Chiron Announces Phase III Clinical Trial Evaluating

Proleukin(R) In Combination With HIV Therapy Emeryville, Calif. (ots-PRNewswire) - Trial to Establish theRole of Proleukin(R) in Delaying the Progression Of AIDS in HIV-Infected Patients. Chiron Corporation (Nasdaq: CHIR) announced today that it isenrolling patients in its international Phase III study, calledSILCAAT, evaluating the safety and effectiveness of Proleukin(R)(aldesleukin) Interleukin-2 in combination with standard anti-HIVtherapy in people living with HIV with low CD4+ T-cell counts.Proleukin, a recombinant form of Interleukin-2, also known as T-cell growth factor, is being administered in combination withanti-HIV therapy in people with HIV infection. This pivotalstudy's primary objective is to determine if intermittent cyclesof Proleukin delay the progression of HIV disease compared towhat is observed with anti-HIV therapy alone in people withadvanced HIV infection. "Using Proleukin to augment the immune system is a novelapproach in the treatment of HIV. Current anti-HIV therapiestarget the virus specifically, while Proleukin is designed tobolster and help rebuild the compromised immune system," saidLewis T. (Rusty) Williams, M.D., Ph.D., Chiron's chief scientificofficer and president of research and development. "Based onpromising preliminary data, we believe that Proleukin inconjunction with anti-HIV therapy may offer people with HIV animproved clinical benefit over current therapies alone." The hallmark of HIV infection is a progressive decrease andfunctional impairment of CD4+ T-cells, which can cause the bodyto become more susceptible to the opportunistic infections andmalignancies associated with AIDS. Phase II studies involvingpeople with HIV infection have shown that Proleukin incombination with standard anti-HIV therapy produced increases inCD4+ T-cell counts and an improvement in other immune functionparameters that appear to be sustained over the course of oneyear. The most common side effects reported in the studies ofProleukin in people with HIV infection were injection sitereaction (i.e., redness or bumps), a flu-like syndrome, edema,rashes, joint and muscle pain, oral discomfort, anorexia andinsomnia. Most of these effects improved after the five-daycourse of Proleukin therapy was completed. "Advances in anti-HIV treatments clearly have improved thelength and quality of life of HIV-positive people. However, insome people, the degree of immune reconstitution and control ofthe virus is incomplete," said Yves Levy, M.D., Hospital HenriMondor in Paris and principal investigator of SILCAAT worldwide."This Phase III trial will help define the potential addedbenefits of Proleukin when combined with anti-HIV therapies andanswer the question of whether Proleukin can augment the immunesystem, resulting in a delay of HIV disease progression." Chiron's international Phase III clinical trial, SILCAAT(Phase III Multicenter Randomized Study of SubcutaneousRecombinant, Human Interleukin-2 in HIV-Infected People with LowCD4+ Counts Under Active Antiretroviral Therapy), will enroll anestimated 1,400 people at more than 100 sites worldwide. "With the breadth of Phase II research illustratingProleukin's ability to increase CD4+ T-cell counts and improvethe immune status in most people with HIV, we are excited to beconducting this large, multicenter, international Phase IIItrial," said Anne-Marie Duliege, M.D., a director of clinicalresearch and clinical development at Chiron. "We hope this studywill establish Proleukin's long-term clinical benefits, enablingChiron to offer people with HIV a unique addition to currenttreatment options." Enrollment Criteria People enrolled in the trial will be randomized into one oftwo treatment groups, receiving either Proleukin plus anti-HIVtherapy, or anti-HIV therapy alone. Researchers will assess thetime to the first AIDS-defining event (illness associated withAIDS) as the primary endpoint, with variations in CD4+ T-cellcounts, changes in viral load and occurrence of adverse events asother important endpoints. People eligible to enroll in the studyare those who have: * HIV infection; * CD4+ T-cell counts between 50 and 300; * Viral load < 10,000 RNA copies/mL; * Been on a stable regimen of at least two anti-HIV therapiesfor at least

four months. Chiron Corporation was granted a license to market Proleukinfor the treatment of metastatic melanoma (advanced skin cancer)in the United States in January 1998 and for the treatment ofmetastatic renal cell carcinoma (advanced kidney cancer) in theUnited States in 1992, in Canada in 1994 and in Europe in 1989. Chiron Corporation, headquartered in Emeryville, California,is a leading biotechnology company that participates in threeglobal healthcare markets: biopharmaceuticals, vaccines and bloodtesting. The company is applying a broad and integratedscientific approach to the development of innovative products forpreventing and treating cancer, infectious diseases andcardiovascular disease. This approach is supported by researchstrengths in recombinant proteins, genomics, small molecules,gene therapy and vaccines. This news release may contain forward-looking statementsregarding Proleukin and its effect on patients with HIV. Actualresults could differ materially from those described in this newsrelease as a result of a number of factors, including, but notlimited to the following: There can be no assurance that finalresults of human clinical trials will be supportive of regulatoryapprovals required to market products, or that final regulatoryapproval will be received in a timely manner, if at all, or thatany product in the Chiron product pipeline will be successfullydeveloped or manufactured, or that patient and physicianacceptance of these products will be achieved, nor can there beany assurance as to the impact of competitive products or othermarket conditions. These and other factors investors shouldconsider are more thoroughly discussed in the company's filingswith the Securities and Exchange Commission, including the Form10-K and Form 10-Q. Chiron does not undertake any obligation torevise or update the statements in this press release to reflectevents or circumstances after the date of this release. NOTE: Proleukin is a registered trademark of ChironCorporation. Product information may be obtained on the Internetat www.chiron.com or by calling 800-chiron8. ots Original TextService: Chiron Corporation Internet: http://www.newsaktuell.deContact: Julie Wood of Chiron Corporation, (USA) 510-923-6686;or Philip N. Swayze of Ketchum Public Relations, (USA) 415-984-2284, for Chiron Corporation Web site: http://www.chiron.com

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