The Mechanism of Action of β- d -2′-Deoxy-2′-Fluoro-2′- C -Methylcytidine Involves a Second Metabolic Pathway Leading to β- d -2′-Deoxy-2′-Fluoro-2′- C -Methyluridine 5′-Triphosphate, a Potent Inhibitor of the Hepatitis C Virus RNA-Dependent RNA Polymerase

β-d -2′-Deoxy-2′-fluoro-2′-C -methylcytidine (PSI-6130) is a potent inhibitor of hepatitis C virus (HCV) RNA replication in an HCV replicon assay. The 5′-triphosphate of PSI-6130 is a competitive inhibitor of the HCV RNA-dependent RNA polymerase (RdRp) and acts as a nonobligate chain terminator. Recently, it has been shown that the metabolism of PSI-6130 also results in the formation of the 5′-triphosphate of the uridine congener, β-d -2′-deoxy-2′-fluoro-2′-C -methyluridine (PSI-6206; RO2433). Here we show that the formation of the 5′-triphosphate of RO2433 (RO2433-TP) requires the deamination of PSI-6130 monophosphate and that RO2433 monophosphate is subsequently phosphorylated to the corresponding di- and triphosphates by cellular UMP-CMP kinase and nucleoside diphosphate kinase, respectively. RO2433-TP is a potent inhibitor of the HCV RdRp; however, both enzymatic and cell-based assays show that PSI-6130 triphosphate is a more potent inhibitor of the HCV RdRp than RO2433-TP.