Optimer Pharmaceuticals and Cubist Pharmaceuticals Announce Collaboration to Co-Promote DIFICIDTM (fidaxomicin) for Clostridium

Optimer Pharmaceuticals and Cubist Pharmaceuticals Announce Collaboration to Co-Promote DIFICIDTM (fidaxomicin) for Clostridium difficile Infection in the United States

Collaboration Intended to Accelerate U.S. Launch of DIFICIDTM to Patients with Clostridium difficile Infection
SAN DIEGO - April 6, 2011 - Optimer Pharmaceuticals, Inc. (NASDAQ: OPTR) and Cubist Pharmaceuticals, Inc. (NASDAQ: CBST) today announced the signing of an exclusive two-year co-promotion agreement to market DIFICIDTM (fidaxomicin), in the United States.  DIFICID is a novel antibiotic currently being reviewed for market approval by the FDA for the treatment of Clostridium difficile infection (CDI), which is also known as Clostridium difficile-associated diarrhea (CDAD).  On April 5, 2011, the FDA's Anti-Infective Drugs Advisory Committee voted unanimously that the clinical evidence submitted by Optimer demonstrated the safety and effectiveness of DIFICID for the treatment of CDI, thereby recommending approval of DIFICID as a treatment for CDI. The FDA is expected to complete its review of DIFICID and deliver a decision on market approval by a Prescription User Fee Act (PDUFA) goal date of May 30, 2011.

Under the terms of the agreement, Optimer and Cubist will co-promote DIFICID to physicians, hospitals, long-term care facilities and other healthcare institutions in the United States. Optimer and Cubist will also jointly provide medical affairs support for the product. Optimer plans to strategically build its sales force and medical affairs team.  Optimer's team will be complemented by Cubist's existing U.S. hospital sales force and medical affairs team which is already dedicated to the antibiotic space for CUBICIN® (daptomycin for injection), Cubist's therapy for Staphylococcus aureus bacteremia, including right-sided endocarditis, and complicated skin infections caused by certain Gram-positive bacteria, including MRSA.

"Cubist has a proven track record and well established relations with all stakeholders involved in the antibiotics space.  We believe this collaboration will maximize the impact of and de-risk DIFICID's U.S. commercial launch, assuming approval," said Pedro Lichtinger, President and CEO of Optimer.  "Optimer plans to hire 100 sales representatives to target the 1,100 hospitals, which comprise about 70% of U.S. hospital CDI cases.  We also believe this collaboration with Cubist will allow us to accelerate formulary adoption, expand our market reach and penetration to up to 2,000 hospitals, comprising over 90% of U.S. hospital CDI cases, and commit resources to long-term care facilities that comprise approximately one-third of all CDI cases.  In addition, this provides the opportunity to capture the undiagnosed CDI market segment which some estimate to be as many as three million cases per year."

Cubist's President and CEO Michael Bonney said, "We believe that the strengths of Cubist's seasoned sales force, medical affairs team and our experience in the hospital will add tremendous value to Optimer's launch strategy and execution. We look forward to working with Optimer to introduce a much needed therapy for seriously ill patients suffering from CDAD."

The agreement expires two years from the first commercial sale of DIFICID.  The agreement can be renewed for additional terms upon agreement of the parties and can be terminated early by either party in certain defined situations; Optimer will be responsible for the manufacturing and distribution of DIFICID in the United States and for recording product revenue.  Optimer will be responsible for obtaining FDA approval, and will retain ownership of the New Drug Application.  For its efforts under the co-promotion agreement, including providing specified levels of resources and activities, Cubist will receive quarterly service fees of $3.75 million ($15 million per year) over the two years beginning with the first commercial sale of DIFICID. Cubist is also eligible to receive an additional $5 million in the first year after first commercial sale and $12.5 million in the second year of the agreement if mutually agreed upon annual sales targets are achieved, as well as a portion of Optimer's gross profits from any sales above the specified annual sales target levels.

Scheduled Conference Call

Optimer will host a conference call today at 8:00 a.m. Eastern Time (5:00 a.m. Pacific Time) to discuss the outcome of the Anti-Infective Drugs Advisory Committee as well as the Cubist collaboration. To participate in the conference call, please dial (877) 280-7280 from the U.S., or (678) 825-8232 for international callers. Please specify to the operator that you would like to join "Optimer's Conference Call." The conference call will be webcast live under the Investors section of Optimer's website at www.optimerpharma.com, where it will be archived for 30 days following the call. Please connect to Optimer's website several minutes prior to the start of the broadcast to ensure adequate time for any software download that may be necessary.

About DIFICIDTM (Fidaxomicin)

DIFICIDTM (fidaxomicin) is an investigational antibiotic with a novel mechanism of action, which inhibits the bacterial enzyme RNA polymerase, resulting in the rapid killing of C. difficile. The narrow-spectrum profile of DIFICID eradicates C. difficile selectively with minimal disruption to the normal intestinal flora, while the alternative antibiotics used to treat CDI, metronidazole and vancomycin, have been shown to disrupt the gut flora. DIFICID facilitates the return of normal physiological conditions in the colon which may be responsible for reducing CDI recurrence rates. In two Phase 3 trials for the treatment of CDI, DIFICID was equally effective in clinical cure when compared to vancomycin, the only FDA approved product for CDI.  DIFICID also demonstrated statistically significant reduction in recurrences and an increase in global cure rate, defined as cure without recurrence. Importantly, DIFICID reduced the risk of recurrence by 47% compared to vancomycin. The New England Journal of Medicine has published results from the first Phase 3 trial in an article titled, "Fidaxomicin versus Vancomycin for Clostridium difficile Infection," which appeared in the February 3, 2011 issue.  Optimer filed a marketing application in the EU for DIFICID for the treatment of CDI and reducing risk of recurrence when used for treatment of initial CDI.

About Clostridium difficile Infection (CDI)

Clostridium difficile infection (CDI), commonly referred to as "C. difficile" or "c-diff", has become a significant medical problem in hospitals, long-term care facilities, and in the community and is estimated to afflict more than 700,000 people each year in the U.S.  It is a serious illness resulting from infection of the inner lining of the colon by C. difficile bacteria, which produce toxins that cause inflammation of the colon, severe diarrhea and, in the most serious cases, death.  Patients typically develop CDI from the use of broad-spectrum antibiotics that disrupt normal gastrointestinal (gut) flora, thus allowing C. difficile bacteria to flourish and produce toxins.

Current therapeutic options for CDI include the off-label use of metronidazole and oral vancomycin, the latter being the only FDA-approved treatment.  However, approximately 20% to 30% of CDI patients who initially respond to these treatments experience a clinical recurrence following cessation of the CDI treatment.

Primary risk factors for CDI include broad-spectrum antibiotic use (such as cephalosporins and fluoroquinolones), older age (over 65) and exposure to emerging hyper-virulent strains (BI/NAP1/027, 078, 001) of C. difficile. The rise in incidence of CDI, along with high rates of both treatment failures and recurrences with current therapies has resulted in greater awareness and concern about CDI among medical professionals and public health officials. You may learn more about CDI at www.cdiinfo.org, a website of Optimer.

About Cubist

Cubist Pharmaceuticals, Inc. is a biopharmaceutical company focused on the research, development, and commercialization of pharmaceutical products that address unmet medical needs in the acute care environment. In the U.S., Cubist markets CUBICIN® (daptomycin for injection), the first antibiotic in a class of anti-infectives called lipopeptides. The Cubist clinical product pipeline currently consists of a Phase 2 program focused on the development of a novel cephalosporin to address certain serious infections caused by multi-drug resistant (MDR) Gram-negative organisms; and a Phase 2 program for the treatment of CDAD (Clostridium difficile-associated diarrhea). Cubist also is working on several pre-clinical programs being developed to address areas of significant medical needs. These include therapies to treat various serious bacterial infections, and agents to treat acute pain. Cubist is headquartered in Lexington, Mass. Additional information can be found at Cubist's web site at www.cubist.com.

About Optimer

Optimer Pharmaceuticals, Inc. is a biopharmaceutical company focused on discovering, developing and commercializing innovative hospital specialty products that have a positive impact on society. Optimer has two anti-infective product candidates in development, DIFICIDTM (fidaxomicin) and PruvelTM (prulifloxacin). DIFICID is a narrow spectrum antibiotic being developed for the treatment of Clostridium difficile infection (CDI). The FDA granted the Company's request for six-month Priority Review of Optimer's NDA for DIFICID, and has assigned a Prescription Drug User Fee Act (PDUFA) goal date of May 30, 2011. The Company also filed a MAA with the European Medicines Agency (EMA) for fidaxomicin. PruvelTM is a prodrug in the fluoroquinolone class of antibiotics being developed as a treatment for infectious diarrhea. Additional information can be found at http://www.optimerpharma.com.

Forward Looking Statements

Statements included in this press release that are not a description of historical facts are forward-looking statements, including without limitation statements related to the potential regulatory approval and commercialization of DIFICID, the expected timing of the FDA's review of the NDA for DIFICID, future activities conducted under the co-promotion agreement, the potential benefits of the co-promotion agreement, including benefits to the commercial launch of DIFICID, the number of hospitals to which DIFICID will be marketed, and the possibility of capturing a portion of the undiagnosed CDI market, Optimer's plans to build its own sales force, potential payments under the co-promotion agreement, and expected trends in the incidence of CDI. Words such as "believes", "would", "anticipates", "plans", "expects", "may", "intend", "will", and similar expressions are intended to identify forward-looking statements. The inclusion of forward-looking statements should not be regarded as a representation by Optimer or Cubist that any of their respective plans will be achieved. Actual results may differ materially from those set forth in this release due to the implementation and continuation of the co-promotion agreement, each party's performance of its respective obligations under the co-promotion agreement, Optimer's and Cubist's ability to commercialize DIFICID, the ability of Optimer and its third party contractors to manufacture and supply sufficient quantities of DIFICID in accordance with Good Manufacturing Practices to meet demand, the ability of Optimer to adequately prepare for the launch of DIFICID in the U.S. and of Cubist and Optimer to work together effectively on the commercial launch, the parties' ability to successfully collaborate with one another under the co-promotion agreement regarding sales and marketing plans and strategy, the parties' ability to terminate the co-promotion agreement, existing competition for DIFICID in the U.S., the development of alternative treatments for or means of preventing CDI, whether and when regulatory authorities will review or approve Optimer's NDA for DIFICID on expected timelines, or at all, whether healthcare professionals will prescribe DIFICID, if approved, whether DIFICID will receive reimbursement coverage from healthcare payors and government agencies, the extent to which DIFICID will be accepted on hospital formularies, the timing of payments by Optimer under the co-promotion agreement, the timing and amount of payments to be received by Cubist, Optimer's ability to successfully recruit and retain sales and marketing personnel, Optimer's ability to successfully manage a sales and marketing organization and other the risks detailed in each of Optimer's and Cubist's  most recent Annual Report on Form 10-K filed with the Securities and Exchange Commission, including those factors discussed under the caption "Risk Factors" in such filings.

Forward-looking statements speak only as of the date of this release, and neither Cubist nor Optimer undertakes any obligation to update or revise these statements,