Stock based compensation (Tables)

3 Months Ended

Mar. 31, 2017

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

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
	March 31,
	2017		2016
	(in thousands)
Research and development	$	1,258	$	1,117
General and administrative		907		305
Total stock-compensation expense	$	2,165	$	1,422

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 plan was as follows:

		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested restricted common stock as of December 31, 2016	1,167,984	$	0.76
Issued	—		—
Vested	(153,685)	$	0.76
Repurchased	(17,157)	$	0.51
Unvested restricted common stock as of March 31, 2017	997,142	$	0.74

Summary of status and changes in stock options

The following is a summary of stock option activity for the three months ended March 31, 2017:

		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2016	1,871,237	$	10.21
Granted	1,043,800	$	11.94
Exercised	(70,938)	$	8.49
Cancelled or forfeited	(60,246)	$	9.93
Outstanding at March 31, 2017	2,783,853	$	10.91	8.9	$	6,858
Exercisable at March 31, 2017	511,643	$	9.30	8.0		2,075
Vested and expected to vest at March 31, 2017	2,783,853	$	10.91	8.9	$	6,858

Assumptions

The assumptions utilized to value the warrants to purchase equity securities as of March 31, 2017 and December 31, 2016 are as follows:

	As of March 31,		As of December 31,
	2017		2016
Risk-free interest rate	1.8	%	1.8	%
Expected dividend yield	—	%	—	%
Expected term (in years)	4.4		4.7
Expected volatility	86.7	%	97.5	%

Employees and directors stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

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,
	2017		2016
Risk-free interest rate	2.0	%	1.5	%
Expected dividend yield	—	%	—	%
Expected term (in years)	6.0		6.0
Expected volatility	73.6	%	73.0	%

Non-employees stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Assumptions

The fair value of each option issued to non‑employees was estimated at each vesting and reporting date using the Black‑Scholes option pricing model. The reporting date fair value was determined using the following weighted‑average assumptions:

	As of March 31,
	2017		2016
Risk-free interest rate	2.0	%	1.5	%
Expected dividend yield	—	%	—	%
Expected term (in years)	8.7		10.0
Expected volatility	83.6	%	84.1	%