Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 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 September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
	(in thousands)
Research and development	$	1,168	$	1,003	$	3,330	$	3,024
General and administrative		653		457		1,772		1,474
Total stock-based compensation	$	1,821	$	1,460	$	5,102	$	4,498

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,
	2018		2017		2018		2017
Expected term (in years)		5.6 – 6.1		6.1		5.3 – 7.0		5.3 –7.0
Expected volatility		88 – 91%		87%		88 – 91%		86 – 96%
Risk-free interest rate		2.7 – 3.0%		1.8 – 2.1%		2.6 – 3.0%		1.8 – 2.3%
Expected dividend rate		—  %		—  %		—  %		—  %

Summary of Stock-Based Compensation Activity

A summary of activity under the 2008 Plan, 2015 Plan and 2018 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 under the 2015 Plan		2,095,808
Awards authorized under the 2018 Plan		1,500,000
Options granted		(3,486,150	)		3,486,150			2.37
Options exercised		—			(118,942	)		1.50
Options forfeited/cancelled		118,072			(118,072	)		4.31
Outstanding — September 30, 2018		1,731,500			10,719,737		$	2.86		8.06	$	1,420
Exercisable — September 30, 2018					5,376,902		$	3.07		7.19	$	1,251
Vested and expected to vest — September 30, 2018					10,719,737		$	2.86		8.06	$	1,420