Stock Incentive Plans (Tables)

3 Months Ended

Mar. 31, 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 March 31,
	2019		2018
	(in thousands)
Research and development	$	558	$	133
General and administrative		685		247
	$	1,243	$	380

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 months ended March 31, 2019 and 2018 are as follows:

	Three months ended March 31,
	2019	2018
Expected volatility	64.03% — 64.68%	52.80% — 60.12%
Weighted-average risk-free interest rate	2.48% — 2.60%	2.33% — 2.69%
Expected dividend yield	— %	— %
Expected term (in years)	6.25	5.5 — 7.6
Underlying common stock fair value	$21.43 — $28.13	$6.63 — $16.00

Summary of Option Activity under Plans

A summary of option activity under the Plans during the three months ended March 31, 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		460,575		$	25.95
Exercised		(18,876	)	$	1.09
Cancelled/Forfeited		(12,000	)	$	21.43
Outstanding at March 31, 2019		3,632,821		$	12.44		8.8	$	55,536
Vested and expected to vest at    March 31, 2019		3,632,821		$	12.44		8.8	$	55,536
Exercisable at March 31, 2019		1,011,546		$	6.36		8.1	$	21,619

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		(31,584	)
Issued		—
Repurchased		—
Unvested shares - March 31, 2019		119,705