Pharmasset Initiates First Time in Human Study of PSI-938 for the Treatment of Hepatitis C

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Source : Pharmasset, Inc.

PRINCETON, N.J., April 8 /PRNewswire-FirstCall/ — Pharmasset, Inc. announced today that dosing has initiated in a phase 1, single ascending dose (SAD) study in healthy volunteers with PSI-938, a third generation purine nucleotide analog polymerase inhibitor of hepatitis C virus (HCV). Pharmasset recently filed an Investigational New Drug Application (IND) with the Food and Drug Administration (FDA).

“PSI-938 is our first purine nucleotide analog to move into clinical development,” stated Dr. Michelle Berrey, Pharmasset’s Chief Medical Officer. “As PSI-938’s resistance profile is different from other nucleoside/tides in development for HCV and it has a different metabolic pathway from pyrimidine analogs, such as RG7128 and PSI-7977, we believe PSI-938 could be potentially combined with other nucleosides in development to deliver a pan-genotype regimen. We look forward to reporting the first antiviral data with PSI-938 in the third quarter of 2010.”

Purine and Pyrimidine Analogs

Purine nucleoside/tide analogs have many of the benefits of pyrimidine nucleoside/tide analogs, such as RG7128 and PSI-7977, in that they have demonstrated in vitro activity across multiple HCV genotypes, have a higher barrier to resistance than other classes of HCV small molecules in development, and have a lower risk of drug interactions when combined with other direct acting antivirals targeting HCV. In addition, Pharmasset’s purine analogs retain activity against the S282T mutation associated with in vitro resistance in other nucleoside/tide analogs in development, and are metabolized to the active triphosphate form through a different phosphorylation pathway than the pyrimidine analogs. Given these characteristics, Pharmasset’s purine and pyrimidine analogs have the potential to be combined as part of a future treatment regimen.

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 C virus (HCV) and, secondarily, on the development of Racivir? for the treatment of human immunodeficiency virus (HIV). Our research and development efforts focus on nucleoside/tide analogs, a class of compounds which act as alternative substrates for the viral polymerase, thus inhibiting viral replication. We currently have three clinical-stage product candidates. RG7128, a nucleoside analog for chronic HCV infection, is in two Phase 2b clinical trials in combination with Pegasys® plus Copegus® and is also in the INFORM studies, the first series of studies designed to assess the potential of combinations of small molecules without Pegasys® and Copegus® to treat chronic HCV. These clinical studies are being conducted through a strategic collaboration with Roche. Our other clinical stage candidates include PSI-7977, an isomer of PSI-7851 and an unpartnered, next-generation HCV nucleotide analog that is in a Phase 2a trial, and the third-generation purine analog PSI-938 in Phase 1. Racivir, for the treatment of HIV, has completed a Phase 2 clinical trial. We also have in our pipeline an additional purine nucleotide analog, PSI-661, an isomer of PSI-879 in advanced preclinical development.

Pegasys® and Copegus® are registered trademarks of Roche.

Contact Richard E. T. Smith, Ph.D. VP, Investor Relations and Corporate Communications Office: +1 (609) 613-4181

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 adverse events could cause the cessation or delay of any of the ongoing or planned clinical trials and/or our development of our product candidates, the risk that the results of previously conducted studies involving our product candidates will not be repeated or observed in ongoing or future studies involving our product candidates, the risk that our collaboration with Roche will not continue or will not be successful, and the risk that any one or more of our product candidates will not be successfully developed and commercialized. For a discussion of these and other 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, 2009 and our Quarterly Report on Form 10Q for the period ended December 31, 2009 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: Richard E. T. Smith, Ph.D., VP, Investor Relations and Corporate
Communications, Office: +1-609-613-4181

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