Summary of OPTIMER PHARMACEUTICALS INC

Item 3.03 Material Modification to Rights of Security Holders.

In connection with the review of strategic alternatives for the Company discussed in Item 8.01 below, on February 26, 2013, the Board of Directors (the "Board") declared a dividend of one right (a "Right") for each outstanding share of common stock, par value $0.001 per share ("Common Stock"), of the Company held of record at the close of business on March 11, 2013 (the "Record Time"), or issued thereafter and prior to the Separation Time (as defined below) or pursuant to options and convertible securities outstanding at the Separation Time, payable upon certification by the NASDAQ Stock Market to the Securities and Exchange Commission that the Rights have been approved for listing and registration. The Rights will be issued pursuant to a Stockholder Protection Rights Agreement, dated as of February 26, 2013 (the "Rights Agreement"), between the Company and American Stock Transfer & Trust Company, LLC, as Rights Agent. Each Right entitles its registered holder to purchase from the Company, after the Separation Time, one one-hundredth of a share of Participating Preferred Stock, par value $0.001 per share ("Participating Preferred Stock"), for $40.00 (the "Exercise Price"), subject to adjustment.

The following is a summary of the Rights Agreement. The following summary is qualified in its entirety by the full text of the Rights Agreement (which includes as Exhibit A the forms of Rights Certificate and Election to Exercise and as Exhibit B the form of Certificate of Designations and Terms of the Participating Preferred Stock), which is attached hereto as Exhibit 4.1 and is hereby incorporated by reference.

The Rights will be evidenced by the Common Stock certificates until the close of business on the earlier of (either, the "Separation Time"): (i) the tenth business day (or such later date as the Board may from time to time fix by resolution adopted prior to the Separation Time that would otherwise have occurred) after the date on which any Person (as defined in the Rights Agreement) commences a tender or exchange offer which, if consummated, would result in such Person's becoming an Acquiring Person (as defined below), and
(ii) the time of the first event causing a Flip-in Date (as defined below) to occur; provided that if the foregoing results in the Separation Time being prior to the Record Time, the Separation Time will be the Record Time; and provided further that if a tender or exchange offer referred to in clause (i) is cancelled, terminated or otherwise withdrawn prior to the Separation Time without the purchase of any shares of stock pursuant thereto, such offer will be deemed never to have been made. A "Flip-in Date" will occur on any Stock Acquisition Date (as defined below) or such later date and time as the Board may from time to time fix by resolution adopted prior to the Flip-in Date that would otherwise have occurred. A "Stock Acquisition Date" means the earlier of
(a) the first date on which the Company announces that a Person has become an Acquiring Person, which announcement makes express reference to such status as an Acquiring Person or (b) the date on which any Acquiring Person has acquired more than 40% of our Common Stock. An "Acquiring Person" is any Person having Beneficial Ownership (as defined in the Rights Agreement) of 15% or more of the outstanding shares of Common Stock, not including (i) the Company, any wholly-owned subsidiary of the Company or any employee stock ownership or other employee benefit plan of the Company, (ii) any person who is the Beneficial Owner of 15% or more of the outstanding Common Stock at the time of the first public announcement of the adoption of the Rights Agreement or who becomes the Beneficial Owner of 15% or more of the outstanding Common Stock solely as a result of an acquisition of Common

Stock by the Company or the exercise or exchange of Rights held by such Person following the occurrence of a Flip-in Date which has not resulted from the acquisition of Beneficial Ownership of Common Stock by such Person, until such time as such Person acquires additional Common Stock, other than through a dividend or stock split, aggregating 0.1% or more of the outstanding Common Stock, (iii) any Person who becomes the Beneficial Owner of 15% or more of the outstanding Common Stock without any plan or intent to seek or affect control of the Company if such Person promptly divests sufficient securities such that such 15% or greater Beneficial Ownership ceases or (iv) any Person who Beneficially Owns shares of Common Stock consisting solely of (A) shares acquired pursuant to the grant or exercise of an option granted by the Company in connection with an agreement to merge with, or acquire, the Company entered into prior to a Flip-in Date, (B) shares owned by such Person and its affiliates and associates at the time of such grant and (C) shares, amounting to less than 1% of the outstanding Common Stock, acquired by affiliates and associates of such Person after the time of such grant. Beneficial Ownership includes, solely for purposes of determining whether any Person is an Acquiring Person, constructive ownership of shares in respect of which a Person has a Synthetic Long Position (as defined in the Rights Agreement). The Rights Agreement provides that, until the Separation Time, the Rights will be transferred with and only with the Common Stock. Common Stock certificates issued after the Record Time but prior to the Separation Time (or the registration of Common Stock in the Company's stock transfer books with respect to uncertificated shares) shall evidence one Right for each share of Common Stock represented thereby and such Certificates (or confirmation of registration with respect to uncertificated shares) shall contain a legend incorporating by reference the terms of the Rights Agreement (as such may be amended from time to time). Notwithstanding the absence of the aforementioned legend, certificates evidencing shares of Common Stock outstanding at the Record Time (or registration) shall also evidence one Right for each share of Common Stock evidenced thereby. Promptly following the Separation Time, separate certificates evidencing the Rights ("Rights Certificates") will be delivered to holders of record of Common Stock at the Separation Time.

The Rights will not be exercisable until the Separation Time. The Rights will expire on the earliest of (i) the Exchange Time (as defined below), (ii) the close of business on February 15, 2014, (iii) the date on which the Rights are redeemed as described below and (iv) upon the merger of the Company into another . . .

Item 5.02 Departure of Directors or Certain Officers; Election of
Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

On February 26, 2013, the Board appointed Henry A. McKinnell, Ph.D., the Chairman of our Board, as our Chief Executive Officer. Dr. McKinnell is 70 years old, has served as a director since January 2011 and served as our lead independent director from February 2012 until he was appointed as the Chairman of the Board in April 2012. Dr. McKinnell served as Chairman of the Board of Pfizer Inc., from May 2001 until his retirement in December 2006 and as Chief Executive Officer from January 2001 to July 2006. Dr. McKinnell currently serves as the Chairman of the Board of Moody's Corporation. Dr. McKinnell also serves as Chairman of the Board of the Accordia Global Health Foundation. He is Chairman Emeritus of the Connecticut Science Center, and is a member of the Academic Alliance for AIDS Care and Prevention in Africa.

Dr. McKinnell has no family relationship with any of the officers or directors of the Company and has not been party to any transactions with the Company during the past fiscal year to the present that would require reporting pursuant to Item 404(a) of Regulation S-K. There is no arrangement or understanding between Dr. McKinnell and any third party pursuant to which he was selected as Chief Executive Officer.

Dr. McKinnell will be entitled to receive (i) a base salary of $1.00 per year,
(ii) stock options to purchase up to 225,000 shares of our Common Stock which will vest over four years from February 26, 2013 and (iii) 60,000 restricted stock units, which vest over time beginning on the date we achieve a specified financial goal. In connection with Dr. McKinnell's appointment, the Board of Directors appointed Mark Auerbach, who has served on Optimer's Board of Directors since June 2005, as Lead Independent Director.

Dr. McKinnell will replace Pedro Lichtinger, who stepped down from his position as the Company's President and Chief Executive Officer and resigned in his capacity as a member of the Board on February 26, 2013. In connection with his resignation, Mr. Lichtinger and the Company entered into a Letter Agreement, dated February 26, 2013, which confirms that his resignation will be treated as a termination without cause for the purposes of his employment arrangements, and provides that Mr. Lichtinger will repay any amounts and benefits he receives in connection with his resignation in the event that it is finally determined by a court that he engaged in any act for which he would not be entitled to indemnification under Section 8.1 of the Company's Amended and Restated Bylaws.

On February 26, 2013, the Board also appointed Meredith Schaum as our General Counsel, Chief Compliance Officer and Secretary. Ms. Schaum has served as our Senior Corporate Counsel since January 2012 and will replace Kurt Hartman, who agreed to step down in his capacity as the Company's General Counsel, Chief Compliance Officer, Senior Vice President and Secretary. Mr. Hartman signed a Letter Agreement, dated February 26, 2013, substantially similar to that signed by Mr. Lichtinger except that Mr. Hartman's Letter Agreement provides that he will be paid amounts due under the Company's severance plan in a lump sum. This summary of the Letter Agreements is qualified in its entirety by the full text of the Letter Agreements, which are attached hereto as Exhibits 10.1 and 10.2 and are hereby incorporated by reference. A copy of the press release announcing Mr. Lichtinger's and Mr. Hartman's resignation is attached hereto as Exhibit 99.1.