On April 20, 2012, Optimer Pharmaceuticals, Inc. ("Optimer") entered into a Compensation Agreement ("Compensation Agreement") with AFOS, LLC ("AFOS"). Pursuant to the terms of the Compensation Agreement, Optimer issued 286,260 shares of Optimer's common stock (the "Shares") to AFOS as consideration for the engagement of an affiliate of AFOS to provide certain services to Optimer, and the affiliate's obligation to provide such services.The Shares are subject to various lock-up periods during which AFOS may not transfer such Shares, except pursuant to certain exceptions. The Compensation Agreement is attached as Exhibit 99.1 to this report and the foregoing description is qualified in its entirety by reference to such exhibit.

The Shares were issued pursuant to Optimer's effective registration statement on Form S-3 (Registration Statement No. 333-177709) previously filed with and declared effective by the Securities and Exchange Commission and a prospectus supplement thereunder. A copy of the opinion of Cooley LLP relating to the legality of the issuance of the Shares is attached as Exhibit 5.1 hereto.

(d) Exhibits.

Exhibit No.                               Description

5.1           Opinion of Cooley.

23.1          Consent of Cooley LLP (included in Exhibit 5.1).

99.1          Compensation Agreement, dated April 20, 2012, by and between Optimer
              Pharmaceuticals, Inc. and AFOS, LLC.