Stock options (Tables)

9 Months Ended

Sep. 30, 2019

Stock options

Schedule of stock option activity under Plan

		Weighted-
	Number	Average
	of Shares	Exercise Price
Options outstanding at January 1, 2019	3,746,567	$	6.58
Granted	1,630,283		3.74
Exercised	(58,964)		2.04
Cancelled	(599,289)		9.42
Options outstanding at September 30, 2019	4,718,597	$	5.29
Options exercisable at September 30, 2019	2,531,612	$	4.36

Schedule of stock-based compensation expense by award type

	Three months ended				Nine months ended
	September 30,				September 30,
		2019		2018		2019		2018
Stock options	$	1,055	$	988	$	3,240	$	2,692
Restricted stock units		184		—		184		—
Employee stock purchase plan		46		65		186		65
Stock-based compensation expense included in Total operating expenses	$	1,285	$	1,053	$	3,610	$	2,757

Schedule of stock-based compensation expense as reflected in the Company’s condensed consolidated statements of operations and comprehensive loss

	Three months ended				Nine months ended
	September 30,				September 30,
		2019		2018		2019		2018
Research and development	$	587	$	495	$	1,636	$	1,302
General and administrative		698		558		1,974		1,455
Stock-based compensation expense included in Total operating expenses	$	1,285	$	1,053	$	3,610	$	2,757

Schedule of weighted average assumptions for estimating fair value of option awards

	Three months ended				Nine months ended
	September 30,				September 30,
	2019		2018		2019		2018
Risk-free interest rate	1.9	%	2.9	%	2.4	%	2.7	%
Expected dividend yield	—	%	—	%	—	%	—	%
Expected term (years)	6.11		5.85		5.99		6.06
Expected stock price volatility	75	%	73	%	74	%	73	%