Filing

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 6, 2014

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:

On January 6, 2014, Mast Therapeutics, Inc. (the “Company”) announced positive data from a nonclinical study of MST-188 in a model of chronic heart failure. The objective of the study was to determine the effect of acute intravenous administration of MST-188 on left ventricular systolic and diastolic cardiac function in a model of chronic heart failure produced by multiple sequential intracoronary microembolizations. A single dose of MST-188 (low or high dose) or placebo was administered over two hours. Hemodynamic, ventriculographic, echocardiographic and electrocardiographic measurements were taken at baseline (prior to study drug administration) and at the following time-points after the start of study drug administration: 2 hours (end of administration), 24 hours, 1 week and 2 weeks. Peripheral venous blood samples were obtained at the same time-points. The Company reported that MST-188 demonstrated a statistically significant improvement in parameters of heart function, including left ventricular ejection fraction and end-systolic volume, stroke volume and cardiac output, and that a single, two-hour infusion of MST-188 resulted in improvements that were significant immediately (at the end of MST-188 administration) and remained significant at one week (and, in some cases, at two weeks) after MST-188 administration. The improvements described above were calculated as the difference between baseline and mean values of each study group at each time-point using a one-way analysis of variance, with p<0.05 considered significant. The study was conducted under the supervision of Dr. Hani N. Sabbah at Henry Ford Health System, a Michigan non-profit corporation. The Company will provide an update on its plans for MST-188 in heart failure later in 2014.

On January 8, 2014, the Company announced the following updates related to EPIC, its pivotal phase 3 study of MST-188 in sickle cell disease:

The Company’s cash, cash equivalents and investment securities as of December 31, 2013 were over $44 million.

The information contained in this report is summary information that is intended to be considered in the context of the Company’s filings with the U.S. Securities and Exchange Commission (the “SEC”), including its Annual Report on Form 10-K filed on March 19, 2013, its Quarterly Report on Form 10-Q filed on November 4, 2013, and other public announcements that the Company makes, by press release or otherwise, from time to time. The Company makes no admission as to the materiality of any information in this report. 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 it believes is appropriate.

Forward Looking Statements

Mast Therapeutics cautions you that this report includes forward-looking statements within the meaning of Section 21E of the Securities Exchange Act of 1934, as amended (the “Exchange Act”). All statements in this report, other than statements of historical fact, are forward-looking statements that are based on the Company’s current expectations and assumptions. The words “expect,” “believe,” “will,” “may,” “plan,” “intend,” “continues,” and similar forward-looking expressions are intended to identify forward-looking statements. Such forward-looking statements include, but are not limited to, statements related to progress with, completion and potential success of the EPIC study, prospects for MST-188 in sickle cell disease and heart failure, and the Company’s development plans for MST-188 in sickle cell disease, heart failure and acute limb ischemia, including the timing of initiation of any clinical studies. Among the factors that could cause or contribute to material differences between the Company’s actual results and the expectations indicated by the forward-looking statements are risks and uncertainties that include, but are not limited to: the uncertainty of outcomes in ongoing and future studies of MST-188 and the risk that MST-188 may not demonstrate adequate safety, efficacy or tolerability in one or more such studies, including EPIC; delays in the commencement or completion of clinical studies, including as a result of difficulties in obtaining regulatory agency agreement on clinical development plans or clinical study design, opening trial sites, enrolling study subjects, manufacturing sufficient quantities of clinical trial material, being subject to a “clinical hold,” and/or suspension or termination of a clinical study, including due to patient safety concerns or lack of funding; the potential for institutional review boards or the FDA or other regulatory agencies to require additional nonclinical or clinical studies prior to initiation of any planned phase 2 clinical study of MST-188; the potential that, even if clinical studies of MST-188 in one indication are successful, clinical studies in another indication may not be successful; the risk that, even if clinical studies are successful, the FDA or other regulatory agencies may determine they are not sufficient to support a new drug application; the Company’s reliance on contract research organizations (CROs), contract manufacturing organizations (CMOs), and other third parties to assist in the conduct of important aspects of development of MST-188, including clinical studies, and regulatory activities for MST-188, and that such third parties may fail to perform as expected; the Company’s ability to obtain additional funding on a timely basis or on acceptable terms, or at all; the potential for the Company to delay, reduce or discontinue current and/or planned development activities, including clinical studies, partner MST-188 at inopportune times or pursue less expensive but higher-risk and/or lower return development paths if it is unable to raise sufficient additional capital as needed; the risk that, even if the Company successfully develops MST-188 in one or more indications, it may not realize commercial success with its products and may never generate revenue sufficient to achieve profitability; the risk that the Company is not able to adequately protect its intellectual property rights relating to the MAST platform and MST-188 and prevent competitors from duplicating or developing equivalent versions of its product candidates, including MST-188; and other risks and uncertainties more fully described in the Company’s press releases and periodic filings with the SEC.

You are cautioned not to place undue reliance on forward-looking statements, which speak only as of the date when made. Mast Therapeutics does not intend to revise or update any forward-looking statement set forth in this report to reflect events or circumstances arising after the date hereof, except as may be required by law. This caution is made under the safe harbor provisions of Section 21E of the Exchange Act.

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.