Novel allosteric covalent inhibitors of bifunctional Cryptosporidium hominis TS-DHFR from parasitic protozoa identified by virtual screening

Protozoans of the genus Cryptosporidium are the causative agent of the gastrointestinal disease, cryptosporidiosis, which can be fatal in immunocompromised individuals. Cryptosporidium hominis (C. hominis) bifunctional thymidylate synthase-dihydrofolate reductase (TS-DHFR) is an essential enzyme in the folate biosynthesis pathway and a molecular target for inhibitor design. Previous studies have demonstrated the importance of the ChTS-DHFR linker region "crossover helix" to the enzymatic activity and stability of the ChDHFR domain. We conducted a virtual screen of a novel non-active site pocket located at the interface of the ChDHFR domain and crossover helix. From this screen we have identified and characterized a noncompetitive inhibitor, compound 15, a substituted diphenyl thiourea. Through subsequent structure activity relationship studies, we have identified a time-dependent inhibitor lead, compound 15D17, a thiol-substituted 2-hydroxy-N-phenylbenzamide, which is selective for ChTS-DHFR, and whose effects appear to be mediated by covalent bond formation with a non-catalytic cysteine residue adjacent to the non-active site pocket.