Stock Based Compensation (Tables)

12 Months Ended

Dec. 31, 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)		Aggregate Intrinsic Value (thousands)
Outstanding — December 31, 2017		821,741		$	13.69		9.17	$	6,376
Options granted		612,050		$	13.13
Options exercised		(12,716	)	$	4.17			$	213
Options canceled/forfeited		(43,315	)	$	7.76
Options expired		(15,783	)	$	158.81
Outstanding — December 31, 2018		1,361,977		$	12.04		8.71	$	2,294
Options granted		450,900		$	7.85
Options exercised		(41,219	)	$	4.57			$	159
Options forfeited		(182,722	)	$	8.36
Options expired		(11,395	)	$	96.49
Outstanding — December 31, 2019		1,577,541		$	10.85		8.15	$	1,350
Exercisable — December 31, 2019		740,356		$	13.30
Shares available to be granted — December 31, 2019		1,014,126

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 year ended December 31, 2019 and 2018:

	Year Ended December 31,
	2019			2018
Employee Stock Options:
Expected term (in years)		5.98			6.04
Risk-free interest rate		2.35	%		2.67	%
Dividend yield		—			—
Volatility		90.81	%		92.61	%
Weighted-average fair value of stock options granted	$	5.85		$	9.97

Summary of Stock-Based Compensation Recognized

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

	Year Ended December 31,
	2019		2018
Research and development	$	1,052	$	606
General and administrative⁽¹⁾		2,152		2,000
Total stock-based compensation	$	3,204	$	2,606

(1)	Included $0.03 and $0.1 million related to modifications of certain Board member stock options in the years ended December 31, 2019 and 2018, respectively. These modifications extended the post-termination exercise period of certain options.

Also included in general and administrative stock-based compensation for the year ended December 31, 2019 and 2018 is expense related to 24,235 and 6,790 shares of common stock issued to certain board members in lieu of their cash compensation, respectively.

As of December 31, 2019, 1,014,126 shares of common stock were available for future grant and 1,577,541 options to purchase shares of common stock were outstanding. As of December 31, 2019, the Company had unrecognized employee stock-based compensation expense of $5.3 million, related to unvested stock option awards, which is expected to be recognized over an estimated weighted-average period of 2.41 years.