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): December 4, 2018

TESARO, Inc.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code:  (339) 970-0900

(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 (see General Instruction A.2. below):

o                          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))

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging Growth Company o

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. o

Item 8.01              Other Events.

On December 4, 2018, TESARO, Inc. (the “Company”) issued a press release pursuant to the terms of the Indenture, dated as of September 29, 2014, between the Company and U.S. Bank National Association, as trustee (the “Trustee”) (as supplemented by that First Supplemental Indenture to the Indenture, dated as of September 29, 2014, between the Company and the Trustee, the “Indenture”), announcing an anticipated “Make-Whole Adjustment Event” (as such term is defined in the Indenture) that will occur as a result of completion of the proposed transactions contemplated by the Agreement and Plan of Merger, dated December 3, 2018, by and between the Company, GlaxoSmithKline plc (“GSK”) and Adriatic Acquisition Corporation, an indirect wholly-owned subsidiary of GSK. A copy of the press release is attached as Exhibit 99.1 hereto and incorporated herein by reference.

Section 9 — Financial Statements and Exhibits

Item 9.01              Financial Statements and Exhibits.

(d)           Exhibits.

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.

Dated: December 4, 2018