Stock Based Compensation (Tables)

6 Months Ended

Jun. 30, 2021

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 2018 Plan and related information:

	Number of Shares Underlying Outstanding Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)
Outstanding — December 31, 2020		2,355,615		$	8.59		8.0
Options granted		1,157,488		$	5.90
Options exercised		(5,000	)	$	4.63
Options forfeited		(270,942	)	$	7.12
Options expired		(569	)	$	582.27
Outstanding — June 30, 2021		3,236,592		$	7.66		8.2
Exercisable — June 30, 2021		1,338,239		$	9.65

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:

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Employee Stock Options:
Risk-free rate	0.96%		0.40%		0.74%		1.29%
Expected term (in years)		5.7		5.9		6.0		5.6
Dividend yield	0.00%		0.00%		0.00%		0.00%
Volatility	92.79%		115.20%		93.64%		111.27%
Weighted-average fair value of stock options granted	$	3.37	$	5.25	$	4.45	$	5.50

Summary of Stock-Based Compensation Expense Recognized

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Research and development	$	394	$	314	$	763	$	658
General and administrative(1)		589		520		1,246		981
Total stock-based compensation expense	$	983	$	834	$	2,009	$	1,639

(1) Included in general and administrative for the three and six months ended June 30, 2021 is stock-based compensation expense related to 13,713 shares and 23,862 shares, respectively, of common stock issued to certain board members in lieu of their cash compensation.