Pharmasset Voluntarily Halts Clinical Studies with Clevudine in Hepatitis B Infected...

Pharmasset Voluntarily Halts Clinical Studies with Clevudine in Hepatitis B
Infected Patients

- Company will increase focus on HCV pipeline

- Conference call at 8:00 AM ET today

PRINCETON, N.J., April 20 /PRNewswire-FirstCall/ -- Pharmasset, Inc. (Nasdaq:
VRUS) announced today that after a discussion with its independent Data Safety
Monitoring Board (DSMB) and the FDA, the company has decided to voluntarily
terminate its Phase III QUASH studies of clevudine for the treatment of
chronic hepatitis B (HBV) infection.

Pharmasset recently became aware of a number of spontaneous Serious Adverse
Event reports and Events of Special Interest in patients receiving clevudine
as prescribed therapy for hepatitis B in South Korea. Though only a small
number of cases of mild to moderate myopathy, or muscle weakness associated
with creatine kinase elevations, were reported in the QUASH studies, many of
the patients in South Korea have had longer exposures to clevudine than
patients in the QUASH studies and have reported more serious myopathy than
have patients in the Pharmasset clinical trials. Given the number and severity
of cases observed in South Korea, Pharmasset concluded it was in the best
interest of patients to terminate the studies at this time.

"The safety of patients in the QUASH studies is our top priority and,
therefore, we have decided to stop treatment with clevudine," stated Michelle
Berrey, MD, MPH, Pharmasset's Chief Medical Officer. "Although the number of
cases of myopathy in the QUASH trials was low and the severity was mild, more
severe reports from other trials and post marketing surveillance led us to
believe the risk benefit ratio for clevudine was insufficient to continue
development. We would like to take this opportunity to thank patients and our
investigators for their participation in the QUASH studies."

As part of the study shut down, Pharmasset plans to continue to collect safety
data and to monitor patients after discontinuation of clevudine in the QUASH
studies, but does not plan to submit study results to regulators as pivotal
studies. Clevudine was licensed from Bukwang Pharmaceuticals of South Korea,
where the drug is marketed under the trade name Levovir.

"While we are obviously disappointed with the outcome, we believe the decision
to stop dosing is an appropriate one," stated Schaefer Price, President and
Chief Executive Officer. "Our full energy and resources in research and
development will now be focused on our promising HCV pipeline. Our partner
Roche will shortly initiate dosing in a phase 2b trial with R7128, and will
report interim data from the INFORM-1 trial at the upcoming EASL meeting.
Pharmasset's unpartnered second generation HCV nucleoside, PSI-7851, is
progressing well in a phase I trial and we expect to report the first
antiviral data in the second half of 2009. In addition, we are collecting
preclinical data from our research efforts to select a purine nucleos(t)ide
for clinical development."

Phase III QUASH Studies
The QUASH studies are international, multi-center, randomized, double-blind
clinical trials of 30mg once daily clevudine compared with 10mg once daily
adefovir for 96 weeks in patients with chronic hepatitis due to infection with
e-antigen positive hepatitis B virus (QUASH 305) or e-antigen positive (QUASH
306) who have never been treated with drugs of the nucleoside class. The
primary objective of the studies are to assess the effect of clevudine on
reducing levels of virus in the blood and normalization of liver function at
48 weeks after initiating therapy.

Conference Call and Webcast
Members of Pharmasset's management team will host a conference call today,
Monday, April 20, 2009, at 8:00 a.m. ET to discuss the decision to terminate
the studies. Investors may listen to the webcast of the conference call live
on the "Events & Presentations" section of Pharmasset's website,
www.pharmasset.com. Alternatively, investors may listen to a replay of the
call by dialing (888) 806-6202 from locations in the U.S. and (913) 312-1502
from outside the U.S. The call-in replay and webcast will be available for at
least 72 hours following the call.

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 human
immunodeficiency virus (HIV). Pharmasset is currently developing three product
candidates. R7128, an oral treatment for chronic HCV infection, has completed
a 4-week clinical trial in combination with PEGASYS plus COPEGUS through a
strategic collaboration with Roche, and is initiating a Phase 2b trial.
Racivir, which is being developed for the treatment of HIV in combination with
other approved HIV drugs, has completed a Phase 2 clinical trial. PSI-7851, an
unpartnered second generation HCV nucleotide analogue recently entered phase 1
studies.

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 December 31, 2008 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., richard.smith@pharmasset.com,
+1-609-613-4181