Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share Based Compensation Expense Related to Issuance of Stock Option Awards to Employees and Non Employees Related to Employee Stock Purchase Plan

The Company recognized stock-based compensation expense related to the issuance of stock option awards to employees and non-employees and related to the Employee Stock Purchase Plan in the consolidated statements of operations as follows (in thousands):

	Years Ended December 31,
	2019		2018		2017
Research and development	$	2,061	$	1,910	$	1,363
General and administrative		3,944		4,291		4,087
Total	$	6,005	$	6,201	$	5,450

Grant Date Fair Values of Options Issued to Employees and Non-employees Estimated Using Black-Scholes Option Pricing Model

The grant date fair values of options issued to employees and non-employees were estimated using the Black-Scholes option-pricing model with the following assumptions:

	Years Ended December 31,
	2019			2018			2017
Expected term (in years)		5.97			5.90			6.02
Volatility rate		76.95	%		76.28	%		73.86	%
Risk-free interest rate		2.29	%		2.69	%		2.02	%
Expected dividend yield		0.00	%		0.00	%		0.00	%

Summary of Employee and Non-Employee Option Activity

A summary of employee and non-employee option activity under the Company’s equity award plans is presented below (in thousands, except share data):

	Numberof Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding—January 1, 2019		4,252,983		$	9.23		7.3	$	66
Granted		1,964,025		$	6.99
Exercised		(25,857	)	$	6.86
Canceled or forfeited		(134,140	)	$	9.85
Outstanding—December 31, 2019		6,057,011		$	8.50		7.2	$	6,306
Exercisable—December 31, 2019		3,620,626		$	8.93		6.1	$	3,114
Options vested, exercisable or expected to    vest—December 31, 2019		5,655,008		$	8.64		7.1	$	5,373