Gilead Sciences Announces
Termination of Its U.S. Development
Program for Adefovir Dipivoxil for

Foster City, CA, December 3, 1999 - Gilead
Sciences (Nasdaq: GILD) announced today the
termination of its U.S. HIV development program
for the investigational agent adefovir dipivoxil 60
mg. This decision follows a U.S. Food and Drug
Administration (FDA) Advisory Committee meeting
on November 1, 1999, at which the Committee
recommended against accelerated approval of
adefovir dipivoxil at this time for the treatment of
HIV patients with disease progression.

In subsequent discussions with the FDA, Agency
officials advised Gilead that additional results
from controlled clinical trials would be required to
further clarify the efficacy and safety of adefovir
dipivoxil 60 mg. Additionally, the FDA indicated
that it intended to issue a “not approvable letter”
on the Company’s new drug application (NDA) by
the December 29, 1999 NDA action date, based
on currently available data. As a result of these
discussions, Gilead has determined that an
additional investment of time and resources in
this product is not appropriate given the other
opportunities in Gilead’s clinical pipeline.

“As indicated by the more than 9,000 patients
with limited treatment options who have enrolled
in the adefovir dipivoxil expanded access program
over the past two years, we believe there is a
significant need among patients for new
treatment options – especially those that may
offer antiviral activity against HIV strains that are
resistant to commonly used medications,” said
John C. Martin, Ph.D., President and Chief
Executive Officer of Gilead Sciences. “Although we
respect the suggestions of the Advisory
Committee and the FDA for further clinical
research, we believe it would be in the best
interests of our shareholders and the HIV
community if we allocate our resources to other
development programs.”

Gilead will focus its development efforts on its
other late-stage clinical candidates. Tenofovir
disoproxil fumarate (tenofovir DF) entered
multinational Phase III clinical studies for the
treatment of HIV infection earlier this week.
Based on preclinical and preliminary clinical data,
tenofovir DF appears to have a better profile than
adefovir dipivoxil for HIV, in terms of both
antiviral activity and safety. Additionally, the
Company is developing adefovir dipivoxil for the
treatment of hepatitis B virus infection (HBV). As
compared to the HIV indication, adefovir dipivoxil
for HBV is administered at lower doses (10 mg or
30 mg once per day) and has been shown to have
potent antiviral activity. Adefovir dipivoxil for HBV
is in long-term Phase III studies at multiple
clinical sites in the United States, Europe and

Adefovir Dipivoxil for HIV - U.S. Clinical Trials
and Expanded Access Program Status

Gilead has decided to halt new enrollment in its
U.S. clinical trials and the expanded access
program for adefovir dipivoxil for HIV. Patients
now receiving adefovir dipivoxil in U.S. clinical
trials will be given the opportunity to enroll in the
expanded access program. Patients enrolled in the
expanded access program will be able to continue
to receive adefovir dipivoxil for as long as they
and their treating physicians believe they are
benefiting from the treatment. Because the
average length of therapy on the expanded access
program has been approximately nine months, the
Company believes that its ongoing obligations
under the program will be minimal by the end of

European Development

In September 1999, Gilead filed a Marketing
Authorisation Application (MAA) for adefovir
dipivoxil for HIV with the European Medicines
Evaluation Agency (EMEA), under the centralized
procedure for approval of human therapeutic
products in all member states of the European
Community. The Company anticipates receiving
initial comments on the MAA from the EMEA
during the first quarter of 2000. At that time,
Gilead will evaluate the prospects for continued
development of adefovir dipivoxil for HIV in

Gilead Sciences, headquartered in Foster City, CA,
is an independent biopharmaceutical company
that seeks to provide accelerated solutions for
patients and the people who care for them. The
Company discovers, develops, manufactures and
commercializes proprietary therapeutics for
challenging infectious diseases (viral, fungal and
bacterial infections) and cancer. Gilead maintains
research, development or manufacturing facilities
in Foster City, CA, Boulder, CO, San Dimas, CA,
and Cambridge, UK, and sales and marketing
organizations in the United States, Europe and