Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2021

Share-based Payment Arrangement [Abstract]

Activity of Shares

The following table summarizes preferred stock conversions and warrant exercises (and the related impact on common stock) under the 2019 SPA and 2020 SPA for the three and nine months ended September 30, 2021 and 2020 (in thousands):

	Class A-1 Convertible Preferred Stock	Class A-2 Convertible Preferred Stock	Class A-3 Convertible Preferred Stock	Warrants	Common Stock
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
Conversions/Exercises	—	(7)	—	(250)	326
Balance at June 30, 2021	257	1,331	259	61,868
Conversions/Exercises	—	—	—	—	—
Balance at September 30, 2021	257	1,331	259	61,868
Balance at December 31, 2019	416	3,288	—	46,774	—
Conversions/Exercises	—	(656)	—	—	6,567
Balance at March 31, 2020	416	2,632	—	46,774
Conversions/Exercises	(159)	(671)	—	—	8,296
Balance at June 30, 2020	257	1,961	—	46,774
Conversions/Exercises	—	(322)	—	—	3,217
Balance at September 30, 2020	257	1,639	—	46,774

Common Stock Reserved for Future Issuance

The following shares of common stock were reserved for future issuance as of September 30, 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
2019 PIPE Initial Closing warrants	12,778
2019 PIPE Milestone Closing warrants	30,595
2020 PIPE warrants	18,495
Common stock options outstanding	9,727
RSUs outstanding	446
Common stock available for future grant under 2019 Equity Incentive Plan	853
Employee Stock Purchase Plan	260
Total common shares reserved for future issuance	91,616

Stock Option Activity

The following table summarizes our stock option and RSU (together Stock Awards) activity under all equity incentive plans for the nine months ended September 30, 2021 (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, 2020	6,813	$	1.10	34	$	1.50
Granted	3,143	$	1.47	446	$	0.95
Exercised (options) or Vested (RSUs)	(28)	$	0.96	(34)	$	1.50
Canceled/forfeited/expired	(201)	$	1.22	—	$	1.50
Stock Awards outstanding at September 30, 2021	9,727	$	1.22	446	$	0.95

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 and the shares purchasable under our Employee Stock Purchase Plan during the periods presented:

	Three months ended September 30,				Nine months ended September 30,
	2021		2020		2021		2020
Stock options
Risk-free interest rate	0.9	%	0.4	%	1.0	%	1.1	%
Volatility	96.0	%	95.3	%	95.8	%	95.4	%
Dividend yield	—		—		—		—
Expected term (years)	6.1		6.1		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)	0		0		6.1		6.1
Employee stock purchase plan shares
Risk-free interest rate	0.1	%	0.3	%	0.1	%	0.7	%
Volatility	103.1	%	94.9	%	104.4	%	98.2	%
Dividend yield	—		—		—		—
Expected term (years)	0.5		0.5		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 September 30,				Nine months ended September 30,
	2021		2020		2021		2020
Research and development	$	277	$	255	$	687	$	571
General and administrative	530		437		1,565		1,573
Total	$	807	$	692	$	2,252	$	2,144