Data Monitoring Committee (DMC) determines RG7128, a first-in-class nucleoside analog polymerase inhibitor continues to have acceptable safety profile

An additional ~300 HCV positive genotype1 and 4 patients to be randomized equally across five treatment arms

Additional RG7128 phase 2 studies to be initiated in first half 2010

Princeton, JN -- November 23, 2009 -- Pharmasset, Inc. (Nasdaq: VRUS) announced today that the enrollment of Cohort 2, led by its partner Roche (SWX: ROG.VX; RO.S, OTCQX: RHHBY), will continue for the remaining 300 genotype 1 and 4 patients in the ongoing phase 2b trial of RG7128, a first-in-class nucleoside analog polymerase inhibitor for the treatment of chronic hepatitis C virus (HCV) infection. The decision was reached after a scheduled review by an independent Data Monitoring Committee (DMC) of all available safety data from the first cohort of approximately 100 patients completing 8 or 12 weeks of RG7128 or matching placebo in combination with pegylated interferon and ribavirin.

The DMC reviewed any potential drug discontinuations, incidence and details of adverse events, and selected laboratory assessments. No safety events in the DMC review were considered significantly different from those expected from HCV patients taking pegylated interferon and ribavirin treatment. The committee expressed no safety concerns that would preclude enrollment of the remaining 300 patients in the ongoing phase 2b study in the HCV positive genotype 1 and 4 population, and have not recommended modification of dose or duration of any RG7128 dosing regimens. Enrollment of these patients pre-screened for this Cohort in the fourth quarter 2009 has begun and is expected to be complete by the end of the first quarter of 2010.

"We welcome Roche's decision to open enrollment of Cohort 2 for the remaining 300 patients in the phase 2b trial," stated Michelle Berrey, MD, MPH, Pharmasset's Chief Medical Officer. "We are encouraged by the safety profile of RG7128 to date, which, when coupled with the higher barrier to resistance offered by nucleoside and nucleotide analog inhibitors has the potential to improve SVR rates over the current standard of care."

Additional RG7128 Studies

Pharmasset's development partner, Roche, plans to initiate a number of additional, longer duration phase 2b trials with RG7128 in the first half of 2010. An additional study in patients infected with HCV genotypes 2 and 3 is being planned to initiate later in 2010.

About the RG7128 Phase 2b Trial

The Phase 2b trial is anticipated to enroll a total of about 400 HCV-infected patients with genotypes 1 or 4 who have never received HCV treatment. The trial is evaluating the dose and duration of treatment with RG7128 in combination with Pegasys [pegylated interferon alfa-2a] plus Copegus (ribavirin). The primary efficacy endpoint of the trial is the proportion of patients achieving an SVR, defined as serum HCV RNA below the limit of detection as measured by the Roche TaqMan assay 24 weeks after completion of treatment. Patients are equally randomized into one of 5 arms:

	24 weeks of total treatment, with RG7128 500 mg BID [twice daily] in combination with Pegasys and ribavirin for 12 weeks, followed by 12 weeks of Pegasys and ribavirin ("12+12")
	24 weeks of total treatment, with RG7128 1000 mg BID in combination with Pegasys and ribavirin for 12 weeks, followed by 12 weeks of Pegasys and ribavirin ("12+12")
	24 weeks of total treatment, with RG7128 1000 mg BID in combination with Pegasys and ribavirin for 8 weeks, followed by a further 16 weeks of Pegasys and ribavirin ("8+16")
	48 weeks of total treatment, with RG7128 1000 mg BID in combination with Pegasys and ribavirin for 12 weeks, followed by a further 36 weeks of Pegasys and ribavirin ("12+36")
	A control arm with Pegasys and ribavirin for 48 weeks.

Patients randomized to the 24-week regimen will discontinue all treatment at week 24 if they have achieved a rapid virological response, defined as serum HCV RNA below the limit of detection at week 4, a strategy known as "RVR-guided" treatment. Patients who do not achieve an RVR will continue on the standard of care therapy (Pegasys and ribavirin) until week 48.

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) and, secondarily, on the development of Racivir(TM) for the treatment of human immunodeficiency virus (HIV). 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 three clinical-stage product candidates. RG7128, a nucleoside analog for chronic HCV infections, is in a Phase 2b clinical trial in combination with Pegasys plus Copegus and is also in 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 candidates include PSI-7851, an unpartnered, next generation HCV nucleotide analog, has completed initial Phase 1 clinical studies which provided supportive safety and efficacy data to initiate a Phase 2a trial in 1Q 2010 and Racivir, for the treatment of HIV, which has completed a Phase 2 clinical trial. We have also recently announced the nomination of two purine nucleotide analogs, PSI-938 and PSI-879, for preclinical development.