Optimer Reports Additional Data on Investigational Compound OPT-80 at Anaerobe 2008 Congress

Business Wire, June 26, 2008

- Epidemic Hypervirulent Strain BI/NAP1/027 Was Well Represented in the Phase 2A Study -

SAN DIEGO -- Optimer Pharmaceuticals, Inc. (NASDAQ: OPTR) presented new data from its Phase 2A clinical study of OPT-80 finding a 42% presence of the super toxin-producing strain of C. difficile known as BI/NAP1/027 strain and concluded that there was no significant difference in the overall cure rates with OPT-80 between subjects with the BI/NAP1/027 strain and subjects without the hypervirulent strain. Data was presented today at the 9th Biennial Congress of the Anaerobe Society of the Americas (Anaerobe 2008) in a poster titled "Typing and Susceptibility of Bacterial Isolates from the OPT-80 (PAR-101) Phase 2A Study for C. Difficile-Associated Diarrhea."

The Phase 2A study was an open-label study involving 45 patients, which compared three different doses of OPT-80 (50 mg, 100 mg, and 200 mg bid x 10 days) for the treatment of patients with mild to moderate C. difficile, infection, or CDI. The results were reported in 2005. All of the 16 subjects in the top dosing group (200 mg bid) achieved clinical cure and this dosage was selected for further investigation in the current Phase 3 trials.

Samples from 38 subjects were sufficient for bacterial isolation. Of these, 16 samples or 42% were the BI/NAP1/027 strain. The distribution of the BI/NAP1/027 strain was nearly equal in all three dosing groups in the Phase 2A study.

The bacterial isolates from the study were tested for susceptibility to OPT-80, vancomycin, and metronidazole. Overall antibiotic susceptibilities were consistent with the previously reported MIC90 values for the three antibiotics. However, OPT-80 showed no difference in MIC values between the BI/NAP1/027 strains and the non-BI/NAP1/027 strains, whereas currently available antibiotics, metronidazole and vancomycin, showed higher MIC values (lower in vitro efficacy) for the hypervirulent strains.

The BI/NAP1/027 strain of C. difficile has been reported as a major cause of epidemic CDI in Canada, Europe, and in 38 U.S. states, as of November 2007. The BI/NAP1/027 strain is resistant to the fluoroquinolone and cephalosporin class of antibiotics, generates approximately 16-23 times more toxin than other strains and is characterized by more severe disease symptoms, more colectomies, longer hospital stays and higher mortality rates. Infection by this strain has been associated with a high risk of acute clinical deterioration and a poor response to the current standard therapy, metronidazole.

OPT-80, currently in two Phase 3 trials, is a first-in-class narrow spectrum antimicrobial agent with excellent activity against many clostridia including C. difficile and moderate activity against certain Gram-positive bacteria, including VRE (Vancomycin-resistant Enterococcus) and MRSA (Methicillin-resistant Staphylococcus aureus). OPT-80, with its narrow spectrum profile, may eradicate C. difficile selectively with minimal disruption to the normal intestinal flora. This may facilitate the return of the normal physiological condition in the colon and reduce the probability of CDI recurrence.

"The rising prevalence of CDI may be attributable to the emergence of the hypervirulent NAP1 strain, which could potentially cause a higher clinical failure rate," said Youe-Kong Shue, Ph.D., Vice President of Clinical Development at Optimer Pharmaceuticals. "The data presented at Anaerobe today provides positive additional information for OPT-80, suggesting a trend toward clinical cure of the epidemic strain with our lead compound."

About Optimer

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, currently in two pivotal Phase 3 clinical trials, is being developed for the treatment of Clostridium difficile infection, the most common hospital-acquired diarrhea. Prulifloxacin, also in two pivotal Phase 3 clinical trials, 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 OPT-80, its effectiveness in treating C. difficile infection, and the rate of incidence of C. difficile and specific strains 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 Optimer's product research and development programs, the development of therapies that compete with Optimer's product candidates, 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.