Pharmasset, Inc. announced the addition of two treatment arms to the Electron trial of PSI-7977, a nucleotide analog polymerase inhibitor, for the treatment of chronic hepatitis C (HCV). The rapid and consistent antiviral effects and high barrier to resistance demonstrated with PSI-7977 to date provided the rationale for additional exploratory regimens in this setting.  The protocol amendment adds one arm exploring 12 weeks of PSI-7977 monotherapy in treatment naive patients with HCV genotype 1 (GT1), and one arm of PSI-7977 and ribavirin (RBV) in treatment-experienced patients with HCV genotype 2 (GT2) or genotype 3 (GT3).

In addition, the previously announced arm in HCV GT1 patients with a prior "null" response to an interferon (IFN) containing regimen, which was planned to assess PSI-7977/IFN/RBV, has been modified to an IFN-free 12-week regimen of PSI-7977/RBV.

“We look forward to reporting SVR12 results from Part 1 and interim data from the PSI-7977 monotherapy arm of Electron on Sunday, November 6th, 2011 at the upcoming 2011 American Association for the Study of Liver Diseases (AASLD) annual meeting. The preliminary results to be discussed at AASLD led us to expand the study to add an arm of PSI-7977 monotherapy for HCV GT1,” said Michelle Berrey, MD, MPH, Pharmasset's chief medical officer. “We and others continue to explore the potential of PSI-7977 for IFN-free and monotherapy treatment regimens in a broader group of individuals living with HCV of all genotypes and regardless of their response to prior treatment.”

In addition to previously announced interferon-free studies of PSI-7977 by Bristol Myers Squibb and Tibotec, The National Institute of Health (NIH) recently initiated an interferon-free 24 week study of PSI-7977 400mg QD with and without ribavirin, in 60 treatment naive patients with HCV genotype 1 (GT1) in the US.

Electron is an exploratory study of PSI-7977 for the treatment of chronic HCV infection. Part 1 of the trial is evaluating 12-week regimens of interferon-free PSI-7977 400mg QD with ribavirin (RBV), and three abbreviated duration peginterferon (Peg-IFN) regimens of 4, 8, or 12 weeks in treatment-naive patients with HCV GT2 or GT3. The primary endpoint of the trial is the safety and tolerability of PSI-7977 400mg QD and RBV for 12 weeks, administered with or without Peg-IFN. On May 11, 2011, Pharmasset announced the completed enrollment of Part 1 of Electron: PSI-7977 400 mg with Peg-IFN and RBV for 12 weeks (GT2/3), PSI-7977 400 mg with RBV for 12 weeks, Peg-IFN weeks 1-8 only (GT2/3), PSI-7977 400 mg with RBV for 12 weeks; Peg-IFN weeks 1-4 only (GT2/3), PSI-7977 400 mg with RBV for 12 weeks (GT2/3).

In Part 2 of Electron, a 5th cohort was added to explore PSI-7977 monotherapy in treatment naive patients with HCV GT2 or GT3: PSI-7977 400 mg monotherapy for 12 weeks (n=10 GT2/3).

Following on the previously reported 100% SVR12 in treatment-naive subjects with HCV GT2/3 (PROTON), a 6th cohort was added to ELECTRON to explore an 8-week duration of PSI-7977Peg-IFN/RBV. The previously announced 7th cohort in HCV GT1 patients with a prior "null" response to Peg-IFN, has been modified to an interferon-free 12-week regimen of PSI-7977/RBV.

PSI-7977 400 mg with Peg-IFN/RBV for 8 weeks (n=10 GT2/3 treatment-naive) PSI-7977 400 mg with RBV for 12 weeks (n=10 GT1 null)

In Part 3 of Electron, two additional IFN-free regimens will be explored in treatment-naive patients with HCV GT1 and in treatment-experienced patients with HCV GT2 or GT3. PSI-7977 400 mg monotherapy for 12 weeks (n=25 GT1 treatment-naive), PSI-7977 400 mg with RBV for 12 weeks (n=25 GT2/3 treatment-experienced).

Pharmasset is a clinical-stage pharmaceutical company committed to discovering, developing, and commercializing novel drugs to treat viral infections. Pharmasset's primary focus is the development of oral therapeutics for the treatment of hepatitis C virus (HCV) infection.