Stock based compensation (Tables)

3 Months Ended

Mar. 31, 2020

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,
	2020		2019
	(in thousands)
Research and development	$	1,685	$	1,416
General and administrative		2,355		2,102
Total stock-based compensation expense	$	4,040	$	3,518

Summary of stock-based compensation expense by classification

	Three Months Ended
	March 31,
	2020		2019
	(in thousands)
Stock options	$	3,208	$	2,776
Restricted stock awards and units		741		683
Employee stock purchase plan awards		91		59
Total stock-based compensation expense	$	4,040	$	3,518

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
Granted	429,103	$	13.05
Vested	(108,600)	$	9.10
Forfeited	(28,480)	$	10.50
Unvested restricted stock units as of March 31, 2020	747,427	$	13.18

Summary of status and changes in stock options

The following is a summary of stock option activity for the three months ended March 31, 2020:

		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	899,085	$	12.88
Exercised	(3,035)	$	11.30
Cancelled or forfeited	(148,221)	$	15.66
Outstanding at March 31, 2020	6,065,155	$	15.53	7.9	$	391
Exercisable at March 31, 2020	2,723,252	$	14.77	6.6	$	350
Vested and expected to vest at March 31, 2020	6,065,155	$	15.53	7.9	$	391

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,
	2020		2019
Risk-free interest rate	1.4	%	2.5	%
Expected dividend yield	—	%	—	%
Expected term (in years)	6.0		6.0
Expected volatility	72.3	%	75.0	%