Current Topics in Medicinal Chemistry - Volume 11, Issue 16, 2011

Infections caused by the invasion of protozoan parasites into human hosts represent one of the most serious public health problems in the world today with approximately one billion people living in areas of risk. Such diseases include: (i) malaria, caused by Plasmodium spp, with falciparum and vivax being the most lethal and prevalent species; (ii) visceral leishmaniasis caused by Leishmania donovani, infantum or chagasi (iii) sleeping sickness (Human African Trypanosomiasis) caused by Trypanosoma brucei; (iv) Chagas disease caused by Trypanosoma cruzi and; (v) schistosomiasis caused by Schistosoma spp. Malaria is the most well documented and is found in Africa, Central and South America and South-East Asia. Statistically malaria is the most lethal of the parasitic diseases with about 1-2 million deaths per year, but other protozoan parasites are also a major source of disease leading to morbidity and mortality. In terms of the number of reported infections, schistosomiasis and malaria are similar, though the number of deaths per annum due to malaria is higher. Schistosomiasis is endemic in more than 70 developing countries predominantly in Africa, with more than 200 million people are infected leading to ∼20,000 deaths per year. Sleeping sickness is reported to affect as many as 66 million people in sub-Saharan Africa accounting for ∼50,000 deaths per year, while 7.7 million currently have Chagas disease which is endemic in 21 countries in Latin and South America. It accounts for ∼20,000 deaths per year. Historically, treatment of parasitic infections has predominantly relied on the application of plant products. For example, the bark of the chinchona tree or the leaves of the herb, Artemisia annua have been used for the treatment of malaria. These natural remedies were discovered fortuitously rather than by sound scientific reasoning. This is highlighted by the fact that the active components were not identified until many centuries later (i.e. quinine and artemisinin). Only a few therapeutic agents have been developed by pharmaceutical companies (e.g. benznidazole by Hoffman-La Roche for Chagas disease and Praziquantel by Bayer and Merck for schistosomiasis). This is understandable given the cost of developing new molecular entities as drugs can be as high as US$1.8 billion. The fact that parasitic diseases are found predominantly in the developing world has contributed to the lack of new cost-effective drugs to reach the market. Pharmaceutical companies generally have not been presented with the financial incentives to support the research required to discover and develop therapies against new targets. However, though funding is still limited, philanthropic societies such as the Bill and Melinda Gates Foundation are providing much needed sources of revenue to take up the challenge of eradicating these diseases. Today, however, the chemical arsenal to treat each of the parasitic diseases is still limited to less than a handful of compounds. Furthermore, other granting bodies are recognizing the challenge and importance of the severe socio-economic burden caused by these diseases. They are also providing some of the much needed support as evidenced by the acknowledgements in this series of reviews. As a result, the over reliance on the use of older medications has resulted in resistance now being commonplace. To partially overcome or delay the on-set of resistance, the idea of combination therapies has been introduced. For example, it is now recommended by the World Health Organization that the artemisisins only be administered in combination with other antimalarial drugs such as mefloquine, lumefantrine, amodiaquine, piperaquine and the antifolates, all of which most likely target alternative metabolic pathways to that of the artemisisins. However, even this approach is also now failing because there are reports that artemisisin resistance to Plasmodium falciparum malaria has developed along the Cambodian-Thailand border. As a result there are now no drugs currently available to treat malaria for which resistant strains have not emerged. The need for new antiparasitic drugs is further highlighted by the fact that medications to treat diseases such as Chagas and sleeping sickness can be painful to administer and the side-effects can be extremely harmful. For example, Melasoprol and Eflornithine are the only two drugs currently available to treat second stage African sleeping sickness. Melasoprol is an arsenic derivative and therefore highly toxic while the dose regime for Eflornithine is strict and difficult to apply to patients. Thus, there is an urgent requirement to develop new antiparasitic therapies that have reduced toxic side effects, are cost-effective, easy to administer and for which resistance is less likely to occur. To identify new antiparasitic candidate drug leads a number of diverse approaches can be adopted. These include random screening using the vast libraries of compound extracted from plants, venoms and other natural sources or the use of chemical libraries synthesized in pharmaceutical companies. However, ultimately the process of drug discovery will need to encompass a fundamental understanding of the biology of the organism and the chemistry of the drug if new therapeutics are to be developed to their maximum potential. Such new drugs should decrease significantly the ability of the parasite to mutate to develop resistance. The objective of this edition of “Current Topics in Medicinal Chemistry” is to indentify promising new antiparasitic drug leads, their targets and the progress that has been made to advance these leads towards developing new pharmaceuticals. Given that protozoan parasites have evolved with different biological characteristics the likelihood of the discovery of a general antiparasitic drug is remote. Nonetheless, some protozoan parasites do posses similar metabolic pathways and physical characteristics, thus it is plausible that drugs that successfully target one parasitic organism may also be effective in other parasites. For example (there are many others), pyrimethamine, a drug that inhibits dihydrofolate reductase, can be used to treat both malaria and immunocompromised patients infected by Toxoplasma gondi. This review therefore offers the opportunity for the potential integration of drug lead data that may have arisen through the studies of one pathogenic organism to be translated into drug lead data for an alternate organism....

Schistosomiasis is a widespread tropical parasitic disease, currently treated with Praziquantel, whose precise molecular target is actually unknown. Several other drugs are known to kill the schistosomes in vivo and in vitro, but these are seldom employed because of toxicity, high cost, complex administration or other reasons. The improvement of known drugs or the development of entirely new ones is a desirable goal, in view of the fact that strains of Schistosoma mansoni with reduced sensitivity to Praziquantel have appeared. In this review, we tried to collect the information available on known or putative macromolecular targets of schistosomicidal drugs; thus we focused on the biochemistry of the parasite, rather than the clinical properties of the drugs. The rationale of this approach is that drug design may become realistic if the mechanism of action of each known drug were known at atomic detail, ideally as the 3D structure of each drug in complex with its target. Important macromolecular targets of known drugs reviewed below are: Thioredoxin Glutathione Reductase; Cyclophilin; Acetyl Cholinesterase; Proteases and Purine Nucleoside Phosphorylase. Moreover, a few enzymes of the parasite are known, or thought, to be “druggable”, and therefore interesting, even though no specific drugs are available as yet: examples of such enzymes are Glutathione Peroxidase and Peroxiredoxins.

The increasing demand for novel anti-parasitic drugs due to resistance formation to well-established chemotherapeutically important compounds has increased the demands for a better understanding of the mechanism(s) of action of existing drugs and of drugs in development. While different approaches have been developed to identify the targets and thus mode of action of anti-parasitic compounds, it has become clear that many drugs act not only on one, but possibly several parasite molecules or even pathways. Ideally, these targets are not present in any cells of the host. In the case of apicomplexan parasites, the unique apicoplast, provides a suitable target for compounds binding to DNA or ribosomal RNA of prokaryotic origin. In the case of intracellular pathogens, a given drug might not only affect the pathogen by directly acting on parasite-associated targets, but also indirectly, by altering the host cell physiology. This in turn could affect the parasite development and lead to parasite death. In this review, we provide an overview of strategies for target identification, and present examples of selected drug targets, ranging from proteins to nucleic acids to intermediary metabolism.

The invasion of host cells by malaria parasites represents an attractive target for therapeutic intervention. The role played by apical membrane antigen 1 (AMA1) in this process has been elucidated recently with the demonstration that AMA1 forms a complex with parasite rhoptry neck (RON) proteins as part of the moving junction that develops between the host cell and the invading parasite. Structural studies of AMA1 alone and in complexes with antibodies that inhibit host cell invasion have identified a conserved hydrophobic cleft that is essential to the assembly of the AMA1/RON complex. AMA1 is already established as an important candidate for inclusion in a malaria vaccine. Here we review both the structural details and functional significance of interactions at the hydrophobic cleft of AMA1, and argue that this feature of the protein represents an excellent target for the development of drugs that would block host cell invasion by malarial parasites.

Two, simple, C5 compounds, dimethylally diphosphate and isopentenyl diphosphate, are the universal precursors of isoprenoids, a large family of natural products involved in numerous important biological processes. Two distinct biosynthetic pathways have evolved to supply these precursors. Humans use the mevalonate route whilst many species of bacteria including important pathogens, plant chloroplasts and apicomplexan parasites exploit the nonmevalonate pathway. The absence from humans, combined with genetic and chemical validation suggests that the nonmevalonate pathway holds the potential to support new drug discovery programmes targeting Gram-negative bacteria and the apicomplexan parasites responsible for causing serious human diseases, and also infections of veterinary importance. The non-mevalonate pathway relies on eight enzyme-catalyzed stages exploiting a range of cofactors and metal ions. A wealth of structural and mechanistic data, mainly derived from studies of bacterial enzymes, now exists for most components of the pathway and these will be described. Particular attention will be paid to how these data inform on the apicomplexan orthologues concentrating on the enzymes from Plasmodium spp. these cause malaria, one the most important parasitic diseases in the world today.

Pathogenic protozoa threaten lives of several hundred million people throughout the world and are responsible for large numbers of deaths globally. The parasites are transmitted to humans by insect vectors, more than a hundred of infected mammalian species forming reservoir. With human migrations, HIV-coinfections, and blood bank contamination the diseases are now spreading beyond the endemic tropical countries, being found in all parts of the world including the USA, Canada and Europe. In spite of the widely appreciated magnitude of this health problem, current treatment for sleeping sickness (Trypanosoma brucei), Chagas disease (Trypanosoma cruzi) and leishmaniasis (Leishmania spp.) remains unsatisfactory. The drugs are decades old, their efficacy and safety profiles are unacceptable. This review describes sterol 14α-demethylase, an essential enzyme in sterol biosynthesis in eukaryotes and clinical target for antifungal azoles, as a promising target for antiprotozoan chemotherapy. While several antifungal azoles have been proven active against Trypanosomatidae and are under consideration as antiprotozoan agents, crystal structures of sterol 14α-demethylases from three protozoan pathogens, Trypanosoma brucei, Trypanosoma cruzi and Leishmania infantum provide the basis for the development of new, highly potent and pathogen-specific drugs with rationally optimized pharmacological properties.

Chagas disease and African sleeping sickness are trypanosomal infections that represent important public health problems in Latin America and Africa, respectively. The restriction of these diseases to the poorer parts of the world has meant that they have been largely neglected and limited progress has been made in their treatment. The nitroheterocyclic prodrugs nifurtimox and benznidazole, in use against Chagas disease for >40 years, remain the only agents available for this infection. In the case of African sleeping sickness, nifurtimox has recently been added to the arsenal of medicines, with the nitroheterocycle fexinidazole currently under evaluation. For a long time, the cytotoxic mechanisms of these drugs were poorly understood: nifurtimox was thought to act via production of superoxide anions and nitro radicals, while the mode of benznidazole action was more obscure. The trypanocidal activity of nitroheterocyclic drugs is now known to depend on a parasite type I nitroreductase (NTR). This enzyme is absent from mammalian cells, a difference that forms the basis for the drug selectivity. The role of this enzyme in drug activation has been genetically and biochemically validated. It catalyses the 2-electron reduction of nitroheterocyclic compounds within the parasite, producing toxic metabolites without significant generation of superoxide. Recognition that this enzyme is responsible for activation of nitroheterocyclic prodrugs has allowed screening for compounds that preferentially target the parasite. This approach has led to the identification of two new classes of anti-trypanosomal agents, nitrobenzylphosphoramide mustards and aziridinyl nitrobenzamides, and promises to yield new, safer, more effective drugs.

Malaria remains the most serious parasitic diseases affecting humans in the world today, resulting in 1-2 million fatalities each year. Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) are the predominant causative agents. Both are responsible for widespread mortality and morbidity and are a serious socio-economic burden, especially for countries in the developing world. One of the most important defences against malaria has been the use of chemotherapeutic drugs (e.g. chloroquine, artemisinins, pyrimethamine) but these have mainly been found by serendipity. Their mechanisms was not understood at the time of their discovery and, even today, are still not unequivocal. For many of these compounds, the parasite is now resistant and, hence, there is an urgent need to develop new therapeutic drugs directed to validated targets. One metabolic pathway crucial for the survival and replication and survival of the parasite is the synthesis of the purine nucleoside monophosphates essential for the production of DNA/RNA molecules. A key enzyme in this pathway is the 6-oxopurine phosphoribosyltransferase (PRTase). The focus of this review is on the identification and characterization of inhibitors of the enzymes from both Pf and Pv as antimalarial drug leads. The acyclic nucleoside phosphonates (ANPs) appear to be excellent candidates because they are good inhibitors of the two Plasmodium enzymes, can be selective compared to the human enzyme, can arrest parasitemia in cell based assays, have low cytotoxicity to the human host cell and, because of their stable carbon-phosphorous bond, are stable within the cell.

Malaria is a leading cause of morbidity and mortality in the tropics. Chemotherapeutic and vector control strategies have been applied for more than a century but have not been efficient in disease eradication. Increased resistance of malaria parasites to drug treatment and of mosquito vectors to insecticides requires the development of novel chemotherapeutic agents. Malaria parasites exhibit rapid nucleic acid synthesis during their intraerythrocytic growth phase. Plasmodium purine and pyrimidine metabolic pathways are distinct from those of their human hosts. Thus, targeting purine and pyrimidine metabolic pathways provides a promising route for novel drug development. Recent developments in enzymatic transition state analysis have provided an improved route to inhibitor design targeted to specific enzymes, including those of purine and pyrimidine metabolism. Modern transition state analogue drug discovery has resulted in transition state analogues capable of binding to target enzymes with unprecedented affinity and specificity. These agents can provide specific blocks in essential pathways. The combination of tight binding with the high specificity of these logically designed inhibitors, results in low toxicity and minor side effects. These features reduce two of the major problems with the current antimalarials. Transition state analogue design is being applied to generate new lead compounds to treat malaria by targeting purine and pyrimidine pathways.