Infectious Disorders - Drug Targets (Formerly Current Drug Targets - Infectious Disorders) - Volume 10, Issue 3, 2010

Malaria is a devastating disease that kills approximately two million people annually. The causative agent of malaria is a protozoan parasite from the genus Plasmodium. Of the species that infect man, Plasmodium falciparum is responsible for the majority of mortalities. Many efforts have been initiated to eliminate or reduce the malaria burden however in the absence of an effective vaccine; drugs must be used for prophylaxis and treatment. Unfortunately P. falciparum has developed resistance to multiple antimalarial drugs and it is envisioned that resistance will continue and undermine the effectiveness of all currently used antimalarials. Drug combination therapies are currently advocated and deployed to slow the development of drug resistance. These combination therapies utilize antimalarial drugs that have been prescribed for decades and therefore the parasites have developed resistance to them when used as monotherapy. Whilst drug combinations using standard antimalarials might slow the development of drug resistance, such combinations only delay the inevitable. The development of novel chemical entities as antimalarial drugs is urgently needed. In this special journal issue, several plasmodial enzymes are discussed as potential antimalarial drug targets. The biological significance of these enzymes is presented with emphasis on target validation, inhibitor design, and antimalarial potential. These enzymes were selected as they are newly characterized targets that have the potential to identify novel chemotypes that can be fed into the antimalarial drug development pipeline. The first set of articles describes enzymes involved in cell cycle control and signalling pathways that directly regulate growth and differentiation of the malarial parasite. The first article by D. Jirage et al. describes the various protein kinases that have been identified and characterized from P. falciparum. The article is followed by E. Pesce et al. that reviews the current understanding of heat shock proteins (HSPs) in the malaria parasite. HSPs play a major role in maintaining cellular homeostasis by regulating signalling pathways, protein folding, assembly and destruction of protein complexes and therefore are attractive drug targets. Regulation of protein function is continued with an article by D-W Chung and K. Le Roch in which they describe the ubiquitin proteasome system (UPS). Although little is known about malarial UPS, conservation of this system and its importance in multiple cellular processes, make it a potential drug target. Finally the theme of cell cycle and signalling is concluded with a review by S. Kozlov et al. which compares and contrasts cellular growth pathways and cell cycle checkpoints between Plasmodium and mammalian cells. Many of the signalling components involved in cell cycle arrest are conserved in Plasmodium, however it is unclear if plasmodial cell cycle checkpoints exist. The second series of articles focus on drug targets directly involved with metabolism of the malaria parasite. An article by T. Donaldson and K. Kim describes efforts to target the purine salvage pathway with particular emphasis on purine nucleoside phosphorylase (PNP). Since the malaria parasite cannot synthesize purines de novo, efforts to target PNP is supported and has resulted in solving the structure and identifying potent inhibitors with antimalarial activity. This is followed by an article which describes the essentiality of panthothenate for parasite growth. Here C. Spry et al. discuss the uptake and metabolism of pantothenate and emphasize its importance in Coenzyme A (CoA) biosynthesis. The article demonstrates that transporters as well as biosynthetic enzymes can be drug targets for the development of novel antimalarials. Aminopeptidases play an essential role during the final stages of hemoglobin digestion to provide free amino acids to the parasite. K. Trenholme et al. review the biology and drug discovery efforts aimed at targeting the eight P. falciparum aminopeptidases. Efforts to characterize the aminopeptidases are a good example of how biological research and inhibitor design can simultaneously be pursued to support antimalarial drug discovery. Finally, an article by M. Phillips and P. Rathod reviews drug discovery efforts targeting the fourth enzyme, dihydroorotate dehydrogenase (DHODH), of the essential pyrimidine biosynthesis pathway. Lead optimization of select DHODH inhibitors has resulted in the identification of potent antimalarial compounds with favourable pharmacological parameters.

Protein kinases are pursued drug targets in numerous diseases including parasitic infections such as malaria. Plasmodium falciparum, the deadliest malarial parasite, relies on numerous protein kinases to regulate growth and differentiation through a complex life cycle that alternates between an invertebrate and vertebrate host. Many of the protein kinases are uncharacterized, however genetic and biochemical approaches have identified homologues of known eukaryotic kinases families as well as unique families of plasmodial kinases. Several classes of protein kinases have been studied, revealing that not only are these kinases essential for parasite viability, but that structure-based drug design strategies can be applicable to identify protein kinase inhibitors as antimalarial agents. In this review, we profile plasmodial protein kinases that have been characterized. Such a profile allows comparison across the plasmodial kinome and aids in placing these kinases within signaling networks responsible for biological activity but also provides a rationale to develop inhibitors that target multiple plasmodial kinases. With widespread malaria drug resistance, coupled by a parasite that can develop resistance quickly to new drugs, the development of multi-kinase inhibitors may be extremely efficacious and reduce the likelihood for resistance.

Ongoing research into the chaperone systems of malaria parasites, and particularly of Plasmodium falciparum, suggests that heat shock proteins (Hsps) could potentially be an excellent class of drug targets. The P. falciparum genome encodes a vast range and large number of chaperones, including 43 Hsp40, six Hsp70, and three Hsp90 proteins (PfHsp40s, PfHsp70s and PfHsp90s), which are involved in a number of fundamental cellular processes including protein folding and assembly, protein translocation, signal transduction and the cellular stress response. Despite the fact that Hsps are relatively conserved across different species, PfHsps do exhibit a considerable number of unique structural and functional features. One PfHsp90 is thought to be sufficiently different to human Hsp90 to allow for selective targeting. PfHsp70s could potentially be used as drug targets in two ways: either by the specific inhibition of Hsp70s by small molecule modulators, as well as disruption of the interactions between Hsp70s and co-chaperones such as the Hsp70/Hsp90 organising protein (Hop) and Hsp40s. Of the many PfHsp40s present in the parasite, there are certain unique or essential members which are considered to have good potential as drug targets. This review critically evaluates the potential of Hsps as malaria drug targets, as well as the use of chaperones as aids in the heterologous expression of other potential malarial drug targets.

The human malarial parasite, Plasmodium falciparum, is responsible for one of the most infectious diseases of the world and is quickly gaining resistance to the commonly used antimalarial treatments. New data are continually reinforcing the idea that biological functions associated with the ubiquitin proteasome system (UPS) are not just limited to non-lysomal degradation of proteins but consist of a wide array of regulatory mechanisms such as cell cycle progression, transcriptional regulation, gene expression and trafficking. While there is much effort in understanding the UPS in many eukaryotic organisms, the Plasmodium UPS has been relatively understudied despite its potential as a therapeutic drug target. However, in vitro proteasome inhibitors studies have confirmed the essentiality of the UPS in Plasmodia with limited toxicity to human cell lines. In addition, computational studies have shown that there are a number of ubiquitinating proteins upstream of the proteasome that may serve as parasite-specific drug targets due to their variety and divergences from other eukaryotic species. In this review, we highlight the major findings about Plasmodium's UPS and discuss its possible implications as an effective and specific antimalarial target.

Cancer and malaria are life threatening diseases killing millions of people each year. In spite of our best efforts, both continue to resist full control and eradication. If untreated, both malaria and cancer can lead to death. Only a few antimalarial drugs have been developed over the last decades and new drugs are urgently needed to combat drug-resistant parasites. Significant progress has been made in understanding the molecular mechanisms of cancer and designing new anticancer therapies. However, similar to malaria, majority of cancers quickly develop resistance to single target-based therapy. Novel cancer therapeutics are being developed with the aim of targeting multiple signalling pathways in tumour cells, an approach that may be applicable to antimalarial therapy. In this review we compare cell signalling pathways targeted by cancer drugs with similar pathways in the malaria parasite. We placed particular emphasis on cell cycle regulation and cell cycle checkpoints since the associated molecular machinery controlling these processes are conserved in Plasmodium. Furthermore, a large number of cancer drugs target cell cycle control mechanisms and, therefore, these compounds may possess antimalarial activity. We tried to demonstrate that promising areas of anticancer drug development can be incorporated in the existing antimalarial drug discovery program as well as deepen our understanding of parasite biology.

New antimalarials are needed due to the rapid development of resistance to currently deployed drugs. Because Plasmodium species are unable to synthesize purines, purine salvage pathways have been proposed as novel anti-malarial targets. The purine salvage pathway in Plasmodium is streamlined with adenosine deaminase (ADA), purine nucleoside phosphorylase (PNP) and hypoxanthine-xanthine-guanine-phosphoribosyltransferase (HXGPRT) representing the major pathway for purine acquisition. Plasmodium falciparum enzymes PfADA and PfPNP have unique dual specificity that enable them to act upon methylthiopurines resulting from polyamine synthesis. Thus Plasmodium ADA and PNP function in both purine salvage and purine recycling. Genetic studies have confirmed the importance of Plasmodium PNP for viability of malaria parasites. Immucillins, powerful picomolar transition state inhibitors of PNP, are active against cultured Plasmodium falciparum and inhibit all Plasmodium PNPs tested. Several immucillins have undergone human clinical trials, and these compounds represent a new class of compounds with potential activity against human malarias.

In the absence of an effective vaccine against malaria suitable for widespread deployment, the control of this lethal infectious disease relies heavily on antimalarial chemotherapies. The most virulent of the parasites that cause malaria (Plasmodium falciparum) has, however, developed resistance to all antimalarial agents in clinical use, and there is a desperate need for new antimalarial agents that target previously unexploited parasite processes. P. falciparum requires an extracellular supply of pantothenate to support its proliferation during the erythrocytic stage of its development in humans. This requirement highlights the mechanisms involved in the utilization (uptake and metabolism) of pantothenate as potential targets for chemotherapeutic attack. Here we review the evidence demonstrating pantothenate to be an essential nutrient for P. falciparum and data from studies investigating whether this parasite has the capacity to utilize exogenous supplies of the cofactor (coenzyme A; CoA) for which pantothenate serves as a precursor. The results of recent studies aimed at characterizing the mechanisms by which pantothenate is taken up by the P. falciparum-infected erythrocyte and intracellular parasite, and metabolized to CoA, are described. The unique properties that may be exploited to develop selective inhibitors of pantothenate utilization by P. falciparum-infected erythrocytes are highlighted. The molecular identities of P. falciparum pantothenate transporter(s) and CoA biosynthesis enzymes remain unconfirmed. We consider the possible identities, and emphasize the importance of generating these proteins in pure, functionally-active form. The tools currently available for identifying inhibitors of pantothenate utilization that may be potent antiplasmodial agents are also discussed.

Novel targets for new drug development are urgently required to combat malaria, a disease that puts half of the world's population at risk. One group of enzymes identified within the genome of the most lethal of the causative agents of malaria, Plasmodium falciparum, that may have the potential to become new targets for antimalarial drug development are the aminopeptidases. These enzymes catalyse the cleavage of the N-terminal amino acids from proteins and peptides. P. falciparum appears to encode for at least nine aminopeptidases, two neutral aminopeptidases, one aspartyl aminopeptidase, one aminopeptidase P, one prolyl aminopeptidase and four methionine aminopeptidases. Recent advances in our understanding of these genes and their protein products are outlined in this review, including their potential for antimalarial drug development.

Malaria remains a globally prevalent infectious disease that leads to significant morbidity and mortality. While there are a number of drugs approved for its treatment, drug resistance has compromised most of them, making the development of new drugs for the treatment and prevention of malaria essential. The completion of the Plasmodium falciparum genome and a growing understanding of parasite biology are fueling the search for novel drug targets. Despite this, few targets have been chemically validated in vivo. The pyrimidine biosynthetic pathway illustrates one of the best examples of successful identification of anti-malarial drug targets. This review focuses on recent studies to exploit the fourth enzyme in the de novo pyrimidine biosynthetic pathway of P. falciparum, dihydroorotate dehydrogenase (PfDHODH), as a new target for drug discovery. Several chemical scaffolds have been identified by high throughput screening as potent inhibitors of PfDHODH and these show strong selectivity for the malarial enzyme over that from the human host. Potent activity against parasites in whole cell models with good correlation between activity on the enzyme and the parasite have also been observed for a number of the identified series. Lead optimization of a triazolopyrimidine-based series has identified an analog with prolonged plasma exposure, that is orally bioavailable, and which shows good efficacy against the in vivo mouse model of the disease. These data provide strong evidence that PfDHODH is a validated target for the identification of new antimalarial chemotherapy. The challenge remains to identify compounds with the necessary combination of potency and metabolic stability to allow identification of a clinical candidate.