Stock based compensation (Tables)

9 Months Ended

Sep. 30, 2019

Stock based compensation

Compensation cost recognized for all stock-based compensation awards

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

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018
	(in thousands)
Research and development	$	1,304	$	1,176	$	5,975	$	3,706
General and administrative		2,045		1,460		6,145		9,366
Total stock-based compensation expense	$	3,349	$	2,636	$	12,120	$	13,072

Summary of stock-based compensation expense by classification

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018
	(in thousands)
Stock options	$	3,008	$	2,559	$	10,312	$	12,428
Restricted stock awards and units		259		24		1,609		480
Employee stock purchase plan awards		82		53		199		164
Total stock-based compensation expense	$	3,349	$	2,636		12,120		13,072

Summary of status and changes in unvested restricted stock

A summary of the status of and changes in unvested restricted stock unit activity under the Company’s equity award plans for the nine months ended September 30, 2019 was as follows:

		Weighted
		Average
		Grant Date
		Fair Value
	Units	Per Unit
Unvested restricted stock units as of December 31, 2018	—		—
Granted	445,337	$	11.65
Vested	—		—
Forfeited	(24,155)	$	9.25
Unvested restricted stock units as of September 30, 2019	421,182	$	11.79

Summary of status and changes in stock options

The following is a summary of stock option activity for the nine months ended September 30, 2019:

		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2018	4,225,152	$	15.91
Granted	1,538,515	$	15.50
Exercised	(239,336)	$	10.89
Cancelled or forfeited	(288,783)	$	15.50
Outstanding at September 30, 2019	5,235,548	$	16.04	8.2	$	16,807
Exercisable at September 30, 2019	2,259,427	$	14.59	7.3	$	9,547
Vested and expected to vest at September 30, 2019	5,235,548	$	16.04	8.2	$	16,807

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				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018
Risk-free interest rate	1.6	%	2.8	%	2.3	%	2.7	%
Expected dividend yield	—	%	—	%	—	%	—	%
Expected term (in years)	6.0		6.0		6.0		6.0
Expected volatility	74.1	%	74.5	%	74.9	%	74.4	%