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)

January 21, 2015

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:

Introductory Note.

As previously disclosed in the Current Report on Form 8-K filed with the Securities and Exchange Commission (the “SEC”) by Cubist Pharmaceuticals, Inc., a Delaware corporation (the “Company”), on December 8, 2014, the Company entered into an Agreement and Plan of Merger, dated December 8, 2014 (the “Merger Agreement”), with Merck & Co., Inc., a New Jersey corporation (“Parent”), and Mavec Corporation, a Delaware corporation and a wholly-owned subsidiary of Parent (“Purchaser”). Pursuant to the Merger Agreement, on December 19, 2014, Purchaser commenced a tender offer (the “Offer”) to acquire all of the outstanding shares of common stock, par value $0.001 per share, of the Company (“Shares”), for $102.00 per Share (the “Offer Price”), net to the seller in cash, without interest, but subject to any applicable withholding taxes, upon the terms and subject to the conditions set forth in the offer to purchase, dated December 19, 2014 (as amended or supplemented), and the related letter of transmittal.

The Offer expired at 11:59 P.M. Eastern time on January 20, 2015 (the “Expiration Date”). Computershare Trust Company, N.A., the depositary for the Offer, advised Parent and Purchaser that, as of the Expiration Date, a total of 58,039,667 Shares had been validly tendered and not properly withdrawn pursuant to the Offer (not including Shares tendered pursuant to notices of guaranteed delivery which had not been delivered to the depositary prior to the Expiration Date), which tendered Shares represented approximately 75.7% of the outstanding Shares on a fully diluted basis. On January 21, 2015, Purchaser accepted for payment all such Shares validly tendered and not properly withdrawn pursuant to the Offer on or prior to the Expiration Date and made payment for such Shares.

On January 21, 2015, as a result of its acceptance of, and payment for, the Shares tendered in the Offer, Purchaser acquired a sufficient number of Shares to complete the merger of Purchaser with and into the Company (the “Merger”) without a vote of the stockholders of the Company pursuant to Section 251(h) of the General Corporation Law of the State of Delaware (the “DGCL”). Accordingly, following consummation of the Offer, Parent and Purchaser effected the Merger pursuant to Section 251(h) of the DGCL. In the Merger, each Share that was issued and outstanding immediately prior to the effective time of the Merger (the “Effective Time”) (other than (i) Shares held in the treasury of the Company, (ii) Shares owned by Parent, Purchaser, the Company or any of their respective direct or indirect wholly-owned subsidiaries and (iii) Shares owned by stockholders who have properly demanded and not withdrawn a demand for, or lost their right to, appraisal pursuant to Section 262 of the DGCL with respect to such Shares) was converted into the right to receive the Offer Price, at the Effective Time.

Item 2.01              Completion of Acquisition or Disposition of Assets.

As described in the Introductory Note above, on January 21, 2015, Purchaser accepted for payment all Shares validly tendered and not properly withdrawn pursuant to the Offer on or prior to the Expiration Date and on January 21, 2015, paid for such Shares. Shortly thereafter, the Merger was completed pursuant to Section 251(h) of the DGCL, with no stockholder vote required to consummate the Merger. At the Effective Time, the Company became a wholly-owned subsidiary of Parent. As a result, a change of control of the Company occurred.

The information contained in the Introductory Note of this Current Report on Form 8-K is incorporated by reference into this Item 2.01. The foregoing description of the Merger is qualified in its entirety by reference to the Merger Agreement, a copy of which is attached to this Current Report on Form 8-K as Exhibit 2.1 and is incorporated herein by reference.

Item 3.01              Notice of Delisting or Failure to Satisfy a Continuing Listing Rule or Standard; Transfer of Listing.

On January 21, 2015, in connection with the consummation of the Offer and the Merger, the Company (i) notified the NASDAQ Stock Market (“NASDAQ”) of the consummation of the Merger and (ii) requested that NASDAQ file with the SEC a Form 25 Notification of Removal from Listing and/or Registration to delist and deregister the Shares under Section 12(b) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”). Trading of Shares on the NASDAQ was suspended as of the close of trading on January 21, 2015. The Company also intends to file with the SEC a Form 15 requesting that the Company’s reporting obligations under Section 13 and 15(d) of the Exchange Act be suspended.

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Item 3.03              Material Modification to Rights of Security Holders.

The information contained in the Introductory Note and Items 2.01 and 5.03 of this Current Report on Form 8-K is incorporated by reference into this Item 3.03.

Item 5.01              Change in Control of Registrant.

The information contained in the Introductory Note and Items 2.01 and 5.03 of this Current Report on Form 8-K is incorporated by reference into this Item 5.01.

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

In connection with the Merger and as contemplated by the Merger Agreement, upon the consummation of the Merger, the directors of Purchaser, John Mustillo and Sunil Patel became the directors of the Company.  Individual appointments to the various committees of the Company’s Board of Directors (the “Board”) have not been determined as of the date hereof.  In connection therewith, each of Michael W. Bonney, Kenneth M. Bate, Mark H. Corrigan, Jane E. Henney, Nancy J. Hutson, Alison Lawton, Kenneth J. Martin, Leon O. Moulder, Jr., Robert J. Perez and Martin H. Soeters tendered their respective resignations as directors from the Board and from all committees of the Board on which such directors served, effective as of the Effective Time.

In accordance with the terms of the Merger Agreement, the directors of Purchaser at the Effective Time were appointed as directors of the Company from and after the Effective Time.

Item 5.03              Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year.

In connection with the consummation of the Merger, the Company’s certificate of incorporation, as amended or restated, and its bylaws, as amended or restated, were each amended and restated in their entirety at the Effective Time. Copies of the certificate of incorporation and bylaws of the Company are filed as Exhibits 3.1 and 3.2 hereto and are incorporated by reference into this Item 5.03.

Item 9.01              Exhibits.

(d)  Exhibits

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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.

Date: January 21, 2015

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EXHIBIT INDEX

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