Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2021

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) under the May 2019 SPA, the December 2020 SPA and the November 2021 SPA for the years ended December 31, 2021 and 2020 (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	Warrants	Common Stock
Balance at December 31, 2019	416	3,288	—	—	46,774
2020 Closing	—	—	273	—	20,303	24,342
Conversions/Exercises	(159)	(1,872)	(14)	—	(1,039)	21,493
Balance at December 31, 2020	257	1,416	259	—	66,038
2021 Closing	—	—	—	3,726	—	58,923
Conversions/Exercises	—	(85)	—	—	(4,170)	3,525
Balance at December 31, 2021	257	1,331	259	3,726	61,868

Common Stock Reserved for Future Issuance

The following shares of common stock were reserved for future issuance as of December 31, 2021 (in thousands):

Class A-1 convertible preferred stock outstanding (as-converted)	2,567
Class A-2 convertible preferred stock outstanding (as-converted)	13,308
Class A-3 convertible preferred stock outstanding (as-converted)	2,587
Class A-4 convertible preferred stock outstanding (as-converted)	37,257
2019 PIPE Initial Closing warrants	12,778
2019 PIPE Milestone Closing warrants	30,595
2020 PIPE warrants	18,495
Common stock options outstanding	8,661
RSUs outstanding	401
Common stock available for future grant under the 2019 Equity Incentive Plan	2,164
Common stock available for future grant under the Inducement Plan	1,800
Employee Stock Purchase Plan	259
Total common shares reserved for future issuance	130,872

Stock Option Activity

The following table summarizes our stock option activity under all equity incentive plans for the year ended December 31, 2021 (shares and aggregate intrinsic value in thousands):

	Number of options	Weighted average exercise price		Weighted average remaining contractual term	Aggregate intrinsic value
Options outstanding at December 31, 2020	6,813	$	1.10
Granted	3,351	$	1.40
Exercised	(28)	$	0.96
Canceled/forfeited/expired	(1,475)	$	1.36
Options outstanding at December 31, 2021	8,661	$	1.17	8.24	$	—
Vested and exercisable at December 31, 2021	3,494	$	1.22	7.93	$	—

Schedule of Restricted Stock Units Activity

The following table summarizes our RSU activity under all equity incentive plans for the year ended December 31, 2021 (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, 2020	34	$	1.50
Granted	446	$	0.95
Vested	(34)	$	1.50
Canceled/forfeited/expired	(45)	$	0.96
RSUs outstanding at December 31, 2021	401	$	0.95	0.39	$	126

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 2012 Equity Incentive Plan, 2015 Inducement Plan, 2019 Equity Incentive Plan, 2021 Inducement Plan and the shares purchasable under our Employee Stock Purchase Plan during the periods presented: 

	Year ended December 31,
	2021		2020
Stock options
Risk-free interest rate	1.0	%	1.1	%
Volatility	95.8	%	95.4	%
Dividend yield	—		—
Expected term (years)	6.1		6.1
Performance stock options
Risk-free interest rate	1.0	%	1.4	%
Volatility	95.7	%	95.4	%
Dividend yield	—		—
Expected term (years)	6.1		6.1
Employee stock purchase plan shares
Risk-free interest rate	0.1	%	0.6	%
Volatility	101.2	%	98.0	%
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,
	2021		2020
Research and development	$	821	$	693
General and administrative	2,102		1,921
Total	$	2,923	$	2,614