Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

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,
	2018		2017
	(in thousands)
Research and development	$	998	$	1,013
General and administrative		501		514
Total stock-based compensation	$	1,499	$	1,527

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,
	2018	2017
Expected term (in years)	5.4 – 6.9	5.4 –7.0
Expected volatility	88 – 91%	89 – 96%
Risk-free interest rate	2.7 – 2.8%	2.0 – 2.3%
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, 2017		1,503,770			7,470,601		$	3.09		8.12	$	12,363
Awards authorized		2,095,808
Options granted		(2,854,350	)		2,854,350			2.36
Options exercised		—			(64,458	)		1.57
Options forfeited/cancelled		57,662			(57,662	)		5.65
Outstanding — March 31, 2018		802,890			10,202,831		$	2.88		8.45	$	3,050
Exercisable — March 31, 2018					4,582,347		$	3.00		7.54	$	2,198
Vested and expected to vest — March 31, 2018					10,202,831		$	2.88		8.45	$	3,050