Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 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 September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Research and development	$	2,335	$	2,571	$	6,996	$	7,621
Selling, general and administrative	2,455		2,552		7,018		8,076
Total stock-based compensation expense	$	4,790	$	5,123	$	14,014	$	15,697

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,
	2023	2022
Expected dividend yield	0%	0%
Expected volatility	76.1% -94.9%	87.8% - 90.8%
Risk-free interest rate	3.5% - 4.4%	1.4% - 3.6%
Expected term	5.88 years	5.95 years

Schedule of Stock Options Roll Forward

The following table summarizes stock option activity during the nine months ended September 30, 2023:

	Shares	Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value  (in thousands)
Outstanding, December 31, 2022	10,098,929	$	18.58	6.5
Granted	3,267,727	4.91
Exercised	(26,146)	3.34
Forfeited	(293,161)	9.78
Expired	(367,054)	19.77
Outstanding, September 30, 2023	12,680,295	15.26		6.7	$	301
As of September 30, 2023:
Exercisable	7,836,655	19.50		5.4	186
Vested and expected to vest	11,733,021	15.75		6.5	278

Schedule of RSU Activity

The following table summarizes RSU activity during the nine months ended September 30, 2023:

	Shares	Weighted-Average Grant Date Fair Value
Outstanding, December 31, 2022	415,084	$	8.83
Granted	696,635	4.84
Vested	(135,396)	8.32
Forfeited	(68,046)	7.53
Outstanding, September 30, 2023	908,277	5.95