Current Pharmaceutical Design - Volume 14, Issue 9, 2008

The high impact of diseases caused by parasitic protozoa shortens working capacity, causing premature disability, shortening of life quality and expectancy of individuals, leading to economical and social losses. In this issue of Current Pharmaceutical Design several articles, written by colleagues with a major contribution on the area, deal with drugs that interfere with specific structures/organelles of the most important protozoa. In the first article, W. de Souza [1] makes a brief review on the structural organization of parasitic protozoa, pointing out the main structures and organelles that make them very special as eukaryotic cells. In the second article, N. Sen and H.M. Majumder [2] review the available information on the mitochondrion of protozoa and its potentiality as drug target. The third article, by M.C.M. Motta [3] analyses the kinetoplast, a defining characteristic of the order Kinetoplastida, in which the Trypanosomatidae family is included, and its potentiality as drug target. In the fourth article J. Wiesner, A. Reichenberg, S. Heinrich, M. Schlitzer and H. Jomma [4] review the new information available on the plastid-like organelle found in apicomplexan parasites. The fifth article by M. Benchimol [5] deals with the hydrogenosome, an organelle characteristic of trichomonads and validated as drug target in view of peculiarities of its metabolic role. The sixth article by R.Docampo and S.N.J. Moreno [6] brings an up to date view of the acidocalcisome, an organelle described and characterized in recent years in a large number of pathogenic protozoa. In the seventh article P.S. Doyle, M. Sajid, T. O'Brien, K. DuBois, J.C. Engel, Z.B. Mackey and S. Reed [7] review information on the lysosome. Fifty years after its first description, this organelle, has pivotal role in several biological functions. The eight article by C.D. Goodeman and G.I. McFadden [8] review the available information on a new and interesting drug target, the fatty acid biosynthesis pathway. In the ninth article, K.M. Grant [9] covers the present day information on the cell cycle of pathogenic protozoa and the potential of its steps as drug targets. Finally, J.C.F.Rodrigues and W. de Souza [10] make a review on the potentiality of electron microscopy for the analysis of the effect of drugs on the structural organization of pathogenic protozoa. References [1] de Souza W. An Introduction to the Structural Organization of Parasitic Protozoa. Curr Pharm Des 2008; 14(9): 822-838. [2] Sen N, Majumder HK. Mitochondrion of Protozoan Parasite Emerges as Potent Therapeutic Target: Exciting Drugs are on the Horizon. Curr Pharm Des 2008; 14(9): 839-846. [3] Motta MCM. Kinetoplast as a Potential Chemotherapeutic Target of Trypanosomatids. Curr Pharm Des 2008; 14(9): 847-854. [4] Wiesner J, Reichenberg A, Heinrich S, Schlitzer M, Jomaa H. The Plastid-Like Organelle of Apicomplexan Parasites as Drug Target. Curr Pharm Des 2008; 14(9): 855-871. [5] Benchimol M. The Hydrogenosome as a Drug Target. Curr Pharm Des 2008; 14(9): 872-881. [6] Docampo R, Moreno SNJ. The Acidocalcisome as a Target for Chemotherapeutic Agents in Protozoan Parasites. Curr Pharm Des 2008; 14(9): 882-888. [7] Doyle PS, Sajid M, O'Brien T, DuBois K, Engel JC, Mackey ZB, Reed S. Drugs Targeting Parasite Lysosomes. Curr Pharm Des 2008; 14(9): 889-900. [8] Goodman CD, McFadden GI. Fatty Acid Synthesis in Protozoan Parasites: Unusual Pathways and Novel Drug Targets. Curr Pharm Des 2008; 14(9): 901-916. [9] Grant KM. Targeting the Cell Cycle in the Pursuit of Novel Chemotherapies Against Parasitic Protozoa. Curr Pharm Des 2008; 14(9): 917-924. [10] Rodrigues JCF, de Souza W. Ultrastructural Alterations in Organelles of Parasitic Protozoa Induced by Different Classes of Metabolic Inhibitors. Curr Pharm Des 2008; 14(9): 925-938.

As eukaryotic cells, protozoa present a classical structural organization where most of the structures and organelles typical of mammalian cells are found. However, even for usual organelles these organisms present structural diversity. In addition, some of the protozoa structures, such as the mitochondria, peroxisomes and even the Golgi complex, are not observed. On the other hand, new organelles such as the hydrogenosomes, mitosomes, Apicoplast, kinetoplast, glycosomes (specialized peroxisomes), rhoptries, micronemes and dense granules, are characteristic features of some protozoa. Also, several unusual cytoskeletal structures, some of them made of yet uncharacterized proteins, are seen in these eukaryotic microorganisms. Further characterization of these structures indicates that they contain special enzymes involved in distinct metabolic pathways making them potential targets for the development of new anti parasite drugs.

Chemotherapy is the primary means of treating protozoan parasitic infections. A problem for chemotherapy is to find a novel and potential molecule in protozoa, which could be exploited as drug target. To reach this goal, mitochondrion of protozoa can be considered as the most valuable and potential organelle because of its unique structure and function compared to their natural host habitat. In fact, the respiratory systems of parasitic protozoa typically show greater diversity in electron pathways than do their host animals. These unique aspects of electron transport chain (ETC) complexes and their related enzymes represent promising targets for chemotherapy. A cytochrome independent Alternative Oxidase (AOX) in parasites is a leading drug target. Topoisomerases play key functions in replication and organization of kDNA, which is present in a specialized region of unique mitochondria known as kinetoplast. They are considered as potential targets for anti-parasitic drugs. Moreover, a novel pathway of type II Fatty acid synthesis in mitochondria of trypanosomatids provides a new array of inhibitors that could be effective against these parasites. Recent studies on the emergence of drug resistance severely limit the arsenal of available drugs against protozoan parasites. Particularly, mutations of cytochrome b gene of ETC or changes in iron homeostasis by mitochondrial enzyme aconitase alter sensitivity of MDR1 and regulate resistance level to anti-parasitic drugs. This review summarizes recent state of our knowledge and understanding of the action of various therapeutically applied substances on mitochondria and their potential application in the future.

Many trypanosomatid protozoa, such as those belonging to the Trypanosoma and Leishmania genera cause serious diseases to man. Such parasites present an unusual feature, a mitochondrial DNA arranged in catenated circles, known as kinetoplast DNA (kDNA). The replication of kDNA network is a complex process, which involves many proteins. Some of them are classified as topoisomerases and play essential biological roles, not only on kDNA synthesis, but also in the dynamics of the network topology, constituting the main target for drugs in kinetoplast. DNA binding drugs are also reported as chemotherapeutic agents against trypanosomatid infections. This review summarizes what is known about kinetoplast as a potential chemotherapeutic target for trypanosomatid protozoa.

Apicomplexan parasites infectious to humans include Plasmodium spp., Babesia spp., Toxoplasma gondii, Cryptosporidium spp., Isospora belli and Cyclospora cayetanensis. With exception of Cryptosporidium spp., these parasites possess a non-photosynthetic plastid-like organelle called apicoplast. The apicoplast possesses a small circular genome and harbours prokaryotic-type biochemical pathways. As the most important metabolic functions, the mevalonate independent 1-deoxy-D-xylulose 5-phosphate pathway of isoprenoid synthesis and the type II fatty acid synthesis system are operative inside the apicoplast. Classical antibacterial drugs such as ciprofloxacin, tetracycline, doxycycline, clindamycin and spiramycin inhibit the apicoplast-located gyrase and translation machinery, respectively, and are currently used in the clinic for the treatment of infections with apicomplexan parasites. As an inhibitor of isoprenoid synthesis, fosmidomycin was proven to be effective against acute P. falciparum malaria in clinical phase II studies. Triclosan, an inhibitor of fatty acid synthesis, was active in a malaria mouse model. In vitro antimalarial activity was shown for inhibitors of peptide deformylase and the import of apicoplast-targeted proteins. Work on various other inhibitors of apicoplast-located biochemical processes is ongoing.

Hydrogenosomes are spherical or slightly elongated organelles found in non-mitochondrial organisms. In Trichomonas hydrogenosomes measure between 200 to 500 nm, but under drug treatment they can reach 2 μm. Like mitochondria hydrogenosomes: (1) are surrounded by two closely apposed membranes and present a granular matrix: (2) divide in three different ways: segmentation, partition and the heart form; (3) they may divide at any phase of the cell cycle; (4) produce ATP; (5) participate in the metabolism of pyruvate formed during glycolysis; (6) are the site of molecular hydrogen formation; (7) present a relationship with the endoplasmic reticulum; (8) incorporate calcium; (9) import proteins post-translationally; (10) present cardiolipin. However, there are differences, such as: (1) absence of genetic material, at least in trichomonas; (2) lack a respiratory chain and cytochromes; (3) absence of the F0- F1 ATPase; (4) absence of the tricarboxylic acid cycle; (5) lack of oxidative phosphorylation; (6) presence of peripheral vesicles. Hydrogenosomes are considered an excellent drug target since their metabolic pathway is distinct from those found in mitochondria and thus medicines directed to these organelles will probably not affect the host-cell. The main drug used against trichomonads is metronidazole, although other drugs such as β-Lapachone, colchicine, Taxol, nocodazole, griseofulvin, cytochalasins, hydroxyurea, among others, have been used in trichomonad studies, showing: (1) flagella internalization forming pseudocyst; (2) dysfunctional hydrogenosomes; (3) hydrogenosomes with abnormal sizes and shapes and with an electron dense deposit called nucleoid; (4) intense autophagy in which hydrogenosomes are removed and further digested in lysosomes.

Acidocalcisomes are acidic organelles rich in calcium and phosphorus that have been conserved from bacteria to man. In parasitic protozoa acidocalcisomes possess enzymes that are absent or different from their mammalian counterparts and could be potential targets for chemotherapy, such as the vacuolar proton translocating pyrophosphatase, and the soluble inorganic pyrophosphatase, both of which are inhibited by pyrophosphate analogs (bisphosphonates). In addition, a number of drugs, including bisphosphonates, and diamidines appear to accumulate in these organelles and/or induce an increase in their numbers. The mechanism of action of bisphosphonates, however, is by inhibition of the isoprenoid pathway and more specifically the prenyl diphosphate synthases.

Lysosomes were first described as vacuolar structures containing various hydrolytic enzymes at acidic pH. Subsequent studies revealed that the lysosome/vacuolar system is complex and composed of distinct membrane-enclosed vesicles including endosomes, primary and mature lysosomes, autophagic vesicles, residual bodies, multivesicular bodies, and digestive lysosomes. Lysosomes express a battery of hydrolytic enzymes including proteases, acid phosphatases, glycosidases, and lipases. Parasitic protozoa also possess complex intracellular lysosomes/endosomes/vesicles involved in digestion, transport and recycling of molecules similar to those of mammalian cells. Unique characteristics are ascribed to lysosomes of different parasites and may even differ between parasite stages. Transport of hydrolases and proteins to parasite lysosomes is directed either from the Golgi complex via endosomal vesicles or from endocytic vesicles originated in the cell surface. Inhibition of lysosomal proteases demonstrated that different proteolytic machineries catabolize distinct classes of proteins, and this selectivity may be exploited for the development of effective antiparasitic drugs. This review describes lysosomal molecules that are either validated or potential drug targets for Chagas' disease, sleeping sickness, leishmaniasis, toxoplasmosis, malaria, amebiasis, and giardiasis.

Fatty acid biosynthesis pathways in protozoan parasites are reviewed with a view to targeting this metabolism for drug therapy. The type II fatty acid biosynthesis pathways derived from bacteria in protozoan relict plastids and mitochondria are examined in different groups with emphasis on apicomplexa. The suitability of different enzymes from the type II fatty acid biosynthesis pathway for drug intervention, and the state-of-play with known and potential inhibitors is explored. The type I acid biosynthesis pathways that occur in select protozoan parasites and their potential for inhibition using anti-tumour and obesity management compounds currently in development are also examined. Pathways used by parasites to scavenge and modify host lipids are also described briefly and their potential for therapeutics discussed.

Protozoan parasites, such as those responsible for malaria and African Sleeping Sickness, represent a huge burden to the developing world. Current chemotherapy to combat these diseases is inadequate: antiquated, toxic and increasingly ineffective due to drug resistance. In this article, the potential usefulness of targeting key regulators of the parasite cell cycle will be discussed, paying particular attention to three families of protein kinases: Cyclin-dependent kinases, glycogen synthase kinases and Aurora kinases. This review shall outline their identification, which has been greatly accelerated by the availability of parasite genome data, their validation as bona fide regulators of the parasite cell cycle and current data on the availability and anti-parasite activity of inhibitors.

Parasitic protozoa such as Leishmania, Trypanosoma, Plasmodium, Toxoplasma gondii, Giardia and Trichomonas are able to cause several diseases affecting millions of people around the world with dramatic consequences to the socio-economic life of the affected countries. Diseases like malaria, leishmaniasis and trypanosomiasis have been classified by the World Health Organization as neglected diseases, because they have been almost completely forgotten by the governments as well as the pharmaceutical companies. The specific chemotherapy currently employed for the treatment of these diseases has serious limitations due to lack of efficacy, toxic side effects, growth of drug-resistance and high costs. Thus, it is urgent to develop new chemotherapeutic agents that are more effective, safe and accessible. In this context, several works have been focused on understanding the effect of different drug-treatments on these parasitic protozoa. Organelles and structures such as mitochondrion, kinetoplast, apicoplast, glycosome, acidocalcisome, hydrogenosome, plasma membrane and the cytoskeleton have been studied using different approaches to identify new targets for the development of new chemotherapeutic agents that are required. Some studies on alterations in the fine structure, as assayed using electron microscopy, have indicated the nature of lesions induced by several drugs, allowing deductions on possible modes of action. Here, we briefly review the available data of the effects of several drugs on the ultrastructure of parasitic protozoa and show how electron microscopy can contribute to elucidate the different mechanisms of these anti-parasitic drugs.