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 9, 2017

Mast Therapeutics, Inc.

(Exact name of Registrant as Specified in Its Charter)

Registrant’s Telephone Number, Including Area Code: (858) 552-0866

Not Applicable

(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 Instructions A.2. below):

Item 8.01 Other Events.

As previously announced, on January 6, 2017, Mast Therapeutics, Inc. (the “Company”), Victoria Merger Corp., a wholly-owned subsidiary of the Company (“Merger Sub”), and Savara Inc. (“Savara”) entered into an Agreement and Plan of Merger and Reorganization (“Merger Agreement”), pursuant to which, among other things, subject to the satisfaction or waiver of the conditions set forth in the Merger Agreement, Merger Sub will merge with and into Savara (the “Merger”), with Savara becoming a wholly-owned subsidiary of the Company and the surviving corporation of the merger.

Attached hereto as Exhibit 99.1 is a transcript of the joint conference call held on January 9, 2017 by the Company and Savara. Exhibit 99.1 is incorporated by reference herein.

The Company makes no admission as to the materiality of any information in this report. The information contained herein is intended to be considered in the context of the Company’s filings with the Securities and Exchange Commission and other public announcements that the Company makes, by press release or otherwise, from time to time. The Company undertakes no duty or obligation to publicly update or revise the information contained in this report, although it may do so from time to time as its management believes is appropriate. Any such updating may be made through the filing of other reports or documents with the Securities and Exchange Commission (“SEC”), through press releases or through other public disclosure.

Additional Information about the Merger and Where to Find It

In connection with the proposed Merger, the Company intends to file relevant materials with the SEC, including a registration statement on Form S-4 that will contain a prospectus, joint proxy and information statement. Investors and security holders of the Company and Savara are urged to read these materials when they become available because they will contain important information about the Company, Savara and the Merger. The joint proxy statement, information statement, prospectus, and other relevant materials (when they become available), and any other documents filed by the Company with the SEC, may be obtained free of charge at the SEC web site at www.sec.gov. In addition, investors and security holders may obtain free copies of the documents filed with the SEC by the Company by directing a written request to: Mast Therapeutics, Inc., 3611 Valley Centre Drive, Suite 500, San Diego, CA 92130, Attention: Investor Relations. Investors and security holders are urged to read the joint proxy statement, prospectus and the other relevant materials when they become available before making any voting or investment decision with respect to the Merger.

This communication shall not constitute an offer to sell or the solicitation of an offer to sell or the solicitation of an offer to buy any securities, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offering of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended.

Participants in the Solicitation

The Company and its directors and executive officers and Savara and its directors and executive officers may be deemed to be participants in the solicitation of proxies from the stockholders of the Company in connection with the proposed transaction. Information regarding the special interests of these directors and executive officers in the merger will be included in the joint proxy statement/prospectus referred to above. Additional information regarding the directors and executive officers of the Company is also included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2015 and the proxy statement for the Company’s 2016 Annual Meeting of Stockholders. These documents are available free of charge at the SEC web site (www.sec.gov) and from the Company, Attn: Investor Relations, at the address described above.

Item 9.01 Financial Statements and Exhibits.

Reference is made to the Exhibit Index included with this Current Report on Form 8-K.

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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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