Mechanism of activation of PSI-7851 and its diastereoisomer PSI-7977.

A phosphoramidate prodrug of 2'-deoxy-2'-α-fluoro-β-C-methyluridine-5'-monophosphate, PSI-7851, demonstrates potent anti-hepatitis C virus (HCV) activity both in vitro and in vivo. PSI-7851 is a mixture of two diastereoisomers, PSI-7976 and PSI-7977, with PSI-7977 being the more active inhibitor of HCV RNA replication in the HCV replicon assay. To inhibit the HCV NS5B RNA-dependent RNA polymerase, PSI-7851 must be metabolized to the active triphosphate form. The first step, hydrolysis of the carboxyl ester by human cathepsin A (CatA) and/or carboxylesterase 1 (CES1), is a stereospecific reaction. Western blot analysis showed that CatA and CES1 are both expressed in primary human hepatocytes. However, expression of CES1 is undetectable in clone A replicon cells. Studies with inhibitors of CatA and/or CES1 indicated that CatA is primarily responsible for hydrolysis of the carboxyl ester in clone A cells, although in primary human hepatocytes, both CatA and CES1 contribute to the hydrolysis. Hydrolysis of the ester is followed by a putative nucleophilic attack on the phosphorus by the carboxyl group resulting in the spontaneous elimination of phenol and the production of an alaninyl phosphate metabolite, PSI-352707, which is common to both isomers. The removal of the amino acid moiety of PSI-352707 is catalyzed by histidine triad nucleotide-binding protein 1 (Hint1) to give the 5'-monophosphate form, PSI-7411. siRNA-mediated Hint1 knockdown studies further indicate that Hint1 is, at least in part, responsible for converting PSI-352707 to PSI-7411. PSI-7411 is then consecutively phosphorylated to the diphosphate, PSI-7410, and to the active triphosphate metabolite, PSI-7409, by UMP-CMP kinase and nucleoside diphosphate kinase, respectively.