Current Pharmaceutical Design - Volume 8, Issue 4, 2002

The African trypanosome, Trypanosoma brucei exhibits a complex, digenetic life cycle that alternates between the tsetse fly vector and the mammalian host. The life cycle is characterised by a complex series of cell type differentiations and variations in metabolism. In addition the trypanosome exhibits a particular cell biology that has become adapted for its role as a parasite. This article places some of these areas in a frame-work that considers the role of cellular processes in parasitism. I rehearse some conclusions from recent studies and provide hyphotheses and suggestions for future work. Areas debated include: cell surface protein expression, cell differentiation, endomembrane trafficking and protein targeting, the cytoskeleton,flagellum functions in motility, attachment and plasma membrane differentiation, organelle specialisations, control of cell cycle, parasite / host, parasite / parasite and parasite / vector interactions. The review also focusses on the likely impact of the genome project and reverse genetics in providing greater insight to these cellular processes and how, if coordinated with some élan by scientists and funding agencies, this may provide novel targets for future drug development.

Human African trypanosomiasis or sleeping sickness is resurgent [1,2]. The disease is caused by subspecies of the parasitic haemoflagellate, Trypanosoma brucei. Infection starts with the bite of an infected tsetse fly (Glossina spp.). Parasites move from the site of infection to the draining lymphatic vessels and blood stream. The parasites proliferate within the bloodstream and later invade other tissues including the central nervous system. Once they have established themselves within the CNS, a progressive breakdown of neurological function accompanies the disease. Coma precedes death during this late phase. Two forms of the disease are recognised, one caused by Trypanosoma brucei rhodesiense, endemic in Eastern and Southern Africa, in which parasites rapidly invade the CNS causing death within weeks if untreated. T. b. gambiense, originally described in West Africa, but also widespread in Central Africa, proliferates more slowly and can take several years before establishing a CNS-involved infection. Many countries are in the midst of epidemics caused by gambiense-type parasites. Four drugs have been licensed to treat the disease [3] two of them, pentamidine and suramin, are used prior to CNS involvement. The arsenic-based drug, melarsoprol is used once parasites are established in the CNS. The fourth, eflornithine, is effective against late stage disease caused by T. b. gambiense, but is ineffective against T. b. rhodesiense. Another drug, nifurtimox is licensed for South American trypanosomiasis but also been used in trials against melarsoprol-refractory late sage disease. This review focuses on what is known about modes of action of current drugs and discusses targets for future drug development.

The paper reviews basic aspects of the biology of Trypanosoma cruzi emphasizing the following topics: (a) developmental stages of the life cycle in the vertebrate and invertebrate hosts (b) the cytoskeleton of the protozoan, especially the sub-pellicular microtubules (c) the flagellum, emphasizing its attachment to the protozoan body through specialized junctions (d) the kinetoplast-mitochondrion complex, describing its structural organization and the replication of the kinetoplast-DNA (e) the peroxisome (glycosome) and its role in the metabolism of the cell (f) the acidocalcisome, describing its morphology, biochemistry and functional role (g) the cytostome and the endocytic pathway (h) the organization of the endoplasmic reticulum-Golgi complex (i) the nucleus, describing its structural organization during interphase and division, and (j) basic aspects of the process of interaction of the parasite with host cells.

In this article we review the current status of chemotherapeutic approaches for the specific treatment of Chagas disease or American Trypanosomiasis, as well as new rational approaches being developed as a consequence on the increased understanding of the biochemistry and physiology of its causative agent, the protozoan parasite Trypanosoma cruzi. Currently available drugs (nitrofurans and nitroimidazoles), developed empirically over three decades ago, are unsatisfactory due to frequent toxic side effects and limited efficacy, particularly in the prevalent chronic form of the disease. Furthermore, studies of their mechanism of action have shown that their antiparasitic activity is inextricably linked to mammalian host toxicity. Recent advances in this field include the demonstration that new triazole derivatives, with selective inhibitory activity on the parasite's de novo sterol biosynthesis and special pharmacokinetic properties, can induce radical parasitological cure of both acute and chronic Chagas experimental disease. These compounds are active against nitrofuran- and nitroimidazole-resistant T.cruzi strains and maintain their activity even if the hosts are immunosuppressed and are thus logical candidates for clinical trials with Chagas disease patients. Inhibitors of cruzipain, a cathepsin L-like protease responsible for the major proteolytic activity in all stages of the life cycle of the parasite, can selectively block the proliferation of T.cruzi, both in vitro and in vivo and have curative activity in murine models of acute Chagas disease a significant effort is being devoted to their development as antiparasitic drugs. Alkyl-lysophospholipids, which selectively block phosphatidyl-choline biosynthesis in T.cruzi, are promising antiparasitic agents with good oral activity and low toxicity. Other biochemical pathways have been identified as potential chemotherapeutic targets, including hypoxanthine-guanine phosphoribosyl transferase and the enzymes involved in the synthesis and metabolism of trypanothione and inorganic pyrophosphate.

Parasitic protozoa of the genus Leishmania infect mammalian mononuclear phagocytic cells causing a potentially fatal disease with a broad spectrum of clinical manifestations. The drugs of choice used in the leishmaniasis therapy are significantly toxic, expensive and faced with a growing frequency of refractory infections. Thus the search for new leishmanicidal compounds is urgently required. In order to perform a proper drug design and to understand the modes of action of such compounds it is necessary to elucidade the intrincate cellular and molecular events that orchestrate the parasite biology. In order to invade the host cell Leishmania are able to recruit different surface receptors which may assist engaging the microbicidal responses. In the intracellular millieu these pathogens can deactivate and / or subvert the phagocyte signal transduction machinery rendering these cells permissive to infection. In the present review we attempted to approach some of the most relevant cellular and biochemical invasion and evasion strategies employed by Leishmania parasites.

Leishmaniasis, in its variety of visceral (VL), cutaneous (CL) and mucocutaneous (MCL) forms, directly affects about 2 million people per annum, with approximately 350 million individuals at risk worldwide. During the last 10 years there have been extensive epidemics of the visceral form of the disease, which is also emerging as an important opportunistic infection in immunocompromised patients, especially those co-infected with HIV. The control of leishmaniasis remains a problem- principally a zoonotic infection, except in epidemics where it is anthroponotic, interruption of transmission is difficult, though not impossible. No vaccines exist for either VL, CL or MCL and chemotherapy is inadequate and expensive. Current regimes use pentavalent antimony as primary therapy, which must be administered parenterally. Should this fail, a number of other drugs may be employed, depending upon the species of Leishmania concerned and the resources available to the health professionals involved. Recommended secondary treatment employs a variety of drugs, again depending on the nature of the infection. The most widely used of these is amphotericin B, which is highly active but has extensive toxicity complications. The newer formulations of this drug are too expensive to use for the majority of endemic countries. Pentamidine and paromomycin are used in some instances, and a new anti-leishmanial, miltefosine, may be used in the future. In short, there remains a pressing need for new anti-leishmanials and this chapter reviews the current status of chemotherapy, the various avenues being investigated by researchers and their potential application in the future.