Hepatitis C virus (HCV) is one of the most important Flaviviridae infections in humans and is responsible for the second most common cause of viral hepatitis. Presently, nearly 2% of the U.S. population, and an estimated 170 million people worldwide, are HCV carriers (2). The only approved therapy for chronic hepatitis C is alpha interferon (IFN-␣), either alone or in combination with ribavirin. Anemia is the most common adverse effect associated with ribavirin treatment, and neuropsychiatric adverse effects of IFN-␣ lead to premature cessation of therapy in 10 to 20% of patients (9, 13).As additional treatment options are urgently needed, there is an ongoing search for more potent antiviral compounds with fewer adverse effects. However, the search for improved antiviral agents is hampered by the limited and cumbersome propagation of HCV in vitro (4). Therefore, surrogate models such as the HCV RNA replicon that replicates in the human hepatoma cell line Huh7 have been developed (6,29). Improved versions of these HCV replicons contain adaptive mutations (25), and their use has facilitated the evaluation of candidate anti-HCV drugs.Bovine viral diarrhea virus (BVDV) is one of the best characterized members of the Flaviviridae family and has one of the largest RNA genomes (12.5 kb) in this family (8). This virus has the remarkable property of existing as noncytopathic and cytopathic (cpBVDV) biotypes, with cpBVDV strains showing insertions or viral genome rearrangements at the junction site between nonstructural protein 2 (NS2) and NS3 (32). BVDV may provide a surrogate model for HCV, both for the molecular study of viral proteins (33) and for the evaluation of antiviral compounds (3,7,47).In the search for therapeutic agents, any element that is essential for viral (or replicon) RNA replication may be considered a drug discovery target. Such elements can be either viral proteins (NS2-NS3 protease, NS3-NS4A serine proteinase, NS3 RNA helicase, or RNA-dependent RNA polymerase [3,24,34,36] [18,31,41,43]). Current knowledge of the human genome, combined with array technology and pathogen infection models, will likely lead to more defined host-pathogen-related targets for future drug design (17, 23). Today, however, the most successful classes of antiviral compounds with clinical utility in combat against other human viral pathogens (human immunodeficiency virus type 1 [HIV-1], hepatitis B virus [HBV], herpes simplex virus [HSV], and cytomegalovirus) are the protease, the nonnucleoside analogue, and the nucleoside analogue inhibitors. As the latter class of compounds is crucial in controlling herpesvirus, HIV-1, and HBV infections, it is likely and anticipated that

) is a potent specific inhibitor of hepatitis C virus (HCV) RNA synthesis in Huh-7 replicon cells. To inhibit the HCV NS5B RNA polymerase, PSI-6130 must be phosphorylated to the 5-triphosphate form. The phosphorylation of PSI-6130 and inhibition of HCV NS5B were investigated. The phosphorylation of PSI-6130 by recombinant human 2-deoxycytidine kinase (dCK) and uridine-cytidine kinase 1 (UCK-1) was measured by using a coupled spectrophotometric reaction. PSI-6130 was shown to be a substrate for purified dCK, with a K m of 81 M and a k cat of 0.007 s ؊1 , but was not a substrate for UCK-1. PSI-6130 monophosphate (PSI-6130-MP) was efficiently phosphorylated to the diphosphate and subsequently to the triphosphate by recombinant human UMP-CMP kinase and nucleoside diphosphate kinase, respectively. The inhibition of wild-type and mutated (S282T) HCV NS5B RNA polymerases was studied. The steady-state inhibition constant (K i ) for PSI-6130 triphosphate (PSI-6130-TP) with the wild-type enzyme was 4.3 M. Similar results were obtained with 2-C-methyladenosine triphosphate (K i ‫؍‬ 1.5 M) and 2-C-methylcytidine triphosphate (K i ‫؍‬ 1.6 M). NS5B with the S282T mutation, which is known to confer resistance to 2-C-methyladenosine, was inhibited by PSI-6130-TP as efficiently as the wild type. Incorporation of PSI-6130-MP into RNA catalyzed by purified NS5B RNA polymerase resulted in chain termination.Hepatitis C virus (HCV) is an RNA virus which possesses a single-stranded positive-sense RNA as the viral genome. This viral RNA plays important roles during viral replication, as it serves as an mRNA for viral protein synthesis, a template for RNA replication, and a nascent RNA genome for a newly formed virus (17). HCV NS5B RNA-dependent RNA polymerase is a key enzyme in viral RNA replication. This enzyme, which does not require a primer for initiation of RNA synthesis, catalyzes de novo RNA synthesis (8, 11). Nucleoside analogs have been used to treat viral infections, such as herpes simplex virus, human immunodeficiency virus, and hepatitis B virus infections (5,6,21). These drugs are designed to inhibit viral polymerases by a process called chain termination, in which DNA synthesis is quenched by incorporating the triphosphate forms of these drugs, which lack the 3Ј-hydroxyl group on the sugar moiety. In order for nucleoside analogs to inhibit a viral polymerase, they must be transported into the cell and converted to the active 5Ј-triphosphate form by cellular kinases. 2Ј-C-Methylnucleosides have been investigated as anti-HCV agents targeting HCV NS5B RNA polymerase (2, 20). 2Ј-C-Methyladenosine (2Ј-C-Me-A) and 2Ј-C-methylguanosine (2Ј-C-Me-G) showed potent anti-HCV activities in a cell-based replicon assay, and their triphosphate forms inhibited replicase and NS5B RNA polymerase in vitro (20). In addition, 2Ј-C-Me-A exhibited significant activity against HCV in a cell culture system which involves complete HCV replication and which produces infectious HCV (16). A resistant replicon has been selected by passage of HCV in the...

2-Deoxy-2-fluorocytidine (FdC) is a potent inhibitor of the hepatitis C virus RNA replicon in culture, and FdC-5-triphosphate is an effective inhibitor of the NS5B polymerase. Dynamic profiling of cell growth in an antiviral assay showed that FdC caused cytostasis due to an S-phase arrest. These observations demonstrate that FdC treatment is affecting both a viral target and a cellular target.Hepatitis C virus (HCV) infection is the leading cause of liver transplantation in the United States, with sequelae including fibrosis, cirrhosis, and hepatocellular carcinoma (1). In vivo, HCV replication occurs mainly in the cytoplasm of infected hepatocytes, but it has been difficult to demonstrate replication in vitro. Replicon-based systems have now been developed that sustain efficient replication of HCV RNA in cell culture. Initially, subgenomic replicons that expressed only nonstructural proteins were constructed; however, recent reports described replicons that can express the entire HCV polyprotein (5, 7).In addition to the currently approved standard treatment options for HCV infections that use interferon and ribavirin, several new antiviral agents are in preclinical or clinical development. Similar to the case with human immunodeficiency virus type 1 treatment, multiple drug targets (e.g., protease, helicase, polymerase, and entry) may be needed to limit the emergence of drug-resistant variants. The HCV subgenomic replicon provides an excellent system for evaluating HCV antiviral agents in cell culture (3,5,6,10,16,18). We report here the antiviral activity of 2Ј-deoxy-2Ј-fluorocytidine (FdC) (Fig. 1) measured in the HCV subgenomic replicon system and in the bovine viral diarrhea virus (BVDV)-Madin-Darby bovine kidney (MDBK) cell system. HCV-replicon RNA-containing Huh-7 cells (Clone A cells; Apath, LLC, St. Louis, Mo.) were kept in exponential growth as described previously (16). Antiviral assays were performed in medium without G418. Cells were seeded in a 96-well plate at 1,000 cells per well, and test compounds were added immediately after seeding. After 96 h of incubation, total cellular RNA was isolated (Rneasy 96 kit; Qiagen, Valencia, Ca.), and HCV replicon RNA and an internal control (TaqMan rRNA Control Reagents; Applied Biosystems, Foster City, Ca.) were amplified in a single-step multiplex reverse transcription-PCR protocol. FdC (obtained from the Pharmasset compound library) was tested in a concentration range of 0.1 to 200 M, and a 90% effective concentration (EC 90 ) for reducing the intracellular HCV replicon RNA levels of 5.0 M was found ( Fig. 2A). FdC was found to be more potent than ribavirin (EC 90 , ϳ100 M) and comparable in potency to ␤-D-N 4 -hydroxycytidine (NHC) (EC 90 ϭ 5 M) (16). The cellular toxicity against Huh-7 and HepG2 cells was measured after 96 h of incubation by using the CellTiter 96 AQ ueous One solution cell proliferation assay (Promega, Madison, Wis.), and the concentration resulting in 50% reduction in cell growth (CC 50 ) was found to be greater than 100 M. This resul...