الرحمن الله بسمالرحيم

Novel Therapeutics in

Parasitic Diseases

By Dr Nadia Mohamed Sabry Fawy

Professor of Medical Parasitology, Faculty of Medicine, Ain Shams

University

Parasitic diseases are major worldwide health problems .

They plague billions of people, killing millions annually and inflicting irreversible debilitating injuries, such as blindness and disfiguration, on additional millions. Also, their economic impact is globally enormous.

Development of new therapeutics are desperately and

urgently needed for the treatment of parasitic diseases.

WHY?????

The magnitude of many parasitic diseases is increasing in endemic areas

•Their impact is also extending into industrialized countries

(due to increasing international concern.)

•In addition, latent parasitic infections evolving to produce acute diseases have been noted with the increased prevalence of immunodeficiency states and the use of immunosuppression in medical therapy.

Current treatment for many parasitic diseases are poorly tolerated, having varying degrees of effectiveness and serious problems with toxicity, drug-resistant strains and lack of selectivity.

Many potential parasite-specific drug targets have been identified in biochemical and molecular studies and some have been validatedNew approaches have been attempted to exploit these targets.

Immuno-therapy

Phytotherapy

Cell therapyChemotherapy

Gene Therapy

Nanotherapy

Novel therapeutics in parasitic diseases

Expansion of the repertoire of possible therapies

The use of combination therapy

New strategies in treating some parasitic diseases

and

are

Parasitic diseases mostly concerned in novel therapeutics include:

-falciparum malaria ,- neglected protozoal diseases

(leishmaniasis, human African trypanosomiasis and Chagas' disease)

- some helminthic infections as schistosomiasis and lymphatic

filariasis .

Chemotherapeutics agents

Novel chemotherapeutics include agents targeting metabolic biosynthesis and enzymatic pathways e.g. inhibitors of cysteine proteases and iron uptake and regulation mechanisms.

Cysteine Protease Inhibitors (CPI) Cysteine Proteases are enzymes known for proteins and polypeptides catabolism through cleavage of peptide bonds.

They have central roles in biological parasite activities as they enable parasites to:

- Bore through tissue barriers- Degrade host proteins for nutrition

- Clude host immune response

For Plasmodium spp.Falcipains are the best characterized

Plasmodium cysteine proteases.

They have a role in: *Hemoglobin hydrolysis

* Erythrocyte rupture * Erythrocyte invasion by erythrocytic

malaria parasites .

Falcipain-2 and falcipain-3 are hemoglobinases that appear to hydrolyse host erythrocyte hemoglobin in the parasite food vacuole, a process essential

for their survival .

This function was recently confirmed for falcipain-2, as disruption of the falcipain-2 gene led to a transient block in

hemoglobin hydrolysis .

Falcipains are highly promising antimalarial drug targets .

E. histolytica CPs are a key virulence factor of E. histolytica .

They play a role in intestinal invasion by:1 .Degrading the extracellular matrix .

2.Circumventing the host immune response through cleavage of secretory IgA , IgG, and activation of complement.

CPs are encoded by at least seven genes, several of which are found in E.histolytica but not E. dispar.

Amoebapain and histolysin are two similar but distinct CPs purified from E. histolytica Inhibition of cysteine proteinase activity with inhibitors or an antisense construct significantly decreased liver abscess formation in SCID mice and

hamsters

Trypanosoma cruzi Cruzain (cruzipain) is the major

papain-like cysteine protease of T cruzi Cruzain is indispensable for the survival and propagation of this protozoan parasite and therefore, it has attracted considerable interest as a potential drug target

Cruzipain participates in the parasite functions to:

-Favour cell invasion - Facilitate host tissues proteolytic

degradation - Trigger the evasion mechanism

from host immune response.

Irreversible cruzipain inhibitors e.g. peptidyl vinyl sulphones are able to block steps in the differentiation of the

parasite's life cycle .

They effectively kill the organism by inducing accumulation of unprocessed cruzipain in the Golgi cisternae, interfering with the secretory pathway..

Polymorphic trypanosomesPotent peptidyl and peptidomimetic inhibitors of brucipain (a.k.a. trypanopain-Tb) the CP of T. brucei have proved trypanocidal.

Azadipeptide nitriles belong to a novel class of extremely potent CPIs

A series of azanitrile-containing compounds were shown to potently inhibit both recombinant cruzain and rhodesain at low nanomolar/picomolar ranges.

This provides a good starting point for further development of these azanitrile-containing compounds as potential anti-parasitic agents.

Use of chemotherapeutics

targeting Iron uptake and regulation mechanisms

-Pathogenic parasites are dependent on iron .

-They have diverse adaptations to exploit host iron resources.

-Iron is essential for growth of Leishmania, Plasmodium, Trichomonas and Trypanosoma

species in vitro.

-Iron is also used by schistosomes for development and reproduction

-This development can be disrupted by administration of iron chelators.

Iron-uptake transporters and receptors are implicated in the pathogen virulence and immunogenicity.

They are generally surface located,

making them favorable drug or vaccine targets.

There is evidence that the antiparasitic activity of artemisinine drug family (effective against both schistosomiasis and malaria) is iron-dependent.

Inhibition of hemozoin formation by malaria parasites

-Hemozoin formation is a validated target for most of the well-known

existing antimalarial drugs .

-It is a suitable target to develop new antimalarials through studying the possible mechanisms of free heme

detoxification.

Inhibitors of ergosterol biosynthesis pathway of T. cruzi Trypanocidal agents capable to block ergosterol biosynthesis are currently investigated.

Posaconazole and ravuconazole are planned to enter in clinical trials in the near future.

Nanotherapy

Nanaparticles have been used to enhance the efficacy of some antiparasitic medications

1 -Nanosilver as a potential drug adjuvant against lymphatic filariasis

Nanoparticles of silver endowed (talented) with antibacterial potency are known to induce apoptosis in eukaryotic cells .

-Nanoparticles of silver ,but not of gold, elicited effective microfilaricidal activity on microfilariae of brugia malayi at relatively low concentrations .

-There is evidence of the induction of apoptosis in microfilariae .

-Nanosilver may act synergistically with DEC:

-either individually -or as an adjunct to this standard drug

(after critical evaluation of safety parameters).

2 -Antileishmanial effect of silver nanoparticles

One of the best-known mechanisms of the antibacterial effects of silver nanoparticles (Ag-NPs) is the production of reactive oxygen species to which Leishmania parasites are

very sensitive.

Ag-NPs demonstrated significant antileishmanial effects by inhibiting the proliferation and metabolic activity of promastigotes.

However, the antiparasitic mechanism was oxidative attack by Ag-NPs.

3 -The use of Np as a delivery device of indinavir against Cryptosporidium parvum

Indinavir (IND) can reduce Cryptosporidium parvum infection in both in vitro and in vivo models .

Limitations are - Renal toxicity

-High rate of metabolism and degradation.

-Tetramethylrhodamine-labelled

Nanoparticles were

-loaded with IND

-modified by conjugation with an anti-Cryptosporidium IgG polyclonal antibody

Simultaneous addition of 50μM of IND loaded NP (Ab-TMR-IND-Np) and excysted oocysts to the cell culture resulted in complete inhibition of the infection.

Therefore, this might represent a novel therapeutic strategy against

Cryptosporidium spp. infection .

The use of Np as an IND delivery device, allows the development of a more appropriate dose formulation reducing the IND side effects.

4 -Solid lipid nanoparticle suspension enhanced the therapeutic efficacy of praziquantel against a tapeworm

PZQ-loaded, hydrogenated castor oil solid lipid nanoparticle (PZQ-HCO-SLN) suspension was

investigated.

The SLN suspension significantly enhanced the pharmacological activity and therapeutic efficacy of

PZQ in E.granulosus infected dogs .

The suspension would be a promising formulation for prophylaxis and therapy of

tapeworm infection

Molecular-based immunotherapy

A number of zoite surface (glyco) proteins are expressed during, and believed to be involved in, invasion and infection of host epithelial cells by c.parvum.

In the absence of protective treatments for this illness, antibodies targeted against these zoite surface (glyco) proteins offer a rational

approach to therapy .

Monoclonal, polyclonal and recombinant antibodies represent useful immunotherapeutic means of combating infection, especially when highly immunogenic C. parvum antigens are utilized as targets.

Interruption of life cycle stages of this parasite via antibodies that target critical surface-exposed proteins can potentially decrease:

*The severity of symptoms*Subsequent re-infection of host

tissues.

Cell Therapy

Stem cellsStem cells are non-specialized cells that

possess two main characteristics :-the ability to self-renew, maintaining a

pool of stem cells ,-the potential for differentiation into

specialised cells .*Two main types of stem cells

considered for tissue regeneration, embryonic and adult stem cells.

The sources commonly used for experimental studies are the bone marrow, adipose tissue, dental pulp, umbilical cord blood and

placenta .

-Chronic chagasic cardiomyopathy (CCC) is the most prevalent form of chronic symptomatic

disease .

-It is characterised by an intense myocarditis, myocytolysis, neuronal destruction and

replacement of lost cardiac tissue by fibrosis .

-These pathological changes may consequently cause heart failure and death.

The discovery of stem cells capable of differentiating into specialised cell types has opened new avenues for the treatment of degenerative disorders, including heart failure. In the case of Chagas disease, this therapy is being developed not to kill the infectious agent, but to ameliorate the heart disease caused by chronic infection with T. cruzi.

-Benznidazole ( the main antiparasitic drug available for the etiological treatment of Chagas’ disease) is mainly effective in the acute phase of infection

But, its use during the chronic phase has been highly debated.

Heart transplantation is the only

effective treatment available for

patients with heart failure due to

Chagas disease

 

Stem cells can promote the degradation of fibrosis although the specific

mechanisms remains to be clarified .

Gene Therapy

Principle:Short strands of "antisense" nucleic acid-like material are combined with a small peptide that can transport those strands through cell membranes and into parasites living within cells (T. gondii, malaria parasites)

They disrupt genetic signals shutting down any of the parasite's genes .

-Example :In treating toxoplasmosis;

- It showed early promise where it prevented the parasites from making selected proteins.

-In newly infected mice, it reduced the number of viable %parasites by more than 90

Production of an enzyme, dihydrofolate reductase (needed by the parasite to make folate and to replicate)was markedly reduced after 48 hours.

Antisense oligomers targeting another enzyme and factors that direct the activity of many genes "transcription factors" were also successful, reducing parasite replication.

Studies to eradicate latent stages of T. gondii, (dormant in retina or brain cells for years), are

underway .Such transductive peptides could bring small molecules into the untreatable dormant phase of

the parasite .

Expansion of the repertoire of possible therapies for treatment of neglected

protozoal diseases

Human African trypanosomiasisCurrently used drugs for HAT are toxic; however, left untreated, the disease is fatal.

For T b gambiense -Pentamidine and suramin are available

for early-stage disease.

-Eflornithine and melarsoprol are used for late-stage (CNS) disease

For T b rhodesiense -There is little new clinical evidence

regarding the treatment .

-Suramin is used for early-stage disease, and melarsoprol is used for late-stage disease

Combination therapyNifurtimox-eflornithine combination therapy and melarsoprol-nifurtimox combination therapy were more effective than monotherapy for treatment of late stage Gambian cases.

However, melarsoprol-nifurtimox had higher death rates

is the first new clinical drug candidate with the potential for treating advanced-stage sleeping sickness in thirty years.

Fexinidazole

Fexinidazole is -a 5-nitroimidazole drug

-currently in phase I clinical trials-rediscovered by the Drugs for

Neglected Diseases initiative (DNDi) .

F exinidazole and its two principal metabolites, sulfoxide and sulfone

Effective against both T. b. gambiense and T. b. rhodesiense

In vitro and in vivo (in mouse model)

In acute and chronic stages of the

disease

-Fexinidazole does not pose a genotoxic hazard to patients .

-It represents a promising drug candidate for HAT .

- Fexinidazole is about to enter Phase II clinical trials.

Screening efforts have led to the identification of two new classes of anti trypanosomal agents:

nitrobenzylphosphoramide mustards and aziridinyl nitrobenzamides, promising to yield new safer and more effective drugs.

Chagas’ Disease The role of therapy in Chagas disease is controversial.

However, most authorities recommend treatment for:

-Acute and congenital infections,- Infections in children ,

-Reactivated infections in immunocompromised patients.

Current chemotherapy (based on the nitroheterocyclic compounds,

benzonidazole and nifurtimox) provides :-unsatisfactory results

-considerable side effects .

Benznidazole is-Better tolerated

-Generally considered the drug of choice . –But contraindicated in pregnancy and

renal and hepatic insufficiency.

However

Benznidazole for indeterminate (asymptomatic acute cases) or

early chronic Chagas’ disease may :-improve parasite clearance rates

-prevent progression to cardiomyopathy.

Several new triazoles, squalene synthase inhibitors, and cysteine protease inhibitors have shown efficacy in animal models.

A phase II clinical trial of posaconazole vs benznidazole for chronic Chagas disease is under way.

Visceral leishmaniasis 1- Pentavalent antimonial agents, such as sodium stibogluconate, were previously the therapeutic choice for VL. Currently: -Their use is limited due to resistance especially in South Asia.

- They are poorly tolerated-Response rates are variable and relapses are common.

2 -Amphotericin Liposomal amphotericin is more effective

and less toxic than amphotericin B .

3 -Miltefosine ,-The only orally available drug for VL,

-Monotherapy may lead to drug resistance .

4 -Paromomycin is an alternative for VL .Miltefosine and Paromomycin were non-inferior to amphotericin

Combination therapy for VL Shorter

treatment duration

Decreased toxicity No resistance

So, it was more effective than monotherapy.

-In Sudan: paromomycin + antimony -In India:

*Single-dose liposomal amphotericin + short-course miltefosine

*Single-dose liposomal amphotericin+ short-course paromomycin

*Short-course miltefosine + paromomycin

Cutaneous leishmaniasis (CL) and Mucocutaneous leishmaniasis (ML)

-The lesions of CL usually heal spontaneously.- The decision to treat depends on:

*Lesion location and size.

*The region of acquisition and infecting spp .

*The risk of progression to ML (limited to some New World CL infecting species) .

*Patient preference.

-Antimonial agents and oral miltefosine gave variable response in different areas of Old World and New World.

-In New World, Combination therapy

with allopurinol or pentoxifylline plus antimonial agents may be more effective

than antimonial agents alone .

Amphotericin (particularly liposomal formulations) has been successful in a growing number of CL patients infected in both the Old

and New World. However, the optimal dosing regimen has not

yet been determined

Topical therapy -Paromomycin(15%)-methyl benzethonium(12% )ointment and intra-lesional antimonial agents, may be an alternative treatment for Old World CL.

-Imiquimod, a topical immunomodulator, improved cure rates in Peru (New World) when given with parenteral antimonial agents.

Novel drugs Sitamaquine (an oral 8-aminoquinoline) cured 50% to 90% of patients with VL in phase 2 trials.

Fexinidazole, a drug currently in phase 1 clinical trials for treating African trypanosomiasis, shows promise for

treating visceral leishmaniasis.

This drug is rapidly oxidized in vivo in mice, dogs and humans to sulfoxide and sulfone metabolites.

Both metabolites were active against Leishmania donovani amastigotes grown in macrophages, whereas the parent compound

was inactive .

Trials with azithromycin, amphotericin, miltefosine, and low-dose antimonial agents for CL are ongoing.

MalariaFalciparum malaria is the most lethal malaria parasite.

Treatment of Uncomplicated P. falciparum malaria

The preferred treatment in areas with chloroquine resistance is combination therapy

1 -Atovaquone-proguanil , 2 -Artemether-lumefantrine (AL) ,

3 -Oral quinine plus doxycycline .

Resistance: -Chloroquine resistance is

widespread .-Atovaquone-proguanil–resistant

P falciparum is rare .-However, WHO recommends

that artemisinins be used exclusively in combination regimens

-More recently, strains of P falciparum have developed decreased sensitivity to artemisinins -These have emerged along the border between Cambodia and Thailand, in part because of long-standing monotherapy practices

Novel antimalarial therapiesArterolane, a synthetic trioxolane derived from artemisinins, is undergoing phase 3 clinical trials in combination with piperaquine for P.

falciparum malaria .

In 2 recent clinical trials, pyronaridine-artesunate and azithromycin plus artesunate were satisfactory for treatment of falciparum malaria.

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