Dr. Kiser's overall career goal is to develop an independent research program specializing in the application of clinical pharmacology research to the treatment of viral hepatitis. Pharmacology research in chronic Hepatitis C virus (HCV) is urgently needed because of the poor response rates with current therapies and the need to investigate the optimal dose and the potential for drug interactions with newer compounds. Through the study proposed, the candidate will use pharmacokinetics (PK) to guide dosing decisions with ribavirin, generate preliminary data on intracellular ribavirin concentrations, and become poised to apply PK techniques to studies with new HCV agents. The specific aims for the study are to (1) demonstrate that concentration-controlled ribavirin dosing can achieve a targeted level of plasma exposure with reduced variability in the steady-state area-under-the-concentration-time curve compared with standard weight-based ribavirin dosing, (2) evaluate the safety and efficacy of concentration-controlled versus standard weight- based ribavirin therapy, (3) quantify the intracellular ribavirin mono- (RBV-MP), di- (RBV-DP), and tri- phosphate (RBV-TP) concentrations in peripheral blood mononuclear cells (PBMCs) and red blood cells, and (4) develop a population PK-pharmacodynamic model to investigate the response and toxicity relationships relative to systemic and intracellular ribavirin exposures. Forty, previously treatment-nave HCV genotype 1 patients, initiating peginterferon alfa 2a and ribavirin will be stratified on weight, degree of liver damage, and race and randomized to 2 groups: standard weight-based ribavirin dosing or concentration-guided ribavirin dosing. If we find that concentration-controlled therapy can maintain concentrations within a target range and that it appears safe and effective, then this strategy can be explored in other populations including non- responders, those with HCV recurrence following liver transplantation, and/or patients with HIV/HCV coinfection. We can also apply this proof of concept concentration-controlled strategy to other compounds including peginterferon and the HCV protease and polymerase inhibitors. This study is consistent with a long-term research goal of NIDDK to evaluate new approaches to therapy for all forms of viral hepatitis.