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, 2022	Dec. 31, 2021
Options balances, number of shares (in shares)	5,996,813
Options balances, weighted average exercise price (in dollars per share)	$ 15.77
Options balances, weighted average remaining life (Year)	6 years 2 months 26 days	7 years 3 months 29 days
Options granted, number of shares (in shares)	1,440,100
Options granted, weighted average exercise price (in dollars per share)	$ 2.97
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)	(1,472,385)
Options canceled, weighted average exercise price (in dollars per share)	$ 14.88
Options expired, number of shares (in shares)	(713,832)
Options expired, weighted average exercise price (in dollars per share)	$ 14.62
Options balances, number of shares (in shares)	5,250,696	5,996,813
Options balances, weighted average exercise price (in dollars per share)	$ 12.67	$ 15.77
Options exercisable, number of shares (in shares)	3,479,531
Options exercisable, weighted average exercise price (in dollars per share)	$ 15.17
Options exercisable, weighted average remaining life (Year)	5 years 21 days