Stock Incentive Plans (Tables)

9 Months Ended

Sep. 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 September 30,				Nine months ended September 30,
	2019		2018		2019		2018
	(in thousands)
Research and development	$	1,124	$	326	$	2,786	$	626
General and administrative		863		393		2,413		1,042
	$	1,987	$	719	$	5,199	$	1,668

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 nine months ended September 30, 2019 and 2018 are as follows:

	Three months ended September 30,		Nine months ended September 30,
	2019	2018	2019	2018
Expected volatility	62.4% — 63.2%	56.6% - 56.7%	62.4% — 64.7%	52.8% - 60.1%
Weighted-average risk-free interest rate	1.36% — 1.94%	2.77% - 3.00%	1.36% — 2.60%	2.33% - 3.00%
Expected dividend yield	— %	— %	— %	— %
Expected term (in years)	6.25	6.25	6.25	5.5 - 7.6
Underlying common stock fair value	$15.92 — $21.45	$15.96 - $20.41	$15.92 — $30.34	$6.63 - $20.98

Summary of Option Activity under Plans

A summary of option activity under the Plans during the nine months ended September 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		854,315		$	24.36
Exercised		(74,709	)	$	3.81
Cancelled/Forfeited		(106,300	)	$	18.17
Outstanding at September 30, 2019		3,876,428		$	13.45		8.4	$	26,517
Vested and expected to vest at    September 30, 2019		3,876,428		$	13.45		8.4	$	26,517
Exercisable at September 30, 2019		1,423,217		$	8.14		7.7	$	15,263

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		(78,712	)
Issued		—
Repurchased		—
Unvested shares - September 30, 2019		72,577