Protozoan Purine Phosphoribosyltransferases as Targets to Treat Malaria, African Trypanosomiasis and Chagas's Disease
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PROJECT SUMMARY/Parasitic protozoa rely on purine salvage pathways for which the enzyme hypoxanthine-guanine-xanthinephosphoribosyltransferase (HGXPRT; hereafter, purine phosphoribosyltransferase (PPRT)) is essential. Thegenus Trypanosoma causes debilitating human diseases of high morbidity. Neither vaccines nor usefultherapies exist. Trypanosoma cruzi causes Chagasâ€™s Disease in Central and South America, with over 40cases recently reported in Texas. Trypanosoma brucei rhodosiense and Trypanosoma brucei gambiensecause African sleeping sickness. Parasitic protozoa including T. cruzi and T. brucei are incapable of purinebiosynthesis de novo and make nucleotides and nucleosides by purine salvage pathways. PPRT catalyzes theformation of GMP, IMP, and XMP from 5-phospho-ribose 1-pyrophosphate (PRPP) and the respective bases,guanine, hypoxanthine, and xanthine. Plasmodium falciparum, causes the most virulent form of malaria, and isalso a purine auxotroph for which the action of PPRT is the only physiological path for hypoxanthineincorporation into the nucleotide pool. Transition-state analogue inhibitors (TSAIs) based on transition statesfor related phosphoribosyltransferases are inhibitors of P. falciparum PPRT. Cell-permeable prodrugs blockedthe proliferation of P. falciparum in culture and showed selectivity vs. human HGPRT, despite the highstructural similarity and active-site conservation of the enzymes.This research will design, synthesize, and characterize both in vitro and in vivo novel inhibitors of the PPRTsfrom P. falciparum, T. brucei ssp. and T. cruzi. Transition-state methodology, quantum computationalchemistry, X-ray structure-based inhibitor design and expert chemical synthesis will be used for inhibitordevelopment. Existing lead compounds of sub-nanomolar potency for PPRT from P. falciparum will initiate andinform our inhibitor program. Crystal structures have been reported for T. cruzi PPRT, but no potent inhibitorshave been defined. No inhibitor development for T. brucei has been reported, and PPRTs have not beenkinetically characterized from T. brucei. The generation of new lead compounds is anticipated to proceedrapidly for P. falciparum PPRT and emerge for Trypanosoma PPRTs once the transition-state structures ofthese enzymes have been solved. Optimized inhibitors which bind each of the target PPRTs will be evaluatedby X-ray crystallography, to provide a refinement guide for chemistry. Potent and selective (vs. humanHGPRT) inhibitors of the PPRTs will be evaluated in cell cultures of P. falciparum, T. cruzi, and T. brucei sspfor parasiticidal activity. The most effective of these will be evaluated in murine models of Malaria, Chagasâ€™sdisease and African sleeping sickness. Importantly, a successful outcome from this proposal could lead to newtherapeutic agents to treat three diseases which comprise unmet or under-met medical needs. Thisdevelopment plan uses transition state theory to meet the NIH goals of reducing the lead time for drugdevelopment.