Current Pharmaceutical Design - Volume 7, Issue 12, 2001

Chagas disease or American trypanosomiasis is considered by the Word Health Organization to be one of the important tropical parasitic diseases worldwide together with malaria and schistosomiasis. The etiologic agent of this illness is the kinetoplastid protozoon Trypanosoma cruzi. The present chemotherapy for the treatment of Chagas disease remains unsolved. The drugs currently in use are old, ineffective and toxic. Bearing in mind the metabolic differences between the parasite and the mammalian host, some attractive interesting molecular targets for drug design are presented.

Trypanothione is the key molecule in the defence mechanism of Trypanosoma and Leishmania against oxidative stress. The uniqueness of trypanothione makes the metabolism of this molecule an attractive target in antitrypanosomal and antileishmanial drug design. It became clear that this antioxidant cascade can be considered as the Achilles heel of these parasites. The following targets and their respective inhibitors will be discussed biosynthesis of trypanothione with glutathionylspermidine synthetase and trypanothione synthetase biosynthesis of glutathione with gama-glutamylcysteine synthetase biosynthesis of spermidine with ornithine decarboxylase trypanothione hydroperoxide metabolism with tryparedoxine peroxidase, tryparedoxine and trypanothione reductase.

Trypanosoma cruzi, the causative agent of the American Trypanosomiasis, Chagas disease, contains a major cysteine proteinase (CP), cruzipain (also known as cruzain, or GP57 / 51). The enzyme is a member of the papain C1 family of CPs, with a specificity intermediate between those of cathepsin L and cathepsin B. The enzyme, which is expressed at different levels by different parasite stages, is encoded by a high number of genes (up to 130 in the Tul2 strain), which code for a pre-pro-enzyme. Mature cruzipain consists of a catalytic moiety with high homology to cathepsins S and L, and a C-terminal domain, characteristic of Type I CPs of Trypanosomatids, and absent in all other C1 family CPs described so far. Irreversible inhibitors of cruzipain (peptidyl diazomethylketones, peptidyl fluoromethylketones, peptidyl vinyl sulphones) are able to block the differentiation steps in the parasites life cycle, and effectively kill the organism. Recently, a vinyl sulphone derivative (N-piperazine-Phe-hPhe-vinyl sulphone phenyl) which is an efficient inhibitor of cruzipain and kills T. cruzi by inducing an accumulation of unprocessed cruzipain in the Golgi cisternae, interfering with the secretory pathway, has been tested in vivo in a mice model (J.H. McKerrow et al.). The curative effects observed, as well as the good bioavailability of the inhibitor and its apparent lack of undesirable side effects, make it a promising lead compound for the development of new drugs for the chemotherapy of Chagas disease.

Chagas disease remains an important health problem in the Americas. Advances are being made in parts of South America in blocking transmission from insect vectors or blood transfusion, but more effective chemotherapy is needed for the millions who are already infected. This is especially important since recent results have indicated that treatment is beneficial for the elimination of the chronic course of the disease. The rational development of new drugs depends on the identification of differences between human metabolism and that of the causative parasite, Trypanosoma cruzi. Recent developments in the study of the basic biochemistry of the parasite have allowed the identification of novel targets for chemotherapy, such as sterol metabolism, protein prenylation, proteases, and phospholipid metabolism, and these are the subject of this review.

The most important glycoproteins of trypanosomatids are anchored by glycoinositolphospholipids (GIPLs) to their plasma membrane. In addition, free GIPLs have been described, for instance the lipopeptidophosphoglycan (LPPG) which is a major component of the surface of T. cruzi epimastigotes. An inositolphosphoceramide (IPC) is part of the LPPG and of glycoproteins present in different stages of T. cruzi. Ceramide was not found in mammal GIPL-anchors. The lipid moieties in T. cruzi anchors can be quite variable. However, no diacylglycerol (DAG) was found in contrast with the African trypanosomes. In GIPLs of epimastigotes collected at the logarithmic phase of growth both, 1-O-hexadecyl- 2-O-palmitoylglycerol and ceramide were identified. Lignoceroylsphinganine is the major ceramide, however, no lignoceric acid was detected when analysing the candidate precursors IPCs, in any of the stages of T. cruzi. An alkylglycerol has been found either as a lyso species in the Tc85 glycoprotein of trypomastigotes or acylated as in the 1G7 anchor of metacyclic forms and in the mucins of epimastigote forms. The lipid in the mucins is replaced by ceramide when the parasite differentiates to metacyclic forms. Also, in the Ssp-4 glycoprotein characteristic of amastigotes, a ceramide was identified as the anchor lipid. These variations suggest that a remodelling mechanism is working in T. cruzi. On the other hand, the oligosaccharide core in the GIPLs of T. cruzi is substituted with galactofuranose. This monosaccharide is found only in the pyranose configuration in mammalian glycoproteins and glycolipids. Thus, the biosynthetic steps for the introduction of galactofuranose and ceramide in the anchors of T. cruzi are good targets for the development of therapeutic agents.