Stock-Based Compensation

9 Months Ended

Sep. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

8. Stock-Based Compensation

Stock Option Valuation

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		—	%		—	%

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

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock for those stock options that had exercise prices lower than the estimated fair value of the Company’s common stock.

The weighted-average grant-date fair value of the Company’s stock options granted during the nine months ended September 30, 2019 and 2018 was $11.77 and $9.05, respectively.

Restricted Common Stock

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

The total fair value of restricted common stock vested during the nine months ended September 30, 2019 and 2018 was $44 and $39, respectively.

Stock-Based Compensation

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

As of September 30, 2019, total unrecognized compensation cost related to the unvested stock-based awards was $22,065, which is expected to be recognized over a weighted-average period of 3.24 years.