Pharmasset Nominates PSI-938 as a New Nucleotide Analog Inhibitor of Hepatitis C...

Pharmasset Nominates PSI-938 as a New Nucleotide Analog Inhibitor of Hepatitis
C for Preclinical Development
- IND or foreign regulatory equivalent submission anticipated in first half
2010

PRINCETON, N.J., July 7 /PRNewswire-FirstCall/ -- Pharmasset, Inc. (Nasdaq:
VRUS) announced today the nomination of PSI-352938 ("PSI-938") as a lead
development candidate from two series of purine analogs for the treatment of
chronic hepatitis C virus (HCV) infection. PSI-938 is a proprietary nucleotide
analog polymerase inhibitor of HCV that is being advanced into studies
required for submission of an Investigational New Drug (IND) application with
the FDA or equivalent foreign regulatory application.

"PSI-938 is particularly interesting to us since it differs from our
pyrimidine analogs, R7128 and PSI-7851, because it has a complementary
resistance profile and is metabolized through a different phosphorylation
pathway," stated Michael Otto, PhD, Pharmasset's Chief Scientific Officer.
"These differences may prove to be particularly important as we explore
combinations of nucleos(t)ides in clinical development in the future."

Purine nucleos(t)ide analogs have many of the benefits of pyrimidine
nucleos(t)ide analogs, like R7128 and PSI-7851, in that they have demonstrated
in vitro activity across multiple genotypes, a higher barrier to resistance
than other classes of HCV small molecules in development, and the potential to
be combined with other direct acting antivirals targeting HCV. In addition,
these purine analogs are also active against the S282T resistant variant
selected in vitro by the pyrimidine analogs, and are metabolized to the active
triphosphate form through a different phosphorylation pathway than the
pyrimidines. Given these characteristics, 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(R) for the treatment of human immunodeficiency
virus (HIV). Our research and development efforts focus on nucleos(t)ide
analogs, a class of compounds which act to inhibit the enzymes required for
viral replication. We currently have three clinical-stage product candidates.
R7128, a nucleoside analog for chronic HCV infections, is in a Phase 2b
clinical trial in combination with Pegasys(R) plus Copegus(R) and is also in
INFORM studies, the first series of studies designed to assess the potential
of combinations of small molecules without Pegasys(R) and Copegus(R) to treat
chronic HCV. These clinical studies are being conducted through a strategic
collaboration with Roche. Our other clinical stage candidates are PSI-7851, an
unpartnered, next generation HCV nucleotide analog which recently began Phase
1 clinical studies and Racivir, for the treatment of HIV, which has completed
a Phase 2 clinical trial.

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

Contact
Richard E. T. Smith, Ph.D.
VP, Investor Relations and Corporate Communications
richard.smith@pharmasset.com
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 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, 2008 and our Quarterly Report on Form
10-Q for the period ended March 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.

SOURCE Pharmasset, Inc.

Richard E. T. Smith, Ph.D., VP, Investor Relations and Corporate
Communications of Pharmasset, Inc., +1-609-613-4181,
richard.smith@pharmasset.com