Note 10 - Stock-based Compensation

3 Months Ended

Mar. 31, 2022

Notes to Financial Statements

Share-Based Payment Arrangement [Text Block]

10. Stock-Based Compensation

Stock Options

The following table summarizes the Company’s stock option activity, including grants to non-employees, for the three months ended March 31, 2022:

	Number of Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)
Balance at December 31, 2021		4,640,660		$	6.64		5.66
Granted		2,173,832			0.82
Forfeited		(4,155	)		1.71
Cancelled		(13,816	)		38.80
Exercised		(30,001	)		0.41
Balance at March 31, 2022		6,766,520		$	4.74		5.91

The following table summarizes information about stock options outstanding at March 31, 2022:

	Number of Shares		Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (Millions)
Exercisable at March 31, 2022		2,392,065	$	10.06		5.41	$	0.2
Vested or expected to vest at March 31, 2022		6,421,165	$	4.91		5.88	$	0.4

Restricted Stock Units

The following is a summary of the RSU activity, including performance restricted stock units, for the three months ended March 31, 2022:

	Number of Restricted Stock Units Outstanding			Weighted- Average Grant Date Fair Value
Unvested December 31, 2021		3,839,030		$	2.36
Granted		5,133,307			0.80
Vested		(1,603,449	)		2.53
Forfeited		(69,777	)		1.49
Unvested March 31, 2022		7,299,111		$	1.24

Performance Restricted Stock Units

In 2022 and 2021, the Company granted performance-based restricted stock units with vesting terms based on our attainment of certain operational targets by October 1, 2023 and October 1, 2022, respectively. The number of shares earnable under the 2022 and 2021 awards were based on achieving designated corporate goals.

Share-based Compensation Expense

The following table summarizes non-cash share-based compensation expense by award type for the three months ended March 31, 2022, and 2021:

	Three Months Ended March 31,
	2022		2021
	(in thousands)
Stock options	$	978	$	1,119
Restricted stock units		883		629
Performance restricted stock units		384		38
	$	2,245	$	1,786

As of March 31, 2022, the Company had future employee stock-based compensation expense of approximately $4.4 million related to unvested stock options, which is expected to be recognized over an estimated weighted-average period of 1.9 years. As of March 31, 2022, the unrecognized stock-based compensation expense related to unvested RSUs was approximately $6.7 million, which is expected to be recognized over a weighted average period of approximately 1.9 years.

The fair value of options granted were estimated using the Black-Scholes-Merton option pricing model based on the assumptions in the table below:

	Three Months Ended March 31,
	2022			2021
Expected dividend yield		0%			0%
Expected volatility	131%	-	133%	118%	-	136%
Risk-free interest rate	1.25%	-	1.94%	0.33%	-	0.58%
Expected life (in years)	4.3	-	4.5	3.8	-	4.5%