Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 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 September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
	(in thousands)
Research and development	$	1,003	$	863	$	3,024	$	2,624
General and administrative		457		436		1,474		1,341
Total stock-based compensation	$	1,460	$	1,299	$	4,498	$	3,965

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 September 30,						Nine Months Ended September 30,
	2017			2016			2017			2016
Expected term (in years)		6.1			5.1 – 9.9			5.3 – 7.0			5.1 – 9.9
Expected volatility		87	%		80 – 86	%		86 – 96	%		77 – 86	%
Risk-free interest rate		1.8 – 2.1	%		1.1 – 1.5	%		1.8 – 2.3	%		1.1 – 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		(1,444,425	)		1,444,425			1.43
Options exercised		—			(49,861	)		0.46
Options forfeited/cancelled		252,769			(252,769	)		3.85
Outstanding — September 30, 2017		1,415,702			7,685,449		$	3.07		8.35	$	1,100
Exercisable — September 30, 2017					3,796,870		$	2.97		7.76	$	942
Vested and expected to vest — September 30, 2017					7,685,449		$	3.07		8.35	$	1,100