Note 10 - Stock-based Compensation

3 Months Ended

Mar. 31, 2023

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, 2023:

	Number of Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)
Balance at December 31, 2022		7,584,967		$	4.22		5.31
Granted		2,748,415			0.73
Forfeited		-			-
Cancelled		(913	)		4.21
Exercised		(13,300	)		0.38
Balance at March 31, 2023		10,319,169		$	3.30		5.54

The following table summarizes information about stock options outstanding at March 31, 2023:

	Number of Shares		Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (Millions)
Exercisable at March 31, 2023		4,843,264	$	5.96		4.80	$	0.4
Vested or expected to vest at March 31, 2023		9,645,630	$	3.47		5.47	$	0.6

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,
	2023			2022
Expected dividend yield		0%			0%
Expected volatility	124%	-	126%	131%	-	133%
Risk-free interest rate	3.73%	-	4.14%	1.25%	-	1.94%
Expected life (in years)	4.3	-	4.5	4.3	-	4.5

Restricted Stock Units

The following is a summary of the restricted stock units activity, including performance restricted stock units, for the three months ended March 31, 2023:

	Number of Restricted Stock Units Outstanding			Weighted- Average Grant Date Fair Value
Unvested December 31, 2022		8,483,491		$	1.04
Granted		7,340,630			0.73
Vested		(3,082,354	)		1.18
Forfeited		(84,673	)		1.13
Unvested March 31, 2023		12,657,094		$	0.83

Performance Restricted Stock Units

In 2023 and 2022, the Company granted performance-based restricted stock units with vesting terms based on our attainment of certain operational targets by December 31, 2023 and October 1, 2023, respectively. The number of shares earnable under the 2023 and 2022 awards are based on achieving designated corporate goals. These operational targets have been achieved for the awards granted in 2022, therefore the 2022 performance-based restricted stock units are fully earned and remain subject to three-year time-based vesting requirements. The Company has not yet achieved the operational targets required for the awards granted in 2023.

Stock-based Compensation Expense

The following table summarizes non-cash stock-based compensation expense by award type for the three months ended March 31, 2023 and 2022:

	Three Months Ended March 31,
	2023		2022
	(in thousands)
Stock options	$	665	$	978
Restricted stock units		859		883
Performance restricted stock units		392		384
	$	1,916	$	2,245

As of March 31, 2023, the Company had future employee stock-based compensation expense of approximately $2.9 million related to unvested stock options, which is expected to be recognized over an estimated weighted-average period of approximately 2.0 years. As of March 31, 2023, the unrecognized stock-based compensation expense related to unvested restricted stock units and performance restricted stock units was approximately $7.4 million, which is expected to be recognized over a weighted average period of approximately 1.9 years.