PRINCETON, N.J. (AP) — Pharmasset Inc. said Monday it is ending development of its most advanced drug candidate, clevudine, over concerns of severe muscle weakness in hepatitis B patients. Pharmasset said it made the decision to end the late-stage Quash studies voluntarily, following discussions with an independent monitoring board and the Food and Drug Administration. The company is maintaining its focus on a separate hepatitis C program with its partner Roche. Both hepatitis B and C are liver conditions. The decision was made based on the rate of South Korean patients with muscle weakness, or myopathy, side effects. "Although the number of cases of myopathy in the Quash trials was low and the severity was mild, more severe reports from other trials and post marketing surveillance led us to believe the risk benefit ratio for clevudine was insufficient to continue development," said Chief Medical Officer Dr. Michelle Berrey in a statement. Pharmasset said it will continue collecting safety data and monitoring patients after the study is discontinued, though it won't submit the results to the FDA for approval. "While we are obviously disappointed with the outcome, we believe the decision to stop dosing is an appropriate one," said Chief Executive Schaefer Price in a statement. With the loss of clevudine, the company's next most advanced drug candidate is Racivir, an HIV treatment in midstage development. The developing hepatitis C treatment R7128 is set for midstage development.