of this Current Report on Form 8-K regarding the issuance of shares of the Company’s Common Stock in connection with the Merger is incorporated herein by reference. The shares of Common Stock issued in connection with the Merger were not registered under the Securities Act of 1933, or any applicable state securities laws and were issued in reliance on the exemption from registration provided by Section 4(a)(2) of the Securities Act and/or Regulation D promulgated thereunder. Forward Looking S…

Unregistered Sales of Equity Securities. The information set forth in

Entry into a Material Definitive Agreement. On September 16, 2026, Lexeo Therapeutics, Inc. (“ Lexeo ” or the “ Company ”) entered into an Agreement and Plan of Merger (the “ Merger Agreement ”) with Mantle Therapeutics Inc. (“ Mantle ”), Magma Merger Sub Inc., a Delaware corporation and a wholly-owned subsidiary of the Company (“ Merger Sub ”), and Shareholder Representative Services LLC, a Colorado limited liability company, solely in its capacity as representative, agent and attorney in fa…

Other Events. On September 22, 2026, the Company issued a press release announcing its entry into the Merger Agreement as well as other strategic collaborations entered into by the Company. As part of the press release, the Company announced that it would be hosting a conference call and webcast at 8:00 a.m. ET on September 22, 2026 to discuss said transactions and provide a business update. A copy of the press release is attached as Exhibit 99.1 to this Current Report on Form 8-K and is inco…

and Exhibit 99.1 hereto shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “ Exchange Act ”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any of the Company’s filings under the Securities Act of 1933, as amended, or the Exchange Act, whether made before or after the date hereof, except as expressly set forth by specific reference in such a filing.

Other Events. On June 15, 2026, Lexeo Therapeutics, Inc. (the "Company") issued a press release a nnouncing the regulatory updates to LX2006 for the treatment of Friedreich ataxia (FA) cardiomyopathy including the pivotal study design to support accelerated approval. As part of the pre ss release, the Company announced that it would be hosting a conference call and webcast at 8:00 a.m. ET on June 15, 2026 to discuss regulatory updates on LX2006 for the treatment of FA cardiomyopathy. The pres…

and Exhibit 99.1 hereto shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “ Exchange Act ”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any of the Company’s filings under the Securities Act of 1933, as amended, or the Exchange Act, whether made before or after the date hereof, except as expressly set forth by specific reference in such a filing.

Director — Dr. Laura Sepp-Lorenzino: Dr. Laura Sepp-Lorenzino was appointed to the board of directors and to the Science and Technology Committee.

and Exhibit 99.1 hereto shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “ Exchange Act ”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any of the Company’s filings under the Securities Act of 1933, as amended, or the Exchange Act, whether made before or after the date hereof, except as expressly set forth by specific reference in such a filing.

Head of Research — Dr. Adler: Dr. Adler stepped down from his role of Head of Research and will provide advisory services until July 31, 2026.

Chief Operating Officer — Jose Manuel Otero, Ph.D.: Dr. Jose Manuel Otero was promoted to Chief Operating Officer.

Other Events. On January 12, 2026, Lexeo Therapeutics, Inc. (the “Company”) issued a press release announcing positive interim Phase 1/2 clinical data of LX2020 for the treatment of PKP2-associated arrhythmogenic cardiomyopathy (“PKP2-ACM”). As part of the press release, the Company announced that it would be hosting a conference call and webcast at 8:00 a.m. ET on January 12, 2026 to discuss the interim Phase 1/2 clinical data of LX2020 for the treatment of PKP2-ACM. The press release is att…

and Exhibit 99.1 hereto shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any of the Company’s filings under the Securities Act of 1933, as amended, or the Exchange Act, whether made before or after the date hereof, except as expressly set forth by specific reference in such a filing.

Entry into a Material Definitive Agreement. Underwriting Agreement On October 16, 2025, Lexeo Therapeutics, Inc. (the “ Company ”) entered into an underwriting agreement (the “ Underwriting Agreement ”) with Leerink Partners LLC, as representative (the “ Representative ”) of the several underwriters named therein (collectively, the “ Underwriters ”), relating to the issuance and sale in a public offering (the “ Public Offering ”) of 15,625,000 shares (the “ Firm Shares ”) of the Company’s com…

of this Current Report on Form 8-K. The Company will sell the Pre-Funded Warrants to an “accredited investor,” as that term is defined in the Securities Act, in reliance on the exemption from registration afforded by Section 4(a)(2) of the Securities Act and corresponding provisions of state securities or “blue sky” laws. The Purchaser represented that it is acquiring the Pre-Funded Warrants for investment only and not with a view towards the resale or distribution thereof in violation of the…

Other Events Clinical Update – LX2006 In October 2025, the Company announced progress in regulatory interactions with the FDA on the LX2006 program. In response to questions from the Company regarding the possibility of a faster path to a Biologics License Application, or BLA, the FDA has indicated openness to a BLA submission for accelerated approval that includes clinical data from the Company’s ongoing Phase I/II studies of LX2006 pooled with new clinical data to be generated in the planne…

Results of Operations and Financial Condition. On October 16, 2025, the Company disclosed in a preliminary prospectus supplement relating to the Public Offering that, as of September 30, 2025, the Company estimates that it had $122.8 million of cash, cash equivalents and investments. The preliminary financial information has been prepared by, and is the responsibility of, the Company’s management and has been prepared in good faith on a consistent basis with prior periods. The Company’s indep…

and Exhibit 99.1 hereto shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any of the Company’s filings under the Securities Act of 1933, as amended, or the Exchange Act, whether made before or after the date hereof, except as expressly set forth by specific reference in such a filing.

Chief Financial Officer — Louis Tamayo: Louis Tamayo was hired as the new Chief Financial Officer, succeeding Dr. Kyle Rasbach.

by reference. The information in this Item 7.01, including Exhibit 99.1 attached hereto, shall not be deemed “filed” for purposes of Section 18 of the Exchange Act, or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any filing under the Securities Act, except as expressly set forth by specific reference in such filing. Forward Looking Statements This report contains certain forward-looking statements regarding the business of the Compa…

Entry into a Material Definitive Agreement. Securities Purchase Agreement On May 27, 2025, Lexeo Therapeutics, Inc. (the “ Company ”) entered into a Securities Purchase Agreement (the “ Purchase Agreement ”) for a private placement (the “ Private Placement ”) with certain qualified institutional buyers and institutional accredited investors (each, a “ Purchaser ” and collectively, the “ Purchasers ”). Pursuant to the Purchase Agreement, the Company agreed to sell to the Purchasers an aggregat…

of this Current Report on Form 8-K. The Company will sell the Securities to “accredited investors,” as that term is defined in the Securities Act, in reliance on the exemption from registration afforded by Section 4(a)(2) of the Securities Act and corresponding provisions of state securities or “blue sky” laws. The Purchasers represented that they are acquiring the Securities for investment only and not with a view towards the resale or distribution thereof in violation of the Securities Act.…

and Exhibit 99.1 hereto shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “ Exchange Act ”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any of the Company’s filings under the Securities Act of 1933, as amended, or the Exchange Act, whether made before or after the date hereof, except as expressly set forth by specific reference in such a filing.

and Exhibit 99.1 hereto shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “ Exchange Act ”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any of the Company’s filings under the Securities Act of 1933, as amended, or the Exchange Act, whether made before or after the date hereof, except as expressly set forth by specific reference in such a filing.

Other Events. On January 13, 2025, Lexeo Therapeutics, Inc. (the “ Company ”) posted on its website an updated corporate presentation (the “ Corporate Presentation ”). The Corporate Presentation will be used from time to time in meetings with investors and analysts. A copy of the Corporate Presentation is attached hereto as Exhibit 99.1 and is incorporated by reference herein. Cautionary Note Regarding Forward-Looking Statements This report contains certain forward-looking statements regardin…