Gilead Begins Multinational Phase III Trial of Once-Daily Oral Anti-HIV Drug, Tenofovir Disoproxil Fumarate, in Treatment-Naïve Patients

Second Pivotal Phase III Study

Foster City, CA -- June 12, 2000

Gilead Sciences, Inc. (Nasdaq: GILD) announced today that it has begun enrolling patients in a second multinational Phase III clinical trial of tenofovir disoproxil fumarate 300 mg (tenofovir DF), its investigational agent for the treatment of human immunodeficiency virus (HIV) infection. Designed to enroll 600 patients at sites in the United States, Europe and South America, this trial will evaluate the safety and efficacy of tenofovir DF as part of a simplified treatment regimen in antiretroviral-naïve patients with HIV infection.

“The initiation of this Phase III trial marks an important milestone in the tenofovir DF development program,” said John C. Martin, Ph.D., President and Chief Executive Officer, Gilead Sciences. “The extent to which a first antiretroviral regimen can reliably and conveniently suppress a patient’s viral load is a strong barometer for long-term treatment success. The antiviral activity and safety profile of tenofovir DF seen in early clinical trials suggest it has the potential to become an important treatment alternative both for patients new to antiretroviral treatment as well as for treatment-experienced patients.”

Study Design
This Phase III trial (Study 903) is a 48-week randomized, double-blind, multicenter, active-controlled trial that will compare a treatment regimen of tenofovir DF, efavirenz and lamivudine (3TC) to a treatment regimen of stavudine (d4T), efavirenz and lamivudine. The study is designed to enroll up to 600 HIV-1-infected patients who have not previously received antiretroviral treatment and who have HIV-1 RNA levels greater than 5,000 copies/mL.

Patients enrolling in Study 903 will be randomized (1:1) to receive one of the two treatment regimens. At the end of 48 weeks, these regimens will be compared according to the proportion of patients with HIV RNA levels less than or equal to 400 copies/mL. In addition, the study will evaluate the ability of both regimens to achieve HIV RNA levels less than or equal to 50 copies/mL and compare the safety and tolerability of the two regimens.

48-Week Phase II Safety and Efficacy Results
In April, Gilead presented preliminary results from a Phase II dose-ranging clinical trial (Study 902) evaluating the safety and efficacy of once-daily tenofovir DF when used to intensify a stable background antiretroviral regimen in 189 highly treatment-experienced HIV patients. In this study, 48 weeks of treatment with 300 mg of tenofovir DF was associated with a mean reduction in HIV RNA of 0.68 log10 copies/mL. These efficacy results were consistent with those seen at 24 weeks, suggesting that the antiviral activity of tenofovir DF can be sustained up to 48 weeks in highly treatment-experienced HIV patients. In the 24-week placebo-controlled portion of the study, the discontinuation rate and incidence of adverse events in the tenofovir DF arms and placebo arms were similar. Beyond 24 weeks, there was no evidence of dose-related toxicities.

Antiviral Activity in Treatment-Naïve Patients
In an earlier dose-ranging Phase I monotherapy trial (Study 901), HIV patients receiving tenofovir DF at the 300 mg dose had a mean reduction in HIV RNA of 1.28 log10 copies/mL at four weeks compared to a reduction of 0.03 log10 copies/mL in the placebo group. An analysis of those patients who were antiretroviral-naïve at the time of study entry showed that they had a mean reduction in HIV RNA of 1.51 log10 copies/mL at four weeks compared to an increase of 0.01 log10 copies/mL in the placebo group. The incidence of side effects was similar across tenofovir DF treatment and placebo groups.

Ongoing Tenofovir DF Phase III Program
To further evaluate the safety and efficacy of tenofovir DF 300 mg in combination with other antiretroviral agents for the treatment of HIV infection, Gilead initiated its first multicenter Phase III clinical trial (Study 907) with treatment-experienced HIV patients in November 1999. Patient enrollment in this study was recently completed.

Patients and physicians who would like more information about enrollment in Study 903 may call the AIDS Clinical Trials Information System (ACTIS) at 1-800-TRIALS-A (1-800-874-2572) or Gilead Sciences Medical Information at 1-800-GILEAD-5 (1-800-445-3235).

Gilead Sciences
Gilead Sciences, Inc., 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. Gilead 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; Cambridge, UK and Dublin, Ireland and sales and marketing organizations in the United States, Europe and Australia.

This press release includes forward-looking statements, within the meaning of the Private Securities Litigation Reform Act of 1995, that are subject to risks, uncertainties and other factors that could cause actual results to differ materially from those referred to in the forward-looking statements. Such statements are subject to certain risks and uncertainties, particularly the risk that the safety and efficacy data in Gilead’s earlier studies and trials may not be observed in Gilead’s more reliable Phase III clinical trials and other risks related to regulatory approval of tenofovir DF. Actual results could differ materially from those projected in this release. The reader is cautioned not to rely on these forward-looking statements. These and other risks are described in detail in the Gilead Annual Report on Form 10-K for the year ended December 31, 1999 on file with the U.S. Securities and Exchange Commission.