Stock-Based Compensation (Tables)

6 Months Ended

Jun. 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 June 30,				Six Months Ended June 30,
	2015		2014		2015		2014
	(in thousands)
Research and development	$	225	$	9	$	299	$	18
General and administrative		195		50		301		63
Total stock-based compensation	$	420	$	59	$	600	$	81

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 June 30,				Six Months Ended June 30,
	2015	2014			2015	2014
Expected term (in years)	5.2 – 10.0		5.7		6.0 – 10.0	5.0 – 5.7
Expected volatility	75 – 79%		83	%	75 – 81%	81 – 83%
Risk-free interest rate	1.7%		1.6	%	1.5 – 1.7%	1.5 – 1.7%
Expected dividend rate	—  %		—	%	—  %	—  %

Summary of Stock Options Activity

A summary of activity under the 2005 Plan and 2008 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		—			—
Options granted		(1,184,566	)		1,184,566			5.62
Options exercised		—			(27,813	)		1.00
Options cancelled		—			—			—
Outstanding — June 30, 2015		365,535			3,294,849		$	2.60		9.19	$	40,849
Exercisable — June 30, 2015					2,115,209		$	0.92		8.81	$	29,783
Vested and expected to vest — June 30, 2015					2,983,562		$	2.71		9.20	$	36,665