9. Stock-based Compensation

In February 2021, the Company adopted the 2021 Plan for the issuance of stock options granted to the Company’s key directors, officers, employees and consultants, as a means to secure the benefits arising from capital stock ownership. In connection with the Company’s IPO in November 2023, the Company adopted the 2023 Plan for the issuance of equity awards granted to the Company's directors, officers, employees, and consultants, and for the issuance of shares purchased under the 2023 ESPP to the Company's employees, as a means to secure the benefits arising from capital stock ownership. The purposes of the Plans and the 2023 ESPP are also to promote the alignment of the interests of directors, officers, employees, and consultants with the success of the Company and to provide compensation opportunities to attract, retain and motivate directors, officers, employees, and consultants of the Company. Upon adoption of the 2023 Plan, no further grants may be made under the 2021 Plan.

The number of shares of common stock reserved for issuance under the 2023 Plan will automatically increase on January 1 of each year, beginning on January 1, 2024, and continuing through and including January 1, 2033, by 5% of the total number of shares of common stock outstanding on December 31 of the immediately preceding calendar year, or a lesser number of shares determined by the Company’s Board of Directors prior to the applicable January 1. The number of shares of common stock reserved for issuance under the 2023 ESPP will automatically increase on January 1 of each calendar year, beginning on January 1, 2024 and continuing through and including January 1, 2033, by the lesser of (i) 1% of the total number of shares of capital stock outstanding on December 31 of the preceding calendar year, (ii) 477,200 shares, and (iii) a number of shares determined by the Company’s Board of Directors. Shares subject to purchase rights granted under the 2023 ESPP that terminate without having been exercised in full do not reduce the number of shares available for issuance under the 2023 ESPP.

On January 1, 2026, the number of shares of common stock reserved for issuance under the 2023 Plan and the 2023 ESPP automatically increased by 3,650,088 shares and 477,200 shares, respectively, to totals of 6,804,770 shares and 1,210,439 shares, respectively. As of March 31, 2026, 3,007,796 shares and 1,158,154 shares were available for future issuance under the 2023 Plan and 2023 ESPP, respectively. A total of 155,122 shares have been issued under the 2023 ESPP through March 31, 2026.

On November 4, 2025, the Company's board of directors adopted the Lexeo Therapeutics, Inc. 2025 Inducement Equity Incentive Plan (the "2025 Inducement Equity Plan") and, subject to the adjustment provisions of the 2025 Inducement Equity Plan, reserved 2,000,000 shares of the Company's common stock for issuance pursuant to equity awards granted under the 2025 Inducement Equity Plan.

Stock option activity

Stock options granted under the 2021 Plan are issued from new shares upon exercise, have a contractual term of 10 years from grant date, and generally (i) are subject to requisite service requirements, (ii) vest over a four-year period with 25% of the options granted vesting after one year and the remainder vesting in equal monthly installments over the following 36 months, and (iii) allow for early exercise subject to repurchase. Stock options granted under the 2021 Plan to certain of the Company’s non-employees vest in equal monthly installments over a four-year period or vested upon the achievement of a certain milestone event. The Company did not repurchase any shares of its common stock during the three months ended March 31, 2026 or the three months ended March 31, 2025.

Stock options granted under the 2023 Plan are issued from new shares upon exercise, have a contractual term of 10 years from grant date, and generally vest over a four-year period with 25% of the options granted vesting after approximately one year and the remainder vesting in equal monthly installments over the following 36 months, subject to requisite service requirements. Stock options granted under the 2023 Plan to certain of the Company's non-employee directors vest in equal annual installments over a three-year period for initial grants or over a one-year period for subsequent annual grants, subject to requisite service requirements.

The weighted-average grant date fair value of stock options granted during the three months ended March 31, 2026 and March 31, 2025 was $5.42 per share and $4.51 per share, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. No stock options were exercised during the three months ended March 31, 2025.

The total grant date fair values of stock options vested during the three months ended March 31, 2026 and March 31, 2025 were $4.0 million and $4.0 million, respectively.

The expected dividend yields are 0.00% as the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

As of March 31, 2026, there was $15.7 million of unrecognized stock-based compensation expense related to stock options estimated to be recognized over a weighted-average period of 3.28 years.

RSU and PRSU activity

RSUs granted under the 2023 Plan generally vest over a four-year period with 25% of the RSUs vesting after approximately one year and the remainder vesting in equal quarterly installments over the following 12 quarters, subject to requisite service requirements. Additionally, during the three months ended March 31, 2026, a total of 243,275 PRSUs were granted to employees under the 2023 Plan that vest upon the achievement of certain performance conditions over periods ranging from approximately four months to approximately 11 months, subject to requisite service requirements.

As of March 31, 2026, there was $6.4 million of unrecognized stock-based compensation expense related to RSUs estimated to be recognized over a weighted-average period of 3.36 years.

Employee Stock Purchase Plan

Under the 2023 ESPP, eligible employees may purchase shares of the Company's common stock through payroll deductions at a price equal to 85% of the lower of the fair market values of the stock as of the beginning of the offering period or the end of each six-month purchase period. Each offering period of two years will consist of four six-month purchase periods, subject to reset on the first day of each purchase period depending on the fair market values of the Company's stock. Employees may not purchase more than 6,000 shares of the Company's common stock during any purchase period. The first purchase period within the first offering period under the 2023 ESPP commenced during the three months ended December 31, 2024 and concluded during the three months ended March 31, 2025; following reset, the first purchase period within the second offering period commenced during the three months ended March 31, 2025 and concluded during the three months ended September 30, 2025; and the second purchase period within the second offering period commenced during the three months ended September 30, 2025 and concluded during the three months ended March 31, 2026. During the three months ended March 31, 2026, a total of 52,285 shares were issued under the 2023 ESPP at a weighted-average purchase price of $2.74 per share with total proceeds of approximately $0.1 million received by the Company. During the three months ended March 31, 2025, a total of 65,342 shares were issued under the 2023 ESPP at a weighted-average purchase price of $2.61 per share with total proceeds of approximately $0.2 million received by the Company. As of and during the three months ended March 31, 2026 and March 31, 2025, stock-based compensation expense recorded, as well as the estimated fair value of unearned and unrecognized stock-based expense expected to be recorded, related to the 2023 ESPP were not material.

Stock-based compensation expense