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<p style="margin-bottom:0pt;margin-top:0pt;text-indent:0%;font-family:Times New Roman;font-size:1pt;">&nbsp;</p>
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<p style="text-align:center;margin-top:6pt;margin-bottom:0pt;text-indent:0%;font-weight:bold;font-size:18pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">UNITED STATES</p>
<p style="text-align:center;margin-bottom:0pt;margin-top:0pt;text-indent:0%;font-weight:bold;font-size:18pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">SECURITIES AND EXCHANGE COMMISSION</p>
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<p style="text-align:center;margin-top:9pt;margin-bottom:0pt;text-indent:0%;font-weight:bold;font-size:18pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">FORM 8-K </p>
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<p style="text-align:center;margin-top:9pt;margin-bottom:0pt;text-indent:0%;font-weight:bold;font-size:14pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">CURRENT REPORT </p>
<p style="text-align:center;margin-top:9pt;margin-bottom:0pt;text-indent:0%;font-weight:bold;font-size:12pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 </p>
<p style="text-align:center;margin-top:9pt;margin-bottom:0pt;text-indent:0%;font-weight:bold;font-family:Times New Roman;font-size:10pt;font-style:normal;text-transform:none;font-variant: normal;">Date of Report (Date of earliest event reported): May 27, 2015</p>
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<p style="text-align:center;margin-top:9pt;margin-bottom:0pt;text-indent:0%;font-weight:bold;font-size:24pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">Mast Therapeutics, Inc.</p>
<p style="text-align:center;margin-bottom:0pt;margin-top:0pt;text-indent:0%;font-weight:bold;font-size:7.5pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">(Exact name of Registrant as Specified in Its Charter) </p>
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<p style="text-align:center;margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;font-weight:bold;font-size:7.5pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">(State or Other Jurisdiction</p>
<p style="text-align:center;margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;font-weight:bold;font-size:7.5pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">of Incorporation)</p></td>
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<p style="margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;font-family:Times New Roman;">&nbsp;</p></td>
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<p style="margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;font-family:Times New Roman;">&nbsp;</p></td>
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<p style="text-align:center;margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;font-weight:bold;font-family:Times New Roman;font-size:10pt;font-style:normal;text-transform:none;font-variant: normal;">3611 Valley Centre Drive, Suite 500,</p>
<p style="text-align:center;margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;font-weight:bold;font-family:Times New Roman;font-size:10pt;font-style:normal;text-transform:none;font-variant: normal;">San Diego, CA</p></td>
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<p style="line-height:2pt;margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;font-size:4pt;font-family:Times New Roman;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">&#160;</p></td>
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<p style="line-height:2pt;margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;font-size:4pt;font-family:Times New Roman;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">&#160;</p></td>
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<p style="text-align:center;margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;font-weight:bold;font-size:7.5pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">(Zip Code)</p></td>
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<p style="text-align:center;margin-top:9pt;margin-bottom:0pt;text-indent:0%;font-weight:bold;font-family:Times New Roman;font-size:10pt;font-style:normal;text-transform:none;font-variant: normal;">Registrant&#8217;s Telephone Number, Including Area Code: (858) 552-0866 </p>
<p style="text-align:center;margin-top:9pt;margin-bottom:0pt;text-indent:0%;font-weight:bold;font-family:Times New Roman;font-size:10pt;font-style:normal;text-transform:none;font-variant: normal;">Not Applicable </p>
<p style="text-align:center;margin-bottom:0pt;margin-top:0pt;text-indent:0%;font-weight:bold;font-size:7.5pt;font-family:Times New Roman;font-style:normal;text-transform:none;font-variant: normal;">(Former Name or Former Address, if Changed Since Last Report) </p>
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<p style="margin-top:9pt;margin-bottom:0pt;text-indent:0%;font-family:Times New Roman;font-size:10pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">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): </p>
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<p style="margin-top:9pt;margin-bottom:0pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;font-family:Times New Roman;font-size:10pt;">Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425) </p></td></tr></table></div>
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<p style="margin-top:9pt;margin-bottom:0pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;font-family:Times New Roman;font-size:10pt;">Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12) </p></td></tr></table></div>
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<p style="margin-top:9pt;margin-bottom:0pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;font-family:Times New Roman;font-size:10pt;">Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b)) </p></td></tr></table></div>
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<p style="margin-top:9pt;margin-bottom:0pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;font-family:Times New Roman;font-size:10pt;">Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c)) </p></td></tr></table></div>
<p style="text-align:center;margin-top:4pt;line-height:1pt;border-top:Solid 0.75pt #000000;padding-top:0pt;margin-bottom:0pt;text-indent:0%;font-size:4pt;font-family:Times New Roman;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">&#160;</p>
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<p style="margin-top:6pt;margin-bottom:0pt;text-indent:0%;font-family:Times New Roman;font-weight:bold;;">&nbsp;</p>
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<p style="margin-top:18pt;margin-bottom:0pt;text-indent:0%;font-weight:bold;font-family:Times New Roman;font-size:10pt;font-style:normal;text-transform:none;font-variant: normal;">Item 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.</p>
<p style="margin-top:6pt;margin-bottom:0pt;text-indent:4.54%;font-family:Times New Roman;font-size:10pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">On May 27, 2015, Jack Lief notified Mast Therapeutics, Inc. (the &#8220;Company&#8221;) of his intent to resign from the Company&#8217;s Board of Directors (the &#8220;Board&#8221;).&nbsp;&nbsp;Mr. Lief&#8217;s decision to resign is not due to any disagreement with the Company known to any of its executive officers on any matter relating to the Company&#8217;s operations, policies or practices.&nbsp;&nbsp;Mr. Lief informed the Company that he will resign to avoid any potential conflict of interest given his position as President and Chief Executive Officer of Arena Pharmaceuticals, Inc. and Arena&#8217;s development of a product candidate for the treatment of vascular diseases, including pulmonary arterial hypertension (&#8220;PAH&#8221;).&nbsp;&nbsp;Mast Therapeutics&#8217; product candidate, AIR001, has been tested in Phase 1 and Phase 2 clinical studies for the treatment of PAH and the Company currently is developing AIR001 for heart failure with preserved ejection fraction.&nbsp;&nbsp;The date of Mr. Lief&#8217;s resignation from the Board has not yet been determined, but is expected to be during the third quarter of 2015.&nbsp;&nbsp;</p>
<p style="margin-top:12pt;margin-bottom:0pt;text-indent:0%;font-family:Times New Roman;">&nbsp;</p>
<p style="margin-top:12pt;margin-bottom:0pt;text-indent:0%;font-family:Times New Roman;">&nbsp;</p>
<p style="text-align:center;margin-top:12pt;margin-bottom:0pt;text-indent:0%;font-family:Times New Roman;font-size:10pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">2</p>

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<p style="margin-bottom:0pt;margin-top:0pt;text-indent:0%;font-family:Times New Roman;font-size:10pt;">&nbsp;</p>
<p style="text-align:center;margin-bottom:0pt;margin-top:0pt;text-indent:0%;font-weight:bold;font-family:Times New Roman;font-size:10pt;font-style:normal;text-transform:none;font-variant: normal;">SIGNATURES</p>
<p style="margin-top:12pt;margin-bottom:0pt;text-indent:0%;font-family:Times New Roman;font-size:10pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">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 thereunto duly authorized.</p>
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<p style="margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;color:#;font-family:Times New Roman;font-size:10pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">&nbsp;</p></td>
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<p style="margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;color:#;font-family:Times New Roman;font-size:10pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">Mast Therapeutics, Inc.</p></td>
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<p style="margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;color:#;font-family:Times New Roman;font-size:10pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">&nbsp;</p></td>
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<p style="margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;color:#;font-family:Times New Roman;font-size:10pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">Date:&nbsp;&nbsp;May 28, 2015</p></td>
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<p style="margin-bottom:0pt;margin-top:0pt;margin-left:16.9pt;;text-indent:0pt;;font-style:italic;color:#;font-family:Times New Roman;font-size:10pt;font-weight:normal;text-transform:none;font-variant: normal;">/s/&#160;Brian M. Culley</p></td>
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<p style="margin-bottom:0pt;margin-top:0pt;margin-left:0pt;;text-indent:0pt;;font-weight:bold;color:#;font-family:Times New Roman;font-size:10pt;font-style:normal;text-transform:none;font-variant: normal;">&nbsp;</p></td>
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<p style="margin-bottom:0pt;margin-top:0pt;margin-left:16.9pt;;text-indent:0pt;;color:#;font-family:Times New Roman;font-size:10pt;font-weight:normal;font-style:normal;text-transform:none;font-variant: normal;">Brian M. Culley</p></td>
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{"company-data": {"conformed-name": "Mast Therapeutics, Inc.", "cik": "0001160308", "assigned-sic": "2834", "irs-number": "841318182", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "8-K", "act": "34", "file-number": "001-32157", "film-number": "15896153"}, "business-address": {"street1": "3611 VALLEY CENTRE DRIVE", "street2": "SUITE 500", "city": "SAN DIEGO", "state": "CA", "zip": "92130", "phone": "8585520866"}, "mail-address": {"street1": "3611 VALLEY CENTRE DRIVE", "street2": "SUITE 500", "city": "SAN DIEGO", "state": "CA", "zip": "92130"}, "former-company": [{"former-conformed-name": "ADVENTRX PHARMACEUTICALS INC", "date-changed": "20030613"}, {"former-conformed-name": "BIOKEYS PHARMACEUTICALS INC", "date-changed": "20010928"}]}
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): May 27, 2015
Mast Therapeutics, Inc.
(Exact name of Registrant as Specified in Its Charter)
Delaware
001-32157
84-1318182
(State or Other Jurisdiction
of Incorporation)
(Commission File Number)
(IRS Employer
Identification No.)
3611 Valley Centre Drive, Suite 500,
San Diego, CA
92130
(Address of Principal Executive Offices)
(Zip Code)
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):
¨
Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)
¨
Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)
¨
Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))
¨
Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))
Item 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.
On May 27, 2015, Jack Lief notified Mast Therapeutics, Inc. (the "Company") of his intent to resign from the Company's Board of Directors (the "Board").  Mr. Lief's decision to resign is not due to any disagreement with the Company known to any of its executive officers on any matter relating to the Company's operations, policies or practices.  Mr. Lief informed the Company that he will resign to avoid any potential conflict of interest given his position as President and Chief Executive Officer of Arena Pharmaceuticals, Inc. and Arena's development of a product candidate for the treatment of vascular diseases, including pulmonary arterial hypertension ("PAH").  Mast Therapeutics' product candidate, AIR001, has been tested in Phase 1 and Phase 2 clinical studies for the treatment of PAH and the Company currently is developing AIR001 for heart failure with preserved ejection fraction.  The date of Mr. Lief's resignation from the Board has not yet been determined, but is expected to be during the third quarter of 2015.
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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 thereunto duly authorized.
Mast Therapeutics, Inc.
Date:  May 28, 2015
By:
/s/ Brian M. Culley
Brian M. Culley
Chief Executive Officer
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