Cubist Pharmaceuticals and Chiron Corporation Announce Plans to File for European Regulatory Approval of Cubicin --Daptomycin for Injection-- by Year-End 2004.

Business Editors/Health/Medical Writers

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LEXINGTON, Mass. & EMERYVILLE, Calif.--(BUSINESS WIRE)--June 7, 2004

Cubist Pharmaceuticals, Inc. (Nasdaq: CBST) and Chiron Corporation (Nasdaq: CHIR) today provided an update on the intended strategy to file for regulatory approval of Cubicin(R) (daptomycin for injection) in Europe. CUBICIN, a first-in-class, bactericidal antibiotic, is currently approved in the U.S. and marketed by Cubist for the indication of complicated skin and skin structure infections caused by certain Gram-positive organisms.

By the end of 2004, Chiron expects to submit a Marketing Authorization Application (MAA) to the European Medicines Agency (EMEA) under the centralized filing procedure for approval to market CUBICIN for the indication of complicated skin and soft tissue infections where the presence of susceptible Gram-positive bacteria is confirmed or suspected. After such filing, the EMEA's Committee for Proprietary Medicinal Products (CPMP) would evaluate the application to determine whether to approve CUBICIN in all 25 member states of the European Union (EU).

"We are excited to move ahead with the European registration plans for CUBICIN, as we believe it would directly address a rising unmet need in an important market," said Craig Wheeler, president of Chiron BioPharmaceuticals. "EU approval for CUBICIN would extend our portfolio of anti-infective products, while leveraging our existing sales and marketing structure. We look forward to working with Cubist to ensure a successful approval and launch for CUBICIN in Europe."

"We are pleased with the plan to proceed with an EU filing of CUBICIN by the end of this year," said Michael W. Bonney, President and Chief Executive Officer of Cubist. "Given the success of CUBICIN in the U.S. to date, we are optimistic about the potential for the product in Europe and other international markets."

In October 2003, Cubist and Chiron completed a license agreement for the development and commercialization of CUBICIN in Western and Eastern Europe, Australia, New Zealand, India and certain Central American, South American and Middle Eastern countries.

About Cubicin(R) (daptomycin for injection)

In the U.S., CUBICIN is approved for the treatment of complicated skin and skin structure infections caused by susceptible strains of the following Gram-positive microorganisms: Staphylococcus aureus (including methicillin-resistant strains), Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus dysgalactiae subsp. equisimilis and Enterococcus faecalis (vancomycin-susceptible strains only). To reduce the development of drug-resistant bacteria and maintain the effectiveness of CUBICIN, CUBICIN should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria susceptible to CUBICIN. For full prescribing information, visit www.Cubicin.com.

About Cubist

Cubist Pharmaceuticals, Inc. is a biopharmaceutical company focused on the research, development and commercialization of antiinfective products that meet unmet medical needs. In the U.S., Cubist markets Cubicin(R) (daptomycin for injection), the first antibiotic in a new class of antiinfectives called lipopeptides, for the indication of complicated skin and skin structure infections caused by certain Gram-positive bacteria. CUBICIN is currently the only once-daily bactericidal antibiotic approved in the U.S. with activity against both methicillin-susceptible and methicillin-resistant Staphylococcus aureus (MSSA and MRSA). Cubist's pipeline includes HepeX-B(TM), a monoclonal antibody biologic currently in a Phase 2b study for the prevention of infection by the Hepatitis B virus (HBV) in liver transplant patients, and research efforts focused on novel members of the lipopeptide class of molecules. Cubist is headquartered in Lexington, MA. For more information about Cubist, please visit www.cubist.com.

About Chiron

Through its global Blood Testing, Vaccines and BioPharmaceuticals businesses, Chiron Corporation addresses human suffering with more than 50 diverse products to detect, prevent and treat disease worldwide. The company's consistent success has come from its pioneering science, skill in delivering innovations in biotechnology and disciplined business approach. Chiron believes that science has the power to improve people's lives and harnesses that power to transform public health. For more information about Chiron, please visit www.chiron.com.

Cubist Safe Harbor Statement

Statements contained herein that are not historical fact may be forward-looking statements within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934, and such statements are subject to a variety of risks and uncertainties. There are a number of important factors that could cause actual results to differ materially from those projected or suggested in any forward-looking statements made by Cubist. These factors include, but are not limited to: (i) the level of acceptance of CUBICIN by physicians, patients, third-party payors, and the medical community generally; (ii) Cubist's ability to continue to develop, secure additional regulatory approvals for, and successfully market, CUBICIN; (iii) Cubist's ability to manufacture CUBICIN on a commercial scale; (iv) commercialization of products that are competitive with CUBICIN; (v) Cubist's ability to discover or in-license drug candidates; (vi) Cubist's ability to successfully develop drug candidates in its pipeline, including HepeX-B; (vii) Cubist's ability to successfully commercialize any product or technology developed by Cubist; (viii) Cubist's ability to establish and maintain successful manufacturing, sales and marketing, distribution, and development collaborations; (ix) legislative or regulatory changes adversely affecting Cubist or the biopharmaceutical industry; (x) Cubist's ability to protect its intellectual property and proprietary technologies; and (xi) Cubist's ability to finance its operations. Additional factors that could cause actual results to differ materially from those projected or suggested in any forward-looking statements are contained in Cubist's recent filings with the Securities and Exchange Commission, including those factors discussed under the caption "Risk Factors" in such filings.

Chiron Safe Harbor Statement

This news release contains forward-looking statements, including statements regarding sales growth, possible new product marketing and in-licensing activities, that involve risks and uncertainties and are subject to change. A full discussion of the company's operations and financial condition, including factors that may affect its business and future prospects, is contained in documents the company has filed with the SEC, including the form 10-Q for the quarter ended March 31, 2004, and the form 10-K for the year ended December 31, 2003, and will be contained in all subsequent periodic filings made with the SEC. These documents identify important factors that could cause the company's actual performance to differ from current expectations, including the outcome of clinical trials, regulatory review and approvals, manufacturing capabilities, intellectual property protections and defenses, stock-price and interest-rate volatility, and marketing effectiveness. In particular, there can be no assurance that Chiron will increase sales of existing products, receive approval to market new products or achieve market acceptance for such new products. There can be no assurance that Chiron's in-licensing activities will fully protect it from claims of infringement by third parties.

Consistent with SEC Regulation FD, both companies do not undertake an obligation to update the forward-looking information given here today.

Cubist and Cubicin are registered trademarks of Cubist Pharmaceuticals, Inc.; HepeX-B is a trademark of XTL Biopharmaceuticals Ltd.