Efficient identification of inhibitors targeting the closed active site conformation of the HPRT from Trypanosoma cruzi

Background: Currently, only two drugs are recommended for treatment of infe ction with Trypanosoma cruzi, the etiologic agent of Chagas' disease. These compounds kill the trypomastigote forms of the parasite circulating in the bloodstream, but are relatively ineffective against the intracellular stag e of the parasite life cycle. Neither drug is approved by the FDA for use i n the US. The hypoxanthine phosphoribosyltransferase (HPRT) from T. cruzi i s a possible new target for antiparasite chemotherapy. The crystal structur e of the HPRT in a conformation approximating the transition state reveals a closed active site that provides a well-defined target for computational structure-based drug discovery. Results: A flexible ligand docking program incorporating a desolvation corr ection was used to screen the Available Chemicals Directory for inhibitors targeted to the closed conformation of the trypanosomal HPRT. Of 22 potenti al inhibitors identified, acquired and tested, 16 yielded K-i's between 0.5 and 17 muM versus the substrate phosphoribosylpyrophosphate. Surprisingly, three of eight compounds tested were effective in inhibiting the growth of parasites in infected mammalian cells. Conclusions: This structure-based docking method provided a remarkably effi cient path for the identification of inhibitors targeting the closed confor mation of the trypanosomal HPRT. The inhibition constants of the lead inhib itors identified are unusually favorable, and the trypanostatic activity of three of the compounds in cell culture suggests that they may provide usef ul starting points for drug design for the treatment of Chagas' disease.