyl or non-acetyl C-10 derivatives Novel C-10 ethersubstituted artemisinin derivatives depicted as in compound

766 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Additional substituted phenyl esters of dihydro-artemisinin such as compound (8) exhibited significant invitro and in vivo antimalarial activity [22] Compound (8)showed better in vivo activity than artemetherdihydroartemisinin and sodium artesunate with a halfmaximal effective dose (ED) value of 212 mgkg in a Pberghei mouse model [23] The addition of N-phenylpiperazine ring system such as in compound (9) [24]also enhances in vitro antimalarial activity [25]

heme andor parasite proteins Successful analogue arteflene(17) is a highly active synthetic endoperoxide andchemically more stable than artemisinin It is effective fortreating mild cases of P falciparum malaria [4344]Arteflene (17) production was discontinued by Hoffman La-Roche since its long and tedious synthesis was not suitablefor industrial production

The antimalarial activity of a synthetic analogTrioxaquine DU1301 (18) has been tested in vitro onchloroquine-sensitive and resistant strains [4546]Trioxaquine DU1301 (18) is also efficient on infected miceby oral route at 12-15 mgkg

C-10 non-acetyl artemisinin derivatives with varyingdegree of in vitro antimalarial activity were synthesized [26]The compound 10-(1-pentanoyl) anhydroartemisinin (10)was the most active compound with an IC50 value of 43nM against P falciparum strain NF54 Compound (11) 10-(phenyl) dihydroartemisinin has been claimed to have IC50value of 031 and 073 ngml against P falciparum strainW2 and D6 [27] Compounds from series (11) are moreactive in vitro and may be more stable [27]

The synthesis of metabolically more stable C-10 carbaanalogues (19) and C-10 aryl analogues (20) of DHA hasbeen the focus of chemists for the past 10 years [47] C-10aryl or heteroaromatic derivatives (20) and (21) were alsosynthesized by the groups of Haynes [48] and Posner [47]C-14 modified analogs (22) were prepared by total synthesisand semi-synthetic routes by Avery and Jung groups [49-51]

Several groups have investigated the effect of C-9substitutions A series of C-9 derivatives represented bycompound (12) [28] exhibited IC50 values of 76-216ngml against different strains of P falciparum [29] Thespecific compound (13) containing a non-acetyl C-10substituent is claimed to have an in vitro IC50 value of 27ngml against W2 strain [30] A series of 11-aza-artemisininscontaining a nitryl group instead of oxygen at position 11have been claimed [31] The compound (14) has an IC50value of 173 and 260 ngml against the W2 and D6 strainsrespectively

Antimalarial evaluation of a C-10 carba analog (23)TDR 40292 demonstrated that it was superior to artemetherand artesunate both in vitro and in vivo [52] It cannot formDHA as a metabolite and contains a side chain that can beformulated as a water-soluble salt Apart from monomericsemi synthetic derivatives C-10 non-acetal carba dimers [53]and representative lead compounds (24) (25) and (26) havebeen synthesized [54-56] The isobutyric acid dimeric analog(26) was found to be safe and displayed a therapeutic indexsix times that of sodium artesunate (5)Some simplified trioxanes with potent antimalarial

activity have also been reported [3233] Water-soluble 3-aryltrioxanes such as specified by compounds (15a) and(15b) demonstrated better in vivo efficacies [3435]Compound (15b) was shown to be more efficacious andhave lower toxicity than artelinic acid (4) [36]

QUINOLINE ANTIMALARIALS

Quinolines have been the most important class ofantimalarial drugs Quinoline-containing antimalarial drugssuch as chloroquine (27) quinine (28) and mefloquine (29)are used widely to treat malaria however the malarialparasite is rapidly becoming resistant to the currentlyavailable drugs in Fig (2)

As artemisinin is a complex molecule and uneconomic tosynthesize efforts have been made to prepare simplerantimalarial molecules based on the trioxane ring system ofartemisinin that contains the endoperoxide group The mostpromising of these compounds known to date is OZ 277(16) which consists of a trioxane ring attached to anadamantane ring and provides stability to the endoperoxidegroup without diminishing its antimalarial action Themolecule also includes a tosylate side chain that confersimproved water solubility and formulation properties[3738] Compound (16) called malperox now under Phase Iclinical trials was initially developed with the support ofthe WHO Tropical diseases research programme and later bythe Medicines for Malaria Venture [39]

Quinine (28) isolated from the bark of Peruviancinchona trees was the mainstay of malaria chemotherapyuntil the Second World War when the new syntheticmolecules were developed to circumvent the problem of drugresistance [57] and 4-substituted amino-quinolineschloroquine (27) and amodiaquine (30) were developed Inthe year 1946 the synthetic quinoline compoundchloroquine (27) was introduced by Loeb et al and provedinvaluable in the fight against the disease [58]

The success of chloroquine led to the hope that malariamight be completely eradicated from the worldAmodiaquine (30) a chloroquine analog containing a 4-hydroxyanilino function in its 4-amino side chain iseffective against many chloroquine resistant strains [5960]and the drug remains useful as a treatment of malaria inmany parts of Africa [61] Chloroquine (27) andamodiaquine (30) are extensively metabolized bydealkylation of their Mannich base side chains [57] Stockset al synthesized a series of short chain chloroquinederivatives (31) and (32) in which the diethyl-amino sidechains were replaced by functional groups more resistant to

The combination of artemisinin derivatives with a seconddrug having a different mode of action is a good way toincrease the efficacy of treatment and to prevent theemergence and spread of drug resistance [4041] Somesynthetic artemisinin derived trioxane dimers have also beendeveloped with high stability and efficacy [42]

New antimalarial endoperoxides were synthesized bycombining two complementary active moieties a 4-aminoquinoline (as in chloroquine) known to easilypenetrate within infected red blood cells and a trioxane (as inartemisinin) which is a potential alkylating agent toward

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 767

NCl

HN

N

N

NH

OHO

N

CF3

CF3

HN

OH

H

NCl

HNN

OH

NCl

HN

N

NCl

HN NH

OH

NCl

HN

N

NCl

HN

OH

NH

NCl

HN

HN

F

NCl

HNN

F

NCl

HNN

FOH

NCl

HN OH

N

N

O

HNNH2

N

O

HNNH

OO

N

O

HNNH2

OCH3

O CH3

CF3

Chloropine 27 Quinine 28 Mefloquine 29Amodiaquine 30

31 32 33 34

35 36 37 Isoquine 3 8

3940

Tafenoquine 41

Fig (2) Quinoline derivatives

dealkylation [6263] Addition of piperidyl and pyrrolidylrings to the 4-amino side chain of chloroquine such as incompounds (31) and (32) resulted in an enhanced activityagainst chloroquine resistant P falciparum

a fluorine atom [64] In these molecules the Mannich sidechain has been replaced with substituent less vulnerable todealkylation and have equivalent in vitro and in vivoantimalarial activity comparable to amodiaquine Theirfluorine substituent makes them potentially much moreexpensive to synthesize

When in amodiaquine (30) 4-hydroxyanilino side chainwas modified through replacement of the 5acute aminosubstituent with alkyl groups and a tert-butyl aminosubstituent was added at the 3acute position as in compounds(33) and (34) they were also found to be more active in vivoagainst P berghei than amodiaquine (30) [60] A patentdescribes the compounds (35) (36) and (37) in which thehydroxyl group at the 4acute position of the ring is replaced with

Another patent claimed a series of analogues which arepotentially cheap to produce and effective againstchloroquine resistant parasites [65] The lead compound inthis series Isoquine (38) is more effective than amodiaquine(30) against chloroquine resistant malaria parasites in vitroand against P berghei in vivo

768 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Two other classes of quinolines currently in clinical useare the 8-aminoquinolines (primaquine 39) [66] andquinoline methanols (mefloquine 29) [67] Mefloquine (29)has been effective against chloroquine-resistant strains ofmalaria parasite [6869] while Primaquine (39) is activeonly against the liver stages of malaria parasites Bulaquine(40) an analog of primaquine is claimed to havegametocidal activity [70] Other 8-aminoquinolines currently

in development are WR-238605 (Tafenoquine) (41) and 5-phenoxy primaquine analogs These were synthesized toimprove the poor therapeutic index of primaquine (39) andits relatively short half-life Tafenoquine (41) is more activeand less toxic than primaquine (39) [7172] Tafenoquine(37) based on its promising broad-spectrum of activities hasbeen selected as a potential antirelapse antimalarial drug toreplace primaquine (39) Moreira et al synthesized

N

H3CO

NH

N NH

OR1

R3R2

F3C

N[(CH2)3CH3]2

OH

Cl Cl

HN NH

ClCl

HN N

N N

N

Cl

N

Cl

N

Cl

N

ClN

N CN

N

Cl

X

O

C

N

O

NN

N

Cl

N

Cl

N N

HN Me

Et2N

HN Me

NEt2

S

N

N C9H19

OOH

N

N OMe

Cl

NH

HO

N

N O

HN

O

HN R1

O

CH3

SN

R1

Z NS

R2 R1R2

42

Halofantrine 43WR 268668 44

45 46

n

47

48

x49

50

51 R= Ethylbenzene52 R= 4-propoxy-1-vinylbenzene

2I- ++

53

Fig (3) Quinoline antimalarials and related compounds

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

760 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Table 1 Patented Synthetic Molecules

Patent Nos Title Structure Authors Pub Date

US06094736WO06046949

Synthesis andantimalarial activity of

pyrrolo[32-F]quinazoline-13-diamine derivatives

N

N

N

F3 C

H2 N

NH2

WR227825

Ai JL Jian G QuanZ Donald RS

2006-05-04

US05038024 Novel artemisininederivatives and usesthereof for treating

malaria

O

O

H

HO

O

R3 R1

R4

R2

Begue J-P BonnetDD Crousse B et al

2005-02-17

US066984640EP1655302

Antiparasitic artemisininderivatives

(endoperoxides)

O

O

H

O

O

Y

H

Haynes R LamWW-L Chan H-W

et al

2006-01-10

2006-05-10

US05119232EP1485389

WO03076446

Antiparasitic artemisininderivatives

(endoperoxides)

O

O

H

O

O

N

H

XR1

R2

Z

Haynes RK 2005-06-022004-12-152003-09-18

WO04028476 Orally activeantimalarial anticancer

artemisinin-derivedtrioxane dimers with highselectivity stability andefficacy and methods of

making the same OCH3

O

H

O

OH

OH

O

O

H

O

OH

H

Posner GH ShapiroTA

2004-04-08

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 761

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

WO03095444 Artemisinin-basedperoxide compounds asbroad spectrum anti-

infective agents O

O

H

O

O

H

X

H(A)o (B)e

R1

R2

R3

R4

(G)I

R5

R9

R6( )

2

O

O

H

O

O

X

H H

(Y)q

R16

R14 R15

Avery M AMuraleedharan KM

2003-11-20

US036649647 Trioxane derivatives

Z

O

H

O

O

Y

CH2X

H

H

Haynes R Chan H-W et al

2003-11-18

WO03082852US04053991

Novel substituted 124-trioxanes useful as

antimalarial agents and aprocess for the

preparation thereof

CH2

O

OO

RR2

R1

Singh CTiwari P Puri SK

2003-10-09

2004-03-18

US026500955 An improved one-potsynthesis of [2 8-bis(trifluoromethyl)-4-

quinolinyl]-2-pyridinylmethanone a mefloquine

intermediate

N

H

OH

HN

CF3

CF3

Chawla HPS JoharPS Mital A Meena

RA Negi VS

2002-12-31

US050165054 Process for thepreparation of quinoline

derivatives

N

HNNH2

OCH3

O

CH3O

CF3Bell D Davies BJ

Kincey PM2005-07-28

762 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

US05131058EP051496056

Novel compounds andantimalarials

O O

O O

R1

Z ( )n

( )n Wataya Y Kim H-SNojima M

2005-06-16

WO05037833 4-Amino-QuinolineOuinolizidinyl-andQuinolizidinylalkyl-

derivatives withantimalarial activity

NR

N

(CH2)n

HN

Sparatore ATaramelli D

Basilico N et al

2005-04-28

US04039008 Spiro and Dispiro 1 2 4-Trioxolane antimalarial

O O

O

R1

R2

Vennerstrom JLDong Y Chollet J

et al

2004-02-26

JP05060396 Spiro and Dispiro 1 2 4-Trioxolane antimalarial

O

O

O

R2 R4

R3R1Vennerstrom J LDong Y Chollet J

et al

2005-03-10

US04192724 Process for preparationof ring-substituted 8-

aminoquinoline analogsas antimalarial agents

N

R

H3 CO

R1

HNNHR3

R2

Jain R Jain M SinghPP et al

2004-09-30

EP1606263WO04085402

Ring-substituted 8-aminoquinolinederivatives as

antimalarial agents

N

R

H3CO

R1

HNNHR3

R2

Jain R Jain M SinghPP et al

2005-12-21

US056949569US04038957

Dual moleculescontaining a peroxide

derivative synthesis andtherapeutic applications

thereof

Z1

R1

R4H Z2

R2

R3A-Y3-U-Y2

Meunier B Robert ADechy-Cabaret O

Benoit-Vical F

2004-02-26

US04180913 Antimalarial pyrimidinederivatives and methods

of making and usingthem N

N

NH2

H2N R2

R1

Yongyuth YBongkoch TSumalee K

2004-09-16

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 763

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

US056844350US02193387

Febrifugineisofebrifugine and

method for producing thesame

N

N

O

O HN

OH

N

N

OO

NHHO

Kobayashi SWataya Y Kim H-S

2005-01-18

2002-12-19

WO04000319 Antimalarial activities offebrifugine analogues

N

N

O

O HN

OH Jiang S Hudson HTMilhous KV

2003-12-31

US04053950 Antimalarial activitiesand therapeutic

properties of febrifugineanalogues

N

N

O

O HN

OH Jiang S Hudson HT 2004-03-18

US056906098US04019200

Mixed steroidal 1 2 45-tetraoxane compoundsand methods of making

and using thereof

O

OO

O

CH3OR

XOH

OR

CH3

H

R1R4

R3

R2

CH3

n

Solaja BOpsenica D MPocsfalvi G et al

2005-06-14

2004-01-29

US026426417 Processes andintermediates useful to

make Antifolates N

N NY

R7

R6HN

R3

R4

O Jackson BCEugene DS et al

2002-07-30

US03203908WO0153276

Dihydrofolate reductaseinhibitors

N

N

N

H2N

NH2

R1

R2

X

Y

N

NH

NH

HN

NH2

R1

R2

X

Y

Gordon L 2001-07-26

WO0170237US03144249

Use oforganophosphorous

compounds forproducing a medicament

for treating infections

O

P

R4

R3B

Jomaa H 2001-09-27

764 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

EP1237579WO0139757

Surfactants as MalarialChloroquine resistance

reversal agentsC9H19

(OCH2 CH2)XOH Kain KC Crandall ICharuk J Reithmeier

R

2002-09-11

US05240034 Artemisinin-basedperoxide compounds asbroad spectrum anti-

infective agents

O

O

O

O

H

H

X

H(A)o (B)e

R1

R2 R4

R3

(G)f

R5

R7

R6n

O

O

O

O

H

HH

X

(Y)q

R16

R1 5R1 4

Mitchell AAKannoth MM

2005-10-27

O

O

O

O

H

H

O

O

O

O

O

H

H

OH

DHA

O

O

O

H

H

OR

O

O

O

O

O

H

O

HHO

O

O

O

H

O

HH

O

O

O

O

O

H

O

HH

F3 C

O

O

O

O

H

O

HH

N

NPhH2 C

O

O

O

O

H

HH

O

O

O

O

O

H

HH

O

O

O

O

H

O

HH

NO O

ONH

OCH3

CH3

NH2

2R = CH33R = C2H54R = CH3C6H4 CO2 H-p5R = C(O)(CH2)2COONa

67

8

9

10 11

Artemisinin (1)

12

16

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 765

Fig (1) contd

O

O

O

O

H

HH

S

O

HN

O

O

H

HH

O

O

O

O

H

O

R

O

O

O

O

H

H

R

O

O

O

O

H

H

R

O

O

O

O

H

H

X

F3C CF3

O

O O

O

O

O

O

H

HR

O O

O

H

HN

NH

N

Cl

CH3

13

14 15a R = F15b R = COOH

19 R=Alkyl

20 R= halogen OMe Alkyl

21 X= O NMe

Arteflene (17)22 R= Alkyl arylalkyl OH

18

O

O

O

O

H

H

N N

CF3 O

O

O

O

H

H

S

O

O

O

O

H

HH H

O O

O

O

O

O

H

HH

O P

OCH3

O

O

O

O

O

O

H

HH O

O

O

O

H

H

H

COOH

O

O

O

O

H

HH

23 TDR 4029224

25 26

Fig (1) Structures of artemisinin derivatives and other semi synthetic peroxides

centered free radicals that alkylate protein and damage themicroorganelles and membranes of the parasites [18] Mostimportantly there is no evidence of drug resistance to anymember of the artemisinin family of drugs [19]

(6) have been reported from Hong Kong University ofScience and Technology [20] This series of compoundsshowed IC50 values in the range of 025-250 ngml againstchloroquine-resistant P falciparum strain W2 Otherderivatives of artemisinin as specified by 10β-dihydroartemisinyl benzoate (7) have IC50 values in therange of 007-119 ngml (7) being most active against bothW2 and chloroquine sensitive strain D6 [21]

Several derivatives of artemisinin (1) were disclosedbetween 2000 and 2006 as world patents many of these areacetyl or non-acetyl C-10 derivatives Novel C-10 ethersubstituted artemisinin derivatives depicted as in compound

766 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Additional substituted phenyl esters of dihydro-artemisinin such as compound (8) exhibited significant invitro and in vivo antimalarial activity [22] Compound (8)showed better in vivo activity than artemetherdihydroartemisinin and sodium artesunate with a halfmaximal effective dose (ED) value of 212 mgkg in a Pberghei mouse model [23] The addition of N-phenylpiperazine ring system such as in compound (9) [24]also enhances in vitro antimalarial activity [25]

heme andor parasite proteins Successful analogue arteflene(17) is a highly active synthetic endoperoxide andchemically more stable than artemisinin It is effective fortreating mild cases of P falciparum malaria [4344]Arteflene (17) production was discontinued by Hoffman La-Roche since its long and tedious synthesis was not suitablefor industrial production

The antimalarial activity of a synthetic analogTrioxaquine DU1301 (18) has been tested in vitro onchloroquine-sensitive and resistant strains [4546]Trioxaquine DU1301 (18) is also efficient on infected miceby oral route at 12-15 mgkg

C-10 non-acetyl artemisinin derivatives with varyingdegree of in vitro antimalarial activity were synthesized [26]The compound 10-(1-pentanoyl) anhydroartemisinin (10)was the most active compound with an IC50 value of 43nM against P falciparum strain NF54 Compound (11) 10-(phenyl) dihydroartemisinin has been claimed to have IC50value of 031 and 073 ngml against P falciparum strainW2 and D6 [27] Compounds from series (11) are moreactive in vitro and may be more stable [27]

The synthesis of metabolically more stable C-10 carbaanalogues (19) and C-10 aryl analogues (20) of DHA hasbeen the focus of chemists for the past 10 years [47] C-10aryl or heteroaromatic derivatives (20) and (21) were alsosynthesized by the groups of Haynes [48] and Posner [47]C-14 modified analogs (22) were prepared by total synthesisand semi-synthetic routes by Avery and Jung groups [49-51]

Several groups have investigated the effect of C-9substitutions A series of C-9 derivatives represented bycompound (12) [28] exhibited IC50 values of 76-216ngml against different strains of P falciparum [29] Thespecific compound (13) containing a non-acetyl C-10substituent is claimed to have an in vitro IC50 value of 27ngml against W2 strain [30] A series of 11-aza-artemisininscontaining a nitryl group instead of oxygen at position 11have been claimed [31] The compound (14) has an IC50value of 173 and 260 ngml against the W2 and D6 strainsrespectively

Antimalarial evaluation of a C-10 carba analog (23)TDR 40292 demonstrated that it was superior to artemetherand artesunate both in vitro and in vivo [52] It cannot formDHA as a metabolite and contains a side chain that can beformulated as a water-soluble salt Apart from monomericsemi synthetic derivatives C-10 non-acetal carba dimers [53]and representative lead compounds (24) (25) and (26) havebeen synthesized [54-56] The isobutyric acid dimeric analog(26) was found to be safe and displayed a therapeutic indexsix times that of sodium artesunate (5)Some simplified trioxanes with potent antimalarial

activity have also been reported [3233] Water-soluble 3-aryltrioxanes such as specified by compounds (15a) and(15b) demonstrated better in vivo efficacies [3435]Compound (15b) was shown to be more efficacious andhave lower toxicity than artelinic acid (4) [36]

QUINOLINE ANTIMALARIALS

Quinolines have been the most important class ofantimalarial drugs Quinoline-containing antimalarial drugssuch as chloroquine (27) quinine (28) and mefloquine (29)are used widely to treat malaria however the malarialparasite is rapidly becoming resistant to the currentlyavailable drugs in Fig (2)

As artemisinin is a complex molecule and uneconomic tosynthesize efforts have been made to prepare simplerantimalarial molecules based on the trioxane ring system ofartemisinin that contains the endoperoxide group The mostpromising of these compounds known to date is OZ 277(16) which consists of a trioxane ring attached to anadamantane ring and provides stability to the endoperoxidegroup without diminishing its antimalarial action Themolecule also includes a tosylate side chain that confersimproved water solubility and formulation properties[3738] Compound (16) called malperox now under Phase Iclinical trials was initially developed with the support ofthe WHO Tropical diseases research programme and later bythe Medicines for Malaria Venture [39]

Quinine (28) isolated from the bark of Peruviancinchona trees was the mainstay of malaria chemotherapyuntil the Second World War when the new syntheticmolecules were developed to circumvent the problem of drugresistance [57] and 4-substituted amino-quinolineschloroquine (27) and amodiaquine (30) were developed Inthe year 1946 the synthetic quinoline compoundchloroquine (27) was introduced by Loeb et al and provedinvaluable in the fight against the disease [58]

The success of chloroquine led to the hope that malariamight be completely eradicated from the worldAmodiaquine (30) a chloroquine analog containing a 4-hydroxyanilino function in its 4-amino side chain iseffective against many chloroquine resistant strains [5960]and the drug remains useful as a treatment of malaria inmany parts of Africa [61] Chloroquine (27) andamodiaquine (30) are extensively metabolized bydealkylation of their Mannich base side chains [57] Stockset al synthesized a series of short chain chloroquinederivatives (31) and (32) in which the diethyl-amino sidechains were replaced by functional groups more resistant to

The combination of artemisinin derivatives with a seconddrug having a different mode of action is a good way toincrease the efficacy of treatment and to prevent theemergence and spread of drug resistance [4041] Somesynthetic artemisinin derived trioxane dimers have also beendeveloped with high stability and efficacy [42]

New antimalarial endoperoxides were synthesized bycombining two complementary active moieties a 4-aminoquinoline (as in chloroquine) known to easilypenetrate within infected red blood cells and a trioxane (as inartemisinin) which is a potential alkylating agent toward

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 767

NCl

HN

N

N

NH

OHO

N

CF3

CF3

HN

OH

H

NCl

HNN

OH

NCl

HN

N

NCl

HN NH

OH

NCl

HN

N

NCl

HN

OH

NH

NCl

HN

HN

F

NCl

HNN

F

NCl

HNN

FOH

NCl

HN OH

N

N

O

HNNH2

N

O

HNNH

OO

N

O

HNNH2

OCH3

O CH3

CF3

Chloropine 27 Quinine 28 Mefloquine 29Amodiaquine 30

31 32 33 34

35 36 37 Isoquine 3 8

3940

Tafenoquine 41

Fig (2) Quinoline derivatives

dealkylation [6263] Addition of piperidyl and pyrrolidylrings to the 4-amino side chain of chloroquine such as incompounds (31) and (32) resulted in an enhanced activityagainst chloroquine resistant P falciparum

a fluorine atom [64] In these molecules the Mannich sidechain has been replaced with substituent less vulnerable todealkylation and have equivalent in vitro and in vivoantimalarial activity comparable to amodiaquine Theirfluorine substituent makes them potentially much moreexpensive to synthesize

When in amodiaquine (30) 4-hydroxyanilino side chainwas modified through replacement of the 5acute aminosubstituent with alkyl groups and a tert-butyl aminosubstituent was added at the 3acute position as in compounds(33) and (34) they were also found to be more active in vivoagainst P berghei than amodiaquine (30) [60] A patentdescribes the compounds (35) (36) and (37) in which thehydroxyl group at the 4acute position of the ring is replaced with

Another patent claimed a series of analogues which arepotentially cheap to produce and effective againstchloroquine resistant parasites [65] The lead compound inthis series Isoquine (38) is more effective than amodiaquine(30) against chloroquine resistant malaria parasites in vitroand against P berghei in vivo

768 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Two other classes of quinolines currently in clinical useare the 8-aminoquinolines (primaquine 39) [66] andquinoline methanols (mefloquine 29) [67] Mefloquine (29)has been effective against chloroquine-resistant strains ofmalaria parasite [6869] while Primaquine (39) is activeonly against the liver stages of malaria parasites Bulaquine(40) an analog of primaquine is claimed to havegametocidal activity [70] Other 8-aminoquinolines currently

in development are WR-238605 (Tafenoquine) (41) and 5-phenoxy primaquine analogs These were synthesized toimprove the poor therapeutic index of primaquine (39) andits relatively short half-life Tafenoquine (41) is more activeand less toxic than primaquine (39) [7172] Tafenoquine(37) based on its promising broad-spectrum of activities hasbeen selected as a potential antirelapse antimalarial drug toreplace primaquine (39) Moreira et al synthesized

N

H3CO

NH

N NH

OR1

R3R2

F3C

N[(CH2)3CH3]2

OH

Cl Cl

HN NH

ClCl

HN N

N N

N

Cl

N

Cl

N

Cl

N

ClN

N CN

N

Cl

X

O

C

N

O

NN

N

Cl

N

Cl

N N

HN Me

Et2N

HN Me

NEt2

S

N

N C9H19

OOH

N

N OMe

Cl

NH

HO

N

N O

HN

O

HN R1

O

CH3

SN

R1

Z NS

R2 R1R2

42

Halofantrine 43WR 268668 44

45 46

n

47

48

x49

50

51 R= Ethylbenzene52 R= 4-propoxy-1-vinylbenzene

2I- ++

53

Fig (3) Quinoline antimalarials and related compounds

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 761

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

WO03095444 Artemisinin-basedperoxide compounds asbroad spectrum anti-

infective agents O

O

H

O

O

H

X

H(A)o (B)e

R1

R2

R3

R4

(G)I

R5

R9

R6( )

2

O

O

H

O

O

X

H H

(Y)q

R16

R14 R15

Avery M AMuraleedharan KM

2003-11-20

US036649647 Trioxane derivatives

Z

O

H

O

O

Y

CH2X

H

H

Haynes R Chan H-W et al

2003-11-18

WO03082852US04053991

Novel substituted 124-trioxanes useful as

antimalarial agents and aprocess for the

preparation thereof

CH2

O

OO

RR2

R1

Singh CTiwari P Puri SK

2003-10-09

2004-03-18

US026500955 An improved one-potsynthesis of [2 8-bis(trifluoromethyl)-4-

quinolinyl]-2-pyridinylmethanone a mefloquine

intermediate

N

H

OH

HN

CF3

CF3

Chawla HPS JoharPS Mital A Meena

RA Negi VS

2002-12-31

US050165054 Process for thepreparation of quinoline

derivatives

N

HNNH2

OCH3

O

CH3O

CF3Bell D Davies BJ

Kincey PM2005-07-28

762 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

US05131058EP051496056

Novel compounds andantimalarials

O O

O O

R1

Z ( )n

( )n Wataya Y Kim H-SNojima M

2005-06-16

WO05037833 4-Amino-QuinolineOuinolizidinyl-andQuinolizidinylalkyl-

derivatives withantimalarial activity

NR

N

(CH2)n

HN

Sparatore ATaramelli D

Basilico N et al

2005-04-28

US04039008 Spiro and Dispiro 1 2 4-Trioxolane antimalarial

O O

O

R1

R2

Vennerstrom JLDong Y Chollet J

et al

2004-02-26

JP05060396 Spiro and Dispiro 1 2 4-Trioxolane antimalarial

O

O

O

R2 R4

R3R1Vennerstrom J LDong Y Chollet J

et al

2005-03-10

US04192724 Process for preparationof ring-substituted 8-

aminoquinoline analogsas antimalarial agents

N

R

H3 CO

R1

HNNHR3

R2

Jain R Jain M SinghPP et al

2004-09-30

EP1606263WO04085402

Ring-substituted 8-aminoquinolinederivatives as

antimalarial agents

N

R

H3CO

R1

HNNHR3

R2

Jain R Jain M SinghPP et al

2005-12-21

US056949569US04038957

Dual moleculescontaining a peroxide

derivative synthesis andtherapeutic applications

thereof

Z1

R1

R4H Z2

R2

R3A-Y3-U-Y2

Meunier B Robert ADechy-Cabaret O

Benoit-Vical F

2004-02-26

US04180913 Antimalarial pyrimidinederivatives and methods

of making and usingthem N

N

NH2

H2N R2

R1

Yongyuth YBongkoch TSumalee K

2004-09-16

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 763

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

US056844350US02193387

Febrifugineisofebrifugine and

method for producing thesame

N

N

O

O HN

OH

N

N

OO

NHHO

Kobayashi SWataya Y Kim H-S

2005-01-18

2002-12-19

WO04000319 Antimalarial activities offebrifugine analogues

N

N

O

O HN

OH Jiang S Hudson HTMilhous KV

2003-12-31

US04053950 Antimalarial activitiesand therapeutic

properties of febrifugineanalogues

N

N

O

O HN

OH Jiang S Hudson HT 2004-03-18

US056906098US04019200

Mixed steroidal 1 2 45-tetraoxane compoundsand methods of making

and using thereof

O

OO

O

CH3OR

XOH

OR

CH3

H

R1R4

R3

R2

CH3

n

Solaja BOpsenica D MPocsfalvi G et al

2005-06-14

2004-01-29

US026426417 Processes andintermediates useful to

make Antifolates N

N NY

R7

R6HN

R3

R4

O Jackson BCEugene DS et al

2002-07-30

US03203908WO0153276

Dihydrofolate reductaseinhibitors

N

N

N

H2N

NH2

R1

R2

X

Y

N

NH

NH

HN

NH2

R1

R2

X

Y

Gordon L 2001-07-26

WO0170237US03144249

Use oforganophosphorous

compounds forproducing a medicament

for treating infections

O

P

R4

R3B

Jomaa H 2001-09-27

764 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

EP1237579WO0139757

Surfactants as MalarialChloroquine resistance

reversal agentsC9H19

(OCH2 CH2)XOH Kain KC Crandall ICharuk J Reithmeier

R

2002-09-11

US05240034 Artemisinin-basedperoxide compounds asbroad spectrum anti-

infective agents

O

O

O

O

H

H

X

H(A)o (B)e

R1

R2 R4

R3

(G)f

R5

R7

R6n

O

O

O

O

H

HH

X

(Y)q

R16

R1 5R1 4

Mitchell AAKannoth MM

2005-10-27

O

O

O

O

H

H

O

O

O

O

O

H

H

OH

DHA

O

O

O

H

H

OR

O

O

O

O

O

H

O

HHO

O

O

O

H

O

HH

O

O

O

O

O

H

O

HH

F3 C

O

O

O

O

H

O

HH

N

NPhH2 C

O

O

O

O

H

HH

O

O

O

O

O

H

HH

O

O

O

O

H

O

HH

NO O

ONH

OCH3

CH3

NH2

2R = CH33R = C2H54R = CH3C6H4 CO2 H-p5R = C(O)(CH2)2COONa

67

8

9

10 11

Artemisinin (1)

12

16

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 765

Fig (1) contd

O

O

O

O

H

HH

S

O

HN

O

O

H

HH

O

O

O

O

H

O

R

O

O

O

O

H

H

R

O

O

O

O

H

H

R

O

O

O

O

H

H

X

F3C CF3

O

O O

O

O

O

O

H

HR

O O

O

H

HN

NH

N

Cl

CH3

13

14 15a R = F15b R = COOH

19 R=Alkyl

20 R= halogen OMe Alkyl

21 X= O NMe

Arteflene (17)22 R= Alkyl arylalkyl OH

18

O

O

O

O

H

H

N N

CF3 O

O

O

O

H

H

S

O

O

O

O

H

HH H

O O

O

O

O

O

H

HH

O P

OCH3

O

O

O

O

O

O

H

HH O

O

O

O

H

H

H

COOH

O

O

O

O

H

HH

23 TDR 4029224

25 26

Fig (1) Structures of artemisinin derivatives and other semi synthetic peroxides

centered free radicals that alkylate protein and damage themicroorganelles and membranes of the parasites [18] Mostimportantly there is no evidence of drug resistance to anymember of the artemisinin family of drugs [19]

(6) have been reported from Hong Kong University ofScience and Technology [20] This series of compoundsshowed IC50 values in the range of 025-250 ngml againstchloroquine-resistant P falciparum strain W2 Otherderivatives of artemisinin as specified by 10β-dihydroartemisinyl benzoate (7) have IC50 values in therange of 007-119 ngml (7) being most active against bothW2 and chloroquine sensitive strain D6 [21]

Several derivatives of artemisinin (1) were disclosedbetween 2000 and 2006 as world patents many of these areacetyl or non-acetyl C-10 derivatives Novel C-10 ethersubstituted artemisinin derivatives depicted as in compound

766 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Additional substituted phenyl esters of dihydro-artemisinin such as compound (8) exhibited significant invitro and in vivo antimalarial activity [22] Compound (8)showed better in vivo activity than artemetherdihydroartemisinin and sodium artesunate with a halfmaximal effective dose (ED) value of 212 mgkg in a Pberghei mouse model [23] The addition of N-phenylpiperazine ring system such as in compound (9) [24]also enhances in vitro antimalarial activity [25]

heme andor parasite proteins Successful analogue arteflene(17) is a highly active synthetic endoperoxide andchemically more stable than artemisinin It is effective fortreating mild cases of P falciparum malaria [4344]Arteflene (17) production was discontinued by Hoffman La-Roche since its long and tedious synthesis was not suitablefor industrial production

The antimalarial activity of a synthetic analogTrioxaquine DU1301 (18) has been tested in vitro onchloroquine-sensitive and resistant strains [4546]Trioxaquine DU1301 (18) is also efficient on infected miceby oral route at 12-15 mgkg

C-10 non-acetyl artemisinin derivatives with varyingdegree of in vitro antimalarial activity were synthesized [26]The compound 10-(1-pentanoyl) anhydroartemisinin (10)was the most active compound with an IC50 value of 43nM against P falciparum strain NF54 Compound (11) 10-(phenyl) dihydroartemisinin has been claimed to have IC50value of 031 and 073 ngml against P falciparum strainW2 and D6 [27] Compounds from series (11) are moreactive in vitro and may be more stable [27]

The synthesis of metabolically more stable C-10 carbaanalogues (19) and C-10 aryl analogues (20) of DHA hasbeen the focus of chemists for the past 10 years [47] C-10aryl or heteroaromatic derivatives (20) and (21) were alsosynthesized by the groups of Haynes [48] and Posner [47]C-14 modified analogs (22) were prepared by total synthesisand semi-synthetic routes by Avery and Jung groups [49-51]

Several groups have investigated the effect of C-9substitutions A series of C-9 derivatives represented bycompound (12) [28] exhibited IC50 values of 76-216ngml against different strains of P falciparum [29] Thespecific compound (13) containing a non-acetyl C-10substituent is claimed to have an in vitro IC50 value of 27ngml against W2 strain [30] A series of 11-aza-artemisininscontaining a nitryl group instead of oxygen at position 11have been claimed [31] The compound (14) has an IC50value of 173 and 260 ngml against the W2 and D6 strainsrespectively

Antimalarial evaluation of a C-10 carba analog (23)TDR 40292 demonstrated that it was superior to artemetherand artesunate both in vitro and in vivo [52] It cannot formDHA as a metabolite and contains a side chain that can beformulated as a water-soluble salt Apart from monomericsemi synthetic derivatives C-10 non-acetal carba dimers [53]and representative lead compounds (24) (25) and (26) havebeen synthesized [54-56] The isobutyric acid dimeric analog(26) was found to be safe and displayed a therapeutic indexsix times that of sodium artesunate (5)Some simplified trioxanes with potent antimalarial

activity have also been reported [3233] Water-soluble 3-aryltrioxanes such as specified by compounds (15a) and(15b) demonstrated better in vivo efficacies [3435]Compound (15b) was shown to be more efficacious andhave lower toxicity than artelinic acid (4) [36]

QUINOLINE ANTIMALARIALS

Quinolines have been the most important class ofantimalarial drugs Quinoline-containing antimalarial drugssuch as chloroquine (27) quinine (28) and mefloquine (29)are used widely to treat malaria however the malarialparasite is rapidly becoming resistant to the currentlyavailable drugs in Fig (2)

As artemisinin is a complex molecule and uneconomic tosynthesize efforts have been made to prepare simplerantimalarial molecules based on the trioxane ring system ofartemisinin that contains the endoperoxide group The mostpromising of these compounds known to date is OZ 277(16) which consists of a trioxane ring attached to anadamantane ring and provides stability to the endoperoxidegroup without diminishing its antimalarial action Themolecule also includes a tosylate side chain that confersimproved water solubility and formulation properties[3738] Compound (16) called malperox now under Phase Iclinical trials was initially developed with the support ofthe WHO Tropical diseases research programme and later bythe Medicines for Malaria Venture [39]

Quinine (28) isolated from the bark of Peruviancinchona trees was the mainstay of malaria chemotherapyuntil the Second World War when the new syntheticmolecules were developed to circumvent the problem of drugresistance [57] and 4-substituted amino-quinolineschloroquine (27) and amodiaquine (30) were developed Inthe year 1946 the synthetic quinoline compoundchloroquine (27) was introduced by Loeb et al and provedinvaluable in the fight against the disease [58]

The success of chloroquine led to the hope that malariamight be completely eradicated from the worldAmodiaquine (30) a chloroquine analog containing a 4-hydroxyanilino function in its 4-amino side chain iseffective against many chloroquine resistant strains [5960]and the drug remains useful as a treatment of malaria inmany parts of Africa [61] Chloroquine (27) andamodiaquine (30) are extensively metabolized bydealkylation of their Mannich base side chains [57] Stockset al synthesized a series of short chain chloroquinederivatives (31) and (32) in which the diethyl-amino sidechains were replaced by functional groups more resistant to

The combination of artemisinin derivatives with a seconddrug having a different mode of action is a good way toincrease the efficacy of treatment and to prevent theemergence and spread of drug resistance [4041] Somesynthetic artemisinin derived trioxane dimers have also beendeveloped with high stability and efficacy [42]

New antimalarial endoperoxides were synthesized bycombining two complementary active moieties a 4-aminoquinoline (as in chloroquine) known to easilypenetrate within infected red blood cells and a trioxane (as inartemisinin) which is a potential alkylating agent toward

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 767

NCl

HN

N

N

NH

OHO

N

CF3

CF3

HN

OH

H

NCl

HNN

OH

NCl

HN

N

NCl

HN NH

OH

NCl

HN

N

NCl

HN

OH

NH

NCl

HN

HN

F

NCl

HNN

F

NCl

HNN

FOH

NCl

HN OH

N

N

O

HNNH2

N

O

HNNH

OO

N

O

HNNH2

OCH3

O CH3

CF3

Chloropine 27 Quinine 28 Mefloquine 29Amodiaquine 30

31 32 33 34

35 36 37 Isoquine 3 8

3940

Tafenoquine 41

Fig (2) Quinoline derivatives

dealkylation [6263] Addition of piperidyl and pyrrolidylrings to the 4-amino side chain of chloroquine such as incompounds (31) and (32) resulted in an enhanced activityagainst chloroquine resistant P falciparum

a fluorine atom [64] In these molecules the Mannich sidechain has been replaced with substituent less vulnerable todealkylation and have equivalent in vitro and in vivoantimalarial activity comparable to amodiaquine Theirfluorine substituent makes them potentially much moreexpensive to synthesize

When in amodiaquine (30) 4-hydroxyanilino side chainwas modified through replacement of the 5acute aminosubstituent with alkyl groups and a tert-butyl aminosubstituent was added at the 3acute position as in compounds(33) and (34) they were also found to be more active in vivoagainst P berghei than amodiaquine (30) [60] A patentdescribes the compounds (35) (36) and (37) in which thehydroxyl group at the 4acute position of the ring is replaced with

Another patent claimed a series of analogues which arepotentially cheap to produce and effective againstchloroquine resistant parasites [65] The lead compound inthis series Isoquine (38) is more effective than amodiaquine(30) against chloroquine resistant malaria parasites in vitroand against P berghei in vivo

768 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Two other classes of quinolines currently in clinical useare the 8-aminoquinolines (primaquine 39) [66] andquinoline methanols (mefloquine 29) [67] Mefloquine (29)has been effective against chloroquine-resistant strains ofmalaria parasite [6869] while Primaquine (39) is activeonly against the liver stages of malaria parasites Bulaquine(40) an analog of primaquine is claimed to havegametocidal activity [70] Other 8-aminoquinolines currently

in development are WR-238605 (Tafenoquine) (41) and 5-phenoxy primaquine analogs These were synthesized toimprove the poor therapeutic index of primaquine (39) andits relatively short half-life Tafenoquine (41) is more activeand less toxic than primaquine (39) [7172] Tafenoquine(37) based on its promising broad-spectrum of activities hasbeen selected as a potential antirelapse antimalarial drug toreplace primaquine (39) Moreira et al synthesized

N

H3CO

NH

N NH

OR1

R3R2

F3C

N[(CH2)3CH3]2

OH

Cl Cl

HN NH

ClCl

HN N

N N

N

Cl

N

Cl

N

Cl

N

ClN

N CN

N

Cl

X

O

C

N

O

NN

N

Cl

N

Cl

N N

HN Me

Et2N

HN Me

NEt2

S

N

N C9H19

OOH

N

N OMe

Cl

NH

HO

N

N O

HN

O

HN R1

O

CH3

SN

R1

Z NS

R2 R1R2

42

Halofantrine 43WR 268668 44

45 46

n

47

48

x49

50

51 R= Ethylbenzene52 R= 4-propoxy-1-vinylbenzene

2I- ++

53

Fig (3) Quinoline antimalarials and related compounds

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

762 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

US05131058EP051496056

Novel compounds andantimalarials

O O

O O

R1

Z ( )n

( )n Wataya Y Kim H-SNojima M

2005-06-16

WO05037833 4-Amino-QuinolineOuinolizidinyl-andQuinolizidinylalkyl-

derivatives withantimalarial activity

NR

N

(CH2)n

HN

Sparatore ATaramelli D

Basilico N et al

2005-04-28

US04039008 Spiro and Dispiro 1 2 4-Trioxolane antimalarial

O O

O

R1

R2

Vennerstrom JLDong Y Chollet J

et al

2004-02-26

JP05060396 Spiro and Dispiro 1 2 4-Trioxolane antimalarial

O

O

O

R2 R4

R3R1Vennerstrom J LDong Y Chollet J

et al

2005-03-10

US04192724 Process for preparationof ring-substituted 8-

aminoquinoline analogsas antimalarial agents

N

R

H3 CO

R1

HNNHR3

R2

Jain R Jain M SinghPP et al

2004-09-30

EP1606263WO04085402

Ring-substituted 8-aminoquinolinederivatives as

antimalarial agents

N

R

H3CO

R1

HNNHR3

R2

Jain R Jain M SinghPP et al

2005-12-21

US056949569US04038957

Dual moleculescontaining a peroxide

derivative synthesis andtherapeutic applications

thereof

Z1

R1

R4H Z2

R2

R3A-Y3-U-Y2

Meunier B Robert ADechy-Cabaret O

Benoit-Vical F

2004-02-26

US04180913 Antimalarial pyrimidinederivatives and methods

of making and usingthem N

N

NH2

H2N R2

R1

Yongyuth YBongkoch TSumalee K

2004-09-16

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 763

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

US056844350US02193387

Febrifugineisofebrifugine and

method for producing thesame

N

N

O

O HN

OH

N

N

OO

NHHO

Kobayashi SWataya Y Kim H-S

2005-01-18

2002-12-19

WO04000319 Antimalarial activities offebrifugine analogues

N

N

O

O HN

OH Jiang S Hudson HTMilhous KV

2003-12-31

US04053950 Antimalarial activitiesand therapeutic

properties of febrifugineanalogues

N

N

O

O HN

OH Jiang S Hudson HT 2004-03-18

US056906098US04019200

Mixed steroidal 1 2 45-tetraoxane compoundsand methods of making

and using thereof

O

OO

O

CH3OR

XOH

OR

CH3

H

R1R4

R3

R2

CH3

n

Solaja BOpsenica D MPocsfalvi G et al

2005-06-14

2004-01-29

US026426417 Processes andintermediates useful to

make Antifolates N

N NY

R7

R6HN

R3

R4

O Jackson BCEugene DS et al

2002-07-30

US03203908WO0153276

Dihydrofolate reductaseinhibitors

N

N

N

H2N

NH2

R1

R2

X

Y

N

NH

NH

HN

NH2

R1

R2

X

Y

Gordon L 2001-07-26

WO0170237US03144249

Use oforganophosphorous

compounds forproducing a medicament

for treating infections

O

P

R4

R3B

Jomaa H 2001-09-27

764 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

EP1237579WO0139757

Surfactants as MalarialChloroquine resistance

reversal agentsC9H19

(OCH2 CH2)XOH Kain KC Crandall ICharuk J Reithmeier

R

2002-09-11

US05240034 Artemisinin-basedperoxide compounds asbroad spectrum anti-

infective agents

O

O

O

O

H

H

X

H(A)o (B)e

R1

R2 R4

R3

(G)f

R5

R7

R6n

O

O

O

O

H

HH

X

(Y)q

R16

R1 5R1 4

Mitchell AAKannoth MM

2005-10-27

O

O

O

O

H

H

O

O

O

O

O

H

H

OH

DHA

O

O

O

H

H

OR

O

O

O

O

O

H

O

HHO

O

O

O

H

O

HH

O

O

O

O

O

H

O

HH

F3 C

O

O

O

O

H

O

HH

N

NPhH2 C

O

O

O

O

H

HH

O

O

O

O

O

H

HH

O

O

O

O

H

O

HH

NO O

ONH

OCH3

CH3

NH2

2R = CH33R = C2H54R = CH3C6H4 CO2 H-p5R = C(O)(CH2)2COONa

67

8

9

10 11

Artemisinin (1)

12

16

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 765

Fig (1) contd

O

O

O

O

H

HH

S

O

HN

O

O

H

HH

O

O

O

O

H

O

R

O

O

O

O

H

H

R

O

O

O

O

H

H

R

O

O

O

O

H

H

X

F3C CF3

O

O O

O

O

O

O

H

HR

O O

O

H

HN

NH

N

Cl

CH3

13

14 15a R = F15b R = COOH

19 R=Alkyl

20 R= halogen OMe Alkyl

21 X= O NMe

Arteflene (17)22 R= Alkyl arylalkyl OH

18

O

O

O

O

H

H

N N

CF3 O

O

O

O

H

H

S

O

O

O

O

H

HH H

O O

O

O

O

O

H

HH

O P

OCH3

O

O

O

O

O

O

H

HH O

O

O

O

H

H

H

COOH

O

O

O

O

H

HH

23 TDR 4029224

25 26

Fig (1) Structures of artemisinin derivatives and other semi synthetic peroxides

centered free radicals that alkylate protein and damage themicroorganelles and membranes of the parasites [18] Mostimportantly there is no evidence of drug resistance to anymember of the artemisinin family of drugs [19]

(6) have been reported from Hong Kong University ofScience and Technology [20] This series of compoundsshowed IC50 values in the range of 025-250 ngml againstchloroquine-resistant P falciparum strain W2 Otherderivatives of artemisinin as specified by 10β-dihydroartemisinyl benzoate (7) have IC50 values in therange of 007-119 ngml (7) being most active against bothW2 and chloroquine sensitive strain D6 [21]

Several derivatives of artemisinin (1) were disclosedbetween 2000 and 2006 as world patents many of these areacetyl or non-acetyl C-10 derivatives Novel C-10 ethersubstituted artemisinin derivatives depicted as in compound

766 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Additional substituted phenyl esters of dihydro-artemisinin such as compound (8) exhibited significant invitro and in vivo antimalarial activity [22] Compound (8)showed better in vivo activity than artemetherdihydroartemisinin and sodium artesunate with a halfmaximal effective dose (ED) value of 212 mgkg in a Pberghei mouse model [23] The addition of N-phenylpiperazine ring system such as in compound (9) [24]also enhances in vitro antimalarial activity [25]

heme andor parasite proteins Successful analogue arteflene(17) is a highly active synthetic endoperoxide andchemically more stable than artemisinin It is effective fortreating mild cases of P falciparum malaria [4344]Arteflene (17) production was discontinued by Hoffman La-Roche since its long and tedious synthesis was not suitablefor industrial production

The antimalarial activity of a synthetic analogTrioxaquine DU1301 (18) has been tested in vitro onchloroquine-sensitive and resistant strains [4546]Trioxaquine DU1301 (18) is also efficient on infected miceby oral route at 12-15 mgkg

C-10 non-acetyl artemisinin derivatives with varyingdegree of in vitro antimalarial activity were synthesized [26]The compound 10-(1-pentanoyl) anhydroartemisinin (10)was the most active compound with an IC50 value of 43nM against P falciparum strain NF54 Compound (11) 10-(phenyl) dihydroartemisinin has been claimed to have IC50value of 031 and 073 ngml against P falciparum strainW2 and D6 [27] Compounds from series (11) are moreactive in vitro and may be more stable [27]

The synthesis of metabolically more stable C-10 carbaanalogues (19) and C-10 aryl analogues (20) of DHA hasbeen the focus of chemists for the past 10 years [47] C-10aryl or heteroaromatic derivatives (20) and (21) were alsosynthesized by the groups of Haynes [48] and Posner [47]C-14 modified analogs (22) were prepared by total synthesisand semi-synthetic routes by Avery and Jung groups [49-51]

Several groups have investigated the effect of C-9substitutions A series of C-9 derivatives represented bycompound (12) [28] exhibited IC50 values of 76-216ngml against different strains of P falciparum [29] Thespecific compound (13) containing a non-acetyl C-10substituent is claimed to have an in vitro IC50 value of 27ngml against W2 strain [30] A series of 11-aza-artemisininscontaining a nitryl group instead of oxygen at position 11have been claimed [31] The compound (14) has an IC50value of 173 and 260 ngml against the W2 and D6 strainsrespectively

Antimalarial evaluation of a C-10 carba analog (23)TDR 40292 demonstrated that it was superior to artemetherand artesunate both in vitro and in vivo [52] It cannot formDHA as a metabolite and contains a side chain that can beformulated as a water-soluble salt Apart from monomericsemi synthetic derivatives C-10 non-acetal carba dimers [53]and representative lead compounds (24) (25) and (26) havebeen synthesized [54-56] The isobutyric acid dimeric analog(26) was found to be safe and displayed a therapeutic indexsix times that of sodium artesunate (5)Some simplified trioxanes with potent antimalarial

activity have also been reported [3233] Water-soluble 3-aryltrioxanes such as specified by compounds (15a) and(15b) demonstrated better in vivo efficacies [3435]Compound (15b) was shown to be more efficacious andhave lower toxicity than artelinic acid (4) [36]

QUINOLINE ANTIMALARIALS

Quinolines have been the most important class ofantimalarial drugs Quinoline-containing antimalarial drugssuch as chloroquine (27) quinine (28) and mefloquine (29)are used widely to treat malaria however the malarialparasite is rapidly becoming resistant to the currentlyavailable drugs in Fig (2)

As artemisinin is a complex molecule and uneconomic tosynthesize efforts have been made to prepare simplerantimalarial molecules based on the trioxane ring system ofartemisinin that contains the endoperoxide group The mostpromising of these compounds known to date is OZ 277(16) which consists of a trioxane ring attached to anadamantane ring and provides stability to the endoperoxidegroup without diminishing its antimalarial action Themolecule also includes a tosylate side chain that confersimproved water solubility and formulation properties[3738] Compound (16) called malperox now under Phase Iclinical trials was initially developed with the support ofthe WHO Tropical diseases research programme and later bythe Medicines for Malaria Venture [39]

Quinine (28) isolated from the bark of Peruviancinchona trees was the mainstay of malaria chemotherapyuntil the Second World War when the new syntheticmolecules were developed to circumvent the problem of drugresistance [57] and 4-substituted amino-quinolineschloroquine (27) and amodiaquine (30) were developed Inthe year 1946 the synthetic quinoline compoundchloroquine (27) was introduced by Loeb et al and provedinvaluable in the fight against the disease [58]

The success of chloroquine led to the hope that malariamight be completely eradicated from the worldAmodiaquine (30) a chloroquine analog containing a 4-hydroxyanilino function in its 4-amino side chain iseffective against many chloroquine resistant strains [5960]and the drug remains useful as a treatment of malaria inmany parts of Africa [61] Chloroquine (27) andamodiaquine (30) are extensively metabolized bydealkylation of their Mannich base side chains [57] Stockset al synthesized a series of short chain chloroquinederivatives (31) and (32) in which the diethyl-amino sidechains were replaced by functional groups more resistant to

The combination of artemisinin derivatives with a seconddrug having a different mode of action is a good way toincrease the efficacy of treatment and to prevent theemergence and spread of drug resistance [4041] Somesynthetic artemisinin derived trioxane dimers have also beendeveloped with high stability and efficacy [42]

New antimalarial endoperoxides were synthesized bycombining two complementary active moieties a 4-aminoquinoline (as in chloroquine) known to easilypenetrate within infected red blood cells and a trioxane (as inartemisinin) which is a potential alkylating agent toward

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 767

NCl

HN

N

N

NH

OHO

N

CF3

CF3

HN

OH

H

NCl

HNN

OH

NCl

HN

N

NCl

HN NH

OH

NCl

HN

N

NCl

HN

OH

NH

NCl

HN

HN

F

NCl

HNN

F

NCl

HNN

FOH

NCl

HN OH

N

N

O

HNNH2

N

O

HNNH

OO

N

O

HNNH2

OCH3

O CH3

CF3

Chloropine 27 Quinine 28 Mefloquine 29Amodiaquine 30

31 32 33 34

35 36 37 Isoquine 3 8

3940

Tafenoquine 41

Fig (2) Quinoline derivatives

dealkylation [6263] Addition of piperidyl and pyrrolidylrings to the 4-amino side chain of chloroquine such as incompounds (31) and (32) resulted in an enhanced activityagainst chloroquine resistant P falciparum

a fluorine atom [64] In these molecules the Mannich sidechain has been replaced with substituent less vulnerable todealkylation and have equivalent in vitro and in vivoantimalarial activity comparable to amodiaquine Theirfluorine substituent makes them potentially much moreexpensive to synthesize

When in amodiaquine (30) 4-hydroxyanilino side chainwas modified through replacement of the 5acute aminosubstituent with alkyl groups and a tert-butyl aminosubstituent was added at the 3acute position as in compounds(33) and (34) they were also found to be more active in vivoagainst P berghei than amodiaquine (30) [60] A patentdescribes the compounds (35) (36) and (37) in which thehydroxyl group at the 4acute position of the ring is replaced with

Another patent claimed a series of analogues which arepotentially cheap to produce and effective againstchloroquine resistant parasites [65] The lead compound inthis series Isoquine (38) is more effective than amodiaquine(30) against chloroquine resistant malaria parasites in vitroand against P berghei in vivo

768 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Two other classes of quinolines currently in clinical useare the 8-aminoquinolines (primaquine 39) [66] andquinoline methanols (mefloquine 29) [67] Mefloquine (29)has been effective against chloroquine-resistant strains ofmalaria parasite [6869] while Primaquine (39) is activeonly against the liver stages of malaria parasites Bulaquine(40) an analog of primaquine is claimed to havegametocidal activity [70] Other 8-aminoquinolines currently

in development are WR-238605 (Tafenoquine) (41) and 5-phenoxy primaquine analogs These were synthesized toimprove the poor therapeutic index of primaquine (39) andits relatively short half-life Tafenoquine (41) is more activeand less toxic than primaquine (39) [7172] Tafenoquine(37) based on its promising broad-spectrum of activities hasbeen selected as a potential antirelapse antimalarial drug toreplace primaquine (39) Moreira et al synthesized

N

H3CO

NH

N NH

OR1

R3R2

F3C

N[(CH2)3CH3]2

OH

Cl Cl

HN NH

ClCl

HN N

N N

N

Cl

N

Cl

N

Cl

N

ClN

N CN

N

Cl

X

O

C

N

O

NN

N

Cl

N

Cl

N N

HN Me

Et2N

HN Me

NEt2

S

N

N C9H19

OOH

N

N OMe

Cl

NH

HO

N

N O

HN

O

HN R1

O

CH3

SN

R1

Z NS

R2 R1R2

42

Halofantrine 43WR 268668 44

45 46

n

47

48

x49

50

51 R= Ethylbenzene52 R= 4-propoxy-1-vinylbenzene

2I- ++

53

Fig (3) Quinoline antimalarials and related compounds

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 763

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

US056844350US02193387

Febrifugineisofebrifugine and

method for producing thesame

N

N

O

O HN

OH

N

N

OO

NHHO

Kobayashi SWataya Y Kim H-S

2005-01-18

2002-12-19

WO04000319 Antimalarial activities offebrifugine analogues

N

N

O

O HN

OH Jiang S Hudson HTMilhous KV

2003-12-31

US04053950 Antimalarial activitiesand therapeutic

properties of febrifugineanalogues

N

N

O

O HN

OH Jiang S Hudson HT 2004-03-18

US056906098US04019200

Mixed steroidal 1 2 45-tetraoxane compoundsand methods of making

and using thereof

O

OO

O

CH3OR

XOH

OR

CH3

H

R1R4

R3

R2

CH3

n

Solaja BOpsenica D MPocsfalvi G et al

2005-06-14

2004-01-29

US026426417 Processes andintermediates useful to

make Antifolates N

N NY

R7

R6HN

R3

R4

O Jackson BCEugene DS et al

2002-07-30

US03203908WO0153276

Dihydrofolate reductaseinhibitors

N

N

N

H2N

NH2

R1

R2

X

Y

N

NH

NH

HN

NH2

R1

R2

X

Y

Gordon L 2001-07-26

WO0170237US03144249

Use oforganophosphorous

compounds forproducing a medicament

for treating infections

O

P

R4

R3B

Jomaa H 2001-09-27

764 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

EP1237579WO0139757

Surfactants as MalarialChloroquine resistance

reversal agentsC9H19

(OCH2 CH2)XOH Kain KC Crandall ICharuk J Reithmeier

R

2002-09-11

US05240034 Artemisinin-basedperoxide compounds asbroad spectrum anti-

infective agents

O

O

O

O

H

H

X

H(A)o (B)e

R1

R2 R4

R3

(G)f

R5

R7

R6n

O

O

O

O

H

HH

X

(Y)q

R16

R1 5R1 4

Mitchell AAKannoth MM

2005-10-27

O

O

O

O

H

H

O

O

O

O

O

H

H

OH

DHA

O

O

O

H

H

OR

O

O

O

O

O

H

O

HHO

O

O

O

H

O

HH

O

O

O

O

O

H

O

HH

F3 C

O

O

O

O

H

O

HH

N

NPhH2 C

O

O

O

O

H

HH

O

O

O

O

O

H

HH

O

O

O

O

H

O

HH

NO O

ONH

OCH3

CH3

NH2

2R = CH33R = C2H54R = CH3C6H4 CO2 H-p5R = C(O)(CH2)2COONa

67

8

9

10 11

Artemisinin (1)

12

16

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 765

Fig (1) contd

O

O

O

O

H

HH

S

O

HN

O

O

H

HH

O

O

O

O

H

O

R

O

O

O

O

H

H

R

O

O

O

O

H

H

R

O

O

O

O

H

H

X

F3C CF3

O

O O

O

O

O

O

H

HR

O O

O

H

HN

NH

N

Cl

CH3

13

14 15a R = F15b R = COOH

19 R=Alkyl

20 R= halogen OMe Alkyl

21 X= O NMe

Arteflene (17)22 R= Alkyl arylalkyl OH

18

O

O

O

O

H

H

N N

CF3 O

O

O

O

H

H

S

O

O

O

O

H

HH H

O O

O

O

O

O

H

HH

O P

OCH3

O

O

O

O

O

O

H

HH O

O

O

O

H

H

H

COOH

O

O

O

O

H

HH

23 TDR 4029224

25 26

Fig (1) Structures of artemisinin derivatives and other semi synthetic peroxides

centered free radicals that alkylate protein and damage themicroorganelles and membranes of the parasites [18] Mostimportantly there is no evidence of drug resistance to anymember of the artemisinin family of drugs [19]

(6) have been reported from Hong Kong University ofScience and Technology [20] This series of compoundsshowed IC50 values in the range of 025-250 ngml againstchloroquine-resistant P falciparum strain W2 Otherderivatives of artemisinin as specified by 10β-dihydroartemisinyl benzoate (7) have IC50 values in therange of 007-119 ngml (7) being most active against bothW2 and chloroquine sensitive strain D6 [21]

Several derivatives of artemisinin (1) were disclosedbetween 2000 and 2006 as world patents many of these areacetyl or non-acetyl C-10 derivatives Novel C-10 ethersubstituted artemisinin derivatives depicted as in compound

766 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Additional substituted phenyl esters of dihydro-artemisinin such as compound (8) exhibited significant invitro and in vivo antimalarial activity [22] Compound (8)showed better in vivo activity than artemetherdihydroartemisinin and sodium artesunate with a halfmaximal effective dose (ED) value of 212 mgkg in a Pberghei mouse model [23] The addition of N-phenylpiperazine ring system such as in compound (9) [24]also enhances in vitro antimalarial activity [25]

heme andor parasite proteins Successful analogue arteflene(17) is a highly active synthetic endoperoxide andchemically more stable than artemisinin It is effective fortreating mild cases of P falciparum malaria [4344]Arteflene (17) production was discontinued by Hoffman La-Roche since its long and tedious synthesis was not suitablefor industrial production

The antimalarial activity of a synthetic analogTrioxaquine DU1301 (18) has been tested in vitro onchloroquine-sensitive and resistant strains [4546]Trioxaquine DU1301 (18) is also efficient on infected miceby oral route at 12-15 mgkg

C-10 non-acetyl artemisinin derivatives with varyingdegree of in vitro antimalarial activity were synthesized [26]The compound 10-(1-pentanoyl) anhydroartemisinin (10)was the most active compound with an IC50 value of 43nM against P falciparum strain NF54 Compound (11) 10-(phenyl) dihydroartemisinin has been claimed to have IC50value of 031 and 073 ngml against P falciparum strainW2 and D6 [27] Compounds from series (11) are moreactive in vitro and may be more stable [27]

The synthesis of metabolically more stable C-10 carbaanalogues (19) and C-10 aryl analogues (20) of DHA hasbeen the focus of chemists for the past 10 years [47] C-10aryl or heteroaromatic derivatives (20) and (21) were alsosynthesized by the groups of Haynes [48] and Posner [47]C-14 modified analogs (22) were prepared by total synthesisand semi-synthetic routes by Avery and Jung groups [49-51]

Several groups have investigated the effect of C-9substitutions A series of C-9 derivatives represented bycompound (12) [28] exhibited IC50 values of 76-216ngml against different strains of P falciparum [29] Thespecific compound (13) containing a non-acetyl C-10substituent is claimed to have an in vitro IC50 value of 27ngml against W2 strain [30] A series of 11-aza-artemisininscontaining a nitryl group instead of oxygen at position 11have been claimed [31] The compound (14) has an IC50value of 173 and 260 ngml against the W2 and D6 strainsrespectively

Antimalarial evaluation of a C-10 carba analog (23)TDR 40292 demonstrated that it was superior to artemetherand artesunate both in vitro and in vivo [52] It cannot formDHA as a metabolite and contains a side chain that can beformulated as a water-soluble salt Apart from monomericsemi synthetic derivatives C-10 non-acetal carba dimers [53]and representative lead compounds (24) (25) and (26) havebeen synthesized [54-56] The isobutyric acid dimeric analog(26) was found to be safe and displayed a therapeutic indexsix times that of sodium artesunate (5)Some simplified trioxanes with potent antimalarial

activity have also been reported [3233] Water-soluble 3-aryltrioxanes such as specified by compounds (15a) and(15b) demonstrated better in vivo efficacies [3435]Compound (15b) was shown to be more efficacious andhave lower toxicity than artelinic acid (4) [36]

QUINOLINE ANTIMALARIALS

Quinolines have been the most important class ofantimalarial drugs Quinoline-containing antimalarial drugssuch as chloroquine (27) quinine (28) and mefloquine (29)are used widely to treat malaria however the malarialparasite is rapidly becoming resistant to the currentlyavailable drugs in Fig (2)

As artemisinin is a complex molecule and uneconomic tosynthesize efforts have been made to prepare simplerantimalarial molecules based on the trioxane ring system ofartemisinin that contains the endoperoxide group The mostpromising of these compounds known to date is OZ 277(16) which consists of a trioxane ring attached to anadamantane ring and provides stability to the endoperoxidegroup without diminishing its antimalarial action Themolecule also includes a tosylate side chain that confersimproved water solubility and formulation properties[3738] Compound (16) called malperox now under Phase Iclinical trials was initially developed with the support ofthe WHO Tropical diseases research programme and later bythe Medicines for Malaria Venture [39]

Quinine (28) isolated from the bark of Peruviancinchona trees was the mainstay of malaria chemotherapyuntil the Second World War when the new syntheticmolecules were developed to circumvent the problem of drugresistance [57] and 4-substituted amino-quinolineschloroquine (27) and amodiaquine (30) were developed Inthe year 1946 the synthetic quinoline compoundchloroquine (27) was introduced by Loeb et al and provedinvaluable in the fight against the disease [58]

The success of chloroquine led to the hope that malariamight be completely eradicated from the worldAmodiaquine (30) a chloroquine analog containing a 4-hydroxyanilino function in its 4-amino side chain iseffective against many chloroquine resistant strains [5960]and the drug remains useful as a treatment of malaria inmany parts of Africa [61] Chloroquine (27) andamodiaquine (30) are extensively metabolized bydealkylation of their Mannich base side chains [57] Stockset al synthesized a series of short chain chloroquinederivatives (31) and (32) in which the diethyl-amino sidechains were replaced by functional groups more resistant to

The combination of artemisinin derivatives with a seconddrug having a different mode of action is a good way toincrease the efficacy of treatment and to prevent theemergence and spread of drug resistance [4041] Somesynthetic artemisinin derived trioxane dimers have also beendeveloped with high stability and efficacy [42]

New antimalarial endoperoxides were synthesized bycombining two complementary active moieties a 4-aminoquinoline (as in chloroquine) known to easilypenetrate within infected red blood cells and a trioxane (as inartemisinin) which is a potential alkylating agent toward

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 767

NCl

HN

N

N

NH

OHO

N

CF3

CF3

HN

OH

H

NCl

HNN

OH

NCl

HN

N

NCl

HN NH

OH

NCl

HN

N

NCl

HN

OH

NH

NCl

HN

HN

F

NCl

HNN

F

NCl

HNN

FOH

NCl

HN OH

N

N

O

HNNH2

N

O

HNNH

OO

N

O

HNNH2

OCH3

O CH3

CF3

Chloropine 27 Quinine 28 Mefloquine 29Amodiaquine 30

31 32 33 34

35 36 37 Isoquine 3 8

3940

Tafenoquine 41

Fig (2) Quinoline derivatives

dealkylation [6263] Addition of piperidyl and pyrrolidylrings to the 4-amino side chain of chloroquine such as incompounds (31) and (32) resulted in an enhanced activityagainst chloroquine resistant P falciparum

a fluorine atom [64] In these molecules the Mannich sidechain has been replaced with substituent less vulnerable todealkylation and have equivalent in vitro and in vivoantimalarial activity comparable to amodiaquine Theirfluorine substituent makes them potentially much moreexpensive to synthesize

When in amodiaquine (30) 4-hydroxyanilino side chainwas modified through replacement of the 5acute aminosubstituent with alkyl groups and a tert-butyl aminosubstituent was added at the 3acute position as in compounds(33) and (34) they were also found to be more active in vivoagainst P berghei than amodiaquine (30) [60] A patentdescribes the compounds (35) (36) and (37) in which thehydroxyl group at the 4acute position of the ring is replaced with

Another patent claimed a series of analogues which arepotentially cheap to produce and effective againstchloroquine resistant parasites [65] The lead compound inthis series Isoquine (38) is more effective than amodiaquine(30) against chloroquine resistant malaria parasites in vitroand against P berghei in vivo

768 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Two other classes of quinolines currently in clinical useare the 8-aminoquinolines (primaquine 39) [66] andquinoline methanols (mefloquine 29) [67] Mefloquine (29)has been effective against chloroquine-resistant strains ofmalaria parasite [6869] while Primaquine (39) is activeonly against the liver stages of malaria parasites Bulaquine(40) an analog of primaquine is claimed to havegametocidal activity [70] Other 8-aminoquinolines currently

in development are WR-238605 (Tafenoquine) (41) and 5-phenoxy primaquine analogs These were synthesized toimprove the poor therapeutic index of primaquine (39) andits relatively short half-life Tafenoquine (41) is more activeand less toxic than primaquine (39) [7172] Tafenoquine(37) based on its promising broad-spectrum of activities hasbeen selected as a potential antirelapse antimalarial drug toreplace primaquine (39) Moreira et al synthesized

N

H3CO

NH

N NH

OR1

R3R2

F3C

N[(CH2)3CH3]2

OH

Cl Cl

HN NH

ClCl

HN N

N N

N

Cl

N

Cl

N

Cl

N

ClN

N CN

N

Cl

X

O

C

N

O

NN

N

Cl

N

Cl

N N

HN Me

Et2N

HN Me

NEt2

S

N

N C9H19

OOH

N

N OMe

Cl

NH

HO

N

N O

HN

O

HN R1

O

CH3

SN

R1

Z NS

R2 R1R2

42

Halofantrine 43WR 268668 44

45 46

n

47

48

x49

50

51 R= Ethylbenzene52 R= 4-propoxy-1-vinylbenzene

2I- ++

53

Fig (3) Quinoline antimalarials and related compounds

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

764 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

(Table 1) contd

Patent Nos Title Structure Authors Pub Date

EP1237579WO0139757

Surfactants as MalarialChloroquine resistance

reversal agentsC9H19

(OCH2 CH2)XOH Kain KC Crandall ICharuk J Reithmeier

R

2002-09-11

US05240034 Artemisinin-basedperoxide compounds asbroad spectrum anti-

infective agents

O

O

O

O

H

H

X

H(A)o (B)e

R1

R2 R4

R3

(G)f

R5

R7

R6n

O

O

O

O

H

HH

X

(Y)q

R16

R1 5R1 4

Mitchell AAKannoth MM

2005-10-27

O

O

O

O

H

H

O

O

O

O

O

H

H

OH

DHA

O

O

O

H

H

OR

O

O

O

O

O

H

O

HHO

O

O

O

H

O

HH

O

O

O

O

O

H

O

HH

F3 C

O

O

O

O

H

O

HH

N

NPhH2 C

O

O

O

O

H

HH

O

O

O

O

O

H

HH

O

O

O

O

H

O

HH

NO O

ONH

OCH3

CH3

NH2

2R = CH33R = C2H54R = CH3C6H4 CO2 H-p5R = C(O)(CH2)2COONa

67

8

9

10 11

Artemisinin (1)

12

16

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 765

Fig (1) contd

O

O

O

O

H

HH

S

O

HN

O

O

H

HH

O

O

O

O

H

O

R

O

O

O

O

H

H

R

O

O

O

O

H

H

R

O

O

O

O

H

H

X

F3C CF3

O

O O

O

O

O

O

H

HR

O O

O

H

HN

NH

N

Cl

CH3

13

14 15a R = F15b R = COOH

19 R=Alkyl

20 R= halogen OMe Alkyl

21 X= O NMe

Arteflene (17)22 R= Alkyl arylalkyl OH

18

O

O

O

O

H

H

N N

CF3 O

O

O

O

H

H

S

O

O

O

O

H

HH H

O O

O

O

O

O

H

HH

O P

OCH3

O

O

O

O

O

O

H

HH O

O

O

O

H

H

H

COOH

O

O

O

O

H

HH

23 TDR 4029224

25 26

Fig (1) Structures of artemisinin derivatives and other semi synthetic peroxides

centered free radicals that alkylate protein and damage themicroorganelles and membranes of the parasites [18] Mostimportantly there is no evidence of drug resistance to anymember of the artemisinin family of drugs [19]

(6) have been reported from Hong Kong University ofScience and Technology [20] This series of compoundsshowed IC50 values in the range of 025-250 ngml againstchloroquine-resistant P falciparum strain W2 Otherderivatives of artemisinin as specified by 10β-dihydroartemisinyl benzoate (7) have IC50 values in therange of 007-119 ngml (7) being most active against bothW2 and chloroquine sensitive strain D6 [21]

Several derivatives of artemisinin (1) were disclosedbetween 2000 and 2006 as world patents many of these areacetyl or non-acetyl C-10 derivatives Novel C-10 ethersubstituted artemisinin derivatives depicted as in compound

766 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Additional substituted phenyl esters of dihydro-artemisinin such as compound (8) exhibited significant invitro and in vivo antimalarial activity [22] Compound (8)showed better in vivo activity than artemetherdihydroartemisinin and sodium artesunate with a halfmaximal effective dose (ED) value of 212 mgkg in a Pberghei mouse model [23] The addition of N-phenylpiperazine ring system such as in compound (9) [24]also enhances in vitro antimalarial activity [25]

heme andor parasite proteins Successful analogue arteflene(17) is a highly active synthetic endoperoxide andchemically more stable than artemisinin It is effective fortreating mild cases of P falciparum malaria [4344]Arteflene (17) production was discontinued by Hoffman La-Roche since its long and tedious synthesis was not suitablefor industrial production

The antimalarial activity of a synthetic analogTrioxaquine DU1301 (18) has been tested in vitro onchloroquine-sensitive and resistant strains [4546]Trioxaquine DU1301 (18) is also efficient on infected miceby oral route at 12-15 mgkg

C-10 non-acetyl artemisinin derivatives with varyingdegree of in vitro antimalarial activity were synthesized [26]The compound 10-(1-pentanoyl) anhydroartemisinin (10)was the most active compound with an IC50 value of 43nM against P falciparum strain NF54 Compound (11) 10-(phenyl) dihydroartemisinin has been claimed to have IC50value of 031 and 073 ngml against P falciparum strainW2 and D6 [27] Compounds from series (11) are moreactive in vitro and may be more stable [27]

The synthesis of metabolically more stable C-10 carbaanalogues (19) and C-10 aryl analogues (20) of DHA hasbeen the focus of chemists for the past 10 years [47] C-10aryl or heteroaromatic derivatives (20) and (21) were alsosynthesized by the groups of Haynes [48] and Posner [47]C-14 modified analogs (22) were prepared by total synthesisand semi-synthetic routes by Avery and Jung groups [49-51]

Several groups have investigated the effect of C-9substitutions A series of C-9 derivatives represented bycompound (12) [28] exhibited IC50 values of 76-216ngml against different strains of P falciparum [29] Thespecific compound (13) containing a non-acetyl C-10substituent is claimed to have an in vitro IC50 value of 27ngml against W2 strain [30] A series of 11-aza-artemisininscontaining a nitryl group instead of oxygen at position 11have been claimed [31] The compound (14) has an IC50value of 173 and 260 ngml against the W2 and D6 strainsrespectively

Antimalarial evaluation of a C-10 carba analog (23)TDR 40292 demonstrated that it was superior to artemetherand artesunate both in vitro and in vivo [52] It cannot formDHA as a metabolite and contains a side chain that can beformulated as a water-soluble salt Apart from monomericsemi synthetic derivatives C-10 non-acetal carba dimers [53]and representative lead compounds (24) (25) and (26) havebeen synthesized [54-56] The isobutyric acid dimeric analog(26) was found to be safe and displayed a therapeutic indexsix times that of sodium artesunate (5)Some simplified trioxanes with potent antimalarial

activity have also been reported [3233] Water-soluble 3-aryltrioxanes such as specified by compounds (15a) and(15b) demonstrated better in vivo efficacies [3435]Compound (15b) was shown to be more efficacious andhave lower toxicity than artelinic acid (4) [36]

QUINOLINE ANTIMALARIALS

Quinolines have been the most important class ofantimalarial drugs Quinoline-containing antimalarial drugssuch as chloroquine (27) quinine (28) and mefloquine (29)are used widely to treat malaria however the malarialparasite is rapidly becoming resistant to the currentlyavailable drugs in Fig (2)

As artemisinin is a complex molecule and uneconomic tosynthesize efforts have been made to prepare simplerantimalarial molecules based on the trioxane ring system ofartemisinin that contains the endoperoxide group The mostpromising of these compounds known to date is OZ 277(16) which consists of a trioxane ring attached to anadamantane ring and provides stability to the endoperoxidegroup without diminishing its antimalarial action Themolecule also includes a tosylate side chain that confersimproved water solubility and formulation properties[3738] Compound (16) called malperox now under Phase Iclinical trials was initially developed with the support ofthe WHO Tropical diseases research programme and later bythe Medicines for Malaria Venture [39]

Quinine (28) isolated from the bark of Peruviancinchona trees was the mainstay of malaria chemotherapyuntil the Second World War when the new syntheticmolecules were developed to circumvent the problem of drugresistance [57] and 4-substituted amino-quinolineschloroquine (27) and amodiaquine (30) were developed Inthe year 1946 the synthetic quinoline compoundchloroquine (27) was introduced by Loeb et al and provedinvaluable in the fight against the disease [58]

The success of chloroquine led to the hope that malariamight be completely eradicated from the worldAmodiaquine (30) a chloroquine analog containing a 4-hydroxyanilino function in its 4-amino side chain iseffective against many chloroquine resistant strains [5960]and the drug remains useful as a treatment of malaria inmany parts of Africa [61] Chloroquine (27) andamodiaquine (30) are extensively metabolized bydealkylation of their Mannich base side chains [57] Stockset al synthesized a series of short chain chloroquinederivatives (31) and (32) in which the diethyl-amino sidechains were replaced by functional groups more resistant to

The combination of artemisinin derivatives with a seconddrug having a different mode of action is a good way toincrease the efficacy of treatment and to prevent theemergence and spread of drug resistance [4041] Somesynthetic artemisinin derived trioxane dimers have also beendeveloped with high stability and efficacy [42]

New antimalarial endoperoxides were synthesized bycombining two complementary active moieties a 4-aminoquinoline (as in chloroquine) known to easilypenetrate within infected red blood cells and a trioxane (as inartemisinin) which is a potential alkylating agent toward

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 767

NCl

HN

N

N

NH

OHO

N

CF3

CF3

HN

OH

H

NCl

HNN

OH

NCl

HN

N

NCl

HN NH

OH

NCl

HN

N

NCl

HN

OH

NH

NCl

HN

HN

F

NCl

HNN

F

NCl

HNN

FOH

NCl

HN OH

N

N

O

HNNH2

N

O

HNNH

OO

N

O

HNNH2

OCH3

O CH3

CF3

Chloropine 27 Quinine 28 Mefloquine 29Amodiaquine 30

31 32 33 34

35 36 37 Isoquine 3 8

3940

Tafenoquine 41

Fig (2) Quinoline derivatives

dealkylation [6263] Addition of piperidyl and pyrrolidylrings to the 4-amino side chain of chloroquine such as incompounds (31) and (32) resulted in an enhanced activityagainst chloroquine resistant P falciparum

a fluorine atom [64] In these molecules the Mannich sidechain has been replaced with substituent less vulnerable todealkylation and have equivalent in vitro and in vivoantimalarial activity comparable to amodiaquine Theirfluorine substituent makes them potentially much moreexpensive to synthesize

When in amodiaquine (30) 4-hydroxyanilino side chainwas modified through replacement of the 5acute aminosubstituent with alkyl groups and a tert-butyl aminosubstituent was added at the 3acute position as in compounds(33) and (34) they were also found to be more active in vivoagainst P berghei than amodiaquine (30) [60] A patentdescribes the compounds (35) (36) and (37) in which thehydroxyl group at the 4acute position of the ring is replaced with

Another patent claimed a series of analogues which arepotentially cheap to produce and effective againstchloroquine resistant parasites [65] The lead compound inthis series Isoquine (38) is more effective than amodiaquine(30) against chloroquine resistant malaria parasites in vitroand against P berghei in vivo

768 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Two other classes of quinolines currently in clinical useare the 8-aminoquinolines (primaquine 39) [66] andquinoline methanols (mefloquine 29) [67] Mefloquine (29)has been effective against chloroquine-resistant strains ofmalaria parasite [6869] while Primaquine (39) is activeonly against the liver stages of malaria parasites Bulaquine(40) an analog of primaquine is claimed to havegametocidal activity [70] Other 8-aminoquinolines currently

in development are WR-238605 (Tafenoquine) (41) and 5-phenoxy primaquine analogs These were synthesized toimprove the poor therapeutic index of primaquine (39) andits relatively short half-life Tafenoquine (41) is more activeand less toxic than primaquine (39) [7172] Tafenoquine(37) based on its promising broad-spectrum of activities hasbeen selected as a potential antirelapse antimalarial drug toreplace primaquine (39) Moreira et al synthesized

N

H3CO

NH

N NH

OR1

R3R2

F3C

N[(CH2)3CH3]2

OH

Cl Cl

HN NH

ClCl

HN N

N N

N

Cl

N

Cl

N

Cl

N

ClN

N CN

N

Cl

X

O

C

N

O

NN

N

Cl

N

Cl

N N

HN Me

Et2N

HN Me

NEt2

S

N

N C9H19

OOH

N

N OMe

Cl

NH

HO

N

N O

HN

O

HN R1

O

CH3

SN

R1

Z NS

R2 R1R2

42

Halofantrine 43WR 268668 44

45 46

n

47

48

x49

50

51 R= Ethylbenzene52 R= 4-propoxy-1-vinylbenzene

2I- ++

53

Fig (3) Quinoline antimalarials and related compounds

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 765

Fig (1) contd

O

O

O

O

H

HH

S

O

HN

O

O

H

HH

O

O

O

O

H

O

R

O

O

O

O

H

H

R

O

O

O

O

H

H

R

O

O

O

O

H

H

X

F3C CF3

O

O O

O

O

O

O

H

HR

O O

O

H

HN

NH

N

Cl

CH3

13

14 15a R = F15b R = COOH

19 R=Alkyl

20 R= halogen OMe Alkyl

21 X= O NMe

Arteflene (17)22 R= Alkyl arylalkyl OH

18

O

O

O

O

H

H

N N

CF3 O

O

O

O

H

H

S

O

O

O

O

H

HH H

O O

O

O

O

O

H

HH

O P

OCH3

O

O

O

O

O

O

H

HH O

O

O

O

H

H

H

COOH

O

O

O

O

H

HH

23 TDR 4029224

25 26

Fig (1) Structures of artemisinin derivatives and other semi synthetic peroxides

centered free radicals that alkylate protein and damage themicroorganelles and membranes of the parasites [18] Mostimportantly there is no evidence of drug resistance to anymember of the artemisinin family of drugs [19]

(6) have been reported from Hong Kong University ofScience and Technology [20] This series of compoundsshowed IC50 values in the range of 025-250 ngml againstchloroquine-resistant P falciparum strain W2 Otherderivatives of artemisinin as specified by 10β-dihydroartemisinyl benzoate (7) have IC50 values in therange of 007-119 ngml (7) being most active against bothW2 and chloroquine sensitive strain D6 [21]

Several derivatives of artemisinin (1) were disclosedbetween 2000 and 2006 as world patents many of these areacetyl or non-acetyl C-10 derivatives Novel C-10 ethersubstituted artemisinin derivatives depicted as in compound

766 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Additional substituted phenyl esters of dihydro-artemisinin such as compound (8) exhibited significant invitro and in vivo antimalarial activity [22] Compound (8)showed better in vivo activity than artemetherdihydroartemisinin and sodium artesunate with a halfmaximal effective dose (ED) value of 212 mgkg in a Pberghei mouse model [23] The addition of N-phenylpiperazine ring system such as in compound (9) [24]also enhances in vitro antimalarial activity [25]

heme andor parasite proteins Successful analogue arteflene(17) is a highly active synthetic endoperoxide andchemically more stable than artemisinin It is effective fortreating mild cases of P falciparum malaria [4344]Arteflene (17) production was discontinued by Hoffman La-Roche since its long and tedious synthesis was not suitablefor industrial production

The antimalarial activity of a synthetic analogTrioxaquine DU1301 (18) has been tested in vitro onchloroquine-sensitive and resistant strains [4546]Trioxaquine DU1301 (18) is also efficient on infected miceby oral route at 12-15 mgkg

C-10 non-acetyl artemisinin derivatives with varyingdegree of in vitro antimalarial activity were synthesized [26]The compound 10-(1-pentanoyl) anhydroartemisinin (10)was the most active compound with an IC50 value of 43nM against P falciparum strain NF54 Compound (11) 10-(phenyl) dihydroartemisinin has been claimed to have IC50value of 031 and 073 ngml against P falciparum strainW2 and D6 [27] Compounds from series (11) are moreactive in vitro and may be more stable [27]

The synthesis of metabolically more stable C-10 carbaanalogues (19) and C-10 aryl analogues (20) of DHA hasbeen the focus of chemists for the past 10 years [47] C-10aryl or heteroaromatic derivatives (20) and (21) were alsosynthesized by the groups of Haynes [48] and Posner [47]C-14 modified analogs (22) were prepared by total synthesisand semi-synthetic routes by Avery and Jung groups [49-51]

Several groups have investigated the effect of C-9substitutions A series of C-9 derivatives represented bycompound (12) [28] exhibited IC50 values of 76-216ngml against different strains of P falciparum [29] Thespecific compound (13) containing a non-acetyl C-10substituent is claimed to have an in vitro IC50 value of 27ngml against W2 strain [30] A series of 11-aza-artemisininscontaining a nitryl group instead of oxygen at position 11have been claimed [31] The compound (14) has an IC50value of 173 and 260 ngml against the W2 and D6 strainsrespectively

Antimalarial evaluation of a C-10 carba analog (23)TDR 40292 demonstrated that it was superior to artemetherand artesunate both in vitro and in vivo [52] It cannot formDHA as a metabolite and contains a side chain that can beformulated as a water-soluble salt Apart from monomericsemi synthetic derivatives C-10 non-acetal carba dimers [53]and representative lead compounds (24) (25) and (26) havebeen synthesized [54-56] The isobutyric acid dimeric analog(26) was found to be safe and displayed a therapeutic indexsix times that of sodium artesunate (5)Some simplified trioxanes with potent antimalarial

activity have also been reported [3233] Water-soluble 3-aryltrioxanes such as specified by compounds (15a) and(15b) demonstrated better in vivo efficacies [3435]Compound (15b) was shown to be more efficacious andhave lower toxicity than artelinic acid (4) [36]

QUINOLINE ANTIMALARIALS

Quinolines have been the most important class ofantimalarial drugs Quinoline-containing antimalarial drugssuch as chloroquine (27) quinine (28) and mefloquine (29)are used widely to treat malaria however the malarialparasite is rapidly becoming resistant to the currentlyavailable drugs in Fig (2)

As artemisinin is a complex molecule and uneconomic tosynthesize efforts have been made to prepare simplerantimalarial molecules based on the trioxane ring system ofartemisinin that contains the endoperoxide group The mostpromising of these compounds known to date is OZ 277(16) which consists of a trioxane ring attached to anadamantane ring and provides stability to the endoperoxidegroup without diminishing its antimalarial action Themolecule also includes a tosylate side chain that confersimproved water solubility and formulation properties[3738] Compound (16) called malperox now under Phase Iclinical trials was initially developed with the support ofthe WHO Tropical diseases research programme and later bythe Medicines for Malaria Venture [39]

Quinine (28) isolated from the bark of Peruviancinchona trees was the mainstay of malaria chemotherapyuntil the Second World War when the new syntheticmolecules were developed to circumvent the problem of drugresistance [57] and 4-substituted amino-quinolineschloroquine (27) and amodiaquine (30) were developed Inthe year 1946 the synthetic quinoline compoundchloroquine (27) was introduced by Loeb et al and provedinvaluable in the fight against the disease [58]

The success of chloroquine led to the hope that malariamight be completely eradicated from the worldAmodiaquine (30) a chloroquine analog containing a 4-hydroxyanilino function in its 4-amino side chain iseffective against many chloroquine resistant strains [5960]and the drug remains useful as a treatment of malaria inmany parts of Africa [61] Chloroquine (27) andamodiaquine (30) are extensively metabolized bydealkylation of their Mannich base side chains [57] Stockset al synthesized a series of short chain chloroquinederivatives (31) and (32) in which the diethyl-amino sidechains were replaced by functional groups more resistant to

The combination of artemisinin derivatives with a seconddrug having a different mode of action is a good way toincrease the efficacy of treatment and to prevent theemergence and spread of drug resistance [4041] Somesynthetic artemisinin derived trioxane dimers have also beendeveloped with high stability and efficacy [42]

New antimalarial endoperoxides were synthesized bycombining two complementary active moieties a 4-aminoquinoline (as in chloroquine) known to easilypenetrate within infected red blood cells and a trioxane (as inartemisinin) which is a potential alkylating agent toward

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 767

NCl

HN

N

N

NH

OHO

N

CF3

CF3

HN

OH

H

NCl

HNN

OH

NCl

HN

N

NCl

HN NH

OH

NCl

HN

N

NCl

HN

OH

NH

NCl

HN

HN

F

NCl

HNN

F

NCl

HNN

FOH

NCl

HN OH

N

N

O

HNNH2

N

O

HNNH

OO

N

O

HNNH2

OCH3

O CH3

CF3

Chloropine 27 Quinine 28 Mefloquine 29Amodiaquine 30

31 32 33 34

35 36 37 Isoquine 3 8

3940

Tafenoquine 41

Fig (2) Quinoline derivatives

dealkylation [6263] Addition of piperidyl and pyrrolidylrings to the 4-amino side chain of chloroquine such as incompounds (31) and (32) resulted in an enhanced activityagainst chloroquine resistant P falciparum

a fluorine atom [64] In these molecules the Mannich sidechain has been replaced with substituent less vulnerable todealkylation and have equivalent in vitro and in vivoantimalarial activity comparable to amodiaquine Theirfluorine substituent makes them potentially much moreexpensive to synthesize

When in amodiaquine (30) 4-hydroxyanilino side chainwas modified through replacement of the 5acute aminosubstituent with alkyl groups and a tert-butyl aminosubstituent was added at the 3acute position as in compounds(33) and (34) they were also found to be more active in vivoagainst P berghei than amodiaquine (30) [60] A patentdescribes the compounds (35) (36) and (37) in which thehydroxyl group at the 4acute position of the ring is replaced with

Another patent claimed a series of analogues which arepotentially cheap to produce and effective againstchloroquine resistant parasites [65] The lead compound inthis series Isoquine (38) is more effective than amodiaquine(30) against chloroquine resistant malaria parasites in vitroand against P berghei in vivo

768 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Two other classes of quinolines currently in clinical useare the 8-aminoquinolines (primaquine 39) [66] andquinoline methanols (mefloquine 29) [67] Mefloquine (29)has been effective against chloroquine-resistant strains ofmalaria parasite [6869] while Primaquine (39) is activeonly against the liver stages of malaria parasites Bulaquine(40) an analog of primaquine is claimed to havegametocidal activity [70] Other 8-aminoquinolines currently

in development are WR-238605 (Tafenoquine) (41) and 5-phenoxy primaquine analogs These were synthesized toimprove the poor therapeutic index of primaquine (39) andits relatively short half-life Tafenoquine (41) is more activeand less toxic than primaquine (39) [7172] Tafenoquine(37) based on its promising broad-spectrum of activities hasbeen selected as a potential antirelapse antimalarial drug toreplace primaquine (39) Moreira et al synthesized

N

H3CO

NH

N NH

OR1

R3R2

F3C

N[(CH2)3CH3]2

OH

Cl Cl

HN NH

ClCl

HN N

N N

N

Cl

N

Cl

N

Cl

N

ClN

N CN

N

Cl

X

O

C

N

O

NN

N

Cl

N

Cl

N N

HN Me

Et2N

HN Me

NEt2

S

N

N C9H19

OOH

N

N OMe

Cl

NH

HO

N

N O

HN

O

HN R1

O

CH3

SN

R1

Z NS

R2 R1R2

42

Halofantrine 43WR 268668 44

45 46

n

47

48

x49

50

51 R= Ethylbenzene52 R= 4-propoxy-1-vinylbenzene

2I- ++

53

Fig (3) Quinoline antimalarials and related compounds

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

766 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Additional substituted phenyl esters of dihydro-artemisinin such as compound (8) exhibited significant invitro and in vivo antimalarial activity [22] Compound (8)showed better in vivo activity than artemetherdihydroartemisinin and sodium artesunate with a halfmaximal effective dose (ED) value of 212 mgkg in a Pberghei mouse model [23] The addition of N-phenylpiperazine ring system such as in compound (9) [24]also enhances in vitro antimalarial activity [25]

heme andor parasite proteins Successful analogue arteflene(17) is a highly active synthetic endoperoxide andchemically more stable than artemisinin It is effective fortreating mild cases of P falciparum malaria [4344]Arteflene (17) production was discontinued by Hoffman La-Roche since its long and tedious synthesis was not suitablefor industrial production

The antimalarial activity of a synthetic analogTrioxaquine DU1301 (18) has been tested in vitro onchloroquine-sensitive and resistant strains [4546]Trioxaquine DU1301 (18) is also efficient on infected miceby oral route at 12-15 mgkg

C-10 non-acetyl artemisinin derivatives with varyingdegree of in vitro antimalarial activity were synthesized [26]The compound 10-(1-pentanoyl) anhydroartemisinin (10)was the most active compound with an IC50 value of 43nM against P falciparum strain NF54 Compound (11) 10-(phenyl) dihydroartemisinin has been claimed to have IC50value of 031 and 073 ngml against P falciparum strainW2 and D6 [27] Compounds from series (11) are moreactive in vitro and may be more stable [27]

The synthesis of metabolically more stable C-10 carbaanalogues (19) and C-10 aryl analogues (20) of DHA hasbeen the focus of chemists for the past 10 years [47] C-10aryl or heteroaromatic derivatives (20) and (21) were alsosynthesized by the groups of Haynes [48] and Posner [47]C-14 modified analogs (22) were prepared by total synthesisand semi-synthetic routes by Avery and Jung groups [49-51]

Several groups have investigated the effect of C-9substitutions A series of C-9 derivatives represented bycompound (12) [28] exhibited IC50 values of 76-216ngml against different strains of P falciparum [29] Thespecific compound (13) containing a non-acetyl C-10substituent is claimed to have an in vitro IC50 value of 27ngml against W2 strain [30] A series of 11-aza-artemisininscontaining a nitryl group instead of oxygen at position 11have been claimed [31] The compound (14) has an IC50value of 173 and 260 ngml against the W2 and D6 strainsrespectively

Antimalarial evaluation of a C-10 carba analog (23)TDR 40292 demonstrated that it was superior to artemetherand artesunate both in vitro and in vivo [52] It cannot formDHA as a metabolite and contains a side chain that can beformulated as a water-soluble salt Apart from monomericsemi synthetic derivatives C-10 non-acetal carba dimers [53]and representative lead compounds (24) (25) and (26) havebeen synthesized [54-56] The isobutyric acid dimeric analog(26) was found to be safe and displayed a therapeutic indexsix times that of sodium artesunate (5)Some simplified trioxanes with potent antimalarial

activity have also been reported [3233] Water-soluble 3-aryltrioxanes such as specified by compounds (15a) and(15b) demonstrated better in vivo efficacies [3435]Compound (15b) was shown to be more efficacious andhave lower toxicity than artelinic acid (4) [36]

QUINOLINE ANTIMALARIALS

Quinolines have been the most important class ofantimalarial drugs Quinoline-containing antimalarial drugssuch as chloroquine (27) quinine (28) and mefloquine (29)are used widely to treat malaria however the malarialparasite is rapidly becoming resistant to the currentlyavailable drugs in Fig (2)

As artemisinin is a complex molecule and uneconomic tosynthesize efforts have been made to prepare simplerantimalarial molecules based on the trioxane ring system ofartemisinin that contains the endoperoxide group The mostpromising of these compounds known to date is OZ 277(16) which consists of a trioxane ring attached to anadamantane ring and provides stability to the endoperoxidegroup without diminishing its antimalarial action Themolecule also includes a tosylate side chain that confersimproved water solubility and formulation properties[3738] Compound (16) called malperox now under Phase Iclinical trials was initially developed with the support ofthe WHO Tropical diseases research programme and later bythe Medicines for Malaria Venture [39]

Quinine (28) isolated from the bark of Peruviancinchona trees was the mainstay of malaria chemotherapyuntil the Second World War when the new syntheticmolecules were developed to circumvent the problem of drugresistance [57] and 4-substituted amino-quinolineschloroquine (27) and amodiaquine (30) were developed Inthe year 1946 the synthetic quinoline compoundchloroquine (27) was introduced by Loeb et al and provedinvaluable in the fight against the disease [58]

The success of chloroquine led to the hope that malariamight be completely eradicated from the worldAmodiaquine (30) a chloroquine analog containing a 4-hydroxyanilino function in its 4-amino side chain iseffective against many chloroquine resistant strains [5960]and the drug remains useful as a treatment of malaria inmany parts of Africa [61] Chloroquine (27) andamodiaquine (30) are extensively metabolized bydealkylation of their Mannich base side chains [57] Stockset al synthesized a series of short chain chloroquinederivatives (31) and (32) in which the diethyl-amino sidechains were replaced by functional groups more resistant to

The combination of artemisinin derivatives with a seconddrug having a different mode of action is a good way toincrease the efficacy of treatment and to prevent theemergence and spread of drug resistance [4041] Somesynthetic artemisinin derived trioxane dimers have also beendeveloped with high stability and efficacy [42]

New antimalarial endoperoxides were synthesized bycombining two complementary active moieties a 4-aminoquinoline (as in chloroquine) known to easilypenetrate within infected red blood cells and a trioxane (as inartemisinin) which is a potential alkylating agent toward

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 767

NCl

HN

N

N

NH

OHO

N

CF3

CF3

HN

OH

H

NCl

HNN

OH

NCl

HN

N

NCl

HN NH

OH

NCl

HN

N

NCl

HN

OH

NH

NCl

HN

HN

F

NCl

HNN

F

NCl

HNN

FOH

NCl

HN OH

N

N

O

HNNH2

N

O

HNNH

OO

N

O

HNNH2

OCH3

O CH3

CF3

Chloropine 27 Quinine 28 Mefloquine 29Amodiaquine 30

31 32 33 34

35 36 37 Isoquine 3 8

3940

Tafenoquine 41

Fig (2) Quinoline derivatives

dealkylation [6263] Addition of piperidyl and pyrrolidylrings to the 4-amino side chain of chloroquine such as incompounds (31) and (32) resulted in an enhanced activityagainst chloroquine resistant P falciparum

a fluorine atom [64] In these molecules the Mannich sidechain has been replaced with substituent less vulnerable todealkylation and have equivalent in vitro and in vivoantimalarial activity comparable to amodiaquine Theirfluorine substituent makes them potentially much moreexpensive to synthesize

When in amodiaquine (30) 4-hydroxyanilino side chainwas modified through replacement of the 5acute aminosubstituent with alkyl groups and a tert-butyl aminosubstituent was added at the 3acute position as in compounds(33) and (34) they were also found to be more active in vivoagainst P berghei than amodiaquine (30) [60] A patentdescribes the compounds (35) (36) and (37) in which thehydroxyl group at the 4acute position of the ring is replaced with

Another patent claimed a series of analogues which arepotentially cheap to produce and effective againstchloroquine resistant parasites [65] The lead compound inthis series Isoquine (38) is more effective than amodiaquine(30) against chloroquine resistant malaria parasites in vitroand against P berghei in vivo

768 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Two other classes of quinolines currently in clinical useare the 8-aminoquinolines (primaquine 39) [66] andquinoline methanols (mefloquine 29) [67] Mefloquine (29)has been effective against chloroquine-resistant strains ofmalaria parasite [6869] while Primaquine (39) is activeonly against the liver stages of malaria parasites Bulaquine(40) an analog of primaquine is claimed to havegametocidal activity [70] Other 8-aminoquinolines currently

in development are WR-238605 (Tafenoquine) (41) and 5-phenoxy primaquine analogs These were synthesized toimprove the poor therapeutic index of primaquine (39) andits relatively short half-life Tafenoquine (41) is more activeand less toxic than primaquine (39) [7172] Tafenoquine(37) based on its promising broad-spectrum of activities hasbeen selected as a potential antirelapse antimalarial drug toreplace primaquine (39) Moreira et al synthesized

N

H3CO

NH

N NH

OR1

R3R2

F3C

N[(CH2)3CH3]2

OH

Cl Cl

HN NH

ClCl

HN N

N N

N

Cl

N

Cl

N

Cl

N

ClN

N CN

N

Cl

X

O

C

N

O

NN

N

Cl

N

Cl

N N

HN Me

Et2N

HN Me

NEt2

S

N

N C9H19

OOH

N

N OMe

Cl

NH

HO

N

N O

HN

O

HN R1

O

CH3

SN

R1

Z NS

R2 R1R2

42

Halofantrine 43WR 268668 44

45 46

n

47

48

x49

50

51 R= Ethylbenzene52 R= 4-propoxy-1-vinylbenzene

2I- ++

53

Fig (3) Quinoline antimalarials and related compounds

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 767

NCl

HN

N

N

NH

OHO

N

CF3

CF3

HN

OH

H

NCl

HNN

OH

NCl

HN

N

NCl

HN NH

OH

NCl

HN

N

NCl

HN

OH

NH

NCl

HN

HN

F

NCl

HNN

F

NCl

HNN

FOH

NCl

HN OH

N

N

O

HNNH2

N

O

HNNH

OO

N

O

HNNH2

OCH3

O CH3

CF3

Chloropine 27 Quinine 28 Mefloquine 29Amodiaquine 30

31 32 33 34

35 36 37 Isoquine 3 8

3940

Tafenoquine 41

Fig (2) Quinoline derivatives

dealkylation [6263] Addition of piperidyl and pyrrolidylrings to the 4-amino side chain of chloroquine such as incompounds (31) and (32) resulted in an enhanced activityagainst chloroquine resistant P falciparum

a fluorine atom [64] In these molecules the Mannich sidechain has been replaced with substituent less vulnerable todealkylation and have equivalent in vitro and in vivoantimalarial activity comparable to amodiaquine Theirfluorine substituent makes them potentially much moreexpensive to synthesize

When in amodiaquine (30) 4-hydroxyanilino side chainwas modified through replacement of the 5acute aminosubstituent with alkyl groups and a tert-butyl aminosubstituent was added at the 3acute position as in compounds(33) and (34) they were also found to be more active in vivoagainst P berghei than amodiaquine (30) [60] A patentdescribes the compounds (35) (36) and (37) in which thehydroxyl group at the 4acute position of the ring is replaced with

Another patent claimed a series of analogues which arepotentially cheap to produce and effective againstchloroquine resistant parasites [65] The lead compound inthis series Isoquine (38) is more effective than amodiaquine(30) against chloroquine resistant malaria parasites in vitroand against P berghei in vivo

768 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Two other classes of quinolines currently in clinical useare the 8-aminoquinolines (primaquine 39) [66] andquinoline methanols (mefloquine 29) [67] Mefloquine (29)has been effective against chloroquine-resistant strains ofmalaria parasite [6869] while Primaquine (39) is activeonly against the liver stages of malaria parasites Bulaquine(40) an analog of primaquine is claimed to havegametocidal activity [70] Other 8-aminoquinolines currently

in development are WR-238605 (Tafenoquine) (41) and 5-phenoxy primaquine analogs These were synthesized toimprove the poor therapeutic index of primaquine (39) andits relatively short half-life Tafenoquine (41) is more activeand less toxic than primaquine (39) [7172] Tafenoquine(37) based on its promising broad-spectrum of activities hasbeen selected as a potential antirelapse antimalarial drug toreplace primaquine (39) Moreira et al synthesized

N

H3CO

NH

N NH

OR1

R3R2

F3C

N[(CH2)3CH3]2

OH

Cl Cl

HN NH

ClCl

HN N

N N

N

Cl

N

Cl

N

Cl

N

ClN

N CN

N

Cl

X

O

C

N

O

NN

N

Cl

N

Cl

N N

HN Me

Et2N

HN Me

NEt2

S

N

N C9H19

OOH

N

N OMe

Cl

NH

HO

N

N O

HN

O

HN R1

O

CH3

SN

R1

Z NS

R2 R1R2

42

Halofantrine 43WR 268668 44

45 46

n

47

48

x49

50

51 R= Ethylbenzene52 R= 4-propoxy-1-vinylbenzene

2I- ++

53

Fig (3) Quinoline antimalarials and related compounds

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

768 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

Two other classes of quinolines currently in clinical useare the 8-aminoquinolines (primaquine 39) [66] andquinoline methanols (mefloquine 29) [67] Mefloquine (29)has been effective against chloroquine-resistant strains ofmalaria parasite [6869] while Primaquine (39) is activeonly against the liver stages of malaria parasites Bulaquine(40) an analog of primaquine is claimed to havegametocidal activity [70] Other 8-aminoquinolines currently

in development are WR-238605 (Tafenoquine) (41) and 5-phenoxy primaquine analogs These were synthesized toimprove the poor therapeutic index of primaquine (39) andits relatively short half-life Tafenoquine (41) is more activeand less toxic than primaquine (39) [7172] Tafenoquine(37) based on its promising broad-spectrum of activities hasbeen selected as a potential antirelapse antimalarial drug toreplace primaquine (39) Moreira et al synthesized

N

H3CO

NH

N NH

OR1

R3R2

F3C

N[(CH2)3CH3]2

OH

Cl Cl

HN NH

ClCl

HN N

N N

N

Cl

N

Cl

N

Cl

N

ClN

N CN

N

Cl

X

O

C

N

O

NN

N

Cl

N

Cl

N N

HN Me

Et2N

HN Me

NEt2

S

N

N C9H19

OOH

N

N OMe

Cl

NH

HO

N

N O

HN

O

HN R1

O

CH3

SN

R1

Z NS

R2 R1R2

42

Halofantrine 43WR 268668 44

45 46

n

47

48

x49

50

51 R= Ethylbenzene52 R= 4-propoxy-1-vinylbenzene

2I- ++

53

Fig (3) Quinoline antimalarials and related compounds

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 769

imidazolidin-4-one derivatives (42) of primaquine (39) aspotential double prodrug gametocidal agents [7374] in Fig(3)

antimalarial drugs until less toxic derivatives can besynthesized

With the aim of maintaining both steric hindrance and areduction of the degrees of freedom new bis- tris- andtetraquinolines (45) and (46) in which 4-amino group isattached to tri- and tetrazamacrocycles have been synthesized[80] An increase in rigidity by cyclization yieldedmolecules that were not more active but differed by absenceof cytotoxic effects where as tetraquinolines were verypotent against chloroquine resistant strains and non-cytotoxic against mammalian cells A series of chloroquine-like bisquinolines were synthesized with a hydrocarbonlinkage at the 2-position and the most active compound(47) was active against chloroquine resistant strain [81]

Another major class of antimalarial compounds is aryl(amino) carbinols in which the quinoline portion of the 4-quinolinemethanols is replaced by a different aromatic ringsystem The subclass of 9-phenanthrenemethanols [7576]such as halofantrine (43) was the most promising but itsusefulness has been restricted by reports of serious cardiotoxicity [77] Bisquinolines heteroalkanediamine compoundscontaining two quinoline nuclei combined through analiphatic or aromatic linker were synthesized to study theeffects of enhanced bulkiness and rigidity on their activityagainst P falciparum and were found to be active againstchloroquine-resistant strains of malaria [78] The mosteffective compound WR 268668 (44) was only six timesmore potent than chloroquine against a chloroquine-resistantstrain but 200 times more effective against a chloroquine-sensitive strain of P falciparum [79] The toxicity of thesebisquinoline derivatives prevents further development as

Development of chloroquine resistance reversal agentsinvolves potentiating its effects using compounds with weakantimalarial activity Malarial parasites can bechemosensitised to quinolines by co-treatment with channelblockers including verapamil and certain antidepressants

N

N

NH

H2N

NH2Cl

N

N

N

H2N

NH2Cl

N

N

N

H2 N

NH2Cl

N

N

N

NH2

H2 N

O O

Cl

Cl

Cl

N

N

NH

NH2

H2 N

O O

Cl

Cl

Cl

H3CO

OCH3

H3CON

N NH2

NH2

N

N

N

NH2

H2NO

Cl N

N

N

NH2

H2N

Cl

O

N

N

N

NH2

H2 N

Cl

O

N N

H2N OEt

O

OMe

OMe

OMe

NH2

N

NH2N R

NH2R2

R1

Cycloguanil 54 Proguanil 55 Pyrimethamine 56

WR-99210 57 PS-15 58

59 60 61

6263

64 R = Me R1 = R2 = Cl65 R = Phenyl R1 = R2 = Cl66 R = Hexyl R1 = R2 = H

Fig (4) DHFR inhibitors

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

770 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

[82] Based on similar mechanisms the derivatives of thecompound (48) were developed to reverse chloroquineresistance [83] Guan et al synthesized a series of newresistance reversal agents against chloroquine resistant Pfalciparum which include derivatives of four differentheterocyclic aromatic ring systems- phenothiazineiminodibenzyl iminostilbene and diphenylamine with sidechain length between four and six carbons [84]

mixtures that act against the parasite-specific enzymesdihydropteroate synthetase and dihydrofolate reductaseAvailable combinations include the sulfa drug-pyrimethamine combinations sulfadoxine (SD)-pyrimethamine (PM) and sulfalene-pyrimethamine theformer being more widely available PM is a competitiveinhibitor of dihydrofolate reductase (DHFR) while SD actsagainst dihydropteroate synthetase (DHPS) The enzymedihydrofolate reductase (DHFR) plays an important role inthymine synthesis The DHFR domain of the bifunctionalP falciparum (Pf) DHFR-TS (thymidiylate synthase) is oneof the well-characterized antimalarial targets A number ofdrugs targeting the DHFR domain have been developedincluding cycloguanil (54) the active metabolite ofproguanil (55) and pyrimethamine PM (56) in Fig (4) Theemergence of resistance has decreased the clinical utility ofthese drugs for prophylaxis and treatment

The uses of surfactants to reverse chloroquine resistancehave also been reported [85] Compounds of the type (49)have antimalarial activity and reverse chloroquinemefloquine and quinine resistance [8687] Aza-acridinequinoline derivative pyronaridine (50) has been synthesizedand currently in phase I clinical trials [88] Wiesner et alhave reported 2 5-diaminobenzophenones as a novel leadstructure of antimalarial agents that are active againstmultiresistant strains of P falciparum [89] Compound (51)was identified as a promising hit that inhibited growth of Pfalciparum by 93 at a concentration of 10microM 4-Propoxycinnamic acid derivative (52) was 10-fold active(IC50 = 340 nM) than (51) [9091] Calas et al synthesizedseries of mono- and bis thiazolium salts (53) in which polarheads are linked either from the nitrogen atom or from theC-5 position of the thiazolium ring [92]

The diaminotriazine based compound WR-99210 (57)was shown to be a potent inhibitor of (Pf) DHFR in vitro[93] WR-99210 inhibits parasite growth in the nanomolarrange but could not be developed as an antimalarial becauseof poor availability and gastrointestinal intolerance Abiguanide precursor PS-15 (58) was developed as a prodrugfor WR-99210 but abandoned due to cross-resistance withcycloguanil and PM Cycloguanil (DHFR-inhibiting activemetabolite of proguanil) and chlorcycloguanil (DHFR-inhibiting active metabolite of chlorproguanil) are morepotent than pyrimethamine Trimethoprim (59) is the leastpotent of the antimalarial DHFR inhibitors

ANTIFOLATE DERIVATIVES

Amongst the antimalarial drugs currently in clinical usethe antifolates have the best defined molecular targetsnamely the enzymes dihydrofolate reductase (DHFR) anddihydropteroate synthase (DHPS) which function in thefolate metabolic pathway Antifolates attack all growingstages of the malaria parasite and are found to inhibit theearly growing stages in the liver and the developing infectivestages in the mosquito The only useful combinations ofantifolate drugs for the treatment of malaria are synergistic

4-Substituted pyrrolo[12-a]quinoxalines derivativesrelated to the moieties present in classical and non classicalantifolic agents were prepared and evaluated in vitro forantiproliferative and antimicrobial activities [94] Newanalogues containing 3-phenoxyphenyl (60) 4-phenoxyphenyl (61) and 4-n-butoxyphenyl (62) have beensynthesized [95] and tested against a chloroquine

H N P

O

OHONa

OH

O

H3 C N P

O

OHONa

OH

OO

O

O

PO

O

ONa

O

O

O

HO OH

OH

O

P

ONaO

ONa

O

NOH

O

NH

PHO

O

HO

N

O

OH

NH

P

O

NH

NN

N

NO

HO HN

O

(H2 C)4 NH

OH

Fosmidomycin 67 FR-900098 68

70

69

72

71

73

Fig (5) DOXP inhibitors

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 771

cycloguanil-resistant strain These analogues gave IC50values in the nanomolar range when tested in vitro againstPf-FCR3 a chloroquinecycloguanil-resistant straincontaining the DHFR mutation

DOXP reductoisomerase and has the advantage to benontoxic Only the lead compound fosmidomycin (67) hasgone into phase II clinical trials but recrudescence wascommon due to the extremely short half-life of the drug

The extension of the hydrophobic side chain on the 5-benzyl moiety of the 5-benzyl-24-diaminopyrimidine suchas in compound (63) [96] led to better binding affinity thanthat of antibacterial trimethoprim (59) The 2 4-diaminopyrimidine analogs bearing a m-Cl group (64) andan unsubstituted 5-phenyl group (65) together with long 6-alkyl substituent show high binding affinity [97] The mostactive compound of this series (66) has an IC50 value of006 microM against the wild type of pf DHFR

Several patents describe organophosphorus compounds asDOXP reductoisomerase inhibitors Phosphonoformic acidderivatives of (69) are active against P falciparum havingIC50 values of 264-946 nM [101102] Several analogues ofcompounds (70) (71) and (72) have been claimed to beactive against P falciparum [103-105] Most of thesecompounds are structural analogs of the lead compoundfosmidomycin

A series of novel 3acute-amido-3acute-deoxy-N6-(1-naphthylmethyl) adenosines were synthesized for this targetand were tested for antimalarial activity verses the D2 strainof P falciparum and DOXP reductoisomerase inhibition[106] The most active compound (73) of this seriesdisplayed an IC50 of 28 microM and 75 of DOXP inhibition

DOXP REDUCTOISOMERASE INHIBITORS

1-Desoxy-D-xylulose 5-phosphate (DOXP) is a keysubstrate for the enzyme DOXP reductoisomerase a keyprotein in the non-mevalonate pathway of isoprenoidbiosynthesis [98] P falciparum utilizes alternativemevalonate-independent DOXPMEP pathway for isoprenoidbiosynthesis The presence of DOXPMEP pathway in themalarial parasite and its absence in humans make it aninteresting target for antimalarial drug discovery The DOXPpathway has been identified as a malarial drug target in Pfalciparum and its inhibition with fosmidomycin leads toantimalarial activity in vivo [98]

FEBRIFUGINE DERIVATIVES

The Chinese herb Chang Shan (Dichroa febriguga Lour)has been used for treatment against fevers caused by malariafor centuries The quinazoline derivative febrifugine (74) andits stereoisomer isofebrifugine (75) have been isolated asactive antimalarials [107] from the roots of this herb[108109] in Fig (6)

The lead compounds in this series of analogues arefosmidomycin (67) and the acetyl derivative FR-900098 (68)in Fig (5) These were developed as inhibitors of microbialDOXP reductoisomerase and were found to be an effectiveantibacterial with minimal toxicity [99100] Compound(68) is a more potent antimalarial in vitro than (67) (IC50 =170 verses 350 nM) but both have similar in vivo activitiesFosmidomycin (67) exerts potent antimalarial activityagainst multidrug resistant parasite strains by inhibition of

However the use of febrifugine (74) has beendiscontinued because of its side effects New analogs withthe chemical modifications in (74) that would decrease thetoxicity of this compound were synthesized withouthampering its antimalarial properties [110-112]

A number of derivatives of (74) for the treatment ofmalaria were also developed by the Walter Reed ArmyInstitute of Research [113114] The most promising

N

N

O

O HN

OH

N

N

OO

HO

NH

N

N

O

O HN

OH

Cl

Br N

N

O

O HN

OH

Cl

Cl

Cl

N

N

HO

O

O

NH

HO

Febrifugine 74 Isofebrifugine 75

Halofuginone 76 WR-222048 77

78

Fig (6) Febrifugine derivatives

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

772 Current Medicinal Chemistry 2007 Vol 14 No 7 Alka Mital

compounds were halofuginone (76) and WR-222048 (77)extremely potent against both chloroquine-resistant Pfalciparum strain W2 and the chloroquine-sensitive Pfalciparum strain D6 These analogs were also less toxicthan febrifugine (74)

[2] Sachs J Malaney P Nature 2002 415 680[3] Wright CW J Ethnopharmacol 2005 100 67[4] Wongsrichanalai C Pichard AL Wernsdorfer WH

Meshnick SR Lancet Infect Dis 2002 2 209[5] Ridley RG Exp Parasitol 1997 87 293[6] Wellems TE Plowe CV J Infect Dis 2001 184 770[7] May J Meyer CG Trends Parasitol 2003 19 432

In vitro antimalarial and cytotoxic tests of syntheticfebrifugines demonstrated that compound (78) hadantimalarial activity against P falciparum of similarpotency to that of natural product febrifugine with highselectivity [108] The results also suggest that basicity ofboth the 1- and the 1 ndash nitrogen atoms of febrifugine iscrucial in conferring antimalarial activity Compound (78)exhibited high in vivo antimalarial activity (ED50 = 06mgkg) against P berghei with no serious side effectsThus the metabolite appears to be a promising leadcompound for the development of new types of antimalarialdrugs

[8] Khac VT Van TN Van ST Bioorg Med Chem Lett 200515 2629

[9] Li Y Yang ZS Zhang H Cao B-J Wang F-D Zhang YShi Y-L Yang J-D Wu B-A Bioorg Med Chem 2003 114363

[10] OrsquoNeill PM Posner GH J Med Chem 2004 47 2945[11] Haynes RK Curr Opin Infect Dis 2001 14 719[12] Hein TT White NJ Lancet 1993 341 603[13] Klayman DL Science 1985 228 1049[14] Balint GA Pharmacol Ther 2001 90 261[15] Gordi T Lepist EI Toxicol Lett 2004 147 99[16] Van Agtmael MA Eggelte TA Van Boxtel CJ Trends

Pharmacol Sci 1999 20 199[17] Ploypradith P Acta Trop 2004 89 329[18] Meshnick SR Med Trop (Mars) 1998 58 13

CONCLUSIONS[19] White NJ Phil Trans R Soc Lond B 1999 354 739[20] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974354 2000[21] Kingston HR Wai-Lun L Ho-Wai C Hing-Wo T

EP0974594 2000Drug discovery efforts for the treatment of malaria in the

past have been focused on the quinoline compounds such aschloroquine mefloquine and quinine antifolate compoundssuch as pyrimethamine sulfadoxine dapsone andchlorcycloguanil or artemisinin derived compounds such asartelinic acid and sodium artesunate Artemisinins andquinolines are established classes of drugs in the patentliterature Artemisinin derivatives are the fastest actingantimalarial drugs despite several drawbacks that havelimited their clinical utility

[22] ONeill PM Ward SA WO0104123 2001[23] OrsquoNeill PM Miller A Bishop LPD Hindley S Maggs JL

Ward SA Roberts SM Scheinmann F Andrew V StachulskiAV Posner GH Park BK J Med Chem 2001 44 58

[24] ONeill PM Higson AP Taylor S Irving E WO030481672003

[25] Hindley S Ward SA Storr RC Searle NL Bray PGPark BK Davies J ONeill PM J Med Chem 2002 451052

[26] Posner GH Murray C ODowd H Xie S Shapiro TAWO0042046 2000

A number of drugs and drug combinations are in variousstages of clinical development The majority arecombinations of existing antimalarial drugs with anartemisinin derivative Chlorproguanil-dapsone-artesunate(CDA) artesunate-pyronaridine fosmidomycin artemisoneand Isoquine are at an early phase of clinical developmentOther combinations such as artesunate-amodiaquineartesunate-sulfadoxinepyrimethamine dihydroartemisinin-piperaquine are in the mid phase of development andChlorproguanil-dapsone a non-artemisinin containing fixed-dose antifolate combination is in the late stages of clinicaldevelopment Coartemether is a fixed-dose combination ofartemether and lumifantrine for uncomplicated malariacaused by mixed P falciparum infections

[27] Haynes RK Chan HW Lam WL Tsang HW CheungMK WO0004024 2000

[28] Avery MA Muraleedharan KM Desai PVBandyopadhyaya AK Furtado MM Tekwani BL J MedChem 2003 46 4244

[29] Avery MA Muraleedharan KM WO03095444 2003[30] Haynes RK Chan HW Lam WL Tsang HW WO0004025

2000[31] Wright CW J Ethnopharmacol 2005 100 67[32] Posner GH Cumming JN Woo SH J Med Chem 1998 41

940[33] Posner GH Wang D Cumming JN Oh CH French AN

Bodley AL Shapiro TA J Med Chem 1995 38 2273[34] Posner GH Jeon HB Parker MH Krasavin K Paik I-H

Shapiro TA J Med Chem 2001 44 3054[35] Posner GH Parker MH Krasavin M Shapiro TA

WO0059501 2000[36] Posner GH Jeon HB Ploypradith P Paik IH Borstnik K

Xie S Shapiro TA J Med Chem 2002 45 3824None of the classes or compounds reported by thesepatents has yet yielded a commercially useful drug althoughmany of the antimalarials are either synthetic or derived fromnatural sources As malaria parasites have become moreresistant to the conventional classes of drugs theidentification of new drugs against novel targets should bethe first priority

[37] Vennerstrom JL Arbe-Barnes S Brun R Chiu FCKChollet J Dong Y Dorn A Hunziker D Matile HMcIntosh K Padmanilayam M Tomas J S Scheurer CScorneaux B Tang Y Urwyler H Wittlin S Charman WNNature 2004 430 900

[38] Haynes RK Lam WL EP0974593 2000[39] Anon 2004 Graduate of the TDR ldquomalperoxrdquo programme

reaches the clinics TDR News No 73 6The development of new tools and technologies has

facilitated the identification of novel drug targets and thedevelopment of new antimalarials The combination of thesetechnologies would enable the development of effective andaffordable new drugs to overcome drug-resistant malaria

[40] White NJ Drug Resist Updates 1998 1 3[41] Nosten F Brasseur P Drugs 2002 2 315[42] Posner GH Paik I-H Sur S McRiner AJ Borstnik K Xie

S Shapiro TA J Med Chem 2003 46 1060[43] (a) Hofheinz W Burgin H Gocke E Jaquet C Masciadri

R Schmid G Stohler H Urwyler H Trop Med Parasitol1994 45 261 (b) Jaquet C Stohler H R Chollet J Peters WTrop Med Parasitol 1994 45 266

REFERENCES [44] Bishop LPD Maggs JL ONeill PM Park BK J PharmExp Ther 1999 289 511

[1] Trape J-F Pison G Speigel A Enel C Rogier C TrendsParasitol 2002 18 224

[45] Dechy CO Benoit VF Robert A Meunier B Chem BiolChem 2000 1 281

Recent Advances in Antimalarial Compounds and their Patents Current Medicinal Chemistry 2007 Vol 14 No 7 773

[46] Dechy CO Benoit VF Robert A Magnaval J-F SeguelaJ-P Meunier BC R Chimie 2003 6 153

[78] Vennerstrom J Ellis WY Ager AL Andersen SL GerenaL Milhous WK J Med Chem 1992 35 2129

[47] Borstnik K Paik IH Shapiro TA Posner GH Int JParasitol 2002 32 1661

[79] Raynes K Inter J Parasitol 1999 29 367[80] Girault S Grellier P Berecibar A Maes L Lemiegravere P

Mouray E Davioud-Charvet E Sergheraert C J Med Chem2001 44 1658

[48] Haynes RK Monti D Taramelli D Basilico N Parapini SOlliaro P Antimicrob Agents Chemother 2003 47 2712

[49] Vroman JA Alvin-Gaston M Avery MA Curr Pharm Des1999 5 101

[81] Ayad F Tilley L Deady LW Bioorg Med Chem Lett 200111 2075

[50] Avery MA Alvin-Gaston M Vroman JA Wu B Ager APeters W Robinson BL Charman W J Med Chem 200245 4321

[82] Ward SA Bray PG Lancet 2001 357 904[83] Lin A J Guan J Kyle D E Milhous W K WO02089810

2002[51] Jung M Lee K Jung H Tetrahedron Lett 2001 42 3997 [84] Guan J Kyle DE Gerene L Zhang Q Milhous WK Lin

AJ J Med Chem 2002 45 2741[52] Hindley S Ward SA Storr RC Searle NL Bray PGPark B K Davies J ONeill PM J Med Chem 2002 451052

[85] Kain K C Crandall I Charuk J Reithmeier R WO01397572001

[53] Posner GH Paik IH Sur S McRiner AJ Borstnik K XieS Shapiro TA J Med Chem 2003 46 1060

[86] Crandall I Charuk J Kain KC Antimicrob AgentsChemother 2000 44 2431

[54] Jung M Lee S Ham J Lee K Kim H Kim SK J MedChem 2003 46 987

[87] Ciach M Zong K Kain KC Crandall I Antimicrob AgentsChemother 2003 47 2393

[55] Jeyadevan JP Bray PG Chadwick J Mercer AE ByrneA Ward SA Park B K Williams DP Cosstick R DaviesJ Higson AP Irving E Posner GH OrsquoNeill PM J MedChem 2004 47 1290

[88] Jomaa H DE19843383 2000[89] Wiesner J Wiszligner P Dahse H-M Jomaa H Schlitzer M

Bioorg Med Chem 2001 9 785[90] Wiesner J Mitsch A Wiszligner P Jomaa H Schlitzer M

Bioorg Med Chem Lett 2001 11 423[56] Posner GH McRiner AJ Paik IH Sur S Borstnik K XieS Shapiro TA Alagbala A Foster B J Med Chem 200447 1299

[91] Wiesner J Kettler K Jomaa H Schlitzer M Bioorg MedChem Lett 2002 12 543

[57] OrsquoNeill PM Bray PG Hawley SR Ward SA Park BKPharmacol Ther 1998 77 29

[92] Hamze A Rubi E Arnal P Boisburn M Carcel C Salom-Roig X Maynadier M Wein S Vial H Calas M J MedChem 2005 48 3639[58] Loeb RF Clarke WM Coateney GR Coggeshall LT

Dieuaide FR Dochez AR Hakansson EG Marshall EKMarvel SC McCoy OR Sapero JJ Serbell WH ShannonJA Carden GA J Am Med Assoc 1946 130 1069

[93] Rieckmann KH Yeo AE Edstein MD Trans R Soc TropMed Hyg 1996 90 568

[94] Alleca S Corona P Loriga M Paglietti G Loddo R MasciaV Busonera B La Colla P IL Farmaco 2003 58 639[59] Smrkovski LL Buck RL Alcantara AK Rodriguez CS

Uylangco CV Trans R Soc Trop Med Hyg 1985 79 37 [95] Gordon L WO0153276 2001[60] OrsquoNeill PM Mukhtar A Stocks PA Randle LE Hindley

S Ward SA Storr RC Bickley JF OrsquoNeil IA Maggs JLHughes RH Winstanley PA Bray PG Park BK J MedChem 2003 46 4933

[96] Sirichaiwat C Intaraudom C Kamchonwongpaisan SVanichtanankul J Thebtaranonth Y Yuthavong Y J MedChem 2004 47 345

[97] Tarnchompoo B Sirichaiwat C Phupong W Intaraudom CSirawaraporn W Kamchonwongpaisan S Vanichtanankul JThebtaranonth Y Yuthavong Y J Med Chem 2002 45 1244

[61] Olliaro P Nevill C Lebras J Ringwald P Mussano PGarner P Brasseur P Lancet 1996 348 1196

[62] Stocks PA Raynes KJ Bray PG Park B K OrsquoNeill P MWard S A J Med Chem 2002 45 4975

[98] Jomaa H Wiesner J Sanderbrand S Altincicek B MullerC Zeidler J Lichtenthaler H K Soldati D Beck E Science1999 285 1573[63] Raynes K J Stocks P A WO0050404 2000

[64] ONeill P M Park B K WO0014070 2000 [99] Ridley RG Science 1999 285 1502-1503[65] Park B K ONeill P M WO02072554 2002 [100] Haemers T Wiesner J Poecke SV Goeman J Henschker

D Beck E Jomaa H Van Calenbergh S Bioorg Med ChemLett 2006 16 1888

[66] Baird JK Fryauff DJ Hoffman SL Clin Infect Dis 200337 1659

[67] Ohnmacht CJ Patel AR Lutz RE J Med Chem 1971 14926

[101] Jomaa H WO0030653 2000[102] Jomaa H WO0030625 2000

[68] Trenholme CM Williams RL Desjardins RE Frischer HCarson PE Rieckmann KH Canfield CJ Science 1975 190792

[103] Jomaa Pharmaka GMBH WO0192288 2001[104] Jomaa H WO0170237 2001[105] Collins D A Hogenkamp H P C WO0192288 2001

[69] Palmer KJ Holliday SM Brogden RN Drugs 1993 45 430 [106] Herforth C Wiesner J Heidler P Sanderbrand S VanCalenbergh S Jomaa H Link A Bioorg Med Chem 200412 755

[70] Pratap R Bhaduri A P EP1055427 2000[71] Obaldia N Rossan RN Cooper RD Kyle DE Nuzum

EO Rieckmann KH Shanks GD Am J Trop Med Hyg1997 56 508

[107] Fishman M Cruckshank PA J Med Chem 1970 13 155[108] Hirai S Kikuchi H Kim H-S Begum K Wataya Y

Tasaka H Miyazawa Y Yamamoto K Oshima Y J MedChem 2003 46 4351

[72] Peters W Robinson BL Milhous WK Ann Trop MedParasitol 1993 87 547

[73] Gomes P Araujo MA Rodrigues M Vale N Azevedo ZIley J Chambel P Morais J Moreira R Tetrahedron 200460 5551

[109] Takaya Y Tasaka H Chiba T Uwai K Tanitsu M KimH-S Wataya Y Miura M Takeshita M Oshima Y J MedChem 1999 42 3163

[74] Araujo MA Bom J Capela R Casimiro C Chambel PGomes P Iley J Lopez F Morais J Moreira R OliveiraED Rosano VD Vale N J Med Chem 2005 48 888

[110] Katoh M Matsune R Nagase H Honda T Tetrahedron Lett2004 45 6221

[111] Takeuchi Y Azuma K Oshige M Abe H Nishioka HSasaki K Harayama T Tetrahedron 2003 59 1639[75] Bryson HM Goa K Drugs 1992 43 236

[76] Colwell WT Brown V Christie P Lange J Yamamoto KHenry DW J Med Chem 1972 15 771

[112] Takeuchi Y Azuma K Takakura K Abe H Kim HSWataya Y Harayama T Tetrahedron 2001 57 1213

[77] Matson PA Luby SP Redd SC Rolka HR MeriwetherRA Am J Trop Med Hyg 1996 54 229

[113] Zhu S Meng L Zhang Q Wei L Bioorg Med Chem Lett2006 16 1854

[114] Jiang S Hudson H T Milhous K V WO000319 2004

Received September 01 2006 Revised November 14 2006 Accepted November 14 2006