Note 6 - Stockholders' Equity and Stock-based Compensation - Summary of Stock Option Activity (Details) - The 2015 Plan, 2017 Plan and Inducement Plan [Member] - $ / shares	3 Months Ended	12 Months Ended
	Mar. 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)	6 years 10 months 24 days	6 years 2 months 26 days
Options granted, number of shares (in shares)	807,300
Options granted, weighted average exercise price (in dollars per share)	$ 2.92
Options exercised, number of shares (in shares)	0
Options exercised, weighted average exercise price (in dollars per share)	$ 0
Options canceled, number of shares (in shares)	(68,894)
Options canceled, weighted average exercise price (in dollars per share)	$ 11.24
Options expired, number of shares (in shares)	(380,892)
Options expired, weighted average exercise price (in dollars per share)	$ 13.07
Options balances, number of shares (in shares)	5,608,210	5,250,696
Options balances, weighted average exercise price (in dollars per share)	$ 11.25	$ 12.67
Options exercisable, number of shares (in shares)	3,434,715
Options exercisable, weighted average exercise price (in dollars per share)	$ 14.72
Options exercisable, weighted average remaining life (Year)	5 years 7 months 28 days