PETX Aratana Therapeutics, Inc.

The following constitutes Amendment No. 3 to the Schedule 13D filed by the undersigned (“Amendment No. 3”). This Amendment No. 3 amends the Schedule 13D as specifically set forth herein.

Item 3 is hereby amended and restated to read as follows:

The Shares purchased by Engaged Capital Flagship Master and held in the Engaged Capital Account were purchased with working capital (which may, at any given time, include margin loans made by brokerage firms in the ordinary course of business). The aggregate purchase price of the 3,500,530 Shares beneficially owned by Engaged Capital Flagship Master is approximately $17,178,977, including brokerage commissions. The aggregate purchase price of the 373,881 Shares held in the Engaged Capital Account is approximately $1,844,862, including brokerage commissions.

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 47,671,117 Shares outstanding as of July 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 August 3, 2018.

As of the close of business on August 20, 2018, Engaged Capital Flagship Master beneficially owned 3,500,530 Shares, constituting approximately 7.3% 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 3,500,530 Shares owned by Engaged Capital Flagship Master, constituting approximately 7.3% of the Shares outstanding.

As of the close of business on August 20, 2018, 373,881 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 3,874,411 Shares owned in the aggregate by Engaged Capital Flagship Master and held in the Engaged Capital Account, constituting approximately 8.1% of the Shares outstanding. Engaged Holdings, as the managing member of Engaged Capital, may be deemed to beneficially own the 3,874,411 Shares owned in the aggregate by Engaged Capital Flagship Master and held in the Engaged Capital Account, constituting approximately 8.1% 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 3,874,411 Shares owned in the aggregate by Engaged Capital Flagship Master and held in the Engaged Capital Account, constituting approximately 8.1% of the Shares outstanding.

(c)       Schedule A annexed hereto lists all transactions in securities of the Issuer by the Reporting Persons since the filing of Amendment No. 2 to the Schedule 13D. All of such transactions were effected in the open market.

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: August 20, 2018

SCHEDULE A

Transactions in Securities of the Issuer Since the Filing of Amendment No. 2 to the Schedule 13D

Engaged Capital Flagship Master Fund, LP

Engaged Capital, LLC

( Through the Engaged Capital Account )