UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of report (Date of earliest event reported): September 18, 2013

OPTIMER PHARMACEUTICALS, INC.

(Exact Name of Registrant as Specified in Its Charter)

Delaware

(State or other jurisdiction of incorporation)

101 Hudson Street, Suite 3501, Jersey City, NJ 07302

(Address of principal executive offices, including zip code)

(201) 333-8819

(Registrant’s telephone number, including area code)

N/A

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

o                                    Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

x                                  Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o                                    Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o                                    Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Item 8.01  Other Events.

On September 18, 2013, Optimer Pharmaceuticals, Inc., a Delaware corporation, distributed a communication to its employees, which is attached hereto as Exhibit 99.1 and hereby incorporated herein by reference.

Item 9.01 Financial Statements and Exhibits.

(d)  Exhibits.

Cautionary Statement Regarding Forward-looking Statements

Statements included in this communication that are not a description of historical facts are forward-looking statements, including, without limitation, statements related to the proposed merger with Cubist Pharmaceuticals, Inc. Words such as “expect,” “anticipate,” “will,” “could,” “would,” “project,” “intend,” “plan,” “believe,” “predict,” “estimate,” “should,” “may,” “potential,” “continue,” “ongoing” or variations of such words 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. These forward-looking statements are based on management’s expectations on the date of this release. 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: uncertainties associated with the proposed merger and related transactions, including uncertainties relating to the anticipated timing of the proposed merger and the satisfaction of the conditions to the consummation of the proposed merger, the ability to complete the proposed merger and the impact of the merger on Optimer’s business, employees, customers, suppliers and commercial partners and other risks detailed in Optimer’s filings with the Securities and Exchange Commission. Forward-looking statements speak only as of the date of this release, and Optimer undertakes no obligation to update or revise these statements, except as may be required by law.

Additional Information and Where To Find It

This communication shall not constitute an offer of any securities for sale. In connection with the proposed merger, Optimer has filed a proxy statement/prospectus with the SEC, which Optimer will be mailing to Optimer’s stockholders. BEFORE MAKING ANY VOTING DECISION, OPTIMER’S STOCKHOLDERS ARE URGED TO READ THE PROXY STATEMENT/PROSPECTUS AND OTHER RELEVANT DOCUMENTS FILED WITH THE SEC REGARDING THE PROPOSED MERGER (INCLUDING ALL AMENDMENTS AND SUPPLEMENTS TO THESE DOCUMENTS), WHEN THEY BECOME AVAILABLE, CAREFULLY AND IN THEIR ENTIRETY BECAUSE THESE DOCUMENTS WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED MERGER. Optimer’s stockholders and other investors may obtain, without charge, a copy of the proxy statement/prospectus and other relevant documents filed with the SEC, when they become available, from the SEC’s website at http://www.sec.gov. In addition, investors and stockholders may obtain free copies of the documents filed with the SEC by directing a request to Optimer, 101 Hudson Street, Suite 3501, Jersey City, NJ 07302, telephone: (201) 333-8819, or from Optimer’s website, http://www.optimerpharma.com.

Participants in Solicitation

Optimer and its directors and executive officers may be deemed to be participants in the solicitation of proxies from Optimer’s stockholders with respect to the proposed merger. Information about Optimer’s directors and executive officers and their ownership of Optimer’s common stock is set forth in the proxy/statement prospectus, as filed with the SEC on Schedule 14A on September 18, 2013.  Additional information regarding the interests of those persons and other persons who may be deemed participants in the proposed merger, which may be different than those of Optimer’s stockholders generally, may also be obtained by reading other relevant documents filed with the SEC regarding the proposed merger, when they become available. You may obtain free copies of the foregoing documents as described in the preceding paragraph.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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EXHIBIT INDEX

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