An approach to the development of new drugs for African trypanosomiasis.

Abstract

African sleeping sickness is caused by Trypanosoma brucei gambiense and T. b. rhodesiense. This disease resulted in the deaths of several million people during the first half of the twentieth century (1) and continues to pose a threat of new epidemics (2). Of even more significance is the fact that animal trypanosomiasis or nagana (T. congolense, T. b. brucei, and T. vivax) makes four million square miles of the African continent unsuitable for the production of cattle and other livestock (3). The first trypanocidal agents were developed by Ehrlich and his collaborators in the early part of this century. Over the next 50 yr, drugs such as tryparsarnide, suramin, pentamidine, berenil, ethidium, Antrycide, and melarsoprol became available for use in the treatment of both sleeping sickness and nagana (4). For the past 20 yr, however, there have been no new chemotherapeutic agents introduced. Moreover, the therapeutic usefulness of the older drugs is diminishing due to the increased incidence of resistant strains (5).We have been attempting to develop new chemotherapeutic agents by elucidating biochemical differences between trypanosomes and their hosts and then designing drugs to take advantage of these differences. The present communication describes such an approach to drug development. The biochemical difference we have exploited is the inability of T. b. brucei to synthesize heine (6). As a result of this deficiency and the avid binding of heine to serum proteins in mammalian hosts, the bloodstream form of this organism has no detectable heine (S. R. Meshnick and S. Sassa, unpublished results) or hemoproteins such as cytochromes (7) or catalase (8). In a previous communication (9) we reported that the lack ofcatalase in T. b. brucei leads to an accumulation of intracellular hydrogen peroxide (H202) in these organisms, which should increase their susceptibility to killing by agents that promote the homolytic cleavage of H202 yielding hydrexy (HO.) or hydroperoxy (HOO.) radicals. Presumably, these radicals would react with unsaturated lipids and other cell constituents, thereby leading to cell destruction. Heme proved to be trypanocidal in vitro, whereas several other porphyrins showed in vivo activity (9). We ascribed this to their acting as initiators of homolytic cleavage. To enhance the efficacy of this therapeutic approach, we have attempted to determine the site of H202 production, the fate of the H202 generated, a means of increasing H202 production, and lastly, ways of rendering trypanosomes more susceptible to radical damage. J. ExP. MED.

Molecular Cloning, Biochemical Characterization, and Partial Protective Immunity of the Heme-Binding GlutathioneS-Transferases from the Human HookwormNecator americanus

“…Due to its oxidative iron in the molecular structure, free heme is a potent enzyme inhibitor and generator of toxic reactive oxygen species, catalyzing the formation of lipid peroxides and damaging DNA via oxidative stress (44). The nematode-specific hemebiding GSTs, such as the N. americanus GSTs in this study, may act as a carrier to bind/detoxify heme by conjugating it to reduced GSH, thereby protecting nematodes from the attack of ROS induced by the excess free heme.…”

“…Heme is released during the breakdown of host hemoglobin. Since heme is a potent enzyme inhibitor and generator of toxic ROS (44), it is possible that blood-feeding parasitic nematodes, like hookworms, employ GST proteins in heme detoxification (69). In contrast, insoluble cofactors like heme must also be transported by lipophilic proteins, such as GSTs, in order to be incorporated in essential enzymes, such as cytochrome c, peroxidases, and the numerous globins known to be present in nematode proteomes (45).…”

Biochemical Characterization and Vaccine Potential of a Heme-Binding Glutathione Transferase from the Adult Hookworm Ancylostoma caninum

“…However, this stepwise degradation of hemoglobin results in the release of heme, which is a potent enzyme inhibitor and generator of toxic reactive oxygen species (29). Adult A. caninum worms may produce Ac-GST-1 to assist in removing and/or scavenging hematin or heme-related compounds generated during hemoglobin digestion.…”

Expression of a Gene Encoding Trypanosoma congolense Putative Abc1 Family Protein is Developmentally Regulated

“…However, mitochondrion of the BSF of trypanosome has a unique electron transport chain mediated by CoQ, glycerol-3 phosphate oxidase and plant-like alternative oxidase, called trypanosome alternative oxidase [2,10]. Since this unique electron transport chain is not present in the host, this has been considered as a possible chemotherapeutic target [2,10,15]. However, except for our study, there are no reports about CoQ synthesis pathway and related information in African trypanosome.…”