Stock Incentive Plans (Tables)

6 Months Ended

Jun. 30, 2020

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,
	2020	2019	2020	2019
Expected volatility	64.8% — 65.4%	62.7% — 63.1%	63.2% — 65.4%	62.7% - 64.7%
Weighted-average risk-free interest rate	0.33% — 0.54%	1.95% — 2.38%	0.33% — 1.73%	1.95% - 2.60%
Expected dividend yield	— %	— %	— %	— %
Expected term (in years)	5.5 - 6.25	6.25	5.5 - 6.25	6.25
Underlying common stock fair value	$12.66 — $18.04	$19.51 — $30.34	$12.66 — $21.75	$19.51 — $30.34

Schedule of Stock-Based Compensation Expense

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

	Three months ended June 30,				Six months ended June 30,
	2020		2019		2020		2019
	(in thousands)
Research and development	$	1,754	$	1,104	$	3,068	$	1,663
General and administrative		1,698		865		3,268		1,550
	$	3,452	$	1,969	$	6,336	$	3,213

Summary of Option Activity under Plans

A summary of option activity under the Plans during the six months ended June 30, 2020 is as follows:

	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, 2020		4,843,018		$	14.78		8.5	$	33,809
Granted		1,006,750		$	19.29
Exercised		(37,887	)	$	3.96
Cancelled/Forfeited		(115,982	)	$	22.86
Outstanding at June 30, 2020		5,695,899		$	15.49		8.3	$	19,188
Vested and expected to vest at    June 30, 2020		5,695,899		$	15.49		8.3	$	19,188
Exercisable at June 30, 2020		2,309,829		$	11.19		7.4	$	14,800

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, 2020		21,666
Vested		(16,553	)
Issued		—
Repurchased		—
Unvested shares - June 30, 2020		5,113