Cubist Pharmaceuticals has signed agreements to acquire both Optimer Pharmaceuticals and Trius Therapeutics. Both transactions have been approved by company boards of directors and are expected to close later this year, subject to stockholder approval.

Cubist will acquire all outstanding shares of Optimer common stock for $10.75 per share in cash, or approximately $535 million. Each Optimer stockholder will receive a Contingent Value Right (CVR), expected to be publicly traded, entitling the holder to receive an additional one-time cash payment of up to $5 for each share owned if certain net sales of DIFICID (fidaxomicin) are achieved, or a total transaction value of up to $801 million.

Optimer received FDA 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 or older. The CDAD market is large, with over 700,000 cases annually in the U.S. alone and a high recurrence rate at 20% to 30%. Hospital stays related to CDAD increased four-fold from 1993 to 2009 and, according to the CDC, 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.

 Cubist will acquire all outstanding shares of Trius for $13.50 per share in cash, or approximately $707 million. Each Trius stockholder will receive one CVR entitling the holder to receive an additional cash payment of up to $2 for each share owned if certain commercial sales milestones are achieved. The total transaction is valued at up to $818 million.

Trius brings to Cubist a late-stage antibiotic candidate, tedizolid phosphate (TR-701), and several preclinical antibiotic programs. Tedizolid phosphate is an orally administered second generation oxazolidinone in development for the treatment of certain Gram-positive infections, including methicillin-resistant Staphylococcus aureus (MRSA). Trius has partnered with Bayer Pharma for the development and commercialization of tedizolid phosphate outside of the U.S., Canada and the E.U. An NDA will be submitted to the FDA during the second half of 2013 and a Marketing Authorization Application will be submitted to the EMA in the first half of 2014.

“Tedizolid is a late-stage antibiotic candidate that we believe has the potential to be an important new tool in the infectious disease community’s battle against resistant infections caused by MRSA,” said Michael Bonney, Cubist chief executive officer.