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Bull. Org. mond. Sante 1971, 44, 363-374 Bull. Wld Hlth Org. Biodegradable Analogues of DDT * ROBERT L. METCALF,1 INDER P. KAPOOR 2 & ASHA S. HIRWE 2 Despite the immense utility of DDT for vector control its usefulness is prejudiced by its stability in the environment and by the low rate at which it can be degraded biologically. Metabolic studies in insects, in mice, and in a model ecosystem with several food chains have shown that DDT analogues with substituent groups readily attacked by multifunction oxidases undergo a substantial degree of biological degradation and do not appear to be stored readily in animal tissues or concentrated in food chains. Detailed metabolic pathways have been workedout and it is clear that comparative biochemistry can be used to develop DD T analogues that are adequately persistent yet biodegradable. A number of new DDT analogues have been evaluated for insecticidal activity against flies and mosquitos and for their potential usefulness as safe, persistent, and biodegradable insecticides. After 20 years of intensive search for new insecti- cides for the control of insect vectors of human disease,3 it must be concluded that none of the newer materials equals or excels DDT from the point of view of the combined properties of dura- bility of residues on different surfaces, safety to humans and higher animals, and low-cost insecti- cidal effectiveness. However, there is growing con- cern about the continuing liberation of vast quanti- ties of DDT into the environment (annual world production is estimated to be about 200 000 tons). In the living world, the very qualities that make DDT such an effective residual insecticide-its stability, very low water solubility (0.002 ppm), and high lipid solubility (ca. 100 000 ppm)-result in its accumulation in the fatty tissues of animals and are responsible for its progressive accumulation and concentration in organisms of a food chain, a process that has been referred to as ecological magnification (Dustman & Stickel, 1969). The prob- lems of the biological accumulation of DDT are aggravated by its conversion to the even more stable * A contribution from the WHO International Insecticide Reference Centre, Departments of Entomology and Zoology, University of Illinois, Urbana-Champaign, Ill., USA. Supported in part by grants from the Rockefeller Foundation and from the World Health Organization. 1 Professor of Zoology and Entomology, University of Illinois, Urbana-Champaign, 1ll., USA. 2 Research Associate, Uniiversity of Illinois, Urbana- Champaign, Ill., USA. 3World Health Organization (1968) Evaluation of insecticides for vector control, part I, Geneva (unpublished document WHO/VBC/68.66). dehydrochlorination product DDE,f which is the principal environmental pollutant, and in many insect control programmes it must be replaced by a persistent, biodegradable substitute. The drug-metabolizing or multifunction oxi- dase enzymes (MFO) are known to play a domi- nant role in determining the absolute toxicity of insecticides to both insects and higher animals (Hodgson, 1968). The action of these enzymes is profoundly related to such factors as insecticide specificity and selectivity, to the genetic selection of insecticide-resistant races of insects, and to biodegradation in the environment. It is evident that DDT and its derivatives DDE f [1,1-dichloro-2,2- bis(p-chlorophenyl)ethylene] and TDE [1,1-dichloro- 2,2-bis(p-chlorophenyl)ethane] are highly resistant to detoxification by MFO enzymes, and this single factor accounts for their storage and accumulation in animal tissues, especially at the higher ends of food chains. On the other hand, DDT analogues such as methoxychlor are readily attacked by MFO enzymes, which bring about O-demethylation and rapid elimination of the compound as mono- and bis-phenols (Kapoor et al., 1970). Thus, methoxy- chlor is an example of a persistent but biodegradable insecticide that does not generally accumulate in animal tissues and that is a more prudent choice for a variety of uses where environmental pollution is an important factor. We believe that the concept of persistent but biodegradable insecticides is parti- t Names against which this symbol appears are identified by chemical name in the Glossary on pages 445-446. 2649 -363-
Transcript
Page 1: Biodegradable Analogues ofDDT*

Bull. Org. mond. Sante 1971, 44, 363-374Bull. Wld Hlth Org.

Biodegradable Analogues of DDT *ROBERT L. METCALF,1 INDER P. KAPOOR 2 & ASHA S. HIRWE 2

Despite the immense utility of DDT for vector control its usefulness is prejudiced byits stability in the environment and by the low rate at which it can be degraded biologically.Metabolic studies in insects, in mice, and in a model ecosystem with several food chainshave shown that DDT analogues with substituent groups readily attacked by multifunctionoxidases undergo a substantial degree of biological degradation and do not appear to bestored readily in animal tissues or concentrated in food chains. Detailed metabolic pathwayshave been workedout and it is clear that comparative biochemistry can be used to developDDTanalogues that are adequately persistent yet biodegradable. A number of new DDTanalogues have been evaluated for insecticidal activity against flies and mosquitos and fortheir potential usefulness as safe, persistent, and biodegradable insecticides.

After 20 years of intensive search for new insecti-cides for the control of insect vectors of humandisease,3 it must be concluded that none of thenewer materials equals or excels DDT from thepoint of view of the combined properties of dura-bility of residues on different surfaces, safety tohumans and higher animals, and low-cost insecti-cidal effectiveness. However, there is growing con-cern about the continuing liberation of vast quanti-ties of DDT into the environment (annual worldproduction is estimated to be about 200 000 tons).In the living world, the very qualities that makeDDT such an effective residual insecticide-itsstability, very low water solubility (0.002 ppm),and high lipid solubility (ca. 100 000 ppm)-resultin its accumulation in the fatty tissues of animalsand are responsible for its progressive accumulationand concentration in organisms of a food chain, aprocess that has been referred to as ecologicalmagnification (Dustman & Stickel, 1969). The prob-lems of the biological accumulation of DDT areaggravated by its conversion to the even more stable

* A contribution from the WHO International InsecticideReference Centre, Departments of Entomology and Zoology,University of Illinois, Urbana-Champaign, Ill., USA.Supported in part by grants from the Rockefeller Foundationand from the World Health Organization.

1 Professor of Zoology and Entomology, University ofIllinois, Urbana-Champaign, 1ll., USA.

2 Research Associate, Uniiversity of Illinois, Urbana-Champaign, Ill., USA.

3World Health Organization (1968) Evaluation ofinsecticides for vector control, part I, Geneva (unpublisheddocument WHO/VBC/68.66).

dehydrochlorination product DDE,f which is theprincipal environmental pollutant, and in manyinsect control programmes it must be replaced by apersistent, biodegradable substitute.The drug-metabolizing or multifunction oxi-

dase enzymes (MFO) are known to play a domi-nant role in determining the absolute toxicity ofinsecticides to both insects and higher animals(Hodgson, 1968). The action of these enzymesis profoundly related to such factors as insecticidespecificity and selectivity, to the genetic selectionof insecticide-resistant races of insects, and tobiodegradation in the environment. It is evident thatDDT and its derivatives DDE f [1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene] and TDE [1,1-dichloro-2,2-bis(p-chlorophenyl)ethane] are highly resistant todetoxification by MFO enzymes, and this singlefactor accounts for their storage and accumulationin animal tissues, especially at the higher ends offood chains. On the other hand, DDT analoguessuch as methoxychlor are readily attacked by MFOenzymes, which bring about O-demethylation andrapid elimination of the compound as mono- andbis-phenols (Kapoor et al., 1970). Thus, methoxy-chlor is an example of a persistent but biodegradableinsecticide that does not generally accumulate inanimal tissues and that is a more prudent choice fora variety of uses where environmental pollution is animportant factor. We believe that the concept ofpersistent but biodegradable insecticides is parti-

t Names against which this symbol appears are identifiedby chemical name in the Glossary on pages 445-446.

2649 -363-

Page 2: Biodegradable Analogues ofDDT*

364 R. L. METCALF AND OTHERS

Table 1Ring-substituted 1,1,1 -trichloro-2,2-diphenylethanes

GENERAL FORMULA

H

--_3C<;C Cl3

Substituents Melting Methodpoint of NMR data b

R I R 2 ( C) synthesis a

CH30 CH30 88-9 A aH: 4.93(S); OCH3: 3.734(S)

C2H50 C2H50 105 A aH: 4.9425(S); OCH2: 3.8-4.099(Q); CH3: 1.242-1.484(T)C3H70 C3H70 62 A aH: 4.933(S); OCH2: 3.775-3.984(T); CH2: 1.5-2.1 (M); CH3: 0.87-1.085(T)

(CH3)2CHO (CH3)2CHO 52 D aH: 4.935(S); OCH: 3.82-4.1 (Q); CH3: 1.23-1.43(D)

C4H9O C4H90 50 A aH: 4.94(S); OCH2: 3.82-4.01 (T); CH2-CH2: 1-2(M); CH3: 0.82-1.0(T)CH30 C2H50 88-9 B aH: 4.95(S); OCH2: 3.8-4.167(Q); CH3: 1.217-1.482(T); OCH3: 3.75(S)CH30 C3H70 65 B aH: 4.966; OCH2: 3.75-4(T); CH2: 1.6-1.95(Q); CH3: 0.867-1(T); OCH3:

3.75(5)CH30 C4H 90 42 B aH :4.958; OCH2: 3.85-4.05(T); CH2-CH2: 1.4-1.87 (M) ; CH 3: 0.835-1.067(T);

OCH3: 3.85(S)

CH30 C5H1 10 30 B aH: 5.059(S); OCH2:3.93-4.13(T); (CH2)3:1.4-1.89(M); CH3:0.933-1.65(M)CH30 C6H130 liquid B aH:4.967(S); OCH2: 3.8-4(T); (CH2)4: 1.67-1.835(M);

CH3: 0.818-0.967(T); OCH3: 3.72(S)

CH30 CBH170 43-5 B aH: 4.93(S); OCH2: 3.818-4.01 (T); (CH2)6: 1.1-1.735(M);CH3: 0.884-0.97(T); OCH3: 3.75(S)

C2H50 C3H70 54-5 B aH: 4.936(S); (OCH2)2: 3.7675-4.167(M); CH2: 1.615-1.95(Q);CH3: 0.885-1.165(T); CH3: 1.258-1.488(T)

C2H50 C4H9Q 53-4 B aH: 4.915(S); (OCH2)2: 3.8-4.15(M); CH2-CH2: 1 -1.835(M);CH3: 1.25-1.4675(T); CH3: 0.815-1.05(T)

CH5S CH30 92-3 B aH: 4.966(S); OCH3: 3.7675(S); SCH3: 2.42(S)

CH3S C2H50 103-4 B aH: 4.95(S); OCH2: 3.8-4.15(Q); CH3: 1.242-1.4675(T); SCH3: 2.416(S)CH3S C3H70 98 B aH: 4.95(S); OCH2: 3.785-4.0(T); CH2: 1.618-1.965 (M);

CH 3 0.884-1.1 65(T); CH 3: 2.35(S)

CH3 CH30 80 B aH: 4.966(S); CH3: 2.267(S); OCH3: 3.72(S)

CH3 C2H50 96 B aH: 4.95(S); OCH2: 3.81-4.167(Q); CH3: 2.3(S); CH3: 1.25-1.5(T)CH3 C3H70 77 B aH: 4.95(S); OCH2: 3.75-3.985(T); CH2: 1.485-1.95(M);

CH3: 0.885-1.133(T); CH3: 2.3(S)

CH3 C4H 90 65-6 B aH: 4.97(S); OCH2: 3.8675-4.0(T); CH2-CH2: 1.33-1.8675(M);CH3: 0.834-1.01 (T); CH3: 2.31 87(S)

CH3 C2H5 liquid C aH: 4.97(S); CH2: 2.384-2.785(Q); CH3: 1.066-1.33(T); CH3: 2.267(S)CH30 OCH20 87 C aH: 4.925(S); OCH2: 5.9(S); OCH3: 3.675(S)C2H50 OCH20 115 C aH: 4.916(S); OCH20: 5.9(S); OCH2: 3.835-4.167(Q); CH3: 3.7685(T)CH30 HC-CCH20 72 D aH: 4.966; CCH20: 4.659-4.69(D); CH: 2.46-2.54(T); OCH3: 3.7675(S)CH30 Cl 95 C aH: 5.04(S); OCH3: 3.86(S)

CH30 F 77-8 B aH: 5.016(S); OCH3: 3.785(S)CH3 Br 104 C aH: 4.965(S); CH3: 2.317(S)

a See text. b NMR data: (S) = singlet, (D) = doublet, (T) = triplet, (Q) = quartet, (M) = multiplet.

Page 3: Biodegradable Analogues ofDDT*

BIODEGRADABLE ANALOGUES OF DDT 365

cularly important for achieving satisfactory residualinsecticidal control of pests of both public-healthand agricultural importance without causing long-term environmental pollution and the accumulationof pesticide residues throughout the biosphere.Therefore we are systematically exploring the attackof the MFO enzymes on a variety ofDDT analogueswith the objective of characterizing mechanismsof biodegradation. We are seeking DDT-like com-pounds that, while effective and persistent insecticidesin inanimate situations, when absorbed into livingorganisms will have weak points for attack by theMFO enzymes, promoting rapid detoxification andelimination as water-soluble metabolites. Such DDTanalogues should have many advantages as safe,relatively stable, and potentially inexpensive residualinsecticides.

MATERIALS AND METHODS

The compounds evaluated were all highly puri-fied chemicals synthesized in our laboratory byvariations of the Baeyer condensation betweenchloral and appropriate substituted benzenes. Fourgeneral methods of synthesis were used for thecompounds listed in Table 1: (A) condensationof 2 moles of substituted benzene with 1 mole ofchloral in excess (10 volumes) of concentratedH2SO4 or 0.6 M AlCl3 (anhydrous) to form the sym-metrical 1,1,1-trichloro-2,2-diphenylethane; (B) con-densation ofp-alkoxyphenyl trichloromethylcarbinol(Kapoor et al., 1970) or p-alkylphenyl trichloro-methylcarbinol (Reeve et al., 1966) with 1.0 M AIC13(anhydrous) in ethanol-free chloroform; (C) con-densation of substituted benzene and the carbinolreferred to above in 10 volumes of concentratedH2S04 to produce the asymmetrical 1,1,1-trichloro-2,2-diphenylethanes; and (D) treatment of 1,1,1-trichloro- 2-(p-hydroxyphenyl)- 2 -(p-methoxyphenyl)ethane in acetone with the appropriate alkyl bromidein the presence of potassium carbonate.The compounds were purified by recrystalli-

zation or distillation until they were at least 99%pure by thin-layer chromatography. The structureswere confirmed by NMR spectrometry as shown inTable 1 (see Kapoor et al., 1970).The insecticidal activities were determined by

the standard methods used in Stage I of the WHOInsecticide Evaluation Programme.' Female house-flies, under CO2 anaesthesia, were treated by thetopical application of 1-,ulitre droplets of standardsolutions of the insecticides in acetone. Three

replicates of 20 flies 2-4 days old were treated onthe pronotum at each dosage, and at least 5 dosageswere used to establish each dosage-mortalitycurve. Mortalities were determined by holding theflies at 22°C with 40% sucrose solution as food.Topical applications were also made to laboratory-reared black blowflies, Phormia regina, in exactlythe same manner. The results of these evaluationsare reported in the tables as LD50 values in micro-grams of toxicant per gram of insect, using theaverage weight of the housefly as 20 mg and of theblowfly as 40 mg.To determine the amount of detoxification of

the insecticides in the insect body, the synergist pipe-ronyl butoxide was applied topically to the ventralportion of the abdomen of the flies 1 hour beforetreatment with the insecticide, applied to the prono-tum in a dosage of 1 ,ulitre of 5% w/v solution inacetone (50 Htg per insect). This dosage producedno observable mortality and did not materiallyaffect the longevity of the flies.The toxicity of the DDT analogues for larvae

and adults of the mosquitos Culex pipiens fatigansand Anopheles albimanus was evaluated by theWHO method 1 (see also Metcalf & Fukuto, 1968).

Studies of the toxicity of some of the DDT ana-logues to female Swiss mice, 6-8 weeks old, arealso reported. The compounds were dissolved inolive oil at 5-10% w/v and the requisite dosagewas administered orally by a micrometer-drivenHamilton syringe. The mice were observed forsymptoms of intoxication and for mortality overa 1-week period.

EXPERIMENTAL RESULTS

DDT analogues as substrates for multifuntctionoxidases

The MFO enzymes utilize molecular oxygen anda unique cytochrome, P-450, to form a " hydroxyl--ating " free radical (-OH or *OOH) which is respon-sible for a large variety of reactions with xenobioticcompounds. There are at least 9 distinct types ofbiochemical metabolizing reaction and at least 4 ofthese can be demonstrated with insecticidally activeDDT analogues, as follows:

(1) hydroxylation of aromatic rings, which shouldtake place most readily in unsubstituted analoguessuch as 1,1,1,-trichloro-2,2-bis(phenyl)ethaneor 1,1,1-

I See footnote 3 page 363.

Page 4: Biodegradable Analogues ofDDT*

R. L. METCALF AND OTHERS

H

C le| OH_

R3

H

RO C\OR3/HR

R3

H

R / \ C / \ CH3

R3

H

C e OH

Cf

R3

H

RO / / OH- I -

C

R3

H

R/ COOH

- -

CfK3

H H 0

RS X3 C e S SR RS Ci SR

- I-- -C C

R3 R3

t^2 10223

trichloro-2-(p-chlorophenyl)-2-phenylethane, form-ing monophenolic and diphenolic compounds;

(2) side-chain oxidation, which is demonstrablein 1,11,1-trichloro-2,2-bis(p-methylphenyl)ethane byoxidation to mono- and di-benzoic acids (Kapooret al, unpublished data, 1971);

(3) O-dealkylation, shown by the conversion ofmethoxychlor, 1, 1, 1 - trichloro - 2, 2 - bis(p-methoxy-phenyl)ethane, to mono- and bis-phenols (Kapooret al., 1970); and

(4) sulfide oxidation, shown by the oxidation ofI, I, I -trichloro-2,2 - bis(p-methylthiophenyl)ethane tosulfoxide and sulfone derivatives (Kapoor et al.,1970).

Typical reactions ofDDT analogues as substratesfor these enzymes are shown in the accompanyingfigure.

Synergistic ratio as a measure of biodegradability

Much evidence indicates that methylenedioxy-benzene derivatives (1,3-benzodioxoles) such aspiperonyl butoxide and sesamex t are effective andspecific inhibitors of MFO enzymes. Inhibitionof these drug-metabolizing enzymes is responsiblefor the pronounced synergistic effects of the methyl-enedioxyphenyl compounds when used togetherwith pyrethroids or carbamates; they almost totallyprevent detoxification of the insecticide and permitit to reach the target site of action. This subjecthas been reviewed by Metcalf (1967) and Casida(1970). The data in Tables 2 and 3 show topicalLD50 values for 3-4-day-old susceptible (SNAmIM) andDDT- and dieldrin-resistant (Rsp) female housefliesas determined (a) with the compounds alone and(b) 1 hour after pretreatment with 50 [g of piperonyl

366

Page 5: Biodegradable Analogues ofDDT*

BIODEGRADABLE ANALOGUES OF DDT

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Page 6: Biodegradable Analogues ofDDT*

368 R. L. METCALF AND OTHERS

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Page 7: Biodegradable Analogues ofDDT*

BIODEGRADABLE ANALOGUES OF DDT

butoxide. The synergistic ratio (SR) gives a quantita-tive measure of the effect of the MFO enzymes in thedetoxification of the DDT analogues. It is apparentfrom Tables 2 and 3 that there is a remarkable rangein SR values for the different compounds against theSNAIDM fly-from 40 for 1,1,1-trichloro-2-(p-chloro-phenyl)-2-phenylethane (XXXIV) to 1.0 for 1,1,1-trichloro-2-(p-butoxyphenyl) - 2 - (p - methoxyphe-nyl)ethane (XXI). Thus, the former is rapidlydetoxified in vivo while the latter seems not to beaffected. Such data suggest that the intrinsic toxicityof the DDT analogue is best measured by thesynergized LD50, as has been demonstrated for car-bamates (Metcalf & Fukuto, 1965), and that the SRvalue affords a quantitative measure of the detoxifica-tion of the compound by susceptible and resistantinsects.The data in Table 2 were obtained with symmetri-

cal DDT analogues. DDT (I) is detoxified onlyslowly by MFO action in S flies and somewhatmore rapidly in R flies. The rates of detoxification ofthe simple p-substituents can be ranked as follows:Cl <H <CH3<CH30<CH3S, and this techniquedemonstrates the reactions described for MFOenzymes as shown in the figure. The rates of detoxifi-cation of the p,p'-dialkoxy analogues can be rankedas follows: C2H50 <C3H70 <CH30 <iso-C3H70.The asymmetrical compounds of Table 2 exhibitgreater overall biodegradability. Compounds withhigh SR values included CH3-H (XXXV) andCH3-Br (XII); those with low SR values wereC2H50-C3H70 (XXV), CH30-C3H70 (XX), CH3-C2H5 (XXX), and C.H50-OCH.O (XXXII)." Ethoxychlor ", 1, 1,1 -trichloro-2,2-bis(p-ethoxy-phenyl)ethane (IV), was outstandingly activeagainst both S and R flies and was in fact moretoxic than DDT both alone and synergized. Itis evident from the magnitude of the LD50 valuesthat the SNAIDM strain, which has been reared formore than 25 years on bran and alfalfa inadvertentlycontaminated with trace amounts of DDT, hasbecome less susceptible to DDT. The LD 50valueoriginally obtained in 1948 for this strain at 15.5°Cwas 0.04 ,ug per female. " Ethoxychlor ", with anLD50 of 0.058 jug per female at 15.5°C (Metcalf &Fukuto, 1968), was originally slightly less toxicthan DDT but is now more toxic. We have recentlyobtained S strains from three other laboratoriesand have found their susceptibility to DDT to beabout equal to or less than that of the SNAIDMI strain.

With respect to DDT resistance as indicated bythe response of the RSp housefly (Tables 2 and 3),

the alkyl- and alkoxy-substituted analogues aresubstantially more effective than DDT. Compoundssuch as " ethoxychlor " and the CH30-C2H,O(XIX), CH30-OCH20 (XXXI), C2H50-C3H70(XXV), and C2H50-C4H,O (XXVI) analogues haveresistance ratios of <2.0, while the CH30-C3H70(XX) and CH30-C4H90 (XXI) analogues are ofnearly equal toxicity to the S and R strains. In theDDT molecule, the inductive and mesomeric effectsof the ring substituents control the rate of dehydro-chlorination in response to DDT-ase (Metcalf &Fukuto, 1968), and this rate is minimal when thesubstituents are electron-donating-e.g., alkyl oralkoxy groups. Therefore, where DDT-resistanceis controlled by DDT-ase, as in the Rsp housefly,such analogues should be substantially more effectivethan DDT.

Autosynergism o.fDDTanaloguies

The term autosynergism has been applied tophenyl methylcarbamate insecticides containing3,4-methylenedioxyphenyl or 2- or 3-propynyloxy-phenyl moieties, which have been shown to inhibitthe multifunction oxidases. These carbamates aremuch more toxic to the housefly than their relativeanticholinesterase activity would indicate, suggestingthat they also function as synergists by self-inhi-bition of the detoxifying enzymes (Metcalf, 1968).The DDT-type compound provides the structural

framework for substitution with similar groupson one or both phenyl rings and these compoundshave been examined for autosynergism. Thedata of Table 4 contrast the toxicity and the syner-gism with piperonyl butoxide of several pairs ofcompounds. The higher toxicity and lower SR valuesfor the methylenedioxyphenyl compounds (XXXI,XXXII), especially at the LD90 level, suggest thatautosynergism occurs. There is also evidence of asmaller degree of autosynergism with the propynyl-oxyphenyl compound (VIII). Further analysis ofthis problem is complicated by the lack of a quantita-tive measure of the interaction of DDT-type com-pounds with the receptor site (equivalent to the 150values for carbamates with cholinesterase). However,the synergized LD,0 values for the fly may providean approximate indication of the efficiency ofreceptor interaction. The data of Table 2 indicatethat methoxychlor (III), whose synergized LD50for Musca is 3.5 tkg/g, is a much more efficient inter-actant than 1,1,1-trichloro-2,2-bis(p-propynyloxy-phenyl)ethane (VIII), whose synergized LD50 is28.0 ,ug/g. A more definitive examination of this

369

Page 8: Biodegradable Analogues ofDDT*

R. L. METCALF AND OTHERS

Table 4Autosynergism of DDT analogues

GENERAL FORMULA

H

RI / \ C / R2

-cc'-13

Musca domestica

SNAIDAI RSp

SR slope LD50a SR slopeP (Mg/g) P

12.8

4.9

10.3

4.3

3.9

2.4

4.0

2.5

4.3

8.1

48

39

105

29

26

10.4

3.8

14.0

2.3

3.3

2.6

2.4

1.6

3.9

2.5

Phormia regina

LD5oaSR slope

(,Lg/g)

10

65

9.5

10

11.5

2.2

2.7

1.1

1.0

1.1

3.5

2.4

3.0

4.2

3.8

a Topical application.

phenomenon as applied to DDT-type compoundsdoes not seem feasible at present.

Intrinsic toxicity of DDT analogues to Phormiaand MuscaLack of specific knowledge about the DDT recep-

tor site makes it difficult to relate precisely the mole-cular structures of DDT analogues to their intrin-sic toxicity. The most promising theory of the modeof action of DDT appears to be that of Mullins(1955), which relates the size and shape ofDDT ana-

logues to their ability to enter into pores or inter-spaces between the macromolecules of the nerve

axonic membrane. The toxic action of DDT, it hasbeen suggested, results from its penetration intothe interspaces with a specific orientation so as toproduce ion leaks and consequent depolarizationof the axon. This theory has been further refinedby Holan (1969) to suggest that the active analoguesof DDT function as molecular wedges within theNa+ cavities of the nerve membrane and cause ionleaks by keeping open the expanded sodium springs(Weiss, 1969).The present evaluation (Tables 2 and 3) provides

some interesting leads to further refinement ofthese theories. It is suggested that the LD-,o for

Phormia and the synergized LD50 for Musca providea rough measure of reactivity at the DDT receptorsite. Phormia differs substantially from Muscain its response to xenobiotics-for example, its topi-cal LD50 of carbaryl is 29 ,ug/g and the SR withpiperonyl butoxide is 6.7, whereas the LD50 forMusca is 900 ,g/g with an SR of 72 (Brattsten& Metcalf, 1970). Tissues from Phormia are onlyabout 0.26 times as active per mg of protein inhydroxylating naphthalene and only about 0.07 timesas active in epoxidizing aldrin to dieldrin (Ter-riere, 1968). D. Nye in our laboratory has foundPhormia to have very low levels of cytochromeP-450 compared with Musca. Therefore, the com-

parisons, in Tables 2 and 3, between topical LD50values and SR values for Phormia and Muscademonstrate the role of MFO detoxification in theintrinsic activities of the various DDT analogues.It is apparent that in Phormia the MFO enzymes

play a very limited role in detoxification, with SRvalues of 1-3, while in Musca they are substantiallyhigher, 2.5-40. Methoxychlor is more toxic toPhormia than DDT, unsynergized, and this-together with its higher synergized LD50 for Musca-suggests that methoxychlor has a greater affinityfor the DDT receptor than does DDT. Almost

Substituents

R I R 2

CH30

CH30

CH30

C2HsO

C2H50

LD50a(/Ag/g)

45

22

34

16

13.5

CH30

OCH20

HC-CCH20

CH30

OCH20

370

Page 9: Biodegradable Analogues ofDDT*

BIODEGRADABLE ANALOGUES OF DDT 371

every DDT analogue was more effective on a body-weight basis against Phormia than against Musca,and these data suggest that the apparent toxicityof these insecticidal compounds is determined bythe levels of activity of the MFO enzymes in thetest organisms.The data of Table 2 suggest that for the symmetri-

cal analogues of DDT, the order of toxicity ofp,p'-substituents is C2H50> CH3O> C1> CH3>CH3S> C3H70. This is contrary to the generalimpression that the Cl substituent (DDT) is themost active. For the asymmetrical analogues ofTable 3, the order of intrinsic toxicity is CH3-C2H50 (XVI) > Br-CH3 (XII) > CH3-C3H70(XVII) > CH3S-C3H70 (XXIX) = CH3S-C2H50(XXVIII) = CH3S-CH30 (XXVII) > CH3-C2H5(XXX) > C2H50-C3H70 (XXV) > CH30-HC-CCH20 (XXXIII) > C2H50-OCH,O (XXXII).The CH3-C2H50 (XVI) analogue was the mostactive compound studied, being slightly more activethan " ethoxychlor " (C2H50-C2H50).The high intrinsic activity of these asymmetrical

compounds together with the very adequate acti-vity of CH30-C4H90 and the considerable activityof CH30-C6H130 suggest a possible lack of symme-try of the receptor site. This is further emphasizedby the high activity of synergized monosubstitutedcompounds. It has been assumed in almost everyinvestigation that p,p'-disubstitution is essentialfor high toxicity and maximum receptor interaction.Yet the monochloro-DDT (XXXIV) is synergized40 times and its synergized toxicity for Musca isgreater than that of DDT. However, it is not highlytoxic to Phormia. Similar results were obtainedwith the monomethyl-DDT (XXXV). Even moreinteresting from the standpoint of DDT mode ofaction is the toxicity of o,p'-DDT, 1,1,1-trichloro-2-(p-chlorophenyl)-2-(o-chlorophenyl)ethane, whoseLD50 is 190 ,ug/g alone and 15 ,ug/g synergized(SR = 12.6). This compound can be hydroxylatedin the para-position by MFO and its alleged in-activity is clearly due to detoxification and notto steric factors, as is generally believed. Theseresponses of the fly clearly suggest a need for sub-stantial revision of current ideas about the interactionof DDT and hypothetical receptors. This will beattempted in a subsequent paper.

Selective toxicity to insects and mammalsDDT obviously is of very low toxicity to mammals,

as demonstrated by its enormous use without sick-ness or fatality. However, this seems to be largely

a function of its low dermal toxicity, since recordedvalues for the oral LD50 for the mouse range from150 ,tg/g to 400 ,ug/g; the oral LD50 for the malerat is 1 13 ,ug/g and for the female rat it is 118 ,ug/g(Hayes, 1963). Oral doses of at least 285 mg/kg havebeen taken by man without fatal result (Hayes,1963). Methoxychlor, with an oral LD50 for therat exceeding 6 000 mg/kg, is much less toxic, pro-bably because it undergoes rapid biodegradation(Kapoor et al., 1970).The difference in toxicity suggests that bio-

degradable analogues of DDT may be substantiallyless toxic to mammals than to insects because ofthe generally higher titres of MFO enzymes in themammalian liver. The data of Table 5 summarizethe available information, including data obtained inour laboratory, on some of the compounds inTables 2 and 3. Compounds with a single methoxy,methyl, or methylthio group were of low toxicity.It appears that these groups should be incorporatedinto appropriate DDT molecules where a highsafety factor for mammals is required. " Ethoxy-chlor " is nearly as toxic as DDT to the mouse andits ethoxy-propoxy analogue is considerably moretoxic. A comparison with methoxychlor illustratesthe rather specific nature of O-dealkylation.

Biodegradation ofDDT anialoguies within an ecosystem

The principal problem involved in the wide-spread use of DDT today is its lack of biodegrad-ability and consequent " ecological magnification " infood chain organisms. Therefore any compoundsproposed as substitutes should be free from thisdefect. Our laboratory has developed a modelecosystem with a seven-element food chain, whichcan be used with radiolabelled compounds toevaluate biodegradability. This technique has beenapplied to DDT, methoxychlor, " methiochlor "(Kapoor et al., 1970), " methylchlor", 1,1,1-tri-chloro-2,2-bis(p methylphenyl)ethane, and " ethoxy-chlor" (unpublished data). An account of thesestudies is beyond the scope of the present paper;'however, the data obtained demonstrate the utilityof this approach in evaluating new candidateinsecticides. As shown in Table 6, substitution in thep,p'-positions with groups that can be attacked bythe multifunction oxidases effectively decreases theconcentration of the DDT-type analogue in food-chain organisms.

1 A full account of these studies will be published else-where.

Page 10: Biodegradable Analogues ofDDT*

R. L. METCALF AND OTHERS

Table 5

Toxicity of some DDT analogues for the mouseTable 6

Accumulation of DDT analogues in an ecosystem

GENER,

R'I/

Substituents

R R

Cl Cl

CH3 CH3

CH30 CH30

C2H50 C2H50

C3H70 C3H70

C4H9O C4H90

CH3S CH3S

CH30 CH3

CH30 CH3S

CH30 C2H5O

CH30 -OCH20-

CH30 C4H90

C2H50 CH3S

C2H50 C3H70

C2H50 -OCH20-

CH3 C2H50

CH3 C3H70

AL FORMULA

H

- \ R2

C Cl3

Oral LD5o (mg/kg) for the mouse

Our laboratory Literature

200 200 (t

400 b

750 c, c

300-325

3 350 b

1 000 "IC

1 850 b

250 a

200 '

1 QQU,c

1 000

>1 000

1 000

300-500

-1 000

500

-1 000

75-100

-1 000

1 000

500

" Van Oettingen & Sharpless (1 946)t' Dornenjoz (1946).c No effcct.

Analoguies as DDT substitutes

Because of increasing public concern aboutenvironmental pollution by DDT and legal restric-tions upon its use, the question of its replacement bypersistent biodegradable substitutes has becomeurgent. Methoxychlor, which has been used toa substantial extent for many years, is a suitableDDT replacement for many uses (Kapoor et al.,1970). " Methylchlor" (II) was discussed as an in-

secticide in the original DDT report (Muller, 1946).

Concentration (ppm) in:Compound

DDT

DDE t

TDE

methoxychlor

" ethoxy-chlor' "

" methio-chlor ''"

" methyl-chlor " "(

Water

0.00022

0.0053

0.0004

0.00011

0.0006

0.0018

0.0001

Physa(snail)

7.6

103.5

3.3

13.2

58.5

0.539

72.16

Gambusia(fish) =

18.6

145.0

33.4

0.17

0.922

0

0.084

Con-centration

ratio,fish/water

85 000

27 000

83 000

1 500

1 500

0

840

" These compounds are identified in the text.

It should be re-examined because of its very lowtoxicity for mamumals, potential low cost, andbiodegradability. These compounds appears tobe particularly effective against adult mosquitos."Ethoxychlor " (IV) was first reported as an insecti-cide by Prill et al. (1945) and subsequently by Ste-phenson & Waters (1946). " Ethoxychlor " wasfound to be an excellent insecticide but has re-ceived little further investigation. It appears to beslightly less acuLtely toxic to the mouse than DDTand is biodegradable to a substantial extent." Propoxychlor " (V) and " butoxychlor " (VI) werealso studied by the above investigators and foundto be less effective. " Isopropoxychlor " (VII)was prepared as a guLm by Stephensen & Waters(1948) but was neither purified nor evaluated.

Prill et al. (1946) investigated a number of asymme-trical analogues of DDT including CH30-C2H50(XIX), CH3 O-OCH,O (XXXI), and C1-CH3 (XI).Schneller & Smith (1948) prepared the CH30-C2H50 (XIX), Cl-Br, and Cl-I analogues ofDDT. However, they were unsuccessful in severalattemiipts to prepare the Cl-CH30 analogue1,1,I-trichloro-2-(p - chlorophenyl) - 2 - (p-methoxy-phenyl)ethane (XIII). The corresponding F-CH3Oanalogue XIV) was patented by Balaban & Sutcliffe(1948), together with the CH3-F and F-Cl ana-logues.We can find no record of the preparation or

insecticidal evaluation of the other disubstituted

372 t

Page 11: Biodegradable Analogues ofDDT*

BIODEGRADABLE ANALOGUES OF DDT 373

DDT analogues of Table 3. The C1-CH30analogue (XIII) was prepared for the first time afternumerous failures but was not outstandingly active.The most promising compounds for further evalu-ation appear to be the CH3-CH30 (XV), CH3-C2H50

(XVI), CH3-C3H70 (XVII), CH30-SCH3 (XXVII),C2H50-CH3S (XXVIII), C2H50-OCH20 (XXXII),and CH3-C2H5 (XXX) analogues, all of whichare highly insecticidal, have low toxicity for mammals,and should readily undergo biodegradation.

ACKNOWLEDGEMENTS

It is a pleasure to acknowledge the skilful assistance of Mrs Ruth Millholin, Mrs Penny Hansen, and Mrs ArlenePyle in carrying out the bioassay evaluations.

REFERENCES

Balaban, E. & Sutcliffe, F. K. (1948) British PatentNo. 597 091, 19 January

Brattsten, L. & Metcalf, R. L. (1970) J. econ. Ent., 63,101

Casida, J. (1970) J. agric. Food Chem., 18, 753Domenjoz, R. (1946) Helv. chini. Acta, 29, 1317Dustman, E. H. & Stickel, L. F. (1969) Ann. N. Y.

Acad. Sci., 160, 162Hayes, W. J. (1963) Clinical handbook on economic

poison?s, Atlanta, Ga., US Department of Health,Education, and Welfare, Communicable DiseaseCenter

Hodgson, E., ed. (1968) Enzymatic oxidation of toxicants,Raleigh, North Carolina State University Press

Holan, G. (1969) Nature (Lond.), 221, 1025Kapoor, 1. P., Metcalf, R. L., Nystrom, R. F. & Sangha,G. K. (1970) J. agric. Food Chiem., 18, 1145

Metcalf, R. L. (1967) Ann. Rev. Ent., 12, 229Metcalf, R. L. (1968) The role of oxidative reactions in

the mode ofaction of insecticides. In: Hodgson, E., ed.,Enzymatic oxidation of toxicants, Raleigh, NorthCarolina State University Press, p. 151

Metcalf, R. L. & Fukuto, T. R. (1965) J. agric. FoodChem., 13, 220

Metcalf, R. L. & Fukuto, T. R. (1968) Bull. Wld HlthOrg., 38, 633

Muller, P. (1946) Helv. chim. Acta, 29, 1560Mullins, L. J. (1955) Science, 122, 118Prill, E. A., Hartzel, A. & Arthur, J. M. (1945) Science,

101, 464Prill, E. A., Synerholm, M. E. & Hartzell, A. (1946)

Contrib. Boyce Thompson Inst., 14, 341Reeve, W., Mutchler, J. P. & Liotta, C. I. (1966) Canad.

J. chem., 44, 575Schneller, G. H. & Smith, G. B. L. (1948) J. Amer.

chem. Soc., 70, 4057Stephenson, 0. & Waters, W. A. (1946) J. chem. Soc., 339Street, J. (1969) Ann. N. Y. Acad. Sci., 160, 274Terriere, L. C. (1968) The oxidation of pesticides, the

comparative approach. In: Hodgson, E., ed., Enzymaticoxidation of toxicants, Raleigh, North Carolina StateUniversity Press, p. 175

Van Oettingen, W. F. & Sharpless, N. (1946) J. Pharmacol.exp. Ther., 88, 400

Weiss, D. E. (1969) Aust. J. biol. Sci., 22, 1355

DISCUSSION

ALDRIDGE: Do you think there are potentialities for thedevelopment of molluscidides, using as a starting pointthose compounds that you have shown to accumulate insnails ?

METCALF: It is possible that some of these DDT-like com-pounds with appropriate solubility in water might bemolluscicidal, and we are investigating this possibility.

WEIDEN: Is the apparent lack of microsomal activity in thesnail used in your micro-ecosystem typical of othermolluscs ?

METCALF: The snail used is the genus Physa, one of thelargest of aquatic snails. We do not know much aboutother species or genera, but the one described in the paperhas a very low level of microsomal enzyme activity.

Page 12: Biodegradable Analogues ofDDT*

R. L. METCALF AND OTHERS

WEIDEN: Were the sulfoxides and the sulfones of thealkylthio DDT analogues active?

METCALF: The bis-sulfoxide and the bis-sulfone de-rivatives of 1, 1, 1 -trichloro-2, 2-bis(p-methylthiophenyl)ethane were completely nontoxic to Musca domesticawhen applied topically at 500 ,uglg. They were not sy-nergized by pretreatment with 50 jug of piperonyl but-oxide.

KENAGA: Why has methoxychlor not met with greatercommercial success for insect control? Does Dr Gysinthink that it may be more widely used in the future as a

replacement for DDT?GYsIN: Methoxychlor was considerably less active for flycontrol in Europe. Owing to this factor and to its greaterproduction costs, it never found wider use in Europe. Itmay have to be reconsidered for use in malaria control,but not for crop protection.

374


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