Optimer Pharmaceuticals and Astellas Announce Collaboration to Commercialize Fidaxomicin for Clostridium difficile infection

Optimer Pharmaceuticals and Astellas Announce Collaboration to Commercialize Fidaxomicin for Clostridium difficile infection (CDI) in Europe and certain other Countries in Middle East, Africa and CIS

Optimer Eligible to Receive Milestone Payments of up to $224 Million including $68 Million Upfront Cash Payment, and Double-Digit Royalties on Sales in the Territory

Optimer to Host Conference Call Today at 6:00 a.m. Pacific Time (9:00 a.m. Eastern Time)

SAN DIEGO & STAINES, UK - February 7, 2011- Optimer Pharmaceuticals, Inc. (Nasdaq:OPTR) and Astellas Pharma Europe Ltd. ("Astellas") announced today the signing of an exclusive collaboration and license agreement to develop and commercialize fidaxomicin, an investigational antibiotic for CDI, in Europe and certain other countries in the Middle East, Africa and the Commonwealth of Independent States (CIS).

In return for an exclusive license to fidaxomicin in the territory, Astellas is obligated to pay Optimer an upfront cash payment of approximately $68 million. Optimer is also eligible to receive additional cash payments totaling up to approximately $156 million upon the achievement of certain regulatory and commercial milestones. Furthermore, Astellas is obligated to pay tiered double-digit royalty payments on net sales of fidaxomicin in the Territory. Astellas will be responsible for all future costs associated with the development, manufacturing, and commercialization of fidaxomicin in the territory including the costs of the ongoing Marketing Authorization Application (MAA) with the European Medicines Agency (EMA).

"We believe the combined strengths of Astellas' world-class anti-infective business capabilities, including established relationships with payers and hospitals in Europe and certain other markets,combined with Optimer's novel therapeutic for CDI, represents the most effective way to address a serious, unmet health need," said Mr. Masao Yoshida, President and CEO of Astellas Pharma Europe Ltd. "We look forward to bringing fidaxomicin to these markets to help patients and providers address this serious life threatening disease."

"We expect the Astellas collaboration will help Optimer realize the full potential of fidaxomicin and will help position this medication in these countries as the first line of treatment, both for treating CDI and reducing recurrences," said Pedro Lichtinger, Optimer's President and CEO. "CDI poses a significant cost burden on the healthcare system and we believe, if approved, fidaxomicin will provide a cost-savings opportunity for hospitals and payers, especially when used in populations at risk of recurrence such as the elderly, patients with a prior episode, those taking concomitant antibiotics, immuno compromised patients or those with renal impairment."

Fidaxomicin is an orally administered macrocyclic antibiotic with a new mechanism and narrow spectrum of action being developed for the treatment of CDI. In two Phase 3 trials for the treatment of CDI, fidaxomicin was equally effective in clinical cure when compared to vancomycin, the only FDA approved product for CDI. Most importantly, fidaxomicin was statistically superior to vancomycin in global cure and in reducing recurrences of CDI by up to 47%. 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 has filed marketing applications in the U.S. and the EU for fidaxomicin.

Optimer's exclusive financial advisor for this transaction was J.P. Morgan Securities LLC while Cooley LLP was its legal advisor. Astellas' legal advisor in the transaction was Wragge & Co LLP.

Scheduled Conference Call

Optimer will host a conference call today at 6:00 a.m. Pacific Time (9:00 p.m. Eastern Time) to discuss this announcement. 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 Clostridium difficile Infection (CDI)

Clostridium difficile infection, 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 increasing incidence of CDI, along with higher rates of both treatment failures and recurrences with current therapies have 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 Astellas

Astellas Pharma Europe Ltd., located in the UK, is a European subsidiary of Tokyo-based Astellas Pharma Inc. Astellas is a pharmaceutical company dedicated to improving the health of people around the world through the provision of innovative and reliable pharmaceuticals. The organisation is committed to becoming a global company by combining outstanding R&D and marketing capabilities and continuing to grow in the world pharmaceutical market. Astellas Pharma Europe Ltd. is responsible for 21 affiliate offices located across Europe, the Middle East and Africa, an R&D site and three manufacturing plants. The company employs approximately 3,900 staff across these regions. For more information about Astellas Pharma Europe, please visit www.astellas.eu.

About Optimer Pharmaceuticals

Optimer Pharmaceuticals, Inc. is a biopharmaceutical company focused on discovering, developing and commercializing hospital specialty products to treat serious infections and address unmet medical needs. Optimer has two anti-infective product candidates in development, fidaxomicin and PruvelTM (prulifloxacin). Fidaxomicin is a narrow spectrum antibiotic being developed for the treatment of Clostridium difficile infection. The FDA granted Optimer's request for a six-month Priority Review of fidaxomicin, and has assigned a Prescription Drug User Fee Act (PDUFA) goal date of May 30, 2011. Optimer has 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.