Validating targets for antiparasite chemotherapy.

The enzymes and receptors in parasites that can be qualified as targets for antiparasite chemotherapy should perform essential functions in the parasites and demonstrate some feasibility for selective inhibition. They can be tentatively identified through detailed analysis of various aspects of metabolisms in the parasites or elucidation of the mechanisms of action among proven antiparasitic agents. Preliminary verifications of these putative targets can be indicated by in vitro antiparasite activity of an inhibitor of the target. However, before a major long-term effort to pursue in-depth structure-activity analysis of the target is to be committed for specific inhibitor design, further validations of the target are essential to insure that future studies are not misguided. One old-fashioned approach to validate a target in the pharmaceutical industry is by correlating target inhibitions with antiparasitic activities among large numbers of drug derivatives. The results are often indicative but hardly ever conclusive. Another method is by comparing the putative drug targets between the drug-sensitive and the drug-resistant parasites for potential discrepancies. Unfortunately, the latter often result from indirect causes, such as reduced drug transport, instead of an alteration of the drug target itself. The third experimental approach is by disrupting the gene encoding the putative target in parasite, which can provide the most conclusive evidence on whether the target plays an indispensible role in the parasite. But special conditions are needed for the gene knockout mutants to survive to exhibit their phenotypes and to allow genetic complementation studies for further verifications. Furthermore, gene knockout experiments are often difficult to perform on cells of multiple ploidy or genes of multiple copies, and are currently applicable only to a limited number of protozoan parasites. In the current article I have tried to take a cursory look at some eleven putative drug targets among various parasites, each supported by well-established antiparasitic agents identified as its inhibitors. I have also considered the evidence for validity of each of them and the potential means of further verifying their validity.