Stock based compensation (Tables)

9 Months Ended

Sep. 30, 2016

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				Nine Months Ended
	September 30,				September 30,
	2016		2015		2016		2015
	(in thousands)
Research and development	$	1,378	$	627	$	3,426	$	1,486
General and administrative		411		230		1,103		575
Total stock-compensation expense	$	1,789	$	857	$	4,529	$	2,061

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, 2015	1,818,261	$	0.76
Issued	—		—
Vested	(468,648)	$	0.77
Repurchased	(48,776)		—
Unvested restricted common stock as of September 30, 2016	1,300,837	$	0.74

Summary of status and changes in stock options

The following is a summary of stock option activity for the nine months ended September 30, 2016:

		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2015	1,022,617	$	8.35
Granted	896,610	$	11.83
Exercised	(19,618)	$	7.50
Cancelled or forfeited	(36,990)	$	10.07
Outstanding at September 30, 2016	1,862,619	$	10.01	9.1	$	4,353
Exercisable at September 30, 2016	388,345	$	8.65	8.7	$	1,311
Vested and expected to vest at September 30, 2016	1,862,619	$	10.01	9.1	$	4,353

Assumptions

The assumptions utilized to value the warrants to purchase equity securities on the acquisition date, September 2, 2016, and as of September 30, 2016 are as follows:

	As of September 2,		As of September 30,
	2016		2016
Risk-free interest rate	1.2	%	1.1	%
Expected dividend yield	—	%	—	%
Expected term (in years)	5.0		4.9
Expected volatility	92.3	%	92.3	%

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				Nine Months Ended
	September 30,				September 30,
	2016		2015		2016		2015
Risk-free interest rate	1.2	%	1.7	%	1.5	%	1.6	%
Expected dividend yield	—	%	—	%	—	%	—	%
Expected term (in years)	6.0		6.0		6.0		6.0
Expected volatility	73.2	%	78.4	%	73.1	%	78.6	%

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 September 30,
	2016		2015
Risk-free interest rate	1.9	%	2.2	%
Expected dividend yield	—	%	—	%
Expected term (in years)	8.9		10.0
Expected volatility	84.0	%	82.6	%