Stock Incentive Plans (Tables)

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

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,
	2019		2018		2019		2018
	(in thousands)
Research and development	$	1,104	$	167	$	1,663	$	300
General and administrative		865		402		1,550		649
	$	1,969	$	569	$	3,213	$	949

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

The assumptions used in the Black-Scholes option pricing model for the three and six months ended June 30, 2019 and 2018 are as follows:

	Three months ended June 30,		Six months ended June 30,
	2019	2018	2019	2018
Expected volatility	62.69% — 63.10%	56.36%	62.69% — 64.68%	52.80% - 60.12%
Weighted-average risk-free interest rate	1.95% — 2.38%	2.84%	1.95% — 2.60%	2.33% - 2.84%
Expected dividend yield	— %	— %	— %	— %
Expected term (in years)	6.25	6.25	6.25	5.5 - 7.6
Underlying common stock fair value	$19.51 — $30.34	$20.98	$19.51 — $30.34	$6.63 - $20.98

Summary of Option Activity under Plans

A summary of option activity under the Plans during the six months ended June 30, 2019 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, 2019		3,203,122		$	10.47		8.9	$	39,018
Granted		760,355		$	25.02
Exercised		(31,280	)	$	2.43
Cancelled/Forfeited		(35,355	)	$	18.16
Outstanding at June 30, 2019		3,896,842		$	13.31		8.5	$	30,939
Vested and expected to vest at    June 30, 2019		3,896,842		$	13.31		8.5	$	30,939
Exercisable at June 30, 2019		1,229,412		$	7.36		7.8	$	15,531

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, 2019		151,289
Vested		(62,608	)
Issued		—
Repurchased		—
Unvested shares - June 30, 2019		88,681