Pharmasset, Inc. and Roche to Present R7128 Data at the 14th International Symposium on Hepatitis C Virus and Related Viruses

PRINCETON, N.J., Sept. 6 /PRNewswire-FirstCall/ -- Pharmasset, Inc. and Roche announced today that four scientific presentations related to R7128 for the treatment of chronic hepatitis C virus (HCV) will be made at the 14th International Symposium on Hepatitis C Virus and Related Viruses being held from September 9-13, 2007 in Glasgow, Scotland. R7128, a prodrug of PSI-6130, is an oral cytidine nucleoside analog polymerase inhibitor of HCV that is being developed through Pharmasset's collaboration with Roche. In addition, Pharmasset will present data on proprietary phosphoramidate HCV RNA inhibitors. The conference abstract information is listed below, and the poster presentations will be available in the "Events & Presentations" section of Pharmasset's website at http://www.pharmasset.com on September 10, 2007.

"The presentations at the HCV Symposium represent our growing knowledge about R7128's preclinical properties, clinical safety and pharmacokinetics," stated Dr. Michael Otto, Pharmasset's Executive Vice President, Pharmaceutical Research. "In addition, Pharmasset's internal HCV discovery efforts have identified additional proprietary compounds that may have complementary or improved properties. We will carefully evaluate these molecules for potential advancement toward future development."

Poster presentations at the 14th International Symposium on Hepatitis C Virus and Related Viruses will include:

-- Abstract P-268: Pharmacokinetics, Safety, and Tolerability of R7128, a Novel Nucleoside Polymerase Inhibitor for HCV Following Single, Ascending Oral Doses in Healthy Volunteers. Otto MJ, Robson R, Rodriguez CA, Beard A, Symonds WT, Hill G and Berrey MM (Pharmasset, Christchurch Clinical Studies Trust and Roche Palo Alto). -- Abstract P-262: The Mechanism of Action of Beta-D-2'-Deoxy-2'-fluoro- 2'-C-methylcytidine Involves a Second Metabolic Pathway Leading to Beta-D-2'-Deoxy-2'-fluoro-2'-C-methyluridine 5'-Triphosphate, a Potent Inhibitor of the HCV RNA-Dependent RNA Polymerase. Murakami E, Niu C, Bao H, Micolochick Steuer HM, Otto MJ and Furman PA (Pharmasset). -- Abstract P-265: The Nucleoside Inhibitors R1479, PSI-6130, and NM107 have a Higher Genetic Barrier to Resistance than the Non-nucleoside Inhibitor HCV-796 and the Protease Inhibitor VX-950. McCown M, Rajyaguru S, Symons J, Cammack N and Najera I (Roche Palo Alto). -- Abstract P-263: In Vitro Selection and Characterization of HCV Replicons Resistant to PSI-6130. Ali S, Leveque V, LePogam S, Ma H, Philipp F, Najera I, Klumpp K, Symons J, Cammack N and Jiang WR (Roche Palo Alto). -- Abstract P-259: Beta-D-2'-Deoxy-2'-fluoro-2'-C-methyluridine Phosphoramidates: Potent and Selective Inhibitors of HCV RNA Replication. Sofia MJ, Wang P, Du J, Micolochick Steuer HM, Niu C, Furman PA and Otto MJ (Pharmasset).

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 B virus (HBV), hepatitis C virus (HCV) and human immunodeficiency virus (HIV).

Pharmasset is currently developing three product candidates. Clevudine, for the treatment of chronic HBV infection, is expected to enter Phase 3 clinical trials for registration in the Americas and Europe. Clevudine is already approved for HBV in South Korea and marketed by Bukwang Pharmaceuticals under the brand name Levovir. R7128, an oral treatment for chronic HCV infection, is in a Phase 1 clinical trial through a strategic collaboration with Roche. Racivir, which is being developed for the treatment of HIV in combination with other approved HIV drugs, has completed a Phase 2 clinical trial.

About R7128

R7128 is being developed for the treatment of chronic hepatitis C. R7128 is a prodrug of PSI-6130, which demonstrated potency in preclinical studies. PSI-6130 is a pyrimidine nucleoside analog inhibitor of HCV RNA polymerase, an enzyme that is necessary for hepatitis C viral replication. Results from an oral single ascending dose study of PSI-6130 in 24 healthy male volunteers showed that PSI-6130 was generally well tolerated with no serious adverse events in doses up to 3000 mg.

R7128 Phase 1 Study Overview

The Phase 1 clinical trial is a multiple center, observer-blinded, randomized and placebo-controlled study to investigate the pharmacokinetics, pharmacodynamics, safety, tolerability and food effect of R7128 in healthy volunteers and in patients chronically infected with HCV genotype 1. This Phase 1 study is comprised of two parts:

-- Part 1 is a single ascending dose study of R7128 conducted in 46 healthy volunteers. The primary objective of Part 1 is to assess the safety, tolerability and pharmacokinetics of R7128 following single ascending doses under fasting conditions. The secondary objective of Part 1 is to explore the effect of food on the pharmacokinetics of R7128. Preliminary data from the single ascending dose portion of the study indicate: -- All doses of R7128 studied were generally safe and well-tolerated. -- All patients completed the study, and none experienced gastrointestinal adverse events or serious adverse events during the study. -- No hematological or laboratory abnormalities of clinical significance were noted. -- Part 2 is a multiple ascending dose study of R7128 conducted in 40 patients chronically infected with HCV genotype 1 who have previously failed interferon therapy. The primary objective of Part 2 is to assess the safety, tolerability and pharmacokinetics of R7128 after once-daily or twice-daily dosing for 14 days. The secondary objective is to assess antiviral efficacy by measuring the change in HCV RNA.

About Hepatitis C

Hepatitis C is a blood-borne infectious disease of the liver and is a leading cause of chronic liver disease and liver transplants. The WHO estimates that nearly 180 million people worldwide, or approximately 3% of the world's population, are infected with hepatitis C virus (HCV). The CDC has reported that almost four million people in the United States have been infected with HCV, of whom 2.7 million are chronically infected.

Contact Alan Roemer, Vice President Investor Relations & Corporate Communications alan.roemer@pharmasset.com Office: (609) 613-4125

Forward-Looking Statements

Pharmasset "Safe Harbor" Statement under the Private Securities Litigation Reform Act of 1995: Statements in this press release regarding our business that are not historical facts are "forward-looking statements" that involve risks and uncertainties, including without limitation the risk that there will be a delay in the presentation of R7128 data at upcoming conferences, the risk that the R7128 data to be presented at upcoming conferences is not accurate or representative, the risk that the preliminary data from the R7128 Phase 1 Study is not accurate or representative, the risk that our collaboration with Roche will not continue or will not be successful, the risk that the on-going or anticipated clinical trials for any one or more of our product candidates will not be successful, the risk that any one or more of our product candidates will not be successfully developed and commercialized, and the risk that our proprietary compounds will not be successfully developed into product candidates or commercialized. For a discussion of these risks and uncertainties, any of which could cause our actual results to differ from those contained in the forward-looking statements, see the section of our Quarterly Report on Form 10-Q for the quarter ended June 30, 2007 filed with the Securities and Exchange Commission entitled "Risk Factors" and discussions of potential risks and uncertainties in our subsequent filings with the Securities and Exchange Commission.

CONTACT: Alan Roemer, Vice President, Investor Relations & CorporateCommunications of Pharmasset, Inc., +1-609-613-4125,alan.roemer@pharmasset.com

Web site: http://www.pharmasset.com//