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): March 6, 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 2.02 Results of Operations and Financial Condition.

On March 6, 2017, Mast Therapeutics, Inc. (“Mast” or the “Company”) issued a press release announcing its financial results for the three months and year ended December 31, 2016. A copy of the press release is furnished as Exhibit 99.1 hereto.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits.

The list of exhibits called for by this Item is incorporated by reference to the Exhibit Index immediately following the signature page of this Current Report.

In accordance with General Instruction B.2. of Form 8-K, the information in Item 2.02 of this report and Exhibit 99.1 hereto shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liability of that section, nor shall it be deemed incorporated by reference in any of the Company’s filings under the Securities Act of 1933, as amended, or the Exchange Act, whether made before or after the date hereof, regardless of any incorporation language in such a filing, except as expressly set forth by specific reference in such a filing.

Additional Information about the Merger and Where to Find It

As previously announced, on January 6, 2017, Mast, Victoria Merger Corp., a Delaware corporation and a wholly-owned subsidiary of Mast (“Merger Sub”), and Savara Inc., a privately-held Delaware corporation focused on the development and commercialization of novel therapies for the treatment serious or life-threatening rare respiratory diseases (“Savara”), entered into an Agreement and Plan of Merger and Reorganization (the “Merger Agreement”), pursuant to which, among other things, subject to approval of the stockholders of Mast and Savara and the satisfaction or waiver of the other conditions set forth in the Merger Agreement, Merger Sub will merge with and into Savara, with Savara becoming a wholly-owned subsidiary of the Company (the “Merger”). The transactions contemplated by the Merger Agreement will result in a change in control of Mast.

In connection with the Merger, the Company has filed relevant materials with the SEC, including a registration statement on Form S-4 that contains a prospectus, proxy statement and information statement. Investors and security holders of the Company and Savara are urged to read these materials when the registration statement becomes effective because they contain important information about the Company, Savara and the Merger. The proxy statement/prospectus/information statement 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 proxy statement/prospectus/information statement 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.

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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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