Current Pharmaceutical Design - Volume 13, Issue 6, 2007

Parasitic diseases are the foremost worldwide health problem today, particularly in the under developed countries. It is estimated that the global prevalence of some of these diseases already exceeds 60% among the more than three billion people living in parasite endemic areas. Parasitic diseases are not confined to humans but also affect many domestic and wild animals causing an enormous economic blight to already poor countries and societies. In spite of the alarming health and economic consequences of parasitic infections, these diseases are still on the rise, largely because of poor sanitation and health education, inadequate measures of control, greater use of irrigation for agricultural development, an increase and redistribution of world population, increased world travel, and the development of resistance to drugs used for chemotherapy or chemicals for the control of vectors. In addition, with the recent advent of AIDS, several parasitic diseases which previously did not constitute a major threat to human health emerged as causative agents of lethal opportunistic infections (e.g., toxoplasmosis, cryptosporidiosis). Furthermore, the high mortality rate of some of the parasitic diseases, such as malaria, cannot be ignored. Malaria causes the death of more than two million children every year. Most parasitic diseases, however, like Ascaris or Ancylostoma infections, remain neglected because their effects on human health are more subtle. At the present time, chemotherapy is still the main stay to control most parasitic diseases, since antiparasitic vaccines are not yet available. Nevertheless, the need for new drugs is crucial to prevent or combat some major parasitic infections, (e.g., trypanosomiasis), as no single effective way of controlling this disease is available, or because some serious parasitic infections (e.g., malaria) have developed resistance to presently available drugs. Most of the currently available antiparasitic drugs have been discovered empirically by screening large numbers of compounds for efficacy against parasites in animal models. Few of these drugs have been rationally designed. This is largely because, until recently, little was known about the basic biochemistry, physiology, and molecular biology of parasites and of their interactions with their hosts. The rational design of a drug is usually based on biochemical and physiological differences between pathogens and their hosts. The ideal drug target is a protein that is essential for the parasite and does not have homologues in the host. The entry of parasites into the post-genomic age raises hopes for the identification of such novel kinds of drug targets and in turn, new treatments for parasitic diseases. However powerful, this functional genomics approach will miss some of the attractive targets for the chemotherapy of parasites as many essential proteins tend to be more highly conserved between species than non-essential ones. The articles in the current issues discuss such topics and elucidate a number of the most striking differences between parasites and their mammalian host that constitute excellent potential targets for the rational design of antiparasitic chemotherapeutic regimens. In the first article, Lüscher et al. [1], using trypanosomiasis as an example, discuss several current, successful parasiticides attack targets that have close homologues in their hosts where a therapeutic window is opened only by subtle differences in the regulation of the targets, which cannot be recognized in silico. They also advocate drug targeting, i.e. uptake or activation of a drug via parasite-specific pathways, as a chemotherapeutic strategy to selectively inhibit enzymes that have equally sensitive counterparts in the host. Like most of the other parasites studied, Trypanosomes are purine auxotrophs incapable of de novo purine biosynthesis. They depend on the salvage pathways for their vital purine requirements. Therefore, selective interruption of the parasite purine transport and/or enzymes that utilize these purines are potential targets for chemotherapy. The article of Baldwin and Coworkers [2] focuses its attention on the critical role of nucleoside transport in providing vital purines for the survival of malarial parasites.........

Trypanosoma brucei rhodesiense and T. b. gambiense are the causative agents of sleeping sickness, a fatal disease that affects 36 countries in sub-Saharan Africa. Nevertheless, only a handful of clinically useful drugs are available. These drugs suffer from severe side-effects. The situation is further aggravated by the alarming incidence of treatment failures in several sleeping sickness foci, apparently indicating the occurrence of drug-resistant trypanosomes. Because of these reasons, and since vaccination does not appear to be feasible due to the trypanosomes' ever changing coat of variable surface glycoproteins (VSGs), new drugs are needed urgently. The entry of Trypanosoma brucei into the post-genomic age raises hopes for the identification of novel kinds of drug targets and in turn new treatments for sleeping sickness. The pragmatic definition of a drug target is, a protein that is essential for the parasite and does not have homologues in the host. Such proteins are identified by comparing the predicted proteomes of T. brucei and Homo sapiens, then validated by large-scale gene disruption or gene silencing experiments in trypanosomes. Once all proteins that are essential and unique to the parasite are identified, inhibitors may be found by high-throughput screening. However powerful, this functional genomics approach is going to miss a number of attractive targets. Several current, successful parasiticides attack proteins that have close homologues in the human proteome. Drugs like DFMO or pyrimethamine inhibit parasite and host enzymes alike - a therapeutic window is opened only by subtle differences in the regulation of the targets, which cannot be recognized in silico. Working against the post-genomic approach is also the fact that essential proteins tend to be more highly conserved between species than non-essential ones. Here we advocate drug targeting, i.e. uptake or activation of a drug via parasite-specific pathways, as a chemotherapeutic strategy to selectively inhibit enzymes that have equally sensitive counterparts in the host. The T. brucei purine salvage machinery offers opportunities for both metabolic and transport- based targeting: unusual nucleoside and nucleobase permeases may be exploited for selective import, salvage enzymes for selective activation of purine antimetabolites.

Malaria constitutes an enormous drain on the health and economies of many countries and causes more than a million deaths annually. Moreover, resistance to existing antimalarial drugs is a growing problem, rendering the search for new targets urgent. Protozoan parasites of the genus Plasmodium that cause malaria lack the ability to synthesise the purine ring de novo and so are reliant upon salvage of purines, including hypoxanthine, inosine and adenosine, from the host. The transport systems responsible for uptake of these precursors are therefore promising targets for novel antimalarial drugs. In humans, purine uptake into many cell types is mediated by members of the Equilibrative Nucleoside Transporter (ENT) family, in particular hENT1 and hENT2. Genome sequencing has revealed that P. falciparum and P. vivax, the species responsible for the majority of malaria cases, each also possesses four members of this family, and in P. falciparum transcripts of each are expressed in the erythrocytic stages of the parasite responsible for clinical disease. One of the proteins, PfENT1, is known to be present in the parasite plasma membrane, and the kinetic properties of the heterologously expressed transporter are consistent with its representing the major purine uptake system in the trophozoite. Importantly, its inhibitor specificity and permeant selectivity differ from those of the host. In this review we discuss the possibility of exploiting these differences to develop novel antimalarial drugs that either selectively inhibit purine uptake into the pararasite or are selectively delivered by the transporter to the parasite cytoplasm.

Toxoplasma gondii is an intracellular parasitic protozoan that infects approximately a billion people worldwide. Infection with T. gondii represents a major health problem for immunocompromised individuals, such as AIDS patients, organ transplant recipients, and the unborn children of infected mothers. Currently available drugs usually do not eradicate infection and as many as 50% of the patients do not respond to this therapy. Furthermore, they are ineffective against T. gondii tissue cysts. In addition, prolonged exposure to these drugs induces serious host toxicity forcing the discontinuation of the therapy. Finally, there is no effective vaccine currently available for the treatment of toxoplasmosis. Therefore, it is necessary to develop new and effective drugs for the treatment and management of toxoplasmosis. The rational design of a drug depends on the exploitation of fundamental biochemical or physiological differences between pathogens and their host. Some of the most striking differences between T. gondii and their mammalian host are found in purine metabolism. T. gondii, like most parasites studied, lack the ability to synthesize purines do novo and depend on the salvage of purines from their host to satisfy their requirements of purines. In this respect, the salvage of adenosine is the major source of purines in T. gondii. Therefore, interference with adenosine uptake and metabolism in T. gondii can be selectively detrimental to the parasite. The host cells, on the other hand, can still obtain their purine requirements by their de novo pathways. This review will focus on the broad aspects of the adenosine transport and the enzyme adenosine kinase (EC 2.7.1.20) which are the two primary routes for adenosine utilization in T. gondii, in an attempt to illustrate their potentials as targets for chemotherapy against this parasite.

Tuberculosis remains a serious health problem throughout the world, and new drugs are needed to help control this disease. We have identified several purine nucleoside analogs that exhibit selective activity against Mycobacterium tuberculosis. The lead compound in this series is 2-methyl-adenosine (methyl-Ado), which is active against proliferating and nonproliferating bacteria due to its ability to inhibit protein synthesis. Methyl-Ado is activated by adenosine kinase that is expressed in M. tuberculosis cells. The primary intracellular metabolite is 2-methyl-AMP, although some methyl- ATP was also produced in the cells. Adenosine kinase has been purified from M. tuberculosis cells and its biochemical activity has been characterized and compared to that of the human homolog. The gene for adenosine kinase has been determined to be Rv2202c, which had been putatively identified as a sugar kinase. Because very little is known about purine metabolism in M. tuberculosis, we have initiated studies to characterize the enzymes that are involved in salvage of purine nucleosides. We believe that enhanced knowledge of the characteristics of the enzymes involved in purine salvage in M. tuberculosis should aid in the rational design of more potent purine analogs that can selectively inhibit M. tuberculosis replication. Compounds in this class should be active against strains of M. tuberculosis that are resistant to current agents used to treat this disease and may also target latent disease.

Opportunistic infections are known to cause morbidity and mortality in immunocompromised individuals. In addition, serious infections due to several parasites are also known to affect the quality and duration of life in normal individuals. The importance of dihydrofolate reductase (DHFR) in parasitic chemotherapy arises from its function in DNA biosynthesis and cell replication. DHFR catalyzes the reduction of dihydrofolate (DHF) to tetrahydrofolate (THF), an essential cofactor in the biosynthesis of thymidylate monophosphate (dTMP). Inhibition of DHFR leads to a deficiency of dTMP since DHF cannot be recycled, and thus causes inhibition of cell growth. Methotrexate (MTX) and aminopterin (AMT) were among the first known classical inhibitors of DHFR. Trimethoprim (TMP) and pyrimethamine (PYR) are among the first known non classical inhibitors of DHFR. TMP and PYR are selective but weak inhibitors of DHFR from several parasitic organisms and coadministration of sulfonamides is required to provide synergistic effects for clinical utility. Unfortunately, the side effects associated with sulfa drugs in this combination often result in cessation of therapy. Trimetrexate (TMQ) and piritrexim (PTX) are two potent non classical inhibitors, neither of which exhibit selectivity for pathogen DHFR and must be used with host rescue. However, the current combination therapy suffers from high cost, in addition, several mutations have been reported in the active site of parasitic DHFR rendering the infections refractive to known DHFR inhibitors. The selectivity of TMP is a hallmark in the development of DHFR inhibitors and several efforts have been made to combine the potency of PTX and TMQ with the selectivity of TMP. Thus the structural requirements for DHFR inhibition are of critical importance in the design of antifolates for parasitic chemotherapy. Structural requirements for inhibition have been studied extensively and novel agents that exploit the differences in the active site of human and parasitic DHFR have been proposed. This review discusses the synthesis and structural requirements for selective DHFR inhibition and their relevance to parasitic chemotherapy, since 1995.

Intracellular protozoan parasites are a great threat to animal and human health. To successfully disseminate through an organism these parasites must be able to enter and exit host cells efficiently and rapidly. The inhibition of invasion or egress of obligate intracellular parasites is regarded as a goal for drug development since these processes are essential for their survival and likely to require proteins unique to the parasites. Thus, a more comprehensive knowledge of invasion and egress proteins will aid in the development of drugs and vaccines against these intracellular pathogens. In recent years, the study of a particular parasite, Toxoplasma gondii, has yielded valuable information on how invasion and egress are achieved by some protozoan parasites. Besides being a good model system for the study of parasite biology, Toxoplasma is an important human pathogen capable of causing devastating disease in both immunocompromised individuals and developing fetuses. The lack of effective, inexpensive and tolerable drugs against Toxoplasma makes the development of new therapies an imperative. The following review describes how the identification and in depth study, using proteomics, forward genetics and pharmacology of the Toxoplasma proteins involved in entering and exiting human cells provide an important starting point in identifying targets for drug discovery.

Natural products have served humankind as drug leads for thousands of years. In the last century natural products have not only served as drugs but have inspired the generation of countless synthetic drugs and drug-leads around natural product pharmacophores. There are no disease targets for which natural products have played a more significant role than in the case of malaria and other parasitic diseases. In this review the significance of the manzamine class of marine alkaloids is presented as an example of the future utility of the oceans in the development of antiparasitics. The manzamines represent one of the few new structural classes identified in recent decades with potential for the control of malaria and tuberculosis. While considerable work remains to successfully optimize this class of drug-leads the novel pharmacophore and significant metabolic stability combined with a rapid onset of action and long half-life all strongly support further investigations of this group of potential drug candidates.