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Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2021

Stock Options

Stock option activity under the Options Plan is set forth below for the year ended December 31, 2021:

	Number of Shares Outstanding			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Balances at December 31, 2020		1,481,100		$	16.13		8.36	$	11,123
Stock options granted		794,095			21.62
Stock options exercised		(120,178	)		9.88
Stock options forfeited		(130,336	)		21.30
Balances at December 31, 2021		2,024,681		$	18.33		8.04	$	38,417
Vested and expected to vest at December 31, 2021		2,024,681		$	18.33		8.04	$	38,417
Exercisable at December 31, 2021		1,028,650		$	14.75		7.41	$	23,142

Summary of RSU Activity

Restricted Stock Units (“RSU”) activity under the 2019 Plan is set forth below for the year ended December 31, 2021:

	Number of Shares Outstanding			Weighted Average Grant Price Fair Value Per Share
RSUs outstanding at December 31, 2020		56,500			44.66
RSUs awarded		—			—
RSUs released		(14,125	)		44.66
RSUs forfeited		(1,875	)		44.66
RSUs outstanding at December 31, 2021		40,500			44.66

Summary of Assumptions Used to Value Under Stock Option Plans

The fair value of the rights granted to employees under the 2019 ESPP was estimated using a Black-Scholes option-pricing model with the following valuation assumptions:

	Year Ended December 31,
	2021	2020
Expected term (in years)	0.25 - 2.00	0.25 - 2.00
Volatility	71.3% - 92.9%	84.0%
Risk-free interest rate	0.05% - 1.55%	0.11% - 1.57%
Dividend yield	—	—

Summary of Stock-based Compensation Expense

Total stock-based compensation expense recognized for stock options and RSUs granted to both employees and non-employees and for the employee stock purchase plan was as follows (in thousands):

	Year Ended December 31,
	2021		2020
Research and development	$	5,310	$	4,637
General and administrative		4,642		4,049
Total stock-based compensation expense	$	9,952	$	8,686

Employee and Non-employees

Summary of Assumptions Used to Value Under Stock Option Plans

The assumptions used to value employee and non-employee stock option awards granted under the Option Plans during the years ended December 31, 2021 and 2020, using the Black-Scholes option pricing model, were as follows:

	Year Ended December 31,
	2021	2020
Expected term (in years)	3.93 - 6.05	5.19 - 6.07
Volatility	81.0% - 86.9%	84.0% - 87.0%
Risk-free interest rate	0.55% - 1.26%	0.28% - 1.43%
Dividend yield	—	—