Stock Based Compensation (Tables)

9 Months Ended

Sep. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Option Activity Under Company's Equity Incentive Plans

The following table summarizes stock option activity under the Company’s equity incentive plans and related information:

	Number of Shares Underlying Outstanding Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)
Outstanding — December 31, 2018		1,361,977		$	12.04		8.71
Options granted		426,500		$	8.00
Options exercised		(41,219	)	$	4.57
Options forfeited		(166,722	)	$	8.27
Options expired		(10,698	)	$	92.53
Outstanding — September 30, 2019		1,569,838		$	10.99		8.38
Exercisable — September 30, 2019		649,116		$	14.14		7.86
Vested and expected to vest — September 30, 2019		1,569,838		$	10.99		8.38

Summary of Weighted Average Valuation Assumptions Used to Estimate Fair Value of Employee Stock Options

The fair value of employee stock options was estimated using the following weighted-average assumptions for the nine months ended September 30, 2019 and 2018:

	Nine Months Ended September 30,
	2019			2018
Employee Stock Options:
Risk-free interest rate		2.39	%		2.66	%
Expected term (in years)		5.98			5.97
Dividend yield		—			—
Volatility		89.76	%		93.60	%
Weighted-average fair value of stock options granted	$	5.93		$	10.28

Summary of Stock-Based Compensation Recognized

Total stock-based compensation recognized was as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	284	$	161	$	805	$	418
General and administrative(1)		517		541		1,728		1,448
Total stock-based compensation	$	801	$	702	$	2,533	$	1,866

(1) Included $0.03 and $0.1 million related to modifications of certain Board member stock options in the nine months ended September 30, 2019 and 2018, respectively.  These modifications extended the post-termination exercise period of certain options.