Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2017

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 March 31,
	2017		2016
	(in thousands)
Research and development	$	1,013	$	954
General and administrative		514		409
Total stock-based compensation	$	1,527	$	1,363

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 March 31,
	2017			2016
Expected term (in years)		5.4 – 7.0			5.4 – 10.0
Expected volatility		89 – 96	%		77 – 84	%
Risk-free interest rate		2.0 – 2.3	%		1.3 – 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, 2016		1,392,521			6,543,654		$	3.44		8.82	$	888
Awards authorized		1,214,837
Options granted		(1,206,775	)		1,206,775			1.47
Options exercised		—			(49,861	)		0.46
Options forfeited/cancelled		85,516			(85,516	)		4.64
Outstanding — March 31, 2017		1,486,099			7,615,052		$	3.13		8.81	$	972
Exercisable — March 31, 2017					3,085,928		$	2.80		8.03	$	899
Vested and expected to vest — March 31, 2017					7,615,052		$	3.13		8.81	$	972