Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

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,
	2019		2018
Research and development	$	1,184	$	998
General and administrative		515		501
Total stock-based compensation	$	1,699	$	1,499

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 March 31,
	2019			2018
Expected term (in years)		5.3 — 6.9			5.4 — 6.9
Expected volatility		91 — 94	%		88 — 91	%
Risk-free interest rate		2.5 — 2.6	%		2.7 — 2.8	%
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		(3,093,300	)		3,093,300			1.93
Options exercised		—			(322,318	)		1.38
Options forfeited/cancelled		77,009			(77,009	)		5.59
Outstanding — March 31, 2019		1,903,372			13,198,385		$	2.66		8.13	$	1,358
Exercisable — March 31, 2019					6,571,244		$	3.09		7.06	$	1,185
Vested and expected to vest — March 31, 2019					13,198,385		$	2.66		8.13	$	1,358