Structural Analysis and Inhibitor Optimization of Cryptosporidium n-myristoyltransferase for Drug Discovery

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ABSTRACT Each year over 525,000 children under age five are killed by diarrhea caused by infectious disease. Cryptosporidiosis, the second most frequent cause of childhood diarrhea, is an infection caused by colonization of the intestines by the eukaryotic parasites, Cryptosporidium parvum or C. hominis, and particularly damages and kills malnourished children. In contrast to other Apicomplexans (such as Plasmodium or Toxoplasma), there is no required insect vector or animal host, since parasites can be transferred directly from human to human through the fecal-oral route. Despite the high incidence and significant impact on malnourished children, there are no effective treatments for cryptosporidiosis. We had previously screened the GSK Tres Cantos proprietary library of ~2 million compounds against P. falciparum N-myristoyltransferase (NMT). NMT is an enzyme which transfers myristate, a 14-carbon fatty acid, to the N-terminal glycine residue of proteins co-translationally, which contributes to targeting the substrate protein to membrane regions. NMT has been validated as a drug target in fungal and parasitic diseases, including malaria and leishmaniasis. We hypothesized that NMT high-throughput screening (HTS) hits effective against Plasmodium would also be active against Cryptosporidium and tested the top eight hits against Cryptosporidium parvum NMT (CpNMT). Of those top eight hits, three were effective against the purified enzyme and one showed activity against the parasite in vitro. A follow-on synthetic chemistry and structure- based drug design program further developed these hits into a lead series (called Series-2) of highly effective (~10nM IC50) inhibitors with 500-1000 X selectivity over the human enzyme. The two most promising leads from the Series-2 scaffold were then tested in a mouse model of infection and found that both molecules completely cleared infection. These data serve to chemically validate NMT as a druggable target for the treatment of Cryptosporidiosis. In this proposal we intend to further develop lead compounds in order to improve drug-like characteristics in preparation for preclinical drug development.