Pharmasset and Roche Obtain FDA Consent to Start a Phase 2b Study with R7128 in Treatment Naive HCV Patients

Princeton, NJ -- January 12, 2009 -- Pharmasset, Inc. (Nasdaq: VRUS) announced today that they and their development partner, Roche, have agreed with the FDA on the final design for a phase 2b trial with R7128, a nucleoside inhibitor of hepatitis C (HCV), slated to initiate in the first quarter of this year.

"We are pleased that R7128 is advancing into a large phase 2b trial," stated Michelle Berrey, MD, MPH, Pharmasset's Chief Medical Officer. "R7128 is the most advanced nucleoside polymerase inhibitor in development and we believe this class of drug brings a number of advantages to HCV-infected patients. R7128's higher barrier to resistance and activity across multiple viral genotypes, as well as the promising short-term safety and tolerability, may bring patients a new option for therapy. We look for this trial to better define the optimal treatment duration with R7128 in combination with the standard of care."

The phase 2b trial is anticipated to enroll about 400 treatment-naive, genotype-1 or genotype 4 HCV-infected patients. The trial will evaluate the dose and duration of treatment of R7128 in combination with Pegasys [pegylated interferon alfa-2a] plus Copegus [ribavirin]. The primary efficacy endpoint of the trial will be the proportion of patients that achieve a sustained virologic response (SVR), defined as undetectable (measured by Roche TaqMan assay) HCV RNA 24 weeks after completion of treatment.

Patients will be enrolled into one of 5 arms:

• 24 weeks of total treatment, with R7128 500 mg bid [twice-daily] in combination with pegylated interferon and ribavirin for 12 weeks, followed by 12 weeks of pegylated interferon and ribavirin

• 24 weeks of total treatment, with R7128 1000 mg bid in combination with pegylated interferon and ribavirin for 12 weeks, followed by 12 weeks of pegylated interferon and ribavirin

• 24 weeks of total treatment, with R7128 1000 mg bid in combination with pegylated interferon and ribavirin for 8 weeks, followed by a further 16 weeks of pegylated interferon and ribavirin

• 48 weeks of total treatment, with R7128 1000 mg bid in combination with pegylated interferon and ribavirin for 12 weeks, followed by a further 36 weeks of pegylated interferon and ribavirin.

• A control arm with pegylated interferon and ribavirin for 48 weeks.

Patients in the 24 week arms will discontinue treatment at week 24 if they achieved a rapid virological response (RVR), defined as undetectable level of HCV RNA at week 4 ("RVR-guided").

Patients that do not achieve an RVR will continue on the standard of care until week 48. According to the current study design, patients will be enrolled as two cohorts, with randomization of the second larger cohort being initiated based on 12 week safety data of the first cohort.

During 2009, we expect to provide updates on the progress of the trial.