Stock Based Compensation (Tables)

3 Months Ended

Mar. 31, 2022

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, 2021		2,603,630		$	7.70		7.46
Options granted		879,200		$	0.52
Options forfeited		(658,947	)	$	5.20
Options expired		(46,173	)	$	9.81
Outstanding — March 31, 2022		2,777,710		$	5.92		8.00
Exercisable — March 31, 2022		1,305,020		$	9.42

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 March 31,
	2022			2021
Employee Stock Options:
Risk-free interest rate		1.87	%		0.69	%
Expected term (in years)		6.1			6.1
Dividend yield		—			—
Volatility		91.54	%		93.83	%
Weighted-average fair value of stock options granted	$	0.39		$	4.70

Summary of Stock-Based Compensation Recognized

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

	Three Months Ended March 31,
	2022		2021
Research and development	$	128	$	369
General and administrative(1)		387		657
Total stock-based compensation expense	$	515	$	1,026

(1) Included in general and administrative stock-based compensation for the three months ended March 31, 2022 is stock-based compensation expense related to 32,684 shares of common stock issued to certain board members in lieu of their cash compensation.