February 8, 2024 BY EMAIL

Exhibit 10.66

February 8, 2024

BY EMAIL

Xxxxxx X. Xxxxx

[Address]

Dear Xxxxx:

Upon your execution of this Amendment, the following amendments will be made to your Separation Agreement with Akebia Therapeutics, Inc. (“Akebia”) dated May 5, 2022 (as previously amended, the “Separation Agreement”). Paragraph 1(ii) of the Separation Agreement shall be replaced in its entirety with the following:

(ii)    Separation Date. Unless your employment is terminated by the Company for Cause or by you for Good Reason (as those terms are defined in your Executive Severance Agreement dated March 3, 2014, the “ESA”), you will remain employed until June 14, 2024 (as may be amended from time to time, the "Separation Date"). The Separation Date may be modified only upon mutual written agreement between you and the Company.

All other terms and conditions of the Separation Agreement shall remain in full force and effect.

Very truly yours,

AKEBIA THERAPEUTICS, INC.    Accepted and Agreed to Under Seal:

/s/ Xxxx X. Butler______________________    /s/ Xxxxxx X. Hadas__________________

President and Chief Executive Officer    Xxxxxx X. Xxxxx

Dated: February 8, 2024