Targeting a Prokaryotic Protein in a Eukaryotic Parasite

Abstract

Cryptosporidium is a major cause of the “vicious cycle of diarrhea and malnutrition”, an important AIDS pathogen and potential bio‐terrorism agent. The currently approved drugs to treat cryptosporidiosis are ineffective, and no vaccines exist. Genomic analysis indicates that inosine monophosphate dehydrogenase (IMPDH) is required for the production of guanine nucleotides. Moreover, the parasite obtained its IMPDH gene from bacteria by lateral gene transfer, so that this enzyme is very different from the host counterpart. We designed a high throughput screen to target the most diverged portion of the IMPDH active site. This screen has identified fifty parasite‐selective IMPDH inhibitors. These compounds bind in the NAD site as expected. The x‐ray crystal structure of the inhibitor complex reveals the structural basis for selectivity as well as the spectrum of inhibition of IMPDHs from bacterial pathogens. We have also developed a novel assay for antiparasitic activity by re‐programming the guanine biosynthetic pathways of Toxoplasma gondii to mimic Cryptosporidium. Medicinal chemistry optimization has produced inhibitors with nanomolar affinities. The best inhibitors display antiparasitic activity with greater potency than paromomycin, the current “gold standard” for anticryptosporidial activity. Supported by NIH RO1 AI055268 (Boris Striepen, UGA) and NIH U01 AI075466 (L.H.).