PSI-7977 Receives Fast Track Designation from the FDA for the Treatment of Chronic Hepatitis C Infection

PRINCETON, N.J., Aug. 12 /PRNewswire-FirstCall/ -- Pharmasset, Inc. (Nasdaq: VRUS) has received fast track designation from the U.S. Food and Drug Administration (FDA) for PSI-7977 for the treatment of chronic hepatitis C virus (HCV) infection. PSI-7977 is an oral uridine nucleotide analog polymerase inhibitor of HCV. Pharmasset recently completed dosing in a 28 day Phase 2a trial to evaluate PSI-7977 in combination with Pegasys (pegylated interferon) plus Copegus (ribavirin) in treatment-naive patients chronically infected with HCV genotype 1. Pharmasset expects to initiate a 12-week Phase 2b study of PSI-7977 in the fourth quarter of 2010.

Under the FDA Modernization Act of 1997, fast track designation may facilitate the development and expedite the review of a drug candidate that is intended for the treatment of a serious and life-threatening condition and demonstrates the potential to address an unmet medical need for such a condition. PSI-7977 was granted the fast track designation primarily due to the need for HCV treatments with novel mechanisms of action, oral administration, different resistance profiles and improved safety and efficacy over the existing standard of care for both treatment-naive and treatment-experienced patients.

"The FDA's fast track designation for PSI-7977 acknowledges the urgent need for new HCV drugs," stated Dr. Michael Rogers, Pharmasset's Chief Development Officer. "Currently, there are no HCV nucleoside/tide inhibitors approved for the treatment of chronic HCV infection. We continue to work closely with the FDA on the development and regulatory review of PSI-7977, which has demonstrated compelling antiviral activity, a high barrier to resistance and has been generally well-tolerated in clinical trials to date."

About Pharmasset

Pharmasset is a clinical-stage pharmaceutical company committed to discovering, developing, and commercializing novel drugs to treat viral infections. Pharmasset's primary focus is on the development of oral therapeutics for the treatment of hepatitis C virus (HCV) and, secondarily, on the development of Racivir(TM) for the treatment of human immunodeficiency virus (HIV). Our research and development efforts focus on nucleoside/tide analogs, a class of compounds which act as alternative substrates for the viral polymerase, thus inhibiting viral replication. We currently have four clinical-stage product candidates. RG7128, a cytosine nucleoside analog for chronic HCV infection, is in two Phase 2b clinical studies in combination with Pegasys(R) plus Copegus(R) and is also in the INFORM studies, the first series of studies designed to assess the potential of combinations of small molecules without Pegasys(R) and Copegus(R) to treat chronic HCV. These clinical studies are being conducted through a strategic collaboration with Roche. Our other clinical stage HCV candidates include PSI-7977, an unpartnered uracil nucleotide analog that has recently completed 28 days of dosing in a Phase 2a study, and PSI-938, an unpartnered guanosine nucleotide analog in a Phase 1 study. We also have in our pipeline an additional purine nucleotide analog, PSI-661, in advanced preclinical development. Racivir, for the treatment of HIV, has completed a Phase 2 clinical study.

Pegasys® and Copegus® are registered trademarks of Roche.

Forward-Looking Statements

Pharmasset "Safe Harbor" Statement under the Private Securities Litigation Reform Act of 1995:  Statements in this press release that are not historical facts are "forward-looking statements," including, without limitation, statements  that involve risks, uncertainties, and other important factors, including, without limitation,  the risk of cessation or delay of any of the ongoing or planned clinical trials and/or our development of our product candidates,  the risk that the results of previously conducted studies involving our product candidates will not be repeated or observed in ongoing or future studies involving our product candidates, the risk that our collaboration with Roche will not continue or will not be successful, and the risk that any one or more of our product candidates will not be successfully developed and commercialized.  For a discussion of risks, uncertainties, and other important factors, any of which could cause our actual results to differ from those contained in the forward-looking statements, see the section entitled "Risk Factors" in our Annual Report on Form 10-K for the fiscal year ended September 30, 2009 and our Quarterly Reports on Form 10-Q for the periods ended December 31, 2009, March 31, 2010 and June 30, 2010  filed with the Securities and Exchange Commission and discussions of potential risks, uncertainties, and other important factors in our subsequent filings with the Securities and Exchange Commission.

SOURCE Pharmasset, Inc.