Stock Incentive Plans (Tables)

3 Months Ended

Mar. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

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

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

	Three months ended March 31,
	2020	2019
Expected volatility	63.2% — 63.5%	64.0% — 64.7%
Weighted-average risk-free interest rate	0.74% — 1.73%	2.48% — 2.60%
Expected dividend yield	— %	— %
Expected term (in years)	6.25	6.25
Underlying common stock fair value	$17.00 — $21.75	$21.43 — $28.13

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 March 31,
	2020		2019
	(in thousands)
Research and development	$	1,313	$	558
General and administrative		1,570		685
	$	2,883	$	1,243

Summary of Option Activity under Plans

A summary of option activity under the Plans during the three months ended March 31, 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		666,550		$	21.40
Exercised		(34,311	)	$	3.93
Cancelled/Forfeited		(27,442	)	$	19.57
Outstanding at March 31, 2020		5,447,815		$	15.63		8.5	$	19,541
Vested and expected to vest at    March 31, 2020		5,447,815		$	15.63		8.5	$	19,541
Exercisable at March 31, 2020		1,925,388		$	10.05		7.5	$	14,249

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		(15,542	)
Issued		—
Repurchased		—
Unvested shares - March 31, 2020		6,124