PSI-938 Receives Fast Track from the FDA for Hepatitis C

Pharmasset, Inc.
PRINCETON, N.J., Aug. 24, 2011 --
/PRNewswire/ -- Pharmasset, Inc. (Nasdaq: VRUS) has received fast track designation from the U.S. Food and Drug Administration (FDA) for PSI-938 for the treatment of chronic hepatitis C virus (HCV) infection. PSI-938 is an oral guanosine nucleotide analog polymerase inhibitor of HCV.

In March 2011, Pharmasset presented data from the NUCLEAR study demonstrating that PSI-938 has potent antiviral activity and is generally safe and well tolerated, both as monotherapy and in combination with Pharmasset's lead nucleotide analog, PSI-7977. The NUCLEAR study was conducted in treatment naive subjects with genotype 1 HCV who were treated for 14 days with either PSI-938 or a combination of PSI-938 and PSI-7977 with 92% achieving HCV RNA <15IU/mL, the limit of detection, in the combination arms.

Pharmasset plans to initiate QUANTUM, an interferon-free combination trial with PSI-938 and PSI-7977 in the third quarter of 2011.

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-938 was granted the fast track designation primarily due to the need for HCV treatments with improved tolerability, safety and efficacy over the existing standard of care for both treatment-naive and treatment-experienced patients.

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 the development of oral therapeutics for the treatment of hepatitis C virus (HCV) infection. Our research and development efforts are focused on nucleoside/tide analogs, a class of compounds which act as alternative substrates for the viral polymerase, thus inhibiting viral replication. We currently have three clinical-stage product candidates advancing in trials in various populations. Our pyrimidine, PSI-7977, an unpartnered uracil nucleotide analog, is currently under study in four Phase 2b trials in patients with HCV genotypes 1 through 6, including abbreviated duration interferon and interferon-free regimens. Our purine, PSI-938, an unpartnered guanosine nucleotide analog, recently reported safety and efficacy data from 14 days of monotherapy as well as 14 days in combination with the pyrimidine, PSI-7977. An SVR-endpoint study of the purine-pyrimidine combination is anticipated to begin in the third quarter of 2011. Mericitabine (RG7128) continues in three Phase 2b trials and one interferon-free trial being conducted through a strategic collaboration with Roche.

ContactRichard E. T. Smith, Ph.D. VP, Investor Relations and Corporate Communicationsrichard.smith@pharmasset.comOffice: +1 (609) 613-4181

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," 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, 2010 and our Quarterly Reports on Form 10-Q 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.
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