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Stock-based compensation (Tables)
 12 Months Ended
Dec. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Schedule of stock-based compensation expense
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 
Summary of stock option activity
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 
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 $— 
Summary of significant valuation assumptions
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%
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%
Summary of RSU activity
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.
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.