Cubist Pharmaceuticals, Inc. (NASDAQ: CBST) and Optimer Pharmaceuticals, Inc. (NASDAQ: OPTR) today announced that they have signed a merger agreement under which Cubist will acquire all of the outstanding shares of Optimer common stock for $10.75 per share in cash, or approximately $535 million on a fully diluted basis. In addition to the upfront cash payment, each stockholder of Optimer will receive a Contingent Value Right (CVR), which is expected to be publicly traded, entitling the holder to receive an additional one-time cash payment of up to $5.00 for each share they own if certain net sales of DIFICID®(fidaxomicin) are achieved, or a total transaction value of up to $801 million on a fully diluted basis. The transaction has been approved by the Boards of Directors of both companies. Cubist will host a conference call and webcast today at 6:00 p.m. ET (details below). Optimer received U.S. Food and Drug Administration approval in May 2011 for DIFICID, the first antibacterial drug approved in more than 25 years to treat Clostridium difficile-associated diarrhea (CDAD) in adults 18 years of age or older. In two large Phase 3 clinical studies, DIFICID 200 mg twice daily was non-inferior to oral vancomycin 125 mg four times daily in clinical response at the end of 10 days of treatment, and was superior to vancomycin in sustained clinical response through 25 days beyond the end of treatment. The CDAD market is large, with over 700,000 cases annually in the U.S. alone and a high recurrence rate at 20-30%. Hospital stays related to CDAD increased four-fold from 1993 to 2009 and, according to the U.S. Centers for Disease Control and Prevention, the disease is estimated to be responsible for approximately 14,000 deaths per year in the U.S. DIFICID was launched in the U.S. in July 2011. In April 2011, Cubist and Optimer entered into a two-year agreement under which Cubist has been co-promoting DIFICID to physicians, hospitals, and other healthcare institutions in the U.S. Concurrently with the merger agreement, the companies have agreed to extend the co-promotion agreement for up to one year.