Final SVR Data from PROTON trial with PSI-7977 in Hepatitis C Genotype 2 or 3

-- 24 out of 24 subjects with HCV genotype 2 or 3 achieved an SVR after 12 weeks of treatment with PSI-7977 400mg QD and pegylated interferon and ribavirin -- SVR12 results from PROTON in HCV genotype 1 subjects are expected in second half of 2011

Press Release Source: Pharmasset On Wednesday July 20, 2011, 6:45 am EDT
PRINCETON, N.J., July 20, 2011 /PRNewswire/ -- Pharmasset, Inc. (Nasdaq:VRUS - News) announced today the final sustained virologic response (SVR) results from its phase 2b PROTON study with PSI-7977 dosed once daily in combination with peginterferon alfa 2a and ribavirin (Peg-IFN/RBV) in subjects with hepatitis C virus (HCV) genotype 2 or 3 who have not been treated previously. Twenty four out of twenty four subjects (100%) who completed treatment achieved an SVR, defined as HCV RNA below the limit of detection (<15 IU/ml) 24 weeks after the completion of treatment. No subject exhibited breakthrough on treatment or relapse after treatment.

Twenty five treatment-naive subjects with HCV genotype 2 or 3 were enrolled in an open label arm of the PROTON trial, receiving PSI-7977 400mg QD with Peg-IFN/RBV for 12 weeks, with no Peg-IFN/RBV follow-up. At the European Association for the Study of the Liver (EASL) in April 2011, Dr J. Lalezari presented interim results from this arm showing that 24 out of 24 subjects achieved an SVR12, defined as HCV RNA below the limit of detection (<15 IU/ml) 12 weeks after the completion of treatment. The combination was generally safe and well tolerated with one subject discontinuing treatment after day 1 and was lost to follow up. Overall PSI-7977 with Peg-IFN/RBV demonstrated potent viral suppression in subjects with HCV genotype 2 or 3 over 12 weeks of treatment.

Pharmasset anticipates reporting the SVR12 results from the PROTON trial in genotype 1 HCV subjects in the second half of 2011.

About the PROTON Trial
The Phase 2b PROTON trial enrolled 121 subjects infected with HCV genotype 1 who have not been treated previously. The primary endpoint of the trial will be the assessment of safety and tolerability of PSI-7977 in combination with Peg-IFN/RBV over 12 weeks with response-guided therapy allowing discontinuation of Peg-IFN/RBV at week 24. The trial was conducted in the U.S. Subjects were randomized (2:2:1) into one of 3 arms:
PSI-7977 200mg QD in combination with Peg-IFN/RBV for 12 weeks, followed by 12 or 36 weeks of Peg-IFN/RBV;
PSI-7977 400mg QD in combination with Peg-IFN/RBV for 12 weeks, followed by 12 or 36 weeks of Peg-IFN/RBV;
A control arm of matching placebo with Peg-IFN/RBV for 48 weeks.
In addition, 25 treatment-naive subjects with HCV genotype 2 or 3 were enrolled in a fourth, open label arm, receiving PSI-7977 400mg QD with Peg-IFN/RBV for 12 weeks, with no Peg-IFN/RBV follow-up. Subjects were followed for an additional 24 weeks after discontinuation of all therapy to assess SVR.

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.

Contact Richard E. T. Smith, Ph.D.
VP, Investor Relations and Corporate Communications Office +1 (609) 865-0693

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.

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