Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2015

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Stock-based Compensation Expense for Employees and Non-employees

In the accompanying condensed consolidated statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
	(in thousands)
Research and development	$	733	$	11	$	1,032	$	29
General and administrative		579		105		880		168
Total stock-based compensation	$	1,312	$	116	$	1,912	$	197

Schedule of Estimated Grant-date Fair Value of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions

The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on assumptions as follows:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2015	2014	2015	2014
Expected term (in years)	6.1	5.9 – 6.3	5.2 – 10	5.0 – 6.3
Expected volatility	79%	82 – 83%	75 – 84%	81 – 83%
Risk-free interest rate	1.7 – 1.9%	1.7 – 1.8%	1.5 – 2.4%	1.5 – 1.8%
Expected dividend rate	— %	— %	— %	— %

Summary of Stock Options Activity

A summary of activity under the 2005 Plan, 2008 Plan and 2015 Plan and related information is as follows:

				Options Outstanding
	Shares Available for Grant			Number of Shares Outstanding			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value of Outstanding Options (in thousands)
Outstanding — December 31, 2014		1,550,101			2,138,096		$	0.91		9.29	$	3,153
Awards authorized		3,400,000			—
Options granted		(1,217,568	)		1,217,568			5.96
Options exercised		—			(41,505	)		0.76
Options cancelled		—			—			—
Outstanding — September 30, 2015		3,732,533			3,314,159		$	2.77		8.96	$	58,866
Exercisable — September 30, 2015					2,127,353		$	0.98		8.59	$	41,564
Vested and expected to vest — September 30, 2015					3,217,100		$	2.72		8.95	$	57,303