Cubist Pharmaceuticals, Inc. (NASDAQ: CBST) today announced the initiation of the Phase 3 CB-5945 opioid-induced constipation (OIC) Ascent program with the enrollment of the first patient in the long-term safety study, which is one of four registrational studies in patients with chronic non-cancer pain.

The trial is a multicenter, randomized (1:1), double-blind, placebo-controlled study that compares CB-5945 (0.25 mg twice a day) with placebo and is expected to enroll 1,400 eligible patients in the US and Canada. The primary objective of the study is to evaluate the safety of CB-5945 (0.25 mg twice a day) compared with placebo over a one year treatment period. Importantly, this study also examines the effects of CB-5945 on quality of life as a secondary objective.

The long-term safety study complements the three planned efficacy studies scheduled to begin in the first half of 2013. Overall enrollment for the Phase 3 CB-5945 program is expected to be approximately 3,200 patients.

Cubist’s Chief Scientific Officer Steve Gilman, PhD, said, “Opioid-induced constipation is a debilitating side effect for millions of people who have chronic non-cancer pain. We are excited to begin the robust Phase 3 development program for CB-5945 and hope one day to make this important therapy available for patients.”

About Opioid-Induced Constipation (OIC) and CB-5945

The American Pain Foundation estimates that 9% of the U.S. adult population suffers from moderate to severe non-cancer related chronic pain, which is commonly defined as pain that lasts longer than the usual course of an injury or illness. Long-term management of chronic pain often includes treatment with opioid analgesics. Many people receiving this treatment will develop constipation, as well as other associated gastrointestinal complications. This is believed to be the result of the stimulation of mu opioid receptors within the gastrointestinal tract by opioid analgesics. The stimulation of these peripheral mu opioid receptors disrupts regulation of motility, secretion, and absorption. CB-5945 is a potent, peripherally acting mu opioid receptor antagonist that is designed to block the adverse effects of opioid analgesics on the gastrointestinal tract without compromising centrally mediated pain relief. Currently, there are no FDA-approved therapies to treat OIC in patients with chronic non-cancer pain.

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 includes forward-looking statements, including, without limitation, statements regarding the clinical development of CB-5945, including plans to run Phase 3 clinical trials for this compound in OIC, our expectations on the anticipated timing of such trials, and the therapeutic potential of CB-5945. Each forward-looking statement is subject to risks and uncertainties that could cause actual results to differ materially from those expressed or implied by such statement. Applicable risks and uncertainties include, among others: CB-5945 may not show sufficient therapeutic effect or an acceptable safety profile in Phase 3 clinical trials; CB-5945 may not act in the way expected based on prior clinical and pre-clinical trials; clinical trials of CB-5945 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-5945 may not be as large as Cubist anticipates; if approved, CB-5945 may also compete with products currently in development which may have superior efficacy and/or safety profiles as CB-5945 or have other attributes that make it difficult for CB-5945 to succeed commercially in such markets; technical difficulties or excessive costs relating to the manufacture or supply of CB-5945; we plan to rely, to a significant extent, on third party contract manufacturers and suppliers to manufacture and supply CB-5945 on our behalf so our ability to obtain adequate supplies of CB-5945 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-5945; and we may encounter other unanticipated or unexpected risks with respect to the development or manufacture of CB-5945; and those additional factors discussed under the caption "Risk Factors" in our recent periodic filings with the Securities and Exchange Commission. 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. We caution investors not to place considerable reliance on the forward-looking statements contained in this press release. These forward-looking statements speak only as of the date of this document, and we undertake no obligation to update or revise any of these statements.

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