Note 6 - Stockholders' Equity and Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2024

Notes Tables

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

	Three Months Ended March 31,
	2024		2023
Research and development		949		258
General and administrative		810		638
Total stock-based compensation expense	$	1,759	$	896

Share-Based Payment Arrangement, Cost by Plan [Table Text Block]

	Three Months Ended March 31,
	2024		2023
Time-based options	$	1,163	$	678
Performance-based options		8		117
Market-based options		507		—
ESPP		81		101
Total stock-based compensation expense	$	1,759	$	896

Share-Based Payment Arrangement, Option [Member]

Notes Tables

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

	Three Months Ended March 31,
	2024		2023
Expected term in years		5.3 - 6.3		6.03
Expected volatility		91% - 95%		94%
Risk-free interest rate		4.0% - 4.5%		3.8%
Dividend yield		—		—

	Three Months Ended March 31,
	2024		2023
Expected term in years		5.5 - 5.8		—
Expected volatility		90%		—
Risk-free interest rate		4.2%		—
Dividend yield		—		—

Market-based Options [Member]

Notes Tables

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

	Stock Options Outstanding
	Number of shares		Weighted average exercise price		Weighted average remaining life (in years)
Balances — December 31, 2023		2,100,000	$	6.78		9.44
Options granted		100,000		9.32
Options exercised		—		—
Options canceled		—		—
Options expired		—		—
Balances — March 31, 2024		2,200,000	$	6.89		9.22
Exercisable — March 31, 2024		—		—		—

Performance Options [Member]

Notes Tables

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

	Stock Options Outstanding
	Number of shares			Weighted average exercise price		Weighted average remaining life (in years)
Balances — December 31, 2023		1,224,130		$	5.51		8.53
Options granted		39,000			8.44
Options exercised		(3,000	)		2.92
Options canceled		(3,000	)		2.92
Options expired		—			—
Balances — March 31, 2024		1,257,130		$	5.61		8.35
Exercisable — March 31, 2024		440,628		$	7.60		7.5

Time-based Options [Member]

Notes Tables

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

	Stock Options Outstanding
	Number of shares			Weighted average exercise price		Weighted average remaining life (in years)
Balances — December 31, 2023		6,141,906		$	10.48		7.07
Options granted		500,000			8.98
Options exercised		—			—
Options canceled		(1,125	)		16.60
Options expired		(1,500	)		17.01
Balances — March 31, 2024		6,639,281		$	10.36		7.05
Exercisable — March 31, 2024		3,580,866		$	14.31		5.15

The 2017 Employee Stock Purchase Plan [Member]

Notes Tables

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

	Three Months Ended March 31,
	2024		2023
Expected term in years		0.5 - 1.0		0.5 - 1.0
Expected volatility		98%		83%
Risk-free interest rate		4.9% - 5.3%		5.1% - 5.2%
Dividend yield		—		—

The 2015 Plan, 2017 Plan and Inducement Plan [Member]

Notes Tables

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

	Stock Options Outstanding
	Number of shares			Weighted average exercise price		Weighted average remaining life (in years)
Balances — December 31, 2023		9,466,036		$	9.01		7.78
Options granted		639,000			9.00
Options exercised		(3,000	)		2.92
Options canceled		(4,125	)		6.65
Options expired		(1,500	)		17.01
Balances — March 31, 2024		10,096,411		$	9.01		7.69
Exercisable — March 31, 2024		4,021,494		$	13.58		5.41