Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Stock Option Valuation

The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees, directors and non- employees:

	Year Ended December 31,
	2019		2018		2017
Risk-free interest rate	2.0	%	2.8	%	2.1	%
Expected term (in years)	5.99		5.94		6.06
Expected volatility	88.5	%	86.0	%	77.3	%
Expected dividend yield	0	%	0	%	0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2019:

	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2018	797,931	$	8.29	8.42	$	6,486
Assumed as part of Merger with Arsanis	271,230	62.60
Granted	589,110	14.69
Exercised	(51,617)	7.05
Forfeited	(309,625)	31.55
Outstanding as of December 31, 2019	1,297,029	$	17.05	8.41	$	1,286
Exercisable as of December 31, 2019	495,740	$	21.66	7.21	$	952
Vested and expected to vest as of December 31, 2019	1,122,542	$	17.15	8.32	$	1,245

Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity

The following table summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2019:

	Number of Shares
Unvested at December 31, 2018	—
Granted	116,689
Vested	—
Forfeited	(14,916)
Unvested at December 31, 2019	101,773

Summary of Stock-Based Compensation Expense Classification

Stock-based compensation expense was classified in the consolidated statements of operations as follows:

	Year Ended December 31,
	2019		2018		2017
Research and development expense	$	909	$	258	$	126
General and administrative expense	1,141		501		366
Total stock-based compensation	$	2,050	$	759	$	492