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 25, 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. (“Mast” or the “Company”), Victoria Merger Corp., a wholly-owned subsidiary of Mast (“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 Mast and the surviving corporation of the Merger and resulting in a change in control of Mast.

Attached hereto as Exhibit 99.1 is a written communication from Savara to its security holders, dated January 25, 2017, with information about the Merger. Exhibit 99.1 is incorporated by reference herein.

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 Securities and Exchange Commission (“SEC”), including a registration statement on Form S-4 that will contain a prospectus, joint proxy and information statement (the “Form S-4”). The Company expects to file the Form S-4 with the SEC on or prior to February 14, 2017. 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.

The Company makes no admission as to the materiality of any information in this report, including information in Exhibit 99.1 hereto. The information contained herein is intended to be considered in the context of the Company’s filings with the SEC 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 SEC, through press releases or through other public disclosure.

Forward Looking Statements

Mast cautions you that statements in this report, including in Exhibit 99.1 hereto, that are not a description of historical fact are forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. Forward-looking statements may be identified by the use of words referencing future events or circumstances such as “expect,” “intend,” “plan,” “anticipate,” “believe,” and “will,” among others. Such statements include, but are not limited to, statements regarding the structure, timing and completion of Mast’s proposed merger with Savara; Mast’s continued listing on NYSE MKT prior to and after the proposed merger; Mast’s expectations regarding the capitalization, resources and ownership structure of the combined organization; Mast’s expectations regarding the sufficiency of the combined organization’s resources to fund the advancement of any development program or the completion of any clinical trial; the nature, strategy and focus of the combined organization; the safety, efficacy and projected development timeline and commercial potential of any product candidates; the executive officer and board structure of the combined organization; and the expectations regarding voting by Mast and Savara stockholders. Mast and/or Savara may not actually achieve

2

the proposed merger, or any plans or product development goals in a timely manner, if at all, or otherwise carry out the intentions or meet the expectations or projections disclosed in our forward-looking statements, and you should not place undue reliance on these forward-looking statements.  Because such statements are subject to risks and uncertainties, actual results may differ materially from those expressed or implied by such forward-looking statements. These forward-looking statements are based upon Mast’s and Savara’s current expectations and involve assumptions that may never materialize or may prove to be incorrect. Actual results and the timing of events could differ materially from those anticipated in such forward-looking statements as a result of various risks and uncertainties, which include, without limitation, risks and uncertainties associated with stockholder approval of and the ability to consummate the proposed merger through the process being conducted by Mast and Savara, the ability to project future cash utilization and reserves needed for contingent future liabilities and business operations, the availability of sufficient resources for combined company operations and to conduct or continue planned clinical development programs, the timing and ability of Mast or Savara to raise additional equity capital to fund continued operations; the ability to successfully develop any of Savara’s product candidates, and the risks associated with the process of developing, obtaining regulatory approval for and commercializing drug candidates that are safe and effective for use as human therapeutics. Risks and uncertainties facing Mast are described more fully in Mast’s periodic reports filed with the SEC available at www.sec.gov. You are cautioned not to place undue reliance on forward-looking statements, which speak only as of the date on which they were made.  Mast undertakes no obligation to update such statements to reflect events that occur or circumstances that exist after the date on which they were made, except as may be required by law.

3

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.

4

Exhibit Index

5