All-Oral Combination Study with PSI-7977 for HCV Genotypes 1, 2 and 3 Initiated

PRINCETON, N.J., May 26, 2011 /PRNewswire/ -- Pharmasset, Inc. (Nasdaq: VRUS) announced today the initiation of a Phase 2a trial investigating the combination of Pharmasset's PSI-7977, a nucleotide polymerase inhibitor, and BMS-790052, Bristol-Myers Squibb Company's (NYSE: BMY) NS5A replication complex inhibitor, for the treatment of chronic hepatitis C (HCV).  This trial is the result of a clinical collaboration agreement between Pharmasset and Bristol-Myers Squibb announced in January 2011.

"We are happy to announce the initiation of this important combination trial," stated William Symonds, Pharmasset's Senior Vice President of Clinical Pharmacology and Translational Medicine. "Recent data from Bristol-Myers Squibb's combination study demonstrated that individuals with HCV can be cured without the traditional interferon and ribavirin, but only if two potent DAAs are used and drug resistance is avoided. We believe Pharmasset's nucleotide analogs have demonstrated potent antiviral activity and a high barrier to resistance and therefore have the potential to be the future backbone of interferon-free treatment."

About the Trial

This Phase 2a trial is planned to enroll approximately 84 patients with chronic HCV genotypes 1, 2 or 3 who have not been treated previously. The primary endpoint of the trial is sustained virologic response (SVR). The trial will be conducted in the U.S. Subjects will be randomized equally across each of the following arms:

• PSI-7977 400mg QD for 7 days, then add BMS-790052 60mg QD for further 23 weeks in genotype 1 subjects;
• PSI-7977 400mg QD for 7 days, then add BMS-790052 60mg QD for further 23 weeks in genotype 2 or 3 subjects;
• PSI-7977 400mg QD and BMS-790052 60mg QD for 24 weeks in genotype 1 subjects;
• PSI-7977 400mg QD and BMS-790052 60mg QD for 24 weeks in genotype 2 or 3 subjects;
• PSI-7977 400mg QD, BMS-790052 60mg QD and ribavirin for 24 weeks in genotype 1 subjects;
• PSI-7977 400mg QD, BMS-790052 60mg QD and ribavirin for 24 weeks in genotype 2 or 3 subjects.

Additional details can be found at www.clinicaltrials.gov.

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 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 three 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 third quarter 2011. Mericitabine (RG7128) continues in two Phase 2b trials and one interferon-free trial 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.

SOURCE Pharmasset, Inc.