Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2020

Share-based Payment Arrangement [Abstract]

Common Stock Reserved for Future Issuance

The following shares of common stock were reserved for future issuance as of June 30, 2020 (in thousands):

Class A-1 convertible preferred stock outstanding (as-converted)	2,567
Class A-2 convertible preferred stock outstanding (as-converted)	19,613
Initial Closing warrants	13,890
Milestone Closing warrants	32,884
Common stock options outstanding	6,327
RSUs outstanding	71
Common stock available for future grant under 2019 Equity Incentive Plan	1,361
Employee Stock Purchase Plan	190
Total common shares reserved for future issuance	76,903

Stock Option Activity

The following table summarizes our stock option and RSU activity (together Stock Awards) under all equity incentive plans for the six months ended June 30, 2020 (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, 2019	3,098	$	1.17	129	$	1.50
Granted	3,309	$	1.27	—	$	—
Exercised (options) or Vested (RSUs)	(1)	$	0.95	(39)	$	1.50
Canceled/forfeited/expired	(79)	$	8.35	(19)	$	1.50
Stock Awards outstanding at June 30, 2020	6,327	$	1.14	71	$	1.50

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 and 2019 Equity Incentive Plan and the shares purchasable under our Employee Stock Purchase Plan during the periods presented:

	Three months ended June 30,				Six months ended June 30,
	2020		2019		2020		2019
Stock options and performance stock options
Risk-free interest rate	3.9	%	2.0	%	1.3	%	2.6	%
Volatility	95.4	%	93.9	%	95.4	%	93.8	%
Dividend yield	—		—		—		—
Expected term (years)	6.1		6.1		6.1		6.1
Performance stock options
Risk-free interest rate	—		—		1.4	%	2.6	%
Volatility	—		—		95.4	%	93.8	%
Dividend yield	—		—		—		—
Expected term (years)	0		0		6.1		6.1
Employee stock purchase plan shares
Risk-free interest rate	0.4	%	2.5	%	0.9	%	2.5	%
Volatility	93.7	%	104.1	%	99.8	%	111.7	%
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 June 30,				Six months ended June 30,
	2020		2019		2020		2019
Research and development	$	159	$	(294)	$	316	$	194
General and administrative	470		613		1,136		1,084
Total	$	629	$	319	$	1,452	$	1,278