Amendment No. 2, dated April 13, 2024, to the TNNI3 Research Agreement, dated December 3, 2021, between LEXEO Therapeutics, Inc. and the Regents of the University of California, as amended

Exhibit 10.28

[ *** ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY BRACKETS, HAS BEEN OMITTED BECAUSE IT IS BOTH (I) NOT MATERIAL AND (II) IS THE TYPE THAT THE REGISTRANT TREATS AS PRIVATE OR CONFIDENTIAL.

SECOND AMENDMENT TO THE RESEARCH AGREEMENT BETWEEN LEXEO THERAPEUTICS, INC.

AND

THE REGENTS OF THE UNIVERSITY OF CALIFORNIA ON BEHALF OF ITS SAN DIEGO CAMPUS

This Second Amendment (“Second Amendment”) by and between Lexeo Therapeutics, Inc. located at 345 Park Avenue South, Sixth Floor, New York, NY 10010 (“Sponsor”) and The Regents of the University of California on behalf of its San Diego Campus, located at 9500 Gilman Drive #0934, La Jolla, CA 92093-0934 (“Institution”) is made and effective as of the last date of signature (“Effective Date”).

WHEREAS, Sponsor and Institution entered into the Research Agreement, effective on December 3, 2021 (“Agreement”), as amended on April 19, 2023 (“First Amendment”), to conduct a research project entitled [***] (“Project”);

WHEREAS, Sponsor and Institution mutually desire to amend the Agreement as set forth below;

NOW THEREFORE, in consideration of the premises and mutual covenants herein, the parties agree as follows:

The term of this Agreement shall be for three years from the Effective Date (the “Term”), unless sooner terminated as herein provided.

IN WITNESS WHEREOF, the parties have caused this Second Amendment to be executed by their duly authorized representatives as of the Effective Date.