Pharmasset Initiates Exploratory Interferon Sparing Clinical Trial of PSI-7977 for Chronic Hepatitis C

PRINCETON, N.J., Dec. 14, 2010 /PRNewswire/ -- Pharmasset, Inc. (Nasdaq: VRUS) announced today that dosing has begun in an exploratory study of PSI-7977, a nucleotide analog polymerase inhibitor, for the treatment of chronic hepatitis C (HCV). The trial will evaluate PSI-7977 400mg QD in combination with ribavirin (RBV), with 0, 4, 8, or 12 weeks of pegylated interferon alfa 2a (Peg-IFN) in treatment-naive patients infected with HCV genotype 2 or 3.

"Based on the encouraging efficacy, safety and resistance data from the Phase 2a trial with PSI-7977 in combination with pegylated interferon and ribavirin, we initiated a study to explore shorter durations of interferon, including an interferon-free regimen in treatment naïve patients with HCV genotype 2 or 3," stated Michelle Berrey, MD, MPH, Pharmasset's Chief Medical Officer. "An important limitation to the current treatment of HCV patients is the use of interferon with its associated side effect profile. We believe that PSI-7977's high barrier to resistance and the lack of a significant effect of IL28B genotype in the Phase 2a trial provide us an opportunity with PSI-7977 to explore shorter durations of interferon, and potentially even removing it from the treatment regimen altogether."

About the Trial

The trial is anticipated to enroll approximately 40 patients infected with HCV genotype 2 or 3 who have not previously been treated. The primary endpoint of the trial will be the assessment of safety and tolerability of PSI-7977 400mg QD and RBV for 12 weeks, administered with or without pegylated interferon (Peg-IFN) in treatment naïve patients with HCV genotypes 2 or 3. The trial will be conducted in New Zealand. Patients will be randomized into one of 4 arms:

-- PSI-7977 400mg QD in combination with RBV for 12 weeks (no interferon);

-- PSI-7977 400mg QD in combination with RBV for 12 weeks, with only 4 weeks of Peg-IFN;

-- PSI-7977 400mg QD in combination with RBV for 12 weeks, with only 8 weeks of Peg-IFN;

-- PSI-7977 400mg QD in combination with Peg-IFN and RBV for 12 weeks.

Patients will be stratified by HCV genotype and IL28B status to ensure balance across cohorts.

Pharmasset anticipates reporting interim data from this trial in the first half of 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 on the development of oral therapeutics for the treatment of hepatitis C virus (HCV). 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 initiated dosing in a Phase 2b study in patients with HCV genotypes 1, 2, or 3, and PSI-938, an unpartnered guanosine nucleotide analog which recently completed a 7-day monotherapy study. We also have in our pipeline an additional purine nucleotide analog, PSI-661, in advanced preclinical development.

Pegasys(R) and Copegus(R) 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," 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 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.