Note 8 - Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 2023

Notes Tables

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2023		2022		2023		2022
Research and development	$	146	$	174	$	229	$	548
General and administrative		1,482		1,270		2,666		2,470
Total	$	1,628	$	1,444	$	2,895	$	3,018

Nonvested Restricted Stock Shares Activity [Table Text Block]

	Shares			Fair Value
Weighted Average Fair Value	(In thousands)			Per Share
Unvested, December 31, 2022		193		$	1.74
Granted		—		$	—
Vested		(122	)	$	1.71
Forfeited or canceled		—		$	—
Unvested, June 30, 2023		72		$	1.79

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

				Weighted		Weighted
	Number of			Average		Average		Aggregate
	Unvested			Exercise		Remaining		Intrinsic
	Options			Price		Contractual		Value
	(In thousands)			Per Share		Term (In years)		(In thousands)
Outstanding at December 31, 2022		5,378		$	3.89		8.24	$	6,316
Granted		2,613		$	3.80		9.64
Exercised		(12	)	$	2.43
Forfeited/canceled		(228	)	$	4.75
Outstanding at June 30, 2023		7,751		$	3.84		8.51	$	1,814
Vested at June 30, 2023		3,233		$	4.34		7.75	$	569

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

	Six Months Ended June 30,
	2023			2022
Expected term (in years)		6.0			5.8
Risk-free rate		3.8	%		1.9	%
Expected volatility		88	%		90	%
Expected dividend yield		—			—

Schedule of Nonvested Restricted Stock Units Activity [Table Text Block]

	Shares (In thousands)		Weighted Average Price Per Share		Weighted Average Fair Value (In thousands)
Unvested, December 31, 2022		900	$	3.83	$	3,447
Granted		900		3.93		3,537
Vested		—		—		—
Forfeited or canceled		—		—		—
Unvested, June 30, 2023		1,800	$	3.88	$	6,984