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Stock-Based Compensation (Tables)
3 Months Ended
Mar. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Stock-Based Compensation Expense
The following stock-based compensation expense was recognized for the periods indicated (in thousands):
Three Months Ended March 31,
20232022
Research and development$2,461 $2,392 
Selling, general and administrative2,372 2,882 
Total stock-based compensation expense$4,833 $5,274 
Valuation Assumptions Using the Black-Scholes Option-Pricing Model The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table for options issued during the period indicated:
Three Months Ended March 31,
20232022
Expected dividend yield0%0%
Expected volatility
92.6% -94.3%
87.9% - 88.3%
Risk-free interest rate
3.6% - 4.2%
1.5% - 2.4%
Expected term5.88 years5.95 years
Schedule of Stock Options Roll Forward
The following table summarizes stock option activity during the three months ended March 31, 2023:
SharesWeighted-
Average
Exercise Price
Weighted-Average
Remaining
Contractual Term
(Years)
Aggregate
Intrinsic
Value
 (in thousands)
Outstanding, December 31, 202210,098,929 $18.58 6.5
Granted2,914,405 4.89 
Exercised(3,531)1.51 
Forfeited(87,556)11.46 
Expired(82,635)22.01 
Outstanding, March 31, 202312,839,612 $15.50 7.1$7,827 
As of March 31, 2023:
Exercisable7,048,779 $20.90 5.4660 
Vested and expected to vest11,763,189 $16.03 6.96,669 
Schedule of RSU Activity
The following table summarizes RSU activity during the three months ended March 31, 2023:
Shares
Weighted-Average
Grant Date Fair Value
Outstanding, December 31, 2022415,084 $8.83 
Granted656,135 4.82 
Vested(62,751)10.31 
Forfeited(27,657)6.94 
Outstanding, March 31, 2023980,811 6.11