Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 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 statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows:

	Three Months Ended March 31,
	2016		2015
	(in thousands)
Research and development	$	954	$	74
General and administrative		409		106
Total stock-based compensation	$	1,363	$	180

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 March 31,
	2016	2015
Expected term (in years)	5.4 – 10.0	6.0 – 10.0
Expected volatility	77 – 84%	75 – 82%
Risk-free interest rate	1.3 – 1.9%	1.5 – 1.9%
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		(821,200	)		821,200			7.28
Options exercised		—			(116,673	)		0.83
Options cancelled		73,880			(73,880	)		5.59
Outstanding — March 31, 2016		3,978,883			4,153,460		$	4.55		8.88	$	12,717
Exercisable — March 31, 2016					2,110,814		$	1.20		8.28	$	11,695
Vested and expected to vest — March 31, 2016					4,005,036		$	4.49		8.87	$	12,431