Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2019

Disclosure of Compensation Related Costs, Share-based Payments [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:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Risk-free interest rate	1.6	%	2.7	%	2.0	%	2.7	%
Expected term (in years)	6.04		5.95		6.01		5.91
Expected volatility	84.5	%	86.7	%	88.0	%	86.7	%
Expected dividend yield	0	%	0	%	0	%	0	%

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 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	481,660	15.48
Exercised	(25,627)	6.90
Forfeited	(254,053)	35.92
Outstanding as of September 30, 2019	1,271,141	$	17.11	8.45	$	2,760
Exercisable as of September 30, 2019	480,025	$	21.49	7.15	$	1,709
Vested and expected to vest as of September 30, 2019	1,092,790	$	17.18	8.35	$	2,595

Summary of Stock-Based Compensation Expense Classification

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development expense	$	301	$	86	$	599	$	175
General and administrative expense	390		112		787		296
Total stock-based compensation	$	691	$	198	$	1,386	$	471