Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2022

Equity [Abstract]

Activity of Shares Issued, Converted or Exercised

The following table summarizes preferred stock conversions and warrant exercises (and the related impact on common stock) for the years ended December 31, 2022 and 2021 (in thousands):

	Class A-1 Convertible Preferred Stock	Class A-2 Convertible Preferred Stock	Class A-3 Convertible Preferred Stock	Class A-4 Convertible Preferred Stock	May 2019 Warrants	December 2019 Warrants	December 2020 Warrants	Common Stock
Balance at December 31, 2020	257	1,416	259	—	1,389	3,185	2,030
2021 Closing	—	—	—	3,726	—	—	—	5,892
Conversions/Exercises	—	(85)	—	—	(111)	(125)	(181)	352
Balance at December 31, 2021	257	1,331	259	3,726	1,278	3,060	1,849
Conversions/Exercises	—	—	—	—	—	—	—	—
Balance at December 31, 2022	257	1,331	259	3,726	1,278	3,060	1,849

Common Stock Reserved for Future Issuance

The following shares of common stock were reserved for future issuance as of December 31, 2022 (in thousands):

Class A-1 convertible preferred stock outstanding (as-converted)	257
Class A-2 convertible preferred stock outstanding (as-converted)	1,331
Class A-3 convertible preferred stock outstanding (as-converted)	259
Class A-4 convertible preferred stock outstanding (as-converted)	3,726
Warrants to purchase Common Stock	6,186
Common stock options outstanding	1,372
RSUs outstanding	70
Common stock available for future grant under the 2019 Equity Incentive Plan	394
Common stock available for future grant under the 2021 Inducement Plan	160
2022 Employee Stock Purchase Plan	129
Total common shares reserved for future issuance	13,884

Stock Option Activity

The following table summarizes our stock option and RSU (together "Stock Awards") activity under all equity incentive plans for the year ended December 31, 2022 (shares and aggregate intrinsic value in thousands):

	Number of options	Weighted average exercise price		Weighted average remaining contractual term	Aggregate intrinsic value
Stock Awards outstanding at December 31, 2021	866	$	11.72
Granted	570	$	2.59
Exercised (options) or Vested (RSUs)	—	$	—
Canceled/forfeited/expired	(64)	$	20.42
Stock Awards outstanding at December 31, 2022	1,372	$	7.53	7.1	$	—
Vested and exercisable at December 31, 2022	742	$	8.69	6.3	$	—

Schedule of Restricted Stock Units Activity

The following table summarizes our RSU activity under all equity incentive plans for the year ended December 31, 2022 (shares and aggregate intrinsic value in thousands): 

	Number of options	Weighted average grant date fair value		Weighted average remaining contractual term	Aggregate intrinsic value
RSUs outstanding at December 31, 2021	40	$	9.50
Granted	85	$	2.57
Vested	(36)	$	9.50
Canceled/forfeited/expired	(19)	$	3.97
RSUs outstanding at December 31, 2022	70	$	2.57	0.8	$	96

Assumptions Used to Estimate Fair Value of Stock Options and Performance Stock and Employee Stock Purchase Plan

The following table summarizes the weighted average assumptions used to estimate the fair value of stock options and performance stock awards granted to employees under our 2019 Equity Incentive Plan, 2021 Inducement Plan and the shares purchasable under our Employee Stock Purchase Plans during the periods presented: 

	Year ended December 31,
	2022		2021
Stock options
Risk-free interest rate	2.0	%	1.0	%
Volatility	96.1	%	95.8	%
Dividend yield	—		—
Expected term (years)	6.1		6.1
Performance stock options
Risk-free interest rate	—		1.0	%
Volatility	—		95.7	%
Dividend yield	—		—
Expected term (years)	0		6.1
Employee stock purchase plan shares
Risk-free interest rate	1.6	%	0.1	%
Volatility	104.7	%	101.2	%
Dividend yield	—		—
Expected term (years)	0.5		0.5

Stock-Based Compensation

The following table summarizes the allocation of our stock-based compensation expense for all stock awards during the periods presented (in thousands): 

	Year ended December 31,
	2022		2021
Research and development	$	594	$	821
General and administrative	1,583		2,102
Total	$	2,177	$	2,923