The metabolic repertoire of Leishmania and implications for drug discovery

The development of new anti-Leishmania compounds and the study of the mode of action of existing drugs has long been hampered by the difficulties of obtaining sufficient numbers of intracellular amastigotes or to culture in the laboratory well-defined parasites representative of intracellular amastigote stages. Most of the early studies have been carried out on promastigotes, of which the metabolism is not necessarily identical to that of amastigotes. Also reseach has been complicated by the fact that amastigotes hide away inside the phagosome of the host cell, the macrophage, which imposes additional diffusion barriers and the possibility of metabolizing or degrading drugs before they reach the target. Only a limited number of drugs is available for the treatment of leishmaniasis and hardly anything is known about their mode of action. The recent completion of the Leishmania major genome sequencing project now provides a more complete insight in the metabolic capacities of this and related parasites. Important differences between Leishmania and trypanosomes have been identified, and these differences could be related to the special adaptations required for the viability of Leishmania inside the macrophage’s phagosomal compartment and for its virulence. Differences that exist between parasite and host could be exploited as potential targets for drugs with high selectivity. Despite the fact that glycolysis is not an essential pathway in the intracellular amastigote, its overall functionality is of vital importance, by the fact that many of its enzymes are involved in the reverse process of gluconeogenesis. Also enzymes responsible for mannan synthesis from either hexoses or gluconeogenic substrates are validated drug targets. Glucose transporters and fructose-bisphosphatase, as an exclusive gluconeogenic enzyme, have been validated as drug target. By virtue of the distinctive structural properties exhibited by the enzymes shared by the glycolytic and gluconeogenic pathways and the fact that the majority of them is ensconced inside glycosomes, organelles which require a complete machinery for their biosynthesis and for which the individual peroxin proteins differ considerably from their mammalian counterparts, both glycolytic enzymes and peroxins are interesting drug targets. The methylglyoxal bypass, although not yet validated as a target, certainly deserves further study. Folate transporters have not yet been exploited and three haem biosynthetic enzymes which are all conserved within Leismania and for which there is no obvious function, deserve further investigation. Other interesting drug targets are NADPdependent fumarate reductase, acetate:succinate CoA transferase, dihydroorotate dehydrogenase and arginase. Finally genomic information has provided a possible explanation for the mode of action of the newly introduced anti-leishmanial drug Miltefosine.