The science of chemotherapy has been built upon the seminal concepts arising from pioneering studies, notably by Paul Ehrlich, carried out nearly 100 years ago. The investigations conducted in the early decades of this century were of major significance for antiparasite chemotherapy as they resulted in the discovery of a variety of drugs for treating human and animal diseases. Derivatives of many of these drugs are still being used today, which, it could be claimed, reflects the outstanding success of the early approaches. However, it also highlights the relatively poor record of drug discovery in more recent decades. While major advances were taking place in the development of drugs and vaccines for bacterial, fungal and viral diseases, the discovery of new drugs for parasitic infections progressed relatively slowly. It would be wrong, however, to give an impression that there were no advances. The discovery of the avermectins and praziquantel has had an enormous impact on anthelmintic therapy, and new classes of antiprotozoal drugs have been developed for both prophylaxis and treatment of infections. Nevertheless, major problems remain and for some parasitic diseases the situation now is worse than it was 20 years ago. Malaria is perhaps the best example in this respect. The advent of chloroquine resistance in Plasmodium falciparum has resulted in a major public health problem. In addition, the increase in importance of some diseases, notably those associated with immunosuppression in AIDS, has again highlighted the limitations of antiparasite chemotherapy. New drugs are needed urgently for a number of parasitic diseases.

Currently used antiparasitic drugs, including benzimidazoles, nitroimidazoles, avermectins, polyene ionophores, hydroxynaphthoquinones and sesquiterpene lactones, were identified through the empirical route to drug discovery. The modern rational approach to drug design is focused upon the structure and function of biochemical and molecular targets. The requisite pharmacological properties for new anti-parasite drugs should not be ignored in this process.

Advances in the life and physical sciences have enabled us to characterize the 3-dimensional structure and the biochemical or biological activity of both small and large molecules. The use of structural chemistry to assist understanding of biological activity provides information relevant to the design, development or identification of new pharmaceuticals. This structure based approach has become an important component of drug research and is in widespread use by the major pharmaceutical companies. A brief historical introduction, to convey how this area of science has reached the present stage, is given. The basis of the structural approach to understanding the chemistry of small and large molecule biological activity is outlined with an emphasis on the use of results derived from X-ray diffraction methods. Developments in other areas are discussed to emphasize the multidisciplinary nature of this research and the benefits of combining different methods. Examples of protein crystallographic studies in the area of molecular parasitology, some of which are directly relevant to antiparasite drug design, are presented. The characterization of the enzyme trypanothione reductase, a project which has benefited from many of the recent developments, is detailed. Future challenges and difficulties, both scientific and economic, are discussed.

The enzymes and receptors in parasites that can be qualified as targets for antiparasite chemotherapy should perform essential functions in the parasites and demonstrate some feasibility for selective inhibition. They can be tentatively identified through detailed analysis of various aspects of metabolisms in the parasites or elucidation of the mechanisms of action among proven antiparasitic agents. Preliminary verifications of these putative targets can be indicated by in vitro antiparasite activity of an inhibitor of the target. However, before a major long-term effort to pursue in-depth structure-activity analysis of the target is to be committed for specific inhibitor design, further validations of the target are essential to insure that future studies are not misguided. One old-fashioned approach to validate a target in the pharmaceutical industry is by correlating target inhibitions with antiparasitic activities among large numbers of drug derivatives. The results are often indicative but hardly ever conclusive. Another method is by comparing the putative drug targets between the drug-sensitive and the drug-resistant parasites for potential discrepancies. Unfortunately, the latter often result from indirect causes, such as reduced drug transport, instead of an alteration of the drug target itself. The third experimental approach is by disrupting the gene encoding the putative target in parasite, which can provide the most conclusive evidence on whether the target plays an indispensible role in the parasite. But special conditions are needed for the gene knockout mutants to survive to exhibit their phenotypes and to allow genetic complementation studies for further verifications. Furthermore, gene knockout experiments are often difficult to perform on cells of multiple ploidy or genes of multiple copies, and are currently applicable only to a limited number of protozoan parasites. In the current article I have tried to take a cursory look at some eleven putative drug targets among various parasites, each supported by well-established antiparasitic agents identified as its inhibitors. I have also considered the evidence for validity of each of them and the potential means of further verifying their validity.

Chemically-modified oligonucleotides are now routinely used to prevent gene expression in cell-free media and in cultured cells. The binding of an antisense sequence to a complementary RNA target may lead to the selective inhibition of the encoded information. This may occur at different levels: splicing; transport of the mature RNA from the nucleus to the cytoplasm; translation. Antisense oligonucleotides constitute an interesting tool to shed some light on gene function. They are also potential new therapeutic agents against pathogenic organisms. This review discusses the rules that guide the design of an antisense oligomer and the choice of a target sequence. Examples of the potential use of antisense oligonucleotides in the fields of virology and parasitology, in particular in relation to trypanosomatids, are described.

Parasite enzymes involved in proteolysis and amino acid metabolism have attracted considerable attention over the last decade. Nevertheless, current knowledge is extensive for just a few parasites and several enzymes. Most enzymes remain largely unexplored. This review concentrates upon a selection of the better studied enzymes and the potentially valuable approaches now being adopted in their study. We present a personal view on the most suitable strategies for exploiting this area of parasite biochemistry with novel antiparasite drugs. The content of the review reflects our own work and interests, but we have aimed to include a sufficiently broad range of topics so that this overview serves as a useful introduction for those new to the subject.

There have been several reviews that provide good coverage of the appropriate literature (Barrett, 1991; McKerrow et al. 1993; North & Lockwood, 1995; Sakanari et al. 1995; Robertson et al. 1996; Vial, 1996; Coombs & Mottram, 1997; Walker & Barrett, 1997), therefore we detail here just some of the publications and refer readers to the reviews quoted for further information. This treatise mainly highlights progress made in studies with parasitic protozoa. Parasitic worms present more difficult problems for drug designers and there has been only limited progress to date in this area of biochemistry; we include here mention of just some of the more exciting advances so far.

A metabolic pathway known as the mannitol cycle has been identified in Eimerian parasites. The pathway is a shunt off of the glycolytic pathway at fructose-6-phosphate (F6P). Two enzymes convert F6P to mannitol and two other enzymes are responsible for converting mannitol back to F6P when it is utilized. Although the pathway is present in various stages of the parasite the most apparent role of this pathway is in the sexual portion of the life cycle, particularly in the formation of oocysts. Extremely high concentrations of mannitol, approaching 0.3 M, are present in unsporulated oocysts. Mannitol functions as the endogenous energy source for oocysts to sporulate in the environment outside of the host. An inhibitory protein which inactivates the first enzyme of the mannitol cycle has been isolated from an oocyst derived inhibited enzyme complex and is believed to prevent the futile cycling of F6P during the maturation of oocysts. Evidence of the vital role of mannitol in the development and maturation of Eimeria tenella oocysts has been facilitated through the use of the drug Nitrophenide™, a known anticoccidial which has now been found to be an inhibitor of one of the enzymes responsible for the biosynthesis of mannitol in the parasite. This compound prevents the formation of oocysts and at lower doses reduces mannitol levels in shed oocysts. In addition, oocysts with reduced mannitol levels fail to complete the sporulation process lending further evidence for this polyol's role in the parasite.

Inhibitors of sterol and phospholipid biosynthesis in kinetoplastid parasites such as Trypanosoma cruzi, the causative agent of Chagas' disease, and different species of Leishmania have potent and selective activity as chemotherapeutic agents in vitro and in vivo. Recent work with the sterol C14α-demethylase inhibitor D0870, a bis triazole derivative, showed that this compound is capable of inducing radical parasitological cure in murine models of both acute and chronic Chagas' disease. Other inhibitors of this type, such as SCH 56592, have also shown curative, rather than suppressive, activity against T. cruzi in these models. Leishmania species have different susceptibilities to sterol biosynthesis inhibitors, both in vitro and in vivo. Leishmania braziliensis promastigotes, naturally resistant to C14α-demethylase inhibitors such as ketoconazole and D0870, were susceptible to these drugs when used in combination with the squalene epoxidase inhibitor terbinafine. Inhibitors of Δ24(25) sterol methyl transferase have been shown to act as potent antiproliferative agents against Trypanosoma cruzi, both in vitro and in vivo. New inhibitors of this type which show enhanced activity and novel mechanisms of action have been synthesized. Recent work has also demonstrated that this type of enzyme inhibitors can block sterol biosynthesis and cell proliferation in Pneumocystis carinii, a fungal pathogen which had previously been found resistant to other sterol biosynthesis inhibitors. Ajoene, an antiplatelet compound derived from garlic, was shown to have potent antiproliferative activity against epimastigotes and amastigotes of Trypanosoma cruzi in vitro; this activity was associated with a significant alteration of the phospholipid composition of the cells with no significant effects on the sterol content. In addition, alkyllsophospholipids such as ilmofosine, miltefosine and edelfosine have been shown to block the proliferation of T. cruzi and Leishmania and alter both the phospholipid and sterol composition. These results indicate the potential of lipid biosynthesis inhibitors as useful therapeutic agents in the treatment of leishmaniasis and Chagas' disease.

Leishmania and other trypanosomatid protozoa require reduced pteridines (pterins and folates) for growth, suggesting that inhibition of these pathways could be targeted for effective chemotherapy. This goal has not yet been realized, indicating that pteridine metabolism may be unusual in this lower eukaryote. We have investigated this possibility using both wild type and laboratory-selected antifolate-resistant strains, and with defined genetic knockouts of several pteridine metabolic genes. In Leishmania, resistance to the antifolate methotrexate is mediated through several mechanisms singly or in combination, including alterations in transport leading to reduced drug influx, overproduction (R-region amplification) or point mutation of dihydrofolate reductase-thymidylate synthase (DHFR-TS), and amplification of a novel pteridine reductase (PTR1, encoded by the H-region). All of the proteins involved are potential targets for antifolate chemotherapy. Notably, parasites in which the gene encoding dihydrofolate reductase (DHFR) has been deleted (dhfr-ts− knockouts) do not survive in animal models, validating this enzyme as a target for effective chemotherapy. However, the properties of pteridine reductase 1 (PTR1) suggest a reason why antifolate chemotherapy has so far not been successful in trypanosomatids. PTR1, by its ability to provide reduced pterins and folates, has the potential to act as a by-pass and/or modulator of DHFR inhibition under physiological conditions. Moreover, PTR1 is less sensitive to many antifolates targeted primarily against DHFR. These findings suggest that successful antifolate chemotherapy in Leishmania will have to target simultaneously both DHFR and PTR1.

This paper reviews sites of action of anthelmintic drugs including: (1) levamisole and pyrantel, which act as agonists at nicotinic acetylcholine receptors of nematodes; (2) the avermectins, which potentiate or gate the opening of glutamate-gated chloride channels found only in invertebrates; (3) piperazine, which acts as an agonist at GABA gated chloride channels on nematode muscle; (4) praziquantel, which increases the permeability of trematode tegument to calcium and results in contraction of the parasite muscle; (5) the benzimidazoles, like thiabendazole, which bind selectively to parasite β-tubulin and prevents microtubule formation; (6) the proton ionophores, like closantel, which uncouple oxidative phosphorylation; (7) diamphenethide and clorsulon, which selectively inhibit glucose metabolism of Fasciola and; (8) diethylcarbamazine, which appears to interfere with arachidonic acid metabolism of filarial parasites and host. The review concludes with brief comments on the development of anthelmintics in the future.

Despite considerable therapeutic success with the antimalarial 4-aminoquinolines such as chloroquine, there is serious doubt about the future of this drug class due mainly to the development and spread of parasite resistance throughout endemic areas. In this article we review the possible biochemical and molecular basis of resistance. Based on our current understanding we have considered the possibility of developing strategies which may allow the aminoquinolines to once again be used effectively against P. falciparum. Our conclusions are that drug resistance is the result of a reduced rate of drug uptake which in turn reduces the amount of drug available to bind the target. The basis for this reduced accumulation could be an altered pH gradient making the food vacuole more alkaline or the parasite cytosol more acidic, an efflux pump removing drug directly from the membrane or any other process which will reduce the rate of drug uptake. Central to the effectiveness of this resistance mechanism is the transient availability of a high affinity, low capacity drug binding site (possibly haem) within the parasite. Resistance reversers such as verapamil influence the apparent Ka for this drug binding phenomenon via an increased drug uptake rate. We demonstrate that by chemical modification of the aminoquinolines, producing predictable alterations in their physicochemical properties, that it is possible to minimise the verapamil sensitive component of resistance and reduce significantly cross-resistance patterns without loss in absolute activity. Based on these views we suggest that the aminoquinoline antimalarials still have a role to play in the cheap, safe and effective chemotherapy of falciparum malaria.

Parasitic helminths (worms) cause serious infectious diseases in humans and domestic animals. Control of these infections relies mostly on chemotherapeutics (the anthelmintics), but resistance has developed against most of these broad-spectrum drugs in many parasite species. These resistant parasites are being used to elucidate the molecular mechanisms of drug resistance and drug action. This has led to the development of sensitive assays to detect resistant parasites, but this has not delayed the emergence of additional drug resistant parasite populations. Therefore, as development of new drugs by pharmaceutical companies is slow, we may have to be prepared for a time when broad-spectrum drugs are no longer effective, especially against worms of sheep.

New chemotherapies are urgently needed for the parasitic infections of animals and for the tropical diseases of man. Rational molecular design approaches to attempt to discover such products require a massive investment of resources up-front of actual chemical synthesis. However, such investment is justified, since chemical synthesis itself is highly resource-consuming. The fact that few targets have yet been validated to justify a rational approach is an argument only to get on and validate more. Not all the components of molecular design can yet be done totally rationally, but this is not an argument against applying this approach where it is possible. Absence of a successful track record is inevitable for any newly emerging technology. It is too early to draw conclusions about the relative costs of rational design versus empirical synthesis, since the former is only now beginning to become reality and the latter is in the middle of a (combinatorial) revolution. Similarly, it is too soon to predict with certainty which of these two approaches will prevail in the long run. However, they lend themselves to parallel tracks, so both may well continue for the foreseeable future. Current concerns about who would develop successful discoveries are not reasons for stopping discovery research. Indeed, a string of putative products held at the discovery/development interface would be useful ammunition to those trying to develop partnerships such as a Tropical Diseases R&D Alliance aimed at carrying out such work and sharing costs.