Optimer Pharmaceuticals Completes Enrollment in OPT-80 Phase 3 Clinical Trial in Patients with Clostridium difficile Infection

SAN DIEGO, CA – July 23, 2008 - Optimer Pharmaceuticals, Inc. (Nasdaq:  OPTR) today announced that the company has completed enrollment in the first of two pivotal Phase 3 clinical trials evaluating the safety and efficacy of OPT-80 for the treatment of Clostridium difficile infection, or CDI. 

“Currently, there are limited treatment options for CDI.  Completing enrollment in our first OPT-80 trial is a significant step in the development of OPT-80 as an innovative therapy which may help address the global CDI health crisis,” said Michael N. Chang, Ph.D., CEO and President of Optimer. “We look forward to analyzing and disclosing the data from this trial in the coming months.”

OPT‑80, also known as PAR-101, 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. 

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.

OPT-80 Phase 3 Clinical Study Design

At the present time, the only FDA-approved antibiotic for the treatment of CDI is oral vancomycin.  In the OPT-80 multi-center, randomized, double-blind clinical trial, the safety and efficacy of oral OPT-80 (200 mg q12h) is being compared to that of oral vancomycin (125 mg q6h) in subjects suffering from CDI.  The study was designed to evaluate safety and compare the response to treatment by subjects during and after a 10-day course of therapy for cure, which is the primary endpoint.  If cured, subjects are followed for a subsequent four-week period to evaluate recurrence, which is the secondary endpoint.

Significant Unmet Medical Need

CDI has become a significant problem in hospitals, long-term care facilities and in the community.  It is a serious illness caused by infection of the inner lining of the colon by C. difficile bacteria, which produce toxins that cause inflammation of the colon, severe diarrhea and, in the most serious cases, death.  CDI typically develops from the use of broad-spectrum antibiotics that disrupt normal gastrointestinal (gut) flora, allowing C. difficile bacteria to flourish.

Approximately 20% and 30% of CDI patients who initially respond to oral vancomycin and metronidazole, respectively, experience a clinical recurrence following the cessation of antibiotic administration.

Higher incidence and severity of CDI, increased treatment failures with standard therapies, and the emergence of the hypervirulent BI/NAP1/027 strain of C. difficile have combined to result in greater awareness of CDI and significant concern among medical professionals and public health officials.

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 OPT-80, the incidence and anticipated effects of CDI, the efficacy of current CDI treatments and OPT-80 and the timing of clinical trials and anticipated results thereof.  Words such as "believes", "anticipates", "plans", "expects", “may”, "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, the development of alternative treatments for CDI, 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