UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

(Rule 13d-101)

INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT

TO § 240.13d-1(a) AND AMENDMENTS THERETO FILED PURSUANT TO

§ 240.13d-2(a)

(Amendment No. 1)¹

Aratana Therapeutics, Inc.

(Name of Issuer)

Common Stock, $0.001 par value per share

(Title of Class of Securities)

03874P101

(CUSIP Number)

glenn w. welling

engaged capital, llc

610 Newport Center Drive, Suite 250

Newport Beach, California 92660

(949) 734-7900

STEVE WOLOSKY

OLSHAN FROME WOLOSKY LLP

1325 Avenue of the Americas

New York, New York 10019

(212) 451-2300

(Name, Address and Telephone Number of Person

Authorized to Receive Notices and Communications)

May 18, 2018

(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 §§ 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 § 240.13d-7 for other parties to whom copies are to be sent.

1              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).

CUSIP NO. 03874P101

	1	NAME OF REPORTING PERSON
			Engaged Capital Flagship 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 ITEM 2(d) OR 2(e)				☐
	6	CITIZENSHIP OR PLACE OF ORGANIZATION
			CAYMAN ISLANDS
NUMBER OF			7	SOLE VOTING POWER
SHARES
BENEFICIALLY					2,189,101
OWNED BY			8	SHARED VOTING POWER
EACH
REPORTING					- 0 -
PERSON WITH			9	SOLE DISPOSITIVE POWER
					2,189,101
			10	SHARED DISPOSITIVE POWER
					- 0 -
	11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
			2,189,101
	12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES				☐
	13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
			4.7%
	14	TYPE OF REPORTING PERSON
			PN

2

CUSIP NO. 03874P101

	1	NAME OF REPORTING PERSON
			Engaged Capital Flagship Fund, 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 ITEM 2(d) OR 2(e)				☐
	6	CITIZENSHIP OR PLACE OF ORGANIZATION
			DELAWARE
NUMBER OF			7	SOLE VOTING POWER
SHARES
BENEFICIALLY					2,189,101
OWNED BY			8	SHARED VOTING POWER
EACH
REPORTING					- 0 -
PERSON WITH			9	SOLE DISPOSITIVE POWER
					2,189,101
			10	SHARED DISPOSITIVE POWER
					- 0 -
	11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
			2,189,101
	12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES				☐
	13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
			4.7%
	14	TYPE OF REPORTING PERSON
			PN

3

CUSIP NO. 03874P101

	1	NAME OF REPORTING PERSON
			Engaged Capital Flagship Fund, Ltd.
	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 ITEM 2(d) OR 2(e)				☐
	6	CITIZENSHIP OR PLACE OF ORGANIZATION
			CAYMAN ISLANDS
NUMBER OF			7	SOLE VOTING POWER
SHARES
BENEFICIALLY					2,189,101
OWNED BY			8	SHARED VOTING POWER
EACH
REPORTING					- 0 -
PERSON WITH			9	SOLE DISPOSITIVE POWER
					2,189,101
			10	SHARED DISPOSITIVE POWER
					- 0 -
	11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
			2,189,101
	12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES				☐
	13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
			4.7%
	14	TYPE OF REPORTING PERSON
			CO

4

CUSIP NO. 03874P101

	1	NAME OF REPORTING PERSON
			Engaged Capital, 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 ITEM 2(d) OR 2(e)				☐
	6	CITIZENSHIP OR PLACE OF ORGANIZATION
			DELAWARE
NUMBER OF			7	SOLE VOTING POWER
SHARES
BENEFICIALLY					2,400,000
OWNED BY			8	SHARED VOTING POWER
EACH
REPORTING					- 0 -
PERSON WITH			9	SOLE DISPOSITIVE POWER
					2,400,000
			10	SHARED DISPOSITIVE POWER
					- 0 -
	11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
			2,400,000
	12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES				☐
	13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
			5.1%
	14	TYPE OF REPORTING PERSON
			OO

5

CUSIP NO. 03874P101

	1	NAME OF REPORTING PERSON
			Engaged Capital Holdings, 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 ITEM 2(d) OR 2(e)				☐
	6	CITIZENSHIP OR PLACE OF ORGANIZATION
			DELAWARE
NUMBER OF			7	SOLE VOTING POWER
SHARES
BENEFICIALLY					2,400,000
OWNED BY			8	SHARED VOTING POWER
EACH
REPORTING					- 0 -
PERSON WITH			9	SOLE DISPOSITIVE POWER
					2,400,000
			10	SHARED DISPOSITIVE POWER
					- 0 -
	11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
			2,400,000
	12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES				☐
	13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
			5.1%
	14	TYPE OF REPORTING PERSON
			OO

6

CUSIP NO. 03874P101

	1	NAME OF REPORTING PERSON
			Glenn W. Welling
	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 ITEM 2(d) OR 2(e)				☐
	6	CITIZENSHIP OR PLACE OF ORGANIZATION
			USA
NUMBER OF			7	SOLE VOTING POWER
SHARES
BENEFICIALLY					2,400,000
OWNED BY			8	SHARED VOTING POWER
EACH
REPORTING					- 0 -
PERSON WITH			9	SOLE DISPOSITIVE POWER
					2,400,000
			10	SHARED DISPOSITIVE POWER
					- 0 -
	11	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
			2,400,000
	12	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES				☐
	13	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
			5.1%
	14	TYPE OF REPORTING PERSON
			IN

7

CUSIP NO. 03874P101

	1	NAME OF REPORTING PERSON
			Craig A. Barbarosh
	2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP				(a) ☐
						(b) ☒
	3	SEC USE ONLY
	4	SOURCE OF FUNDS
	5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) OR 2(e)				☐
	6	CITIZENSHIP OR PLACE OF ORGANIZATION
			USA
NUMBER OF			7	SOLE VOTING POWER
SHARES
BENEFICIALLY					- 0 -
OWNED BY			8	SHARED VOTING POWER
EACH
REPORTING					- 0 -
PERSON WITH			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

8

CUSIP NO. 03874P101

	1	NAME OF REPORTING PERSON
			Eric J. Ende
	2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP				(a) ☐
						(b) ☒
	3	SEC USE ONLY
	4	SOURCE OF FUNDS
	5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) OR 2(e)				☐
	6	CITIZENSHIP OR PLACE OF ORGANIZATION
			USA
NUMBER OF			7	SOLE VOTING POWER
SHARES
BENEFICIALLY					- 0 -
OWNED BY			8	SHARED VOTING POWER
EACH
REPORTING					- 0 -
PERSON WITH			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

9

CUSIP NO. 03874P101

	1	NAME OF REPORTING PERSON
			Lowell W. Robinson
	2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP				(a) ☐
						(b) ☒
	3	SEC USE ONLY
	4	SOURCE OF FUNDS
	5	CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEM 2(d) OR 2(e)				☐
	6	CITIZENSHIP OR PLACE OF ORGANIZATION
			USA
NUMBER OF			7	SOLE VOTING POWER
SHARES
BENEFICIALLY					- 0 -
OWNED BY			8	SHARED VOTING POWER
EACH
REPORTING					- 0 -
PERSON WITH			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

10

CUSIP NO. 03874P101

The following constitutes Amendment No. 1 to the Schedule 13D filed by the undersigned (“Amendment No. 1”). This Amendment No. 1 amends the Schedule 13D as specifically set forth herein.

Item 2. Identity and Background.

Item 2 is hereby amended to add the following:

In connection with the Cooperation Agreement, as defined and described in Item 4 below, Craig A. Barbarosh, Eric J. Ende and Lowell W. Robinson are no longer members of the Section 13(d) group and shall cease to be Reporting Persons immediately upon the filing of this Amendment No. 1 to the Schedule 13D. The remaining Reporting Persons will continue filing statements on Schedule 13D with respect to their beneficial ownership of securities of the Issuer to the extent required by applicable law. Each of the remaining Reporting Persons is party to the Joint Filing Agreement, as further described in Item 6 below.

Item 4. Purpose of Transaction.

Item 4 is hereby amended to add the following:

On May 18, 2018, Engaged Capital and certain of its affiliates (collectively, “Engaged”) entered into a Cooperation Agreement (the “Cooperation Agreement”) with the Issuer. Pursuant to the Cooperation Agreement, the Issuer increased the size of the Board of Directors (the “Board”) by one member, accepted the resignation of Robert “Rip” B. Gerber, Jr. as a Class I director, and then appointed (i) Lowell W. Robinson as a Class I director to fill the vacancy created by Mr. Gerber’s resignation (with a term expiring at the Issuer’s 2020 annual meeting of stockholders) and (ii) Craig A. Barbarosh as a new Class III director (with a term expiring at the Issuer’s 2019 annual meeting of stockholders (the “2019 Annual Meeting”)). During the Standstill Period (as defined below), the Issuer agreed not to expand the Board beyond ten members. In addition, the Issuer has agreed to establish an ad hoc Strategic Review Committee of the Board by July 1, 2018 to conduct a strategic review of the Issuer’s business and make recommendations to the Board with respect to the Issuer’s strategy and opportunities to enhance stockholder value. The Strategic Review Committee will be comprised of three independent directors, including one of the newly appointed directors, and the Issuer’s Chief Executive Officer.

Pursuant to the Cooperation Agreement, Engaged withdrew its nomination of director candidates for election to the Board at the Issuer’s 2018 annual meeting of stockholders (the “2018 Annual Meeting”) and is subject to certain customary standstill restrictions from the date of the Cooperation Agreement until the earlier to occur of (i) the date that is thirty calendar days prior to the first anniversary of the deadline for submission of stockholder nominations of director candidates for the 2018 Annual Meeting and (ii) the date that is thirty calendar days prior to the deadline for submission of stockholder nominations of director candidates for the 2019 Annual Meeting (the “Standstill Period”). During the Standstill Period, Engaged also agreed to vote its Shares (a) in favor of the slate of directors recommended by the Board and (b) in accordance with the Board’s recommendations with respect to any other matter presented to stockholders of the Issuer; provided, however, that in the event that Institutional Shareholder Services Inc. (“ISS”) or Glass, Lewis & Co., LLC (“Glass Lewis”) recommends otherwise with respect to any proposals (other than the election or removal of directors), Engaged shall be permitted to vote in accordance with ISS’s or Glass Lewis’ recommendation; provided, further, that Engaged shall be permitted to vote in its sole discretion with respect to any publicly announced proposals relating to a merger, acquisition, disposition of all or substantially all of the assets of the Issuer or other business combination involving the Issuer requiring a vote of stockholders of the Issuer. During the Standstill Period, Engaged also agreed not to acquire beneficial ownership of more than 9.9% of the outstanding Shares without receiving prior Board approval.

11

CUSIP NO. 03874P101

The foregoing description of the Cooperation Agreement does not purport to be complete and is qualified in its entirety by reference to the Cooperation Agreement, which is attached as Exhibit 99.1 hereto and is incorporated herein by reference.

Item 5. Interest in Securities of the Issuer.

Items 5(a) and (c) are hereby amended and restated to read as follows:

(a)       The aggregate percentage of Shares reported owned by each person named herein is based upon 46,844,907 Shares outstanding as of April 30, 2018, which is the total number of Shares outstanding as reported in the Issuer’s quarterly report on Form 10-Q filed with the Securities and Exchange Commission on May 4, 2018.

As of the date hereof, Engaged Capital Flagship Master beneficially owned 2,189,101 Shares, constituting approximately 4.7% of the Shares outstanding. Each of Engaged Capital Fund and Engaged Capital Offshore, as feeder funds of Engaged Capital Flagship Master, may be deemed to beneficially own the 2,189,101 Shares owned by Engaged Capital Flagship Master, constituting approximately 4.7% of the Shares outstanding.

As of the date hereof, 210,899 Shares were held in the Engaged Capital Account, constituting less than 1% of the Shares outstanding.

Engaged Capital, as the general partner and investment adviser of Engaged Capital Flagship Master and the investment adviser of the Engaged Capital Account, may be deemed to beneficially own the 2,400,000 Shares owned in the aggregate by Engaged Capital Flagship Master and held in the Engaged Capital Account, constituting approximately 5.1% of the Shares outstanding. Engaged Holdings, as the managing member of Engaged Capital, may be deemed to beneficially own the 2,400,000 Shares owned in the aggregate by Engaged Capital Flagship Master and held in the Engaged Capital Account, constituting approximately 5.1% of the Shares outstanding. Mr. Welling, as the Founder and CIO of Engaged Capital and sole member of Engaged Holdings, may be deemed to beneficially own the 2,400,000 Shares owned in the aggregate by Engaged Capital Flagship Master and held in the Engaged Capital Account, constituting approximately 5.1% of the Shares outstanding.

As of the date hereof, Messrs. Barbarosh, Ende and Robinson did not beneficially own any Shares, constituting 0% of the Shares outstanding.

(c)       There have been no transactions in securities of the Issuer by the Reporting Persons since the filing of the Schedule 13D.

Item 6. Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer.

Item 6 is hereby amended to add the following:

On May 18, 2018, Engaged and the Issuer entered into the Cooperation Agreement as defined and described in Item 4 above and attached as Exhibit 99.1 hereto.

12

CUSIP NO. 03874P101

On May 18, 2018, the Reporting Persons entered into a Joint Filing Agreement in which the Reporting Persons who will remain Reporting Persons subsequent to this Amendment No. 1 to the Schedule 13D agreed to the joint filing on behalf of each of them of statements on Schedule 13D with respect to the securities of the Issuer to the extent required by applicable law. A copy of the Joint Filing Agreement is attached hereto as Exhibit 99.2 and is incorporated herein by reference.

Item 7. Material to be Filed as Exhibits.

Item 7 is hereby amended to add the following exhibits:

99.1 Cooperation Agreement, dated May 18, 2018.

99.2 Joint Filing Agreement, dated May 18, 2018.

13

CUSIP NO. 03874P101

SIGNATURES

After reasonable inquiry and to the best of his knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

Dated: May 21, 2018

Engaged Capital Flagship Master Fund, LP
By:	Engaged Capital, LLC General Partner
By:	/s/ Glenn W. Welling
	Name:	Glenn W. Welling
	Title:	Founder and Chief Investment Officer

Engaged Capital Flagship Fund, LP
By:	Engaged Capital, LLC General Partner
By:	/s/ Glenn W. Welling
	Name:	Glenn W. Welling
	Title:	Founder and Chief Investment Officer

Engaged Capital Flagship Fund, Ltd.
By:	/s/ Glenn W. Welling
	Name:	Glenn W. Welling
	Title:	Director

Engaged Capital, LLC
By:	/s/ Glenn W. Welling
	Name:	Glenn W. Welling
	Title:	Founder and Chief Investment Officer

Engaged Capital Holdings, LLC
By:	/s/ Glenn W. Welling
	Name:	Glenn W. Welling
	Title:	Sole Member

/s/ Glenn W. Welling

Glenn W. Welling

Individually and as attorney-in-fact for Craig A. Barbarosh, Eric J. Ende and Lowell W. Robinson

14