Note 6 - Stockholders' Equity and Stock-based Compensation - Summary of Stock Option Activity (Details) - The 2015 Plan, 2017 Plan and Inducement Plan [Member] - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Options balances, number of shares (in shares)	5,250,696
Options balances, weighted average exercise price (in dollars per share)	$ 12.67
Options balances, weighted average remaining life (Year)	7 years 9 months 10 days	6 years 2 months 26 days
Options granted, number of shares (in shares)	5,338,386
Options granted, weighted average exercise price (in dollars per share)	$ 5.26
Options exercised, number of shares (in shares)	(301,254)
Options exercised, weighted average exercise price (in dollars per share)	$ 3.89
Options canceled, number of shares (in shares)	(316,494)
Options canceled, weighted average exercise price (in dollars per share)	$ 5.42
Options expired, number of shares (in shares)	(505,298)
Options expired, weighted average exercise price (in dollars per share)	$ 12.71
Options balances, number of shares (in shares)	9,466,036	5,250,696
Options balances, weighted average exercise price (in dollars per share)	$ 9.01	$ 12.67
Options exercisable, number of shares (in shares)	3,833,663
Options exercisable, weighted average exercise price (in dollars per share)	$ 13.67
Options exercisable, weighted average remaining life (Year)	5 years 6 months 21 days