Optimer Pharmaceuticals Completes $14.8 Million Registered Direct Offering

SAN DIEGO– July 28, 2008 – Optimer Pharmaceuticals, Inc. (Nasdaq: OPTR) today announced that it completed a registered direct offering of 1,743,396 shares of common stock at a price of $8.48 per share, for total gross proceeds of $14.8 million.  The closing involved the previously announced sale of 1,443,396 shares of Optimer’s common stock as well as an additional 300,000 shares of Optimer’s common stock which another institutional investor subsequently committed to purchase.

Optimer expects that the proceeds from the transaction will be used in the further development of its ongoing programs, as well as for other general corporate purposes.

All of the shares of Optimer common stock were issued pursuant to an effective registration statement previously filed with the U.S. Securities and Exchange Commission.  The offering and sale of these securities were made only by means of a prospectus supplement and accompanying prospectus.  This press release shall not constitute an offer to sell, or a solicitation of an offer to buy, nor shall there be any sale of these securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to the registration or qualification under the securities laws of any such jurisdiction.

About Optimer Pharmaceuticals

Optimer Pharmaceuticals, Inc. is a biopharmaceutical company focused on discovering, developing and commercializing innovative anti-infective products for the treatment of serious infections. Optimer has two late-stage anti-infective product candidates. OPT-80 is being developed for the treatment of Clostridium difficile infection, the most common hospital-acquired diarrhea. Prulifloxacin is an antibiotic being developed for the treatment of travelers’ diarrhea, a form of infectious diarrhea. Additional information regarding Optimer 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 all statements related to Optimer’s anticipated use of the offering proceeds.  Words such as "believes," "anticipates," "plans," "expects," "intend," "will," "goal" 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 that any of its plans will be achieved. Actual results may differ materially from those set forth in this release due to the risks and uncertainties inherent in Optimer's business including, without limitation, risks relating to: the fact that Optimer will maintain broad discretion in the actual use of the proceeds from the offering, the timing, progress and likelihood of success of Optimer’s product research and development programs, the timing and status of Optimer’s preclinical and clinical development of potential drugs and other risks detailed in Optimer's filings with the Securities and Exchange Commission.

Contacts

Optimer Pharmaceuticals, Inc.

Christina Donaghy, Corporate Communications Manager

John D. Prunty, Chief Financial Officer & VP Finance

858-909-0736

Porter Novelli Life Sciences

Jason I. Spark, Vice President

619-849-6005