View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Publications of the IAS Fellows J Parasit Dis DOI 10.1007/s12639-010-0006-3 REVIEW ARTICLE Drug targets in Leishmania Bhavna Chawla • Rentala Madhubala Received: 30 April 2010 / Accepted: 22 June 2010 Ó Indian Society for Parasitology 2010 Abstract Leishmaniasis is a major public health problem Leishmania. It constitutes of a wide spectrum of diseases and till date there are no effective vaccines available. The ranging in severity from simple cutaneous lesions to the control strategy relies solely on chemotherapy of the infec- usually fatal visceral form. The parasite is transmitted ted people. However, the present repertoire of drugs is through the bite of sandfly to the mammalian hosts. The limited and increasing resistance towards them has posed a disease is endemic in 88 countries and affects as many as major concern. The first step in drug discovery is to identify 12 million people around the globe with an incidence of a suitable drug target. The genome sequences of Leishmania 0.5 million cases of the visceral form of the disease and major and Leishmania infantum has revealed immense 1.5–2.0 million cases of the cutaneous form of the disease, amount of information and has given the opportunity to causing extensive mortality and morbidity. identify novel drug targets that are unique to these parasites. Since, effective vaccines against leishmaniasis are still Utilization of this information in order to come up with a under development, the current control measures rely candidate drug molecule requires combining all the tech- solely on chemotherapy. Pentavalent antimonials are the nology and using a multi-disciplinary approach, right from standard first line of treatment but emergence of resistance characterizing the target protein to high throughput screen- towards them, has limited their usefulness. Alternative ing of compounds. Leishmania belonging to the order ki- chemotherapeutic treatments with amphotericin B and its netoplastidae emerges from the ancient eukaryotic lineages. lipid formulation, miltefosine and paromomycin are They are quite diverse from their mammalian hosts and there available but their use is limited either due to toxicity or are several cellular processes that we are getting to know of, high cost of treatment. The current challenges in the che- which exist distinctly in these parasites. In this review, we motherapy include availability of very few drugs, emer- discuss some of the metabolic pathways that are essential gence of resistance to the existing drugs, their toxicity and and could be used as potential drug targets in Leishmania. lack of cost-effectiveness. Therefore, it is of utmost importance to look for effective drugs and new drug targets Keywords Leishmaniasis Á Leishmania Á for the treatment of leishmaniasis. Metabolic pathways Á Drug targets There seems to be a welcome change in terms of flow of funds for antiparasitic drug discovery. Some of the or- ganisations like Institute of One World Health (IOWH), Introduction Drugs for Neglected Diseases Initiative (DNDi), Bill and Melinda Gates foundation have had a significant impact on Leishmaniasis is a group of parasitic diseases caused by at working towards the drug development for tropical dis- least 20 different species of the protozoan parasite eases. This has enabled technological advances in the field, which includes publicly funded sequencing of the gen- omes, thus fastening up the course towards drug develop- & B. Chawla Á R. Madhubala ( ) ment. A major breakthrough in the field is the availability School of Life Sciences, Jawaharlal Nehru University, New Delhi 110067, India of the complete genome sequence of various species of e-mail: [email protected] Leishmania like, L. major (Ivens et al. 2005), L. infantum 123 J Parasit Dis and L. braziliensis. It has provided us with enormous structure. Unlike mammalian cells, which have cholesterol information about the parasite genome for our better as the major membrane sterol, trypanosomatids synthesize understanding of these organisms. With the availability and ergosterol and other 24-methyl sterols that are required for easily accessible genome sequences, the conventional their growth and viability. These sterols are absent from the method of drug target identification, which was done on the mammalian cells. Therefore, the sterol biosynthetic path- basis of the biochemical and physiological differences way from Leishmania is considered to be an important drug between the pathogen and host, is now changing. Micro- target (Fig. 1). arrays and proteome analysis make use of the genome One of the enzymes that is being studied deeply is sequences and has allowed us to find genes unique to the squalene synthase (SQS) (EC 2.5.1.21) that catalyzes the parasite, some species-specific genes that can help us first committed step of sterol synthesis by coupling two understand the pathogen better. Several bioinformatic farnesyl molecules to form squalene. Zaragozic acids and approaches have also been proposed which will fasten the quinuclidines are known to inhibit SQS. Two quinuclidine pace to come up with anti-leishmanial drugs (Myler 2008). derivatives, ER-119884 and E5700 have been shown to be The analysis of the complete genome sequence may sig- potent anti-Leishmania (Fernandes Rodrigues et al. 2008) nificantly contribute in drug development by revealing the and Trypanosoma (Urbina et al. 2004) agents. The inhibi- presence of novel enzymes and receptors. Furthermore, the tion of SQS by these compounds decreased the parasite’s comparison of the parasite genome with the human genome endogenous sterol levels which had an antiproliferative sequence will make the identification of genes, unique to effect on the parasite. In L. amazonensis,IC50 value of the parasite, easier. ER-119884 and E5700 for promastigotes was found to be 10 and 30 nM respectively (Fernandes Rodrigues et al. 2008). Squalene is converted to 2,3-oxidosqualene by the Identification of drug targets enzyme squalene epoxidase (EC 1.14.99.7). It converts One of the characteristic features in the process of drug development is target identification in a biological path- Acetyl-CoA way. In theory, during identification of a target in a path- HMG-CoA reductase ogen, it is important that the putative target should be either absent in the host or substantially different from the host 3-hydroxy-3-methyl-glutaryl-CoA homolog so that it can be exploited as a drug target. Try- panosomatids, phylogenetically, branch out quite early Mevalonate from the higher eukaryotes. In fact, their cell organization Isopentyl-PP Dimethylallyl-PP is significantly different from the mammalian cells and thus, it is possible to find targets that are unique to these Farnesyl-PP Synthase Isoprenoid Pathway pathogens. Secondly, the target selected should be abso- lutely necessary for the survival of the pathogen. It is also Geranyl-PP important to consider the stage of the life-cycle of the Farnesyl-PP Synthase pathogen in which the target gene is expressed. It is crucial Farnesyl-PP to look at the biochemical properties of the protein; it Squalene Synthase should have a small molecule binding pocket, so that Squalene specific inhibitors can be designed and if the target protein Squalene epoxidase is an enzyme, its inhibition should lead to loss in cell 2,3-oxidosqualene viability. It is of high importance that the target selected should be assayable. Inexpensive and specific assay system Lanosterol should be available for high-throughput screening of mol- ecules (Pink et al. 2005; Barrett et al. 1999). Zimosterol Sterol Methyltransferase Potential drug targets in Leishmania Ergosterol Sterol biosynthetic pathway Fig. 1 Sterol biosynthetic pathway in Leishmania. The pathway shows the important steps and the enzymes involved in sterol Sterols are important components of the cell membrane biosynthesis. The final product in trypanosomatids is ergosterol as that are vital to cellular function and maintenance of cell opposed to cholesterol in mammalian cells 123 J Parasit Dis squalene chain to tetracyclic sterol skeleton. Terbinafine, being employed to obtain compounds that bind to the an allylamine, is known to inhibit squalene epoxidase. It enzymes with high affinity. The 3-D structures are available has been shown that terbinafine inhibits the growth of for some of the trypanosomatid enzymes: glyceraldehydes- promastigotes and intracellular amastigotes and leads to 3-phosphate dehydrogenase (EC 1.2.7.6) (Vellieux et al. changes in structural organization in mitochondrion (Van- 1993; Kim et al. 1995), triosephosphate isomerase (5.3.1.1) nier-Santos et al. 1995). When it is used in combination (Wierenga et al. 1991; Williams et al. 1999), phosphoglyc- with ketoconazole, another inhibitor of ergosterol biosyn- erate kinase (EC 2.7.2.10) (Bernstein et al. 1997), pyruvate thesis, the effect is synergistic. Another class of inhibitors, kinase (2.7.1.40) (Rigden et al. 1999), fructose-1,6-bis- bisphosphonates, inhibits the isoprenoid pathway that is phosphate aldolase (EC 4.1.2.13) (Chudzik et al. 2000) and catalyzed by the enzyme farnesyl diphosphate synthase glycerol-3-phosphate dehydrogenase (EC 1.1.1.8) (Suresh (FPPS). They have been tested in vivo and in vitro against et al. 2000). The differences in the 3-D structures of the the protozoan parasites (Martin et al. 2001; Docampo parasite and host enzymes can be used to design specific and Moreno 2008). A potent inhibition of the cell growth inhibitors (Verlinde and Hol 1994). and suppression of the activity of isolated enzymes (from Specific inhibitors have been designed for the glycolytic L. major) was observed thus validating the isoprenoid enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) pathway as a drug target. based on its crystal structure. Adenosine analogs were syn- Another important putative target in ergosterol biosyn- thesized as tight binding inhibitors that occupy the pocket on thesis is the enzyme D24,25-sterol methyltransferase (SMT) the enzyme that accommodates adenosyl moiety of NAD? (EC 2.1.1.41).