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Stock-based compensation (Tables)
9 Months Ended
Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]  
Schedule of Stock Option Activity Under Plan A summary of stock option activity is as follows:
Number
of Shares
Weighted-
Average
Exercise Price
Outstanding at January 1, 202310,051,283 $9.84 
Granted3,972,166 $4.80 
Exercised(97,596)$4.37 
Cancelled/forfeited(1,747,727)$10.62 
Outstanding at September 30, 202312,178,126 $8.13 
Exercisable at September 30, 20237,130,744 $9.25 
Schedule of Restricted Stock Activity A summary of the RSU activity is as follows:
Number of Shares
Unvested at January 1, 20231,870,791 
Granted3,982,567 
Vested(532,192)
Forfeited(1,198,801)
Unvested at September 30, 20234,122,365 
Schedule of Stock-Based Compensation Expense by Award Type
The following table presents stock-based compensation expense by award type included within the Company’s condensed consolidated statements of operations and comprehensive loss:
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2023202220232022
Stock options$3,211 $3,901 $11,718 $11,952 
Restricted stock units694 1,365 4,647 3,832 
Employee stock purchase plan96 109 686 424 
Stock-based compensation expense included in total operating expenses$4,001 $5,375 $17,051 $16,208 
Schedule of Stock-Based Compensation Expense as Reflected in the Company's Condensed Consolidated Statements of Operations and Comprehensive Loss The following table presents stock-based compensation expense as reflected in the Company’s condensed consolidated statements of operations and comprehensive loss:
Three Months Ended September 30,
Nine Months Ended September 30,
(in thousands)2023202220232022
Research and development$2,153 $2,890 $8,881 $8,569 
General and administrative1,848 2,485 8,170 7,639 
Stock-based compensation expense included in total operating expenses$4,001 $5,375 $17,051 $16,208 
Schedule of Weighted Average Assumptions for Estimating Fair Value of Option Awards The fair value of each option award is estimated on the date of grant using the Black-Scholes option pricing model with the following weighted average assumptions:
Three Months Ended
September 30,
Nine Months Ended September 30,
2023202220232022
Risk-free interest rate4.4 %2.9 %3.8 %2.0 %
Expected dividend yield— %— %— %— %
Expected term (years)6.096.116.065.99
Expected stock price volatility113 %94 %103 %88 %