September 2, 2008

Optimer Pharmaceuticals Completes Enrollment in Second Prulifloxacin Phase 3 Clinical Trial

Optimer Pharmaceuticals, Inc. (Nasdaq:OPTR) today announced that the company has completed enrollment in the second of two planned phase 3 clinical trials evaluating the safety and efficacy of Prulifloxacin for the treatment of infectious diarrhea in travelers. The Company previously reported positive top-line data from the first Phase 3 trial which showed that Prulifloxacin met the primary endpoint of Time to Last Unformed Stool (TLUS), a measure of the time to resolution of diarrhea, compared to placebo with a p-value of less than 0.0001. If the results of this second study are also positive, the Company expects to use these studies as a basis for a New Drug Application filing in 2009.

Travelers' diarrhea is infectious diarrhea caused by bacteria, viruses or parasites and is contracted by the ingestion of contaminated food or water. This infection is characterized by diarrhea along with other related symptoms including nausea, vomiting, abdominal pain or cramping, and fecal urgency.

"We are very excited to have completed enrollment in the second trial," said Michael N. Chang, Ph.D., CEO and President of Optimer. "We look forward to completing the analysis of the data from this trial and we expect top-line results in the fourth quarter."

This multi-center, double-blind, randomized and placebo-controlled clinical trial compares the safety and efficacy of Prulifloxacin versus placebo in adult travelers suffering from acute bacterial gastroenteritis. Clinical trial sites included locations in India, Guatemala, and Mexico. The primary endpoint is TLUS and secondary endpoints include clinical cure based on relief of symptoms and microbiological eradication rates.

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 can be found at

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 Prulifloxacin and the timing of clinical trials and regulatory filings and anticipated results thereof. 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 timing, progress and likelihood of success of its product research and development programs, the timing and status of its preclinical and clinical development of potential drugs and other risks detailed in Optimer's filings with the Securities and Exchange Commission.