Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2020

Stock-based compensation

Compensation cost recognized for all stock-based compensation awards

	Year ended December 31,
	2020		2019		2018
	(in thousands)
Research and development	$	6,357	$	7,383	$	4,717
General and administrative		8,577		8,257		10,993
Total stock-based compensation expense	$	14,934	$	15,640	$	15,710

Summary of stock-based compensation expense by classification

	Year ended December 31,
	2020		2019		2018
	(in thousands)
Stock options	$	11,387	$	13,380	$	14,956
Restricted stock awards and units		3,110		1,935		482
Employee stock purchase plan awards		437		325		272
Total stock-based compensation expense	$	14,934	$	15,640	$	15,710

Summary of status and changes in unvested restricted stock

		Weighted
		Average
		Grant Date
		Fair Value
	Units	Per Unit
Unvested restricted stock units as of December 31, 2019	455,404	$	12.16
Awarded	494,604	$	12.67
Vested	(150,759)	$	12.12
Forfeited	(160,778)	$	11.47
Unvested restricted stock units as of December 31, 2020	638,471	$	12.74

Summary of status and changes in stock options

		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2019	5,317,326	$	15.98
Granted	1,560,575	$	11.87
Exercised	(228,436)	$	10.15
Cancelled or forfeited	(1,164,387)	$	17.33
Outstanding at December 31, 2020	5,485,078	$	14.77	7.5	$	1,313
Exercisable at December 31, 2020	3,027,116	$	15.24	6.5	$	—

Summary of weighted-average assumptions

	Year ended December 31,
	2020		2019		2018
Risk-free interest rate	1.0	%	2.2	%	2.8	%
Expected dividend yield	—	%	—	%	—	%
Expected term (in years)	6.0		6.0		6.0
Expected volatility	73.7	%	74.7	%	74.4	%