Stock Incentive Plans (Tables)

6 Months Ended

Jun. 30, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [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 June 30,		Six months ended June 30,
	2021	2020	2021	2020
Expected volatility	64.6% - 66.9%	64.8% — 65.4%	64.6% - 67.1%	63.2% — 65.4%
Weighted-average risk-free interest rate	0.90% - 1.20%	0.33% — 0.54%	0.50% - 1.20%	0.33% — 1.73%
Expected dividend yield	— %	— %	— %	— %
Expected term (in years)	5.5 - 6.25	5.5 - 6.25	5.5 - 6.25	5.5 - 6.25
Underlying common stock fair value	$6.53 - $9.62	$12.66 — $18.04	$6.53 - $13.91	$12.66 — $21.75

Summary of Option Activity under Plans

The following table summarizes the Company’s stock option activity for the six months ended June 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		1,902,700		$	12.84
Exercised		(59,348	)	$	2.30
Cancelled/Forfeited		(158,374	)	$	18.62
Outstanding at June 30, 2021		7,525,802		$	14.62		8.0	$	5,300
Vested and expected to vest at    June 30, 2021		7,525,802		$	14.62		8.0	$	5,300
Exercisable at June 30, 2021		3,716,324		$	13.21		7.0	$	5,131

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		(2,022	)
Issued		—
Repurchased		—
Unvested shares - June 30, 2021		1,069

Summary of Company's RSU Activity

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

	Number of Restricted Stock Units			Weighted- Average Grant Date Fair Value
Outstanding at January 1, 2021		—		$	—
Granted		277,400			13.85
Vested		—			—
Forfeited		(4,050	)		13.91
Outstanding at June 30, 2021		273,350		$	13.85

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 June 30,				Six months ended June 30,
	2021		2020		2021		2020
	(in thousands)
Research and development	$	2,303	$	1,754	$	4,159	$	3,068
General and administrative		2,170		1,698		4,197		3,268
	$	4,473	$	3,452	$	8,356	$	6,336