BUSINESS WIRE; Wednesday, July 1, 1998

In the therapeutics area, Chiron is investigating the use of its proprietary biologic, interleukin-2 (IL-2), for the treatment of patients infected with HIV. The company's increasing research effort on next generation HIV vaccine candidates is complemented by ongoing clinical research on HIV vaccines.

Immune Therapy for HIV

Proleukin(R) (aldesleukin), Chiron's recombinant IL-2, is an effective immunostimulant that has been shown to increase the CD4+ cell counts of patients infected with HIV. Chiron has supported a number of studies investigating the utility of Proleukin in the treatment of these patients. In these trials, Proleukin has been used in combination with antiretroviral therapy.

In studies conducted to date two basic dosing strategies have been assessed, intermittent administration of Proleukin by the intravenous or subcutaneous route, and low-dose daily subcutaneous injections. The greatest amount of experience has been derived from trials exploring the intermittent dosing strategy.

Data from a phase II ANRS (French National Agency on AIDS Research) trial presented at the twelfth World AIDS conference in Geneva suggest that Proleukin administered intermittently produces roughly comparable CD4+ cell count elevations by the subcutaneous and intravenous routes.

Also presented at the conference were results from long-term follow-up of participants in a phase I/II study indicating that infrequent administration of Proleukin is sufficient to sustain increases in CD4+ cell count number after a six-month course of intermittent administration.

"Chiron has significant clinical and research activity ongoing for both treatment and prevention of HIV," said Anne-Marie Duliege, M.D., M.S., director of clinical research at Chiron.

"We are evaluating Proleukin as an adjunct therapy intended to help restore the function of the immune system. Our clinical strategy in the vaccines area will be to move into the clinic aggressively in order to bring both treatments and preventive vaccines into clinical use more rapidly."

Innovative HIV Vaccines Research

Research now underway at Chiron, and with outside collaborators, is focused on better methods to stimulate both cellular and antibody responses. Scientists at Chiron are examining ways to improve the potency of gene-based vaccines. These include methods of packaging the DNA to prevent its degradation and ways to better target it to certain cell types.

There are also multiple vectors, or delivery systems, being examined for the purpose of increasing protein production. Bacterial vectors and alpha virus vectors have both shown promise as unique approaches with potential systemic benefits, such as mucosal immunity.

Chiron has a strong proprietary position and significant internal expertise for a number of vaccine and adjuvant delivery systems which appear promising for generating the forms of immunity considered critical for an AIDS vaccine.

According to Margaret Liu, M.D., vice president of vaccine research, "Chiron's broad-based vaccine research program integrates an antibody approach and a cell-mediated approach. While there has been some important clinical research conducted with antibody-based vaccines, like gp120, there are still significant clinical questions that need to be addressed.

"Many AIDS vaccine researchers now believe that an antibody component of a vaccine may require more of the envelope protein to be effective in stimulating an adequate immune response. At Chiron we are also exploring issues related to cross-clade neutralization, and the use of primary isolates, which are most closely related to patient-derived viruses."

Chiron currently has two clinical studies underway examining the safety and immunogenicity of two gp120 envelope proteins (a portion of the whole envelope protein) using Chiron's proprietary adjuvant MF59. In a phase II National Institutes of Health (NIH)-sponsored U.S. trial, enrollment is complete and it is anticipated that the immunizations and six-month follow up will be finalized by the beginning of next year.

The other study, being conducted in Thailand in collaboration with the Walter Reed Army Institute of Research (WRAIR), is evaluating different combinations of two gp120 proteins, one derived from a clade B strain (SF2) and the other from a clade E (CM235, "Thai E").

Enrollment has recently been completed in this phase I/II trial and analysis is also anticipated to be completed early next year. Chiron also has plans to participate in a "prime-boost" phase I study in Thailand in collaboration with Pasteur-Merieux Connaught and WRAIR at the beginning of next year.

About Vaccines

Viruses are composed of an outer protein coat, sometimes called an envelope, and an inner genetic core that is generally not infective. Antibody-derived approaches for HIV have focused on using the envelope to bestow immunity to the cell's surface.

In contrast, cellular-based vaccines penetrate the cell to deliver a gene encoding pieces of the HIV directly. These DNA or gene-based vaccines, which contain no infective viral particles, stimulate both an antibody and cellular immune response. Adjuvants are often used with vaccines to boost their effectiveness.

About Chiron

Chiron Corp., headquartered in Emeryville, is a leading biotechnology company that participates in three global healthcare markets: diagnostics, therapeutics and vaccines. Chiron also conducts research and development in the fields of biological proteins, gene therapy and combinatorial chemistry.

Note to Editors: This news release contains forward-looking statements that involve risks and uncertainties which might affect the initiation and completion of clinical trials and the launch of products. A full discussion of the companies' operations and financial condition, including factors that may affect their business and future prospects, is contained in documents the companies file with the SEC, such as form 10-Q and 10-K and 40F reports. These documents identify important factors that could cause the companies' actual performance to differ from current expectations, including the outcome of clinical trials, regulatory review, manufacturing capabilities and marketing effectiveness.

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CONTACT: Chiron Corp. Jim Knighton, 510/923-6055

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