Pharmasset Initiates Dosing in a Combination Study of PSI-7977 and PSI-938 for Chronic Hepatitis C

PRINCETON, N.J., Nov. 30, 2010 /PRNewswire-FirstCall/ -- Pharmasset, Inc. (Nasdaq: VRUS) announced today that dosing has begun in Part 2 of a Phase 1 study. This is the first clinical study combining a purine (PSI-938) and a pyrimidine (PSI-7977) nucleotide analog for HCV, and is designed to evaluate once daily doses of PSI-7977 and PSI-938 in patients with HCV who have not been treated previously.

"We are excited to be initiating this combination study with two proprietary nucleotide analogs for HCV," stated Michelle Berrey, MD, MPH, Pharmasset's Chief Medical Officer. "Based on our in vitro data, we believe the combination of two nucleotide analogs could provide potent antiviral activity across multiple HCV genotypes and could also have a higher barrier to resistance compared to other DAA combinations. We believe nucleotide analogs have a number of key attributes that may make them ideal partners for other DAA combinations, in addition to a 'nuc-nuc' combination."

About the Phase 1 Trial

In Part 1 of the Phase 1 multiple ascending dose study of PSI-938, suppression of HCV RNA below the limit of detection (LOD, <15 IU/mL) was observed in over half of the patients who received PSI-938 at daily doses of 200 mg or 300 mg for seven days. Part 2 of the study is designed to evaluate the combination of PSI-938 and PSI-7977. The primary objective is to assess the safety, tolerability and pharmacokinetics of PSI-938 administered alone for 14 days, and in combination with PSI-7977 for 7 to 14 days. The secondary objective is to evaluate viral kinetics of HCV RNA during monotherapy and combination nucleotide dosing. Approximately forty patients are expected to be enrolled into four cohorts as follows:

• PSI-938 QD administered as monotherapy for 14 days, followed by;
• PSI-938 QD for 7 days followed by PSI-938 plus PSI-7977 QD for 7 days and
• PSI-7977 QD for 7 days followed by PSI-938 plus PSI-7977 QD for 7 days, followed by;
• PSI-938 plus PSI-7977 QD for 14 days

We expect to report preliminary results from Part 2 of this Phase 1 study during the first quarter of calendar year 2011. We also expect to initiate a Phase 2 study of PSI-938 in combination with PSI-7977 during mid-2011. This Phase 2 study proposes to explore durations of PSI-938 and PSI-7977 in interferon-free combinations with an SVR endpoint.

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.

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