Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2022

Equity [Abstract]

Activity of Shares

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 three months ended March 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	Warrants	Common Stock
Balance at December 31, 2021	257	1,331	259	3,726	61,868
Conversions/Exercises	—	—	—	—	—
Balance at March 31, 2022	257	1,331	259	3,726	61,868
Balance at December 31, 2020	257	1,416	259	—	66,038
Conversions/Exercises	—	(78)	—	—	(3,920)	3,199
Balance at March 31, 2021	257	1,338	259	—	62,118

Common Stock Reserved for Future Issuance

The following shares of common stock were reserved for future issuance as of March 31, 2022 (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	12,846
RSUs outstanding	1,226
Common stock available for future grant under the 2019 Equity Incentive Plan	4,453
Common stock available for future grant under the 2021 Inducement Plan	1,800
Employee Stock Purchase Plan	291
Total common shares reserved for future issuance	138,203

Stock Option Activity

The following table summarizes our stock option and RSU (together Stock Awards) activity under all equity incentive plans for the three months ended March 31, 2022 (shares in thousands): 

	Number of options	Weighted average exercise price		Number of RSUs	Weighted average grant date fair value
Stock Awards outstanding at December 31, 2021	8,661	$	1.17	401	$	0.95
Granted	4,357	$	0.26	848	$	0.26
Canceled/forfeited/expired	(172)	$	0.90	(23)	$	0.95
Stock Awards outstanding at March 31, 2022	12,846	$	0.87	1,226	$	0.47

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:

	Three months ended March 31,
	2022		2021
Stock options
Risk-free interest rate	1.7	%	0.9	%
Volatility	95.7	%	95.7	%
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)	0.0		6.1
Employee stock purchase plan shares
Risk-free interest rate	0.3	%	0.1	%
Volatility	97.8	%	101.3	%
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): 

	Three months ended March 31,
	2022		2021
Research and development	$	270	$	181
General and administrative	730		510
Total	$	1,000	$	691