Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Assumptions that Used to Determine Grant-Date Fair Value of Stock Options Granted to Employees and Members of Board

The assumptions that the Company used to determine the grant-date fair value of stock options granted to employees and members of the Board were as follows, presented on a weighted-average basis:

	Nine Months Ended September 30,
	2019			2018
Expected option life (years)		5.97			6.05
Risk-free interest rate		2.40	%		2.73	%
Expected volatility		81.25	%		83.42	%
Expected dividend yield		—	%		—	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the nine months ended September 30, 2019:

	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2018		2,164,100		$	7.75		8.77	$	23,083
Granted		1,315,317		$	16.76
Exercised		(232,758	)	$	2.14
Cancelled or forfeited		(101,231	)	$	10.26
Outstanding as of September 30, 2019		3,145,428		$	11.86		8.63	$	15,491
Exercisable as of September 30, 2019		790,343		$	3.99		7.51	$	8,759
Vested and expected to vest as of September 30, 2019		3,145,428		$	11.86		8.63	$	15,491

Summary of Restricted Stock Unit Activity

The following table summarizes the Company’s restricted common stock activity for the nine months ended September 30, 2019:

	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2018		153,275		$	0.42
Granted		2,300		$	15.65
Vested		(92,262	)	$	0.42
Forfeited, canceled or expired		—		$	—
Issued and unvested as of September 30, 2019		63,313		$	0.42

Schedule of Stock-based Compensation Expense

Stock-based compensation expense was allocated as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	889	$	230	$	2,649	$	417
General and administrative		908		320		2,265		700
Total stock-based compensation expense	$	1,797	$	550	$	4,914	$	1,117