Pharmasset Enters into a Clinical Collaboration Agreement with Tibotec Pharmaceuticals for a Combination Study in Patients Chronically Infected with Hepatitis C

PRINCETON, N.J., July 6, 2011 /PRNewswire/ -- Pharmasset, Inc. (NASDAQ: VRUS), announced today that it has entered into a clinical collaboration agreement with Tibotec Pharmaceuticals to evaluate in a Phase 2 study the safety and efficacy of PSI-7977, Pharmasset's investigational nucleotide polymerase inhibitor, in combination with TMC435, Tibotec Pharmaceuticals' investigational protease inhibitor, for the treatment of chronic hepatitis C virus (HCV).

This phase 2 proof of concept study will evaluate the potential to achieve sustained virologic response 12 weeks post treatment with an all oral, once-daily, interferon-free treatment regimen in patients infected with genotype 1 HCV.  Specifically, the study will assess the safety, pharmacokinetics and pharmacodynamics of 12 and 24 weeks of PSI-7977 in combination with TMC435, with and without ribavirin, in patients chronically infected with HCV genotype 1 who had a prior null response to peginterferon alfa and ribavirin treatment. The study is planned to start in the second half of 2011.  

"We are excited to be working with Tibotec to simplify and improve HCV treatment," stated Bill Symonds, PharmD, Pharmasset's Senior Vice President of Clinical Pharmacology and Translational Medicine. "PSI-7977 is now being studied in interferon-free combinations with each of the three leading classes of direct-acting antivirals: Tibotec's protease inhibitor, Bristol-Myers Squibb's NS5a inhibitor, and our own nucleotide, PSI-938. This advances one of our key goals at Pharmasset: to develop our nucleotide analogs as the backbone of interferon-free HCV therapy."

About PSI-7977

PSI-7977 is a prodrug of a uracil nucleotide analog polymerase inhibitor we are developing for the treatment of chronic HCV infection. PSI-7977 has completed a 28 day phase 2a trial in combination with peg-interferon and ribavirin (Peg-IFN/RBV) and is currently being tested in four phase 2b studies: the PROTON trial in combination with peg-IFN/RBV in HCV genotype 1, 2 or 3 patients; the ATOMIC trial with peg-IFN/RBV in HCV genotypes 1,4,5,6; the ELECTRON trial, an interferon sparing /interferon free study in HCV genotypes 1,2 and 3 and a study with Bristol-Myers Squibb's NS5a inhibitor, BMS-790052, as part of an interferon free regimen. Pharmasset also anticipates initiating its own interferon free trial with PSI-7977 and PSI-938, a guanine nucleotide polymerase inhibitor in the third calendar quarter 2011.  

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 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 in the third quarter of 2011. Mericitabine (RG7128) continues in two Phase 2b trials and one interferon-free trial being conducted through a strategic collaboration with Roche.

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

Pharmasset 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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