Pharmasset, Inc., announced today that it has entered into a
clinical collaboration agreement with Tibotec Pharmaceuticals to evaluate in a Phase
2 study the safety and efficacy of PSI-7977, Pharmasset’s investigational
nucleotide polymerase inhibitor, in combination with TMC435, Tibotec Pharmaceuticals’
investigational protease inhibitor, for the treatment of chronic hepatitis C
virus (HCV).

This phase 2 proof of concept study will evaluate the
potential to achieve sustained virologic response 12 weeks post treatment with
an all oral, once-daily, interferon-free treatment regimen in patients infected
with genotype 1 HCV. Specifically, the study will assess the safety,
pharmacokinetics and pharmacodynamics of 12 and 24 weeks of PSI-7977 in
combination with TMC435, with and without ribavirin, in patients chronically
infected with HCV genotype 1 who had a prior null response to peginterferon
alfa and ribavirin treatment. The study is planned to start in the second half
of 2011.

“We are excited to be working with Tibotec to simplify
and improve HCV treatment,” stated Bill Symonds, PharmD, Pharmasset’s
Senior Vice President of Clinical Pharmacology and Translational Medicine.

“PSI-7977 is now being studied in interferon-free
combinations with each of the three leading classes of direct-acting
antivirals: Tibotec’s protease inhibitor, Bristol-Myers Squibb’s NS5a inhibitor,
and our own nucleotide, PSI-938. This advances one of our key goals at Pharmasset:
to develop our nucleotide analogs as the backbone of interferon-free HCV
therapy.”