Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2016

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 statements 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,
	2016		2015		2016		2015
	(in thousands)
Research and development	$	807	$	225	$	1,761	$	299
General and administrative		496		195		905		301
Total stock-based compensation	$	1,303	$	420	$	2,666	$	600

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

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

	Three Months Ended June 30,		Six Months Ended June 30,
	2016	2015	2016	2015
Expected term (in years)	5.3 – 7.0	5.2 – 10.0	5.3 – 9.9	6.0 – 10.0
Expected volatility	77 – 85%	75 – 79%	77 – 85%	75 – 81%
Risk-free interest rate	1.1 – 1.7%	1.7%	1.1 – 1.9%	1.5 – 1.7%
Expected dividend rate	—  %	—  %	—  %	—  %

Summary of Stock-Based Compensation Activity

A summary of activity under the 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, 2015		3,523,879			3,522,813		$	3.81		8.78	$	40,425
Awards authorized		1,202,324
Options granted		(1,294,880	)		1,294,880			5.82
Options exercised		—			(161,978	)		0.74
Options forfeited/cancelled		245,991			(245,991	)		8.39
Outstanding — June 30, 2016		3,677,314			4,409,724		$	4.26		8.77	$	1,935
Exercisable — June 30, 2016					2,248,919		$	1.78		8.14	$	1,935
Vested and expected to vest — June 30, 2016					4,269,833		$	4.21		8.76	$	1,897