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Stock-Based Compensation (Tables)
9 Months Ended
Sep. 30, 2022
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 September 30,Nine Months Ended September 30,
2022202120222021
Research and development$2,571 $3,109 $7,621 $8,894 
Selling, general and administrative2,552 3,249 8,076 8,820 
Total stock-based compensation expense$5,123 $6,358 $15,697 $17,714 
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:
Nine Months Ended September 30,
20222021
Expected dividend yield0%0%
Expected volatility
87.8% -90.8%
86.2% - 87.4%
Risk-free interest rate
1.4% - 3.6%
0.6% - 1.4%
Expected term5.95 years6.25 years
Schedule of Stock Options Roll Forward
The following table summarizes stock option activity during the nine months ended September 30, 2022:
SharesWeighted-
Average
Exercise Price
Weighted-Average
Remaining
Contractual Term
(Years)
Aggregate
Intrinsic
Value
 (in thousands)
Outstanding, December 31, 20218,373,921 $21.47 6.6
Granted2,730,597 9.40 
Exercised(82,963)1.46 
Forfeited(427,074)16.05 
Expired(267,238)22.76 
Outstanding, September 30, 202210,327,243 $18.63 6.7$124 
As of September 30, 2022:
Exercisable6,455,853 $21.77 5.596 
Vested and expected to vest9,622,594 $18.96 6.6120 
Schedule of RSU Activity
The following table summarizes RSU activity during the nine months ended September 30, 2022:
Shares
Weighted-Average
Grant Date Fair Value
Outstanding, December 31, 202121,500 $25.97 
Granted476,772 8.42 
Exercised(10,445)25.65 
Forfeited(41,000)10.15 
Outstanding, September 30, 2022446,827 8.70