Note 10 - Equity Incentive Plans (Tables)

3 Months Ended

Mar. 31, 2023

Notes Tables

Share-Based Payment Arrangement, Activity [Table Text Block]

							Weighted					Weighted
	Shares			Number of			Option Average		Unvested			RSU Average
	Available			Options			Exercise		RSU Shares			Grant Date
	For Grant			Outstanding			Price		Outstanding			Fair Value
Balance at January 1, 2023		12,074,692			14,725,261		$	4.48		808,310		$	3.57
Granted		(10,071,992	)		6,960,943		$	0.78		3,111,049		$	0.78
Exercised		—			—		$	—		—		$	—
Released		—			—		$	—		(49,220	)	$	5.09
Forfeited		1,887,633			(1,726,202	)	$	5.43		(161,431	)	$	3.49
Canceled		650,435			(654,783	)	$	3.72		—		$	—
Balance at March 31, 2023		4,540,768			19,305,219		$	3.09		3,708,708		$	1.21

As of March 31, 2023, there were 19,305,219 options outstanding with a weighted average exercise price of $3.09, a weighted average remaining term of 8.28 years and an aggregate intrinsic value of $72,000. Of these options, 6,185,977 were vested, with a weighted average exercise price of $4.11, a weighted average remaining term of 6.06 years and an aggregate intrinsic value of $64,000. 

The aggregate intrinsic value represents the total pre-tax value (i.e., the difference between the Company’s stock price and the exercise price) of stock options outstanding as of March 31, 2023, based on the Company’s common stock closing price of $0.76, which would have been received by the option holders had all their in-the-money options been exercised as of that date.

There were no options exercised during the three months ended March 31, 2023. The Company received $48,000 for the 30,418 options exercised during the three months ended March 31, 2022, which had an intrinsic value of $101,000. 

Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	Three Months Ended March 31,
	2023			2022
Risk-free interest rate		3.45% - 3.56%			1.62% - 2.55%
Expected term (in years)		6.00			6.02 - 6.08
Expected volatility		128	%		125% - 126%
Dividend yield		—	%		—	%

Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block]

	Three Months Ended March 31,
	2023		2022
Research and development	$	1,372	$	2,027
General and administrative		1,275		1,103
Total stock-based compensation	$	2,647	$	3,130