Cubist Pharmaceuticals Announces Completion of the Fourth Safety Review of Endocarditis/Bacteremia Data for Cubicin by Data Monitoring Committee.

LEXINGTON, Mass. -- DMC Again Recommends Continuation of Trial

Cubist Pharmaceuticals, Inc. (Nasdaq: CBST) today announced the completion of the fourth planned safety review of patients enrolled in Cubist's ongoing international Phase 3 clinical trial examining the safety and efficacy of Cubicin(R) (daptomycin for injection) in the treatment of infective endocarditis and bacteremia caused by Staphylococcus aureus.

Due to the high mortality rate associated with endocarditis, blinded clinical data from patients enrolled in the study are provided to an independent data monitoring committee (DMC) at pre-determined intervals to identify any safety-related issues. Based upon this review of data from more than 135 treated patients, the DMC has recommended continuation of the trial.

Patients with blood cultures positive for the presence of S. aureus are currently being enrolled at sites in the U.S. and certain Western European countries. In April 2004 the study was expanded to include patients with left-sided endocarditis. Previously the protocol included patients with right-sided endocarditis and complicated or uncomplicated bacteremia. In the study, patients receive either CUBICIN, at 6 mg/kg once daily, or the current standards of care, vancomycin in patients with confirmed methicillin-resistant S. aureus (MRSA) infection or a semi-synthetic penicillin if susceptible S. aureus is confirmed. Cubist continues to expect to report results from this trial in mid 2005.

About Cubicin(R) (daptomycin for injection)

CUBICIN is currently the only once-daily bactericidal antibiotic approved in the U.S. indicated 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, S. agalactiae, S. dysgalactiae subsp equisimilis and Enterococcus faecalis (vancomycin-susceptible strains only). CUBICIN is not indicated for the treatment of pneumonia. Most adverse events reported in clinical trials were mild or moderate in intensity and the most common were constipation, nausea, injection site reactions, and headache. 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. Cubist's pipeline includes HepeX-B(TM), a monoclonal antibody biologic currently in the second of two Phase 2 trials to determine its potential 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.

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.

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

Additional information can be found at Cubist's web site at www.cubist.com.