Cubist Pharmaceuticals, Inc. (NASDAQ: CBST) today announced the initiation of its pivotal Phase 3 studies evaluating the efficacy and safety of CB-315 in patients with Clostridium difficile-associated diarrhea, or CDAD, with enrollment of a patient in the first of two planned identical global trials. These are randomized, double-blind, global studies in which CB-315 (250 mg BID) is being compared with the active comparator oral vancomycin (125 mg QID). Each trial is expected to enroll 608 eligible patients. These studies are designed to evaluate the difference in clinical response rates at the end-of-therapy (EOT) in patients treated with CB-315 versus oral vancomycin, as well as the safety of CB-315 in subjects with CDAD. In addition, these studies will evaluate sustained clinical response after treatment. Cubist’s Chief Scientific Officer Steve Gilman, PhD, said, “The rates and severity of CDAD are increasing due in part to the spread of a new strain of C. difficile with increased virulence. We are very excited to be able to advance CB-315 as a potential therapy, and today’s announcement marks an important milestone as we continue to build a portfolio of potential new therapies for acutely ill patients.” About CDAD CDAD is a disease caused by an overgrowth of, and subsequent toxin production by, Clostridium difficile, a resident anaerobic spore-forming Gram-positive bacterium of the lower gastrointestinal tract. This overgrowth is caused by the use of antibiotics for the treatment of common community and hospital acquired infections (HAIs). Although they treat the underlying infection, many antibiotics disrupt the natural gut flora and allow Clostridium difficile to proliferate. Clostridium difficile produces enterotoxin and cytotoxin, which can lead to severe diarrhea, sepsis and even death. While most types of HAIs are declining, the infection caused by Clostridium difficile remains at historically high levels. According to the latest data from the Centers for Disease Control, C. difficile continues to be the leading cause of death associated with gastroenteritis in the US. For CDAD alone, there was more than a five-fold increase in deaths between 1999 and 2007. C. difficile causes diarrhea linked to 14,000 American deaths each year. About 25% of C. difficile infections first show symptoms in hospital patients; 75% first show in nursing home patients or in people recently cared for in doctors’ offices and clinics. C. difficile infections cost at least $1 billion in extra health care costs annually. About Cubist Cubist Pharmaceuticals, Inc. is a biopharmaceutical company focused on the research, development, and commercialization of pharmaceutical products that address significant unmet medical needs in the acute care environment. Cubist is headquartered in Lexington, Mass. Additional information can be found at Cubist’s web site at www.cubist.com. Cubist Safe Harbor Statement This press release contains forward-looking statements regarding the clinical development of CB-315, including plans to run a Phase 3 clinical trial for this compound in CDAD and the therapeutic potential of CB-315. There are many factors that could cause actual results to differ materially from those in these forward-looking statements. These factors include the following: CB-315 may not show sufficient therapeutic effect or an acceptable safety profile in Phase 3 clinical trials; CB-315 may not act in the way expected based on prior clinical and pre-clinical trials; clinical trials of CB-315 may not be successful or initiated or conducted in a timely manner and the timing of initiation and conduct of subsequent trials is dependent on our ability to successfully work with regulatory authorities, including the FDA on the design of the trials, among other things; we plan to rely, to a significant extent, on third party clinical research organizations, or CROs, to help us conduct clinical trials so the success and timing of the trial is dependent our ability to work with such CROs and their performance; the commercial market for the intended use of CB-315 may not be as large as Cubist anticipates; if approved, CB-315 will compete with products currently on the market and may also compete with products currently in development which may have superior efficacy and/or safety profiles as CB-315 or have other attributes that make it difficult for CB-315 to succeed commercially in such markets; technical difficulties or excessive costs relating to the manufacture or supply of CB-315; we plan to rely, to a significant extent, on third party contract manufacturers and suppliers to manufacture and supply CB-315 on our behalf so our ability to obtain adequate supplies of CB-315 is dependent on our ability to work with such third parties and on their performance; we may not be able to maintain and enforce the intellectual property protecting CB-315; and we may encounter other unanticipated or unexpected risks with respect to the development or manufacture of CB-315. Drug development involves a high degree of risk. Success in pre-clinical trials or early stage clinical trials does not mean that later stage trials will be successful. 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 periodic filings with the Securities and Exchange Commission, including those factors discussed under the caption "Risk Factors" in such filings. These statements speak only as of the date of this release, and Cubist undertakes no obligation to update or revise these statements, except as may be required by law.