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Stock Incentive Plans
 3 Months Ended
Mar. 31, 2025
Share-Based Payment Arrangement [Abstract]  
Stock Incentive Plans Stock Incentive Plans
The Company maintained the 2008 Stock Incentive Plan, or the 2008 Plan, for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non-qualified stock option and restricted stock awards as determined by the Board of Directors. In connection with the Merger, all outstanding awards issued under the 2008 Plan were cancelled, and the Board of Directors formally terminated the 2008 Plan.
In June 2016, the Company’s stockholders approved the 2016 Incentive Award Plan, or the 2016 Plan, which authorized 40,341 shares of common stock for future issuance under the 2016 Plan and the Company ceased granting awards under the 2008 Plan. Upon the effective date of the 2016 Plan, awards issued under the 2008 Plan remained subject to the terms of the 2008 Plan. Awards granted under the 2008 Plan that expired, lapsed or terminated became available under the 2016 Plan as shares available for future grants.
Additionally, pursuant to the terms of the 2016 Plan, the Board of Directors is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board of Directors or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board of Directors established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. In June 2024, the Company’s stockholders approved an amendment and restatement of the 2016 Plan to reserve an additional 3,466,544 shares of the Company’s common stock for issuance. In January 2025, the number of shares of common stock that may be issued under the 2016 Plan was increased by 1,030,694. As of March 31, 2025, 3,598,149 shares remain available for future issuance under the 2016 Plan.
In September 2018, the Company’s 2018 Employment Inducement Incentive Award Plan, or the 2018 Inducement Incentive Award Plan, was adopted by the Board of Directors without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules, which authorized 39,166 shares of its common stock for issuance. In March 2019, the Board of Directors approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 66,667 shares of the Company’s common stock for issuance thereunder. In December 2023, the Board of Directors approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 60,833 shares of the Company’s common stock for issuance thereunder. In June and December 2024, the Board of Directors approved amendments and restatements of the 2018 Inducement Incentive Award Plan to reserve an additional 360,000 and 450,000 shares, respectively, of the Company’s common stock for issuance thereunder. As of March 31, 2025, there are 489,683 shares available for future grant under the 2018 Inducement Incentive Award Plan.
In accordance with the Merger Agreement, the Company assumed Old Cartesian’s 2016 Stock Incentive Plan, or the Old Cartesian Plan. The Old Cartesian Plan permits the granting of options or restricted stock to employees, officers, directors, consultants and advisors to the Company. The unvested common stock options and Series A Preferred Stock options assumed by the Company in connection with the Merger generally vest over a four-year period. Additionally, the stock options granted have a contractual term of ten years and only full shares can be exercised as per the individual award agreements. As of March 31, 2025, there are 36,179 shares available for future grant under the Old Cartesian Plan.
In connection with the Merger, the outstanding stock options to purchase Old Cartesian common stock were converted into stock options to purchase 776,865 shares of common stock and 14,112.299 shares of Series A Preferred Stock of the Company. These replacement awards were revalued at their acquisition-date fair value and then attributed to pre- and post-combination service. This resulted in $2.6 million attributed to post-combination service to be recognized as stock-based compensation expense over the remaining terms of the replacement awards, of which $0.2 million and $0.4 million was recognized as research and development expense in the consolidated statements of operations and comprehensive loss during the three months ended March 31, 2025 and 2024, respectively. Following the stockholder approval of the conversion of the Series A Preferred Stock into shares of common stock, the options exercisable for shares of Series A Preferred Stock became exercisable for shares of common stock.
Stock-Based Compensation Expense
Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss, was as follows (in thousands):
 Three Months Ended
March 31,
 20252024
Research and development$1,275 $712 
General and administrative1,233 719 
Total stock-based compensation expense$2,508 $1,431 

Stock Options
The estimated grant date fair values of stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model based on the following weighted-average assumptions:
Three Months Ended
March 31,
 20252024
Risk-free interest rate4.46 %3.95 %
Dividend yield— — 
Expected term (in years)6.206.20
Expected volatility97.44 %95.37 %
Weighted-average fair value of common stock$17.46 $19.78 
The expected term of the Company’s stock options granted has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Under the simplified method, the expected term is presumed to be the midpoint between the vesting date and the end of the contractual term. The Company utilizes this method due to lack of historical exercise data and the plain nature of its stock-based awards. Expected volatilities are based on the Company’s historical volatility.
The weighted-average grant date fair value of stock options granted during the three months ended March 31, 2025 and 2024 was $14.03 and $15.79, respectively.
As of March 31, 2025, total unrecognized compensation expense related to unvested common stock options was $17.9 million, which is expected to be recognized over a weighted average period of 3.2 years.
The following table summarizes the stock option activity under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and the Old Cartesian Plan for options for common stock:
   Weighted-average 
 Number ofremainingAggregate
 common stockWeighted-averagecontractual termintrinsic value
 optionsexercise price ($)(in years)(in thousands)
Outstanding at December 31, 20241,706,035 $11.99 7.59$12,025 
Granted944,909 $17.46   
Exercised(55,690)$3.29   
Forfeited(120,231)$18.49   
Outstanding at March 31, 20252,475,023 $13.96 8.17$7,376 
Vested at March 31, 2025781,188 $6.23 5.67$6,490 
Vested and expected to vest at March 31, 20252,181,547 $13.38 8.00$7,376 

Restricted Stock Units
During the three months ended March 31, 2025, the Company granted 256,790 restricted stock unit awards with a weighted-average fair value of $16.93 per share based on the closing price of the Company’s common stock on the date of grant under the 2016 Plan, which generally vest over a four-year term. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% for restricted stock unit awards based on historical experience.
Unrecognized compensation expense related to the restricted stock units was $8.0 million as of March 31, 2025, which is expected to be recognized over a weighted-average period of 3.0 years.
The following table summarizes the Company’s restricted stock units under the 2016 Plan and the Old Cartesian Plan:
 Number of sharesWeighted-average
grant date
fair value ($)
Unvested at December 31, 2024
444,238 $19.86 
Granted256,790 16.93 
Vested(113,042)19.66 
Forfeited(15,381)17.15 
Unvested at March 31, 2025
572,605 $18.63