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Stock-based compensation
 12 Months Ended
Dec. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Stock-based compensation Stock-based compensation
2015 Incentive Stock Option and Grant Plan
On December 28, 2015, the Company’s board of directors adopted the 2015 Incentive Stock Option and Grant Plan, or the 2015 Plan, and reserved 2,525,327 shares of common stock for issuance under this plan.
The 2015 Plan authorized the board of directors or a committee of the board to grant incentive stock options, nonqualified stock options and restricted stock awards to eligible employees, outside directors and consultants of the Company. Options granted under the 2015 Plan generally vest over a period of five or eight years with a cliff vesting at one year and quarterly vesting thereafter and options that lapse or are forfeited are available to be granted again. The contractual life of all options granted under the 2015 Plan is ten years from the date of grant.
2020 Stock Option and Incentive Plan
On September 8, 2020, the Company’s board of directors adopted the C4 Therapeutics, Inc. 2020 Stock Option and Incentive Plan, or the 2020 Plan, which became effective on September 30, 2020. Upon adoption there were 6,567,144 shares of common stock reserved for issuance under the 2020 Plan. The Company’s Board of Directors, the Compensation Committee of the Board of Directors, and, in certain contexts, the Chief Executive Officer of the Company are authorized to grant a broad range of stock-based awards under the 2020 Plan, including stock options, stock appreciation rights, or SARs, restricted stock awards, or RSAs, restricted stock units, or RSUs, performance awards and stock bonus awards to the Company’s officers, employees, directors and other key persons, including consultants.
Following the effectiveness of the 2020 Plan, the Company ceased making grants under the 2015 Plan. However, the 2015 Plan continues to govern the terms and conditions of the outstanding awards granted under it. Shares of common stock subject to awards granted under the 2015 Plan that cease to be subject to such awards by forfeiture or otherwise after the termination of the 2015 Plan will be available for issuance under the 2020 Plan. As of December 31, 2024, there were 3,281,795 shares available for future grant under the 2020 Plan.
The 2020 Plan provides for an annual increase, to be added on the first day of each fiscal year, beginning with January 1, 2021 and continuing until the expiration of the 2020 Plan, equal to the lesser of (i) 5% of the outstanding shares of common stock on the immediately preceding December 31st, or (ii) lesser number of shares determined by the administrator of the 2020 Plan, which is the Company’s Board of Directors or the Compensation Committee of the Board of Directors. On January 1, 2025, the annual increase for the 2020 Plan resulted in an additional 3,531,294 shares authorized for issuance being added to the 2020 Plan.
On March 7, 2024, the Company approved an option repricing program applicable to outstanding option awards granted to current employees of the Company under the 2020 Plan with an exercise price per share greater than or equal to $22.00. The repriced awards have new exercise prices of $11.88 per share for awards held by employees generally and $19.00 per share for awards held by members of the Company's senior leadership team. To receive the benefit of this reduced exercise price, holders of repriced option awards must not, prior to March 7, 2025, (i) voluntarily leave employment with the Company or (ii) exercise the repriced options. The repriced options otherwise remain on their existing terms and conditions as set forth in the 2020 Plan and applicable award agreements. During the year ended December 31, 2024, the Company recorded an incremental non-cash charge of $1.2 million related to this option repricing. On October 7, 2024, the 2020 Plan was amended to prohibit the plan's administrator from reducing the exercise price of outstanding stock options or stock appreciation rights or effecting repricing through cancellation and re-grant or cancellation in exchange for cash or other awards without prior stockholder approval.
Stock-based compensation expense was as follows (in thousands):
Years Ended December 31,
20242023
Stock-based compensation expense:
Research and development$12,539 $11,826 
General and administrative17,123 15,409 
Total stock-based compensation expense$29,662 $27,235 
The following table summarizes the stock option activity under the Company’s equity awards plans for the year ended December 31, 2024:
Number of
Options		Weighted-Average
Exercise Price		Weighted-Average
Remaining
Contractual
Term (in years)		Aggregate
Intrinsic
Value
(in thousands)
Outstanding as of December 31, 20239,475,009 $14.34 7.61$8,261 
Granted3,262,687 6.83 
Exercised(149,818)5.19 
Forfeited/expired(1,079,728)17.82 
Outstanding as of December 31, 202411,508,150 $12.00 7.70$2,877 
Exercisable as of December 31, 20245,579,212 $17.81 6.66$150 
Vested and expected to vest as of December 31, 202411,508,150 $12.00 7.70$2,877 
Other information related to the option activity of the Company was as follows:
Years Ended December 31,
20242023
Weighted-average fair value of options granted$5.59 $2.84 
Intrinsic value of options exercised (in thousands)$407 $22 
As of December 31, 2024, the unrecognized compensation cost related to outstanding options was $25.0 million, which is expected to be recognized over a weighted-average period of 2.0 years.
The following table summarizes assumptions used in the Black-Scholes option pricing model to determine the fair value of stock options granted to employees:
Years Ended December 31,
20242023
Expected option life (years)
5.50 - 6.11
5.50 - 6.11
Risk-free interest rate
3.56% - 4.46%
3.45% - 4.73%
Expected volatility
102.82% - 104.41%
81.99% - 86.15%
Expected dividend yield0.00%0.00%
Performance-based restricted stock units
In January and February 2022, the Company’s Board of Directors authorized an issuance of 563,500 performance-based restricted stock units, or PSUs, to certain employees, including members of the Company’s leadership team under the 2020 Plan. Vesting of the PSUs are contingent upon the determination of achievement of certain discovery and clinical milestones, or as specified market conditions are met. Upon vesting, each PSU converts automatically into one share of the Company’s common stock.
The following table summarizes the activity under the Company’s equity plans with respect to PSUs for the year ended December 31, 2024:
 SharesWeighted-Average
Grant Date
Fair Value
Outstanding as of December 31, 2023154,000 $19.47 
Granted— — 
Vested (1)
(56,000)21.74 
Forfeited— — 
Outstanding as of December 31, 202498,000 $18.17 
(1) Vested PSUs include 17,694 shares retained by the Company to cover statutory minimum withholding taxes.
As of December 31, 2024, the unrecognized compensation cost related to outstanding PSUs was $1.2 million which is expected to be recognized over a weighted-average period of 0.2 years.
Time-based restricted stock units
In January and February 2023, the Company’s Board of Directors authorized an issuance of 824,600 time-based restricted stock units, or RSUs, to employees under the 2020 Plan. Vesting of the RSUs are contingent to time-based vesting conditions. These RSUs are valued on the grant date using the grant date market price of the underlying shares. Upon vesting, each RSU automatically converts into one share of the Company’s common stock.
The following table summarizes the activity under the Company’s equity plans with respect to RSUs for the year ended December 31, 2024:
 SharesWeighted-Average
Grant Date
Fair Value
Outstanding as of December 31, 2023750,210 $5.35 
Granted1,090,150 7.22 
Vested(161,862)5.29 
Forfeited(173,209)6.19 
Outstanding as of December 31, 20241,505,289 $6.61 
As of December 31, 2024, the unrecognized compensation cost related to outstanding RSUs was $7.7 million which is expected to be recognized over a weighted-average period of 2.9 years.
Inducement grants
On three separate dates in April, September, and October 2024, the Company's Board of Directors approved the grant of non-qualified stock options to purchase shares of the Company's common stock to three new employees. Options granted under these these inducement grants vest over a period of four years with 25% of the award vesting on the first anniversary of the vesting start date and the balance vesting in equal quarterly installments thereafter.
Number of
Options		Weighted-Average
Exercise Price		Weighted-Average
Remaining
Contractual
Term (in years)		Aggregate
Intrinsic
Value
(in thousands)
Outstanding as of December 31, 2023351,000 $3.49 9.53$759,150 
Granted639,360 6.00 
Exercised— — 
Forfeited/expired— — 
Outstanding as of December 31, 2024990,360 $5.11 9.28$40 
Exercisable as of December 31, 2024109,687 $3.49 8.52$12 
Vested and expected to vest as of December 31, 2024990,360 $5.11 9.28$40 
Other information related to the option activity of the Company was as follows:
Years Ended December 31,
20242023
Weighted-average fair value of options granted$4.94 $— 
As of December 31, 2024, the unrecognized compensation cost related to outstanding options was $3.5 million, which is expected to be recognized over a weighted-average period of 3.4 years.
The following table summarizes assumptions used in the Black-Scholes option pricing model to determine the fair value of stock options granted to employees:
Years Ended December 31,
20242023
Expected option life (years)
6.08 - 6.11
6.11
Risk-free interest rate
3.63% - 4.64%
3.96% - 4.19%
Expected volatility
103.11% - 103.85%
83.35% - 83.50%
Expected dividend yield0.00%0.00%
2020 Employee Stock Purchase Plan
In September 2020, the Company’s Board of Directors adopted the C4 Therapeutics, Inc. 2020 Employee Stock Purchase Plan, or the 2020 ESPP. Eligible employees may authorize payroll deductions of up to 15% of their eligible compensation during an offering period. The Company may hold one or more offering periods each year during which employees will be able to purchase shares under the 2020 ESPP. The Company issued 110,895 shares during the year ended December 31, 2024.
As of December 31, 2024, the Company had 2,166,905 shares available for future issuance under the 2020 ESPP. The 2020 ESPP provides for an annual increase to be added on the first day of each fiscal year, beginning with January 1, 2021 and continuing thereafter through January 1, 2030, equal to the lesser of (i) 1% of the outstanding shares of common stock on the immediately preceding December 31st, (ii) 656,714 shares, or (ii) lesser number of shares determined by the administrator of the 2020 ESPP. On January 1, 2025, the annual increase for the 2020 ESPP resulted in an additional 656,714 shares authorized for issuance being added to the 2020 ESPP.