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Stock-based Compensation
 6 Months Ended
Jun. 30, 2024
Share-Based Payment Arrangement [Abstract]  
Stock-based Compensation Stock-based Compensation
2017 Stock Incentive Plan
The Company adopted the CARISMA Therapeutics Inc. 2017 Stock Incentive Plan, as amended (the Legacy Carisma Plan), that provided for the grant of incentive stock options to employees, directors, and consultants. The maximum term of options granted under the Legacy Carisma Plan was ten years, and stock options typically vested over a four-year period. The Company’s stock options vest based on the terms in the awards agreements and generally vest over four years. Upon completion of the Merger, the Company assumed the Legacy Carisma Plan and the outstanding and unexercised options issued thereunder, and ceased granting awards under the Legacy Carisma Plan.
2014 Stock Incentive Plan
Stock options outstanding under the 2014 Amended and Restated Stock Incentive Plan (the 2014 Plan) generally vest over a four-year period and are exercisable for a period of ten years from the date of grant. As of June 30, 2024, approximately 3.8 million shares of common stock remained available for issuance.
2014 Employee Stock Purchase Plan
The Carisma Therapeutics Inc. 2014 Employee Stock Purchase Plan had 0.2 million shares of common stock available for issuance as of June 30, 2024.
The following table summarizes stock option activity for the six months ended June 30, 2024:
OptionsWeighted
average
exercise
price 		Weighted
average
remaining
contractual
term (years)		Aggregate
Intrinsic
Value (in
thousands)
Outstanding as of December 31, 20236,023,370$3.94 
Exercised(3,810)0.99 $4 
Granted3,674,3651.77 
Forfeited (1,008,687)4.70 
Outstanding as of June 30, 20248,685,238$2.93 8.14$2,018 
Exercisable as of June 30, 20243,487,670$2.19 6.40$1,517 
The weighted-average grant-date per share fair values of options granted during the six months ended June 30, 2024 and 2023 were $1.45 and $4.43, respectively. The fair values in the six months ended June 30, 2024 and 2023 were estimated using the Black-Scholes option-pricing model based on the following assumptions:
Six Months Ended June 30,
20242023
Risk-free interest rate
3.77% - 4.59%
2.92% - 4.03%
Expected term6 years6 years
Expected volatility
103.00% - 107.10%
57.77% - 65.00%
Expected dividend yield— — 
Stock-Based Compensation Expense
The Company recorded stock-based compensation expense in the following expense categories in its accompanying unaudited interim consolidated statements of operations:
Three Months Ended June 30,Six Months Ended June 30,
2024202320242023
Research and development$(44)$24 $392 $34 
General and administrative669 306 1,290 561 
$625 $330 $1,682 $595 
In connection with the revised operating plan, 869,800 options were forfeited during the three months ended June 30, 2024, resulting in a reduction in stock-based compensation expense related to research and development and general and administrative employees. The Company recognized stock-based compensation expense of $0.2 million related to the modification of Sesen Bio options assumed in connection with the Merger during six months ended June 30, 2023. Compensation cost for awards not vested as of June 30, 2024 was $11.8 million and will be expensed over a weighted-average period of 2.9 years