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 13, 2013

CUBIST PHARMACEUTICALS, INC.

(Exact name of registrant as specified in charter)

65 Hayden Avenue, Lexington, Massachusetts 02421

(Address of Principal Executive Offices, including Zip Code)

(781) 860-8660

(Registrant’s telephone number, including area code)

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:

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

o      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))

8.01 Other Events.

On September 13, 2013, Cubist Pharmaceuticals, Inc., or the Company, issued a press release announcing the expiration of the waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976 in connection with the Company’s previously announced merger agreement with Optimer Pharmaceuticals, Inc.  A copy of the press release has been filed as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference.

Item 9.01 Financial Statements and Exhibits.

 (d)    Exhibits

99.1        Press Release dated September 13, 2013.

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SIGNATURE

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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