Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Share-based Payment Arrangement [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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
	(in thousands)
Research and development	$	1,229	$	1,164	$	2,413	$	2,162
General and administrative		529		618		1,044		1,119
Total stock-based compensation	$	1,758	$	1,782	$	3,457	$	3,281

Schedule of Estimated Grant-date Fair Value 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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Expected term (in years)		5.3 – 6.1		5.3 – 7.0		5.3 – 6.9		5.3 – 7.0
Expected volatility		90 – 92%		88 – 91%		90 – 94%		88 – 91%
Risk-free interest rate		1.9 – 2.3%		2.6 – 2.9%		1.9 – 2.6%		2.6 – 2.9%
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, 2018		1,945,025			10,504,412		$	2.86		7.80	$	545
Awards authorized		2,974,638
Options granted		(3,321,300	)		3,321,300			1.85
Options exercised		—			(322,318	)		1.38
Options forfeited/cancelled		241,340			(241,340	)		3.27
Outstanding — June 30, 2019		1,839,703			13,262,054		$	2.64		7.91	$	3
Exercisable — June 30, 2019					7,173,953		$	3.09		6.90	$	3
Vested and expected to vest — June 30, 2019					13,262,054		$	2.64		7.91	$	3