Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2022

Stock-based compensation

Compensation cost recognized for all stock-based compensation awards

	Year ended December 31,
	2022		2021		2020
	(in thousands)
General and administrative	$	6,398	$	7,191	$	8,577
Research and development		2,946		4,133		6,357
Total stock-based compensation expense	$	9,344	$	11,324	$	14,934

Summary of stock-based compensation expense by type of award

	Year ended December 31,
	2022		2021		2020
	(in thousands)
Stock options	$	5,938	$	7,438	$	11,387
Restricted stock awards and units		3,215		3,551		3,110
Employee stock purchase plan awards		191		335		437
Total stock-based compensation expense	$	9,344	$	11,324	$	14,934

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, 2021	806,379	$	7.26
Awarded	964,480	$	4.29
Vested	(436,611)	$	6.92
Forfeited	(221,685)	$	5.05
Unvested restricted stock units as of December 31, 2022	1,112,563	$	5.27

Summary of stock option activity

		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2021	5,013,193	$	12.69
Granted	3,681,075	$	5.17
Exercised	(89,012)	$	7.07
Cancelled or forfeited	(2,405,685)	$	13.15
Outstanding at December 31, 2022	6,199,571	$	8.12	7.9	$	6,095
Exercisable at December 31, 2022	2,744,489	$	10.56	6.5	$	2,287

Summary of weighted-average assumptions

	Year ended December 31,
	2022		2021		2020
Risk-free interest rate	2.2	%	0.9	%	1.0	%
Expected dividend yield	—	%	—	%	—	%
Expected term (in years)	6.0		6.0		6.0
Expected volatility	79.4	%	75.0	%	73.7	%