DESCRIPTION (provided by investigator): The long-term objective of this application is to identify a potent, efficacious and safe drug for the treatment of hepatitis C virus (HCV) through the use of our proprietary HepDirect prodrug technology. It is estimated that >170 million people are infected with Hepatitis C virus worldwide. Despite recent improvements in HCV drug formulations, a large segment of the patient population is still under treated. In the first phase of our SBIR grant support we demonstrated the potential of HepDirect prodrugs in selectively delivering high levels of phosphorylated nucleoside analogues to the liver. The work led to the identification of a development candidate (HepDirect-adefovir), which recently completed its initial Phase I clinical study and is expected to begin Phase 1/2 in early 2003. We now propose to continue the exploration of the HepDirect technology and its applications to a large and structurally-diverse set of nucleosides, with the goal of identifying a potent and selective inhibitor of HCV replication. Most nucleosides designed to be specific inhibitors of viral replication as the nucleoside triphosphate (NTP) fail as a result of poor recognition by the nucleoside kinases responsible for converting the nucleoside to the NTP. The HepDirect technology overcomes this limitation and allows us to pursue nucleoside analogues that are easily overlooked or discarded by companies pursuing standard drug discovery approaches. The specific aims of this application include the synthesis of a large number of HepDirect prodrugs of diverse nucleoside analogues, the production of the corresponding NTP library via the biological conversion of the prodrugs in hepatocytes, and the screening of this library for the inhibition of HCV replication. To support the antiviral activity determinations, a key aim is the development of a cellular assay compatible with the prodrug technology as well as a novel human tissue based HCV replication model. Following the identification of a lead inhibitor, the HepDirect prodrug moiety is optimized for intracellular activation efficiency, oral bioavailability, pharmacokinetics, as well as liver targeting. The final aim is to complete the requisite studies to launch the compound identified into full development for the treatment of HCV including second species pharmacokinetics, pilot animal toxicology, pilot in vitro and in vivo genetic toxicology and general safety pharmacology.