Stock Incentive Plans (Tables)

9 Months Ended

Sep. 30, 2021

Share-based Payment Arrangement [Abstract]

Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model

The assumptions used in the Black-Scholes option pricing model are as follows:

	Three months ended September 30,			Nine months ended September 30,
	2021	2020	2021		2020
Expected volatility	66.3% - 71.7%	64.6% - 65.2%	64.6% - 71.7%		63.2% - 65.4%
Weighted-average risk-free interest rate	0.81% - 1.03%	0.31% - 0.40%	0.50% - 1.20%		0.31% - 1.73%
Expected dividend yield	— %	— %	— %		— %
Expected term (in years)	6.25	6.0 - 6.25	5.5 - 6.25		5.5 - 6.25
Underlying common stock fair value	$6.36-$7.75	$10.10-$16.48	$6.36-$13.91		$10.10-$21.75

Summary of Option Activity under Plans

The following table summarizes the Company’s stock option activity for the nine months ended September 30, 2021:

	Number of Options			Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (in Years)		Aggregate Intrinsic Value
								(in thousands)
Outstanding at January 1, 2021		5,840,824		$	15.18		7.8	$	12,278
Granted		2,053,375		$	12.39
Exercised		(59,465	)	$	2.29
Cancelled/Forfeited		(303,825	)	$	17.59
Outstanding at September 30, 2021		7,530,909		$	14.43		7.7	$	6,607
Vested and expected to vest at    September 30, 2021		7,530,909		$	14.43		7.7	$	6,607
Exercisable at September 30, 2021		4,113,985		$	13.46		6.8	$	6,092

Summary of Unvested Common Stock from Early Exercises Subject to Repurchase

A summary of the Company’s unvested common stock from early exercises that is subject to repurchase by the Company is as follows:

	Shares
Unvested shares - January 1, 2021		3,091
Vested		(2,795	)
Unvested shares - September 30, 2021		296

Summary of Company's RSU Activity

The following table summarizes the Company’s RSU activity for the nine months ended September 30, 2021:

	Number of Restricted Stock Units			Weighted- Average Grant Date Fair Value
Outstanding at January 1, 2021		—		$	—
Granted		313,700			12.98
Vested		—			—
Forfeited		(13,500	)		13.63
Outstanding at September 30, 2021		300,200		$	12.95

Schedule of Stock-Based Compensation Expense

The Company recorded stock-based compensation expense related to stock options, shares purchased under the 2018 ESPP and restricted stock units as follows:

	Three months ended September 30,				Nine months ended September 30,
	2021		2020		2021		2020
	(in thousands)
Research and development	$	2,145	$	1,572	$	6,304	$	4,640
General and administrative		2,081		1,774		6,278		5,042
	$	4,226	$	3,346	$	12,582	$	9,682