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Pharmasset Nominates PSI-7851 as a Lead Development Candidate for the Treatment of Chronic Hepatitis C

- IND or foreign regulatory equivalent filing anticipated in 1Q 2009 -

PRINCETON, N.J., May 19 /PRNewswire-FirstCall/ -- Pharmasset, Inc. (Nasdaq: VRUS) has nominated PSI-7851 as a lead development candidate for the treatment of chronic hepatitis C virus (HCV). PSI-7851 is a proprietary nucleotide analogue polymerase inhibitor of HCV that is being advanced into Good Laboratory Practice (GLP) animal toxicity studies required for submission of an Investigational New Drug (IND) application with the FDA or an equivalent foreign regulatory filing. PSI-7851 has demonstrated in vitro potency that is approximately 15- to 20-fold greater than the in vitro potency of R7128, a nucleoside analog polymerase inhibitor of HCV that Pharmasset is developing through its collaboration with Roche.

"Our in-house research team has, once again, discovered a new potential product candidate for the treatment of chronic HCV," stated Dr. Michael Otto, Pharmasset's Chief Scientific Officer. "Our goal continues to be the identification of anti-HCV compounds with improved potency, equivalent or improved safety and oral bioavailability. In addition, we have focused on increasing intrahepatic triphosphate levels, which may lead to a higher level of active drug in the liver. We anticipate completing the required animal toxicity studies and filing an IND in the first quarter of 2009."

Nucleotide analogs have shown the ability to deliver higher levels of the active triphosphate form of the drug into infected cells. PSI-7851 has demonstrated in vitro anti-HCV activity with EC90 values of 0.31 +/- 0.12 uM, which is approximately 15- to 20-fold more potent than the active metabolite of R7128, PSI-6130. In vitro studies of PSI-7851 have not shown evidence of any mitochondrial or other cellular toxicities that may be associated with some nucleoside analogs. The half-life of the triphosphate in primary human hepatocytes is approximately 38 hours, which suggests the possibility for once-daily dosing. Early animal studies indicate that PSI-7851 can achieve concentrations of active triphosphate in the liver of up to 1,000 times higher than PSI-6130 at equivalent doses.

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 enrolling Phase 3 clinical trials for registration in North, Central and South America and Europe. Clevudine is already approved for HBV in South Korea and marketed by Bukwang Pharmaceuticals in South Korea under the brand name Levovir. R7128, an oral treatment for chronic HCV infection, is in a 4-week Phase 1 clinical trial in combination with Pegasys(R) plus Copegus(R) 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.

Pegasys(R) and Copegus(R) are registered trademarks of Roche.


Alan Roemer, Vice President

Investor Relations & Corporate Communications

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 our results from early studies with PSI-7851 are not repeated in future studies, the risk that we are not able to gather the data and information necessary to, or that we otherwise cannot, file an IND for PSI-7851, the risk that PSI-7851 never becomes a product candidate or is never successfully developed and commercialized, the risk that adverse events could cause the cessation or delay of any of the ongoing or planned clinical trials and/or our development of any or all of our product candidates, the risk that we cannot enroll enough patients for the Phase 3 registration studies for clevudine, the risk that our collaboration with Roche will not continue or 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 we will not be able to further develop any of the compounds from the new series of anti-HCV molecules currently being investigated by us. 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 Annual Report on Form 10-K for the fiscal year ended September 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.

SOURCE Pharmasset, Inc.
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