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Stock-based compensation (Tables)
 12 Months Ended
Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Schedule of stock-based compensation expense
Stock-based compensation expense was as follows (in thousands):
Years Ended December 31,
20232022
Stock-based compensation expense:
Research and development$11,826 $13,108 
General and administrative15,409 16,908 
Total stock-based compensation expense$27,235 $30,016 
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, 2023:
Number of
Options		Weighted-Average
Exercise Price		Weighted-Average
Remaining
Contractual
Term (in years)		Aggregate
Intrinsic
Value
(in thousands)
Outstanding as of December 31, 20226,993,052 $21.57 7.78$1,031 
Granted4,180,270 3.87 
Exercised(13,096)4.46 
Forfeited/expired(1,685,217)18.48 
Outstanding as of December 31, 20239,475,009 $14.34 7.61$8,261 
Exercisable as of December 31, 20233,838,707 $20.54 6.68$482 
Vested and expected to vest as of December 31, 20239,475,009 $14.34 7.61$8,261 
Other information related to the option activity of the Company was as follows:
Years Ended December 31,
20232022
Weighted-average fair value of options granted$2.84 $13.44 
Intrinsic value of options exercised (in thousands)$22 $1,796 
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,
20232022
Expected option life (years)
5.50 - 6.11
5.50 - 6.11
Risk-free interest rate
3.45% - 4.73%
1.47% - 4.22%
Expected volatility
81.99% - 86.15%
81.36% - 88.75%
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, 2023:
 SharesWeighted-Average
Grant Date
Fair Value
Outstanding as of December 31, 2022465,291 $25.09 
Granted— — 
Vested (1)
(135,301)26.52 
Forfeited(175,990)28.91 
Outstanding as of December 31, 2023154,000 $19.47 
(1) Vested PSUs include 17,556 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, 2023:
 SharesWeighted-Average
Grant Date
Fair Value
Outstanding as of December 31, 2022— $— 
Granted824,600 5.37 
Vested— — 
Forfeited(74,390)5.67 
Outstanding as of December 31, 2023750,210 $5.35 
As of December 31, 2023, the unrecognized compensation cost related to outstanding RSUs was $3.5 million which is expected to be recognized over a weighted-average period of 3.2 years.