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Stock-Based Compensation (Tables)
6 Months Ended
Jun. 30, 2023
Share-Based Payment Arrangement [Abstract]  
Summary of Option Activity
A summary of option activity under the Plans for the six months ended June 30, 2023 follows:
(in thousands, except for share and per share amounts)Number of
Shares
Weighted Average
Exercise Price at
Grant Date
Weighted Average
Remaining Term
(Years)
Aggregate
Intrinsic
Value
Outstanding at December 31, 20224,232,210 $14.19 5.81$— 
Granted944,776 6.92 
Expired(130,417)12.38 
Outstanding at June 30, 20235,046,569 12.88 6.2026 
Exercisable at June 30, 20233,321,471 14.26 4.76— 
Vested and expected to vest at June 30, 20234,763,321 13.15 6.0120 
Summary of RSU Activity
A summary of RSU activity for the Plans for the six months ended June 30, 2023 is as follows:
Number of
Shares
Weighted
Average
Grant Date Fair Value
Unvested at December 31, 20221,915,546 $14.41 
Granted852,243 6.97 
Forfeited(53,969)13.13 
Vested(713,149)15.10 
Unvested at June 30, 20232,000,671 11.03 
Stock-Based Compensation Expense
Stock-based compensation expense recognized for the three and six months ended June 30, 2023 and 2022 was comprised of the following:
 Three Months EndedSix Months Ended
(in thousands)June 30,
2023
June 30,
2022
June 30,
2023
June 30,
2022
Research and development$733 $899 $1,799 $2,059 
Selling, general and administrative2,516 2,931 5,801 6,549 
Total stock-based compensation expense$3,249 $3,830 $7,600 $8,608 
Black-Scholes-Merton Option Pricing Model for Employee and Director Stock Options Granted Assumptions used in the Black-Scholes-Merton option pricing model for employee and director stock options granted during the six months ended June 30, 2023 and 2022 were as follows:
 Six Months Ended
June 30, 2023June 30, 2022
Expected dividend yield%%
Weighted average expected volatility47 %46 %
Weighted average expected term (years)6.166.05
Weighted average risk-free rate3.89 %2.03 %