Pharmasset Starts Trial of HCV Polymerase Inhibitor PSI-7977 without Interferon

		SUMMARY: Pharmasset, Inc. announced last month that it has started an exploratory trial to test its experimental hepatitis C virus (HCV) polymerase inhibitor PSI-7977 in combination regimens with ribavirin and varying durations of pegylated interferon alfa-2a (Pegasys). Participants in one arm will receive an all-oral regimen with no ribavirin.

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

	Exploratory phase 2 trial in patients with HCV genotype 2 or 3 to receive PSI-7977 400 mg QD and ribavirin, with 0-12 weeks of pegylated interferon
	Interim data expected in first half of 2011

Princeton, NJ -- December 14, 2010 -- 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 400 mg QD [once-daily] 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 naive 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 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."

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 400 mg QD and RBV for 12 weeks, administered with or without pegylated interferon (Peg-IFN) in treatment naive 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 400 mg QD in combination with RBV for 12 weeks (no interferon);
	PSI-7977 400 mg QD in combination with RBV for 12 weeks, with only 4 weeks of Peg-IFN;
	PSI-7977 400 mg QD in combination with RBV for 12 weeks, with only 8 weeks of Peg-IFN;
	PSI-7977 400 mg 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 plus Copegus and is also in the INFORM studies, the first series of studies designed to assess the potential of combinations of small molecules without Pegasys and Copegus 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.