Filing
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13D
Under the Securities Exchange Act of 1934
(Amendment No. 1)*
Mast Therapeutics, Inc.
(Name of Issuer)
Common Stock, par value $0.001 per share
(Title of Class of Securities)
576314108
(CUSIP Number)
Andrew Langham
Icahn Capital LP
767 Fifth Avenue, 47th Floor
New York, New York 10153
(212) 702-4300
(Name, Address and Telephone Number of Person Authorized to
Receive Notices and Communications)
December 29, 2015
(Date of Event which Requires Filing of this Statement)
If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of Section 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box / /.
NOTE: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d‑7 for other parties to whom copies are to be sent.
*The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.
The information required on the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
High River Limited Partnership
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
WC
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
PN
2
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Hopper Investments LLC
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
OO
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
OO
3
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Barberry Corp.
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
OO
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
CO
4
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Icahn Partners Master Fund LP
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
WC
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
PN
5
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Icahn Offshore LP
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
OO
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
PN
6
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Icahn Partners LP
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
WC
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
PN
7
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Icahn Onshore LP
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
OO
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
PN
8
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Icahn Capital LP
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
OO
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
PN
9
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
IPH GP LLC
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
OO
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
OO
10
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Icahn Enterprises Holdings L.P.
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
OO
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
PN
11
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Icahn Enterprises G.P. Inc.
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
OO
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
CO
12
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Beckton Corp.
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
OO
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
CO
13
SCHEDULE 13D
CUSIP No. 576314108
1 NAME OF REPORTING PERSON
Carl C. Icahn
2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
(a) / /
(b) / /
3 SEC USE ONLY
4 SOURCE OF FUNDS
OO
| 5 | CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) or 2(e)/ / |
6 CITIZENSHIP OR PLACE OF ORGANIZATION
United States of America
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:
7 SOLE VOTING POWER
0
8 SHARED VOTING POWER
0
9 SOLE DISPOSITIVE POWER
0
10 SHARED DISPOSITIVE POWER
0
11 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
0
| 12 | CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES/ / |
13 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
0%
14 TYPE OF REPORTING PERSON
IN
14
SCHEDULE 13D
Item 1. Security and Issuer
The Schedule 13D filed with the Securities and Exchange Commission on August 5, 2005 by the Reporting Persons (the "Schedule 13D") with respect to the shares of Common Stock, par value $0.001 (the "Shares") issued by Mast Therapeutics, Inc. (f/k/a Adventrx Pharmaceuticals Inc.) (the "Issuer") is hereby amended to furnish the additional information set forth herein. All capitalized terms contained herein but not otherwise defined shall have the meanings ascribed to such terms in the Schedule 13D.
This Amendment No. 1 to the Schedule 13D reports that the Reporting Persons ceased to be the beneficial owner of more than five percent (5%) of the Shares.
15
SIGNATURE
After reasonable inquiry and to the best of each of the undersigned knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.
Dated: December 30, 2015
ICAHN PARTNERS MASTER FUND LP
ICAHN OFFSHORE LP
ICAHN PARTNERS LP
ICAHN ONSHORE LP
BECKTON CORP.
HOPPER INVESTMENTS LLC
BARBERRY CORP.
HIGH RIVER LIMITED PARTNERSHIP
By: Hopper Investments LLC, general partner
By: Barberry Corp.
By: /s/ Edward E. Mattner
Name: Edward E. Mattner
Title: Authorized Signatory
ICAHN CAPITAL LP
By: IPH GP LLC, its general partner
By: Icahn Enterprises Holdings L.P., its sole member
By: Icahn Enterprises G.P. Inc., its general partner
IPH GP LLC
By: Icahn Enterprises Holdings L.P., its sole member
By: Icahn Enterprises G.P. Inc., its general partner
ICAHN ENTERPRISES HOLDINGS L.P.
By: Icahn Enterprises G.P. Inc., its general partner
ICAHN ENTERPRISES G.P. INC.
By: /s/ SungHwan Cho
Name: SungHwan Cho
Title: Chief Financial Officer
/s/ Carl C. Icahn
CARL C. ICAHN
[Signature Page of Schedule 13D Amendment No. 1 – Mast Therapeutics, Inc.]
16
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