Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2021

Stock-based compensation

Compensation cost recognized for all stock-based compensation awards

Total compensation cost recognized for all stock-based compensation awards in the condensed consolidated statements of operations and comprehensive loss is as follows:

	Three Months Ended
	March 31,
	2021		2020
	(in thousands)
Research and development	$	1,371	$	1,685
General and administrative		2,230		2,355
Total stock-based compensation expense	$	3,601	$	4,040

Summary of stock-based compensation expense by classification

	Three Months Ended
	March 31,
	2021		2020
	(in thousands)
Stock options	$	2,523	$	3,208
Restricted stock awards and units		975		741
Employee stock purchase plan awards		103		91
Total stock-based compensation expense	$	3,601	$	4,040

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, 2020	638,471	$	12.74
Granted	1,203,684	$	6.28
Vested	(184,217)	$	11.55
Forfeited	(50,607)	$	9.40
Unvested restricted stock units as of March 31, 2021	1,607,331	$	8.15

Summary of stock option activity

The following is a summary of stock option activity for the three months ended March 31, 2021:

		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2020	5,485,078	$	14.77
Granted	1,031,704	$	7.42
Exercised	(3,811)	$	7.27
Cancelled or forfeited	(261,109)	$	14.01
Outstanding at March 31, 2021	6,251,862	$	13.59	7.7	$	—
Exercisable at March 31, 2021	3,165,450	$	15.18	6.5	$	—

Summary of weighted-average assumptions

The fair value of each option issued to employees and directors was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:

	Three Months Ended
	March 31,
	2021		2020
Risk-free interest rate	0.7	%	1.4	%
Expected dividend yield	—	%	—	%
Expected term (in years)	6.0		6.0
Expected volatility	74.8	%	72.3	%