Stock based compensation (Tables)

3 Months Ended

Mar. 31, 2018

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,
	2018		2017
	(in thousands)
Research and development	$	1,198	$	1,258
General and administrative		1,124		907
Total stock-based compensation expense	$	2,322	$	2,165

Summary of status and changes in unvested restricted stock

		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested restricted common stock as of December 31, 2017	557,979	$	0.70
Issued	—		—
Vested	(145,829)	$	0.77
Repurchased	—
Unvested restricted common stock as of March 31, 2018	412,150	$	0.68

Summary of status and changes in stock options

The following is a summary of stock option activity for the three months ended March 31, 2018:

		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2017	3,143,566	$	11.82
Granted	926,091	$	24.76
Exercised	(132,733)	$	10.64
Cancelled or forfeited	(128,389)	$	14.52
Outstanding at March 31, 2018	3,808,535	$	14.92	8.5	$	21,020
Exercisable at March 31, 2018	1,039,408	$	10.61	7.4		8,548
Vested and expected to vest at March 31, 2018	3,808,535	$	14.92	8.5	$	21,020

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,
	2018		2017
Risk-free interest rate	2.7	%	2.0	%
Expected dividend yield	—	%	—	%
Expected term (in years)	6.0		6.0
Expected volatility	74.3	%	73.6	%

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 March 31,
	2018		2017
Risk-free interest rate	2.7	%	2.0	%
Expected dividend yield	—	%	—	%
Expected term (in years)	8.4		8.7
Expected volatility	76.1	%	83.6	%