Stock-Based Compensation Expense (Tables)

12 Months Ended

Dec. 31, 2019

Stock-Based Compensation Expense

Summary of Stock option activity under the 2015 and 2018 Plan

				Weighted-
		Weighted-		Average
		Average		Remaining	Aggregate
	Number of	Exercise		Contractual	Intrinsic
	Options	Price		Term (Years)	Value (in thousands)
Outstanding as of December 31, 2018	1,375,730	$	6.54	8.66	$	426
Granted	1,144,100		5.83
Exercised	(12,554)		4.17
Forfeited	(118,876)		6.74
Outstanding as of December 31, 2019	2,388,400	$	6.20	8.44	$	906
Vested or expected to vest as of December 31, 2019	2,388,400	$	6.20	8.44	$	906
Exercisable as of December 31, 2019	799,871	$	5.54	7.35	$	581

Schedule of stock-based compensation expense

Stock‑based compensation expense was classified in the consolidated statement of operations as follows (in thousands):

	Year Ended
	December 31,
	2019		2018
Research and development	$	891	$	479
General and administrative		1,530		722
Total stock-based compensation expense	$	2,421	$	1,201

Schedule of Black-Scholes option-pricing model

	Year Ended
	December 31,
	2019		2018
Expected stock price volatility	77.8	%	65.8	%
Risk-free interest rate	2.3	%	3.0	%
Expected annual dividend yield	—		—
Expected life of options	6.1	years	6.1	years