Phase 3 Data on CUBICIN Published in Journal Clinical Infectious Diseases; CUBICIN Efficacy Comparable to Standards of Care Against MSSA and MRSA.

LEXINGTON, Mass. -- Cubist Pharmaceuticals, Inc. (Nasdaq: CBST) today announced that data from its two pivotal Phase 3 trials studying the safety and efficacy of its flagship antibiotic product Cubicin(R) (daptomycin for injection) have been published in the June 15 issue of Clinical Infectious Diseases (CID). CUBICIN, the first in a new class of antibiotics, was approved in September 2003 by the U.S. Food & Drug Administration (FDA) for the indication of complicated skin and skin structure (cSSS) infections caused by certain Gram-positive organisms.

The manuscript presents detailed data from the two randomized, international, multi-center Phase 3 trials that demonstrated the safety and efficacy of CUBICIN in the treatment of cSSS infections.

Key data from the manuscript are summarized below:

--  Of 1,092 patients enrolled in the two trials, 534 received
       CUBICIN and 558 received the comparator agents, either a
       semi-synthetic penicillin or vancomycin.

   --  Patients, aged 18-85 years, were enrolled with Gram-positive
       cSSS infections that necessitated hospitalization, including
       wound infections, major abscesses, and infected ulcers; minor
       or superficial infections (e.g., simple abscesses and
       uncomplicated cellulitis) were excluded.

   --  Both studies, individually and collectively, met the
       pre-defined statistical criteria of non-inferiority
       (comparability) across all efficacy populations; the results
       were robust and consistent across all pre-defined patient
       populations, across different species of infecting
       Gram-positive organisms, across different types of infection
       and for primary clinical outcomes (test of cure) and
       post-treatment relapse rates;

       --  CUBICIN demonstrated comparable efficacy to comparator
           against methicillin-susceptible Staphylococcus aureus
           (MSSA).

       --  CUBICIN demonstrated comparable efficacy to comparator
           against methicillin-resistant Staphylococcus aureus
           (MRSA).

   --  CUBICIN was safe and well tolerated; the frequency,
       distribution and severity of adverse events were similar for
       CUBICIN and standard therapy.

Robert J. Perez, Senior Vice President of Sales & Marketing at Cubist commented on the publication: "For the first time ever, physicians have available to them a once-daily, rapidly bactericidal antibiotic that is as efficacious as the currently marketed standards of therapy in treating cSSS infections, regardless of the resistance profile of the Gram-positive pathogen present. With the incidence of MRSA rising at an alarming rate both in the hospital and in the community, physicians increasingly need reliable and safe broad-spectrum Gram-positive agents. We believe CUBICIN directly addresses this growing unmet medical need. In addition," Mr. Perez continued, "due to its safety profile and convenient once-daily dosing, we believe CUBICIN may confer economic benefits to the medical community as well."

About Cubicin(R) (daptomycin for injection)

CUBICIN is 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, 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.

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.