Pharmasset, Inc. (Nasdaq: VRUS) announced that dosing has begun in a Phase 2b study of PSI-7977, a nucleotide analog polymerase inhibitor for the treatment of chronic hepatitis C (HCV). The trial will evaluate PSI-7977 200mg QD and 400mg QD in combination with pegylated interferon alfa 2a and ribavirin, the current standard of care (SOC) in patients with HCV genotype 1 who have not been treated previously.

"We look for the Phase 2b to further support the efficacy, safety and resistance profile of PSI-7977 over 12 weeks, and to confirm a dose for continued development," stated Michelle Berrey, MD, MPH, Pharmasset's Chief Medical Officer. "Given the potent antiviral activity observed in the Phase 2a study and in vitro evidence for pan-genotype activity, we have also decided to include an exploratory, open label arm of patients with HCV genotype 2 and 3 who will receive a 12 week treatment regimen of PSI-7977 in combination with pegylated interferon and ribavirin. If successful, this shorter treatment regimen would be a first step toward defining a new treatment option for these patients."

About the Phase 2b Trial

The Phase 2b trial is anticipated to enroll approximately 125 patients infected with HCV genotype 1 who have not been treated previously. The primary endpoint of the trial will be the assessment of safety and tolerability of PSI-7977, in combination with SOC over 12 weeks with response-guided therapy allowing discontinuation of SOC at week 24. The trial will be conducted in the U.S. Patients will be randomized (2:2:1) into one of 3 arms:

-- PSI-7977 200mg QD in combination with SOC for 12 weeks, followed by 12 or 36 weeks of SOC;

-- PSI-7977 400mg QD in combination with SOC for 12 weeks, followed by 12 or 36 weeks of SOC;

-- A control arm of matching placebo with SOC for 48 weeks.

Patients receiving PSI-7977 will discontinue treatment at week 24 if they achieve an extended RVR (eRVR), defined as HCV RNA below the limit of detection (