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Note 1 - Organization and Operations (Details Textual) - USD ($)
12 Months Ended
Aug. 02, 2017
Nov. 12, 2015
Dec. 31, 2018
Dec. 31, 2017
Cash, Cash Equivalents, and Short-term Investments, Total     $ 21,900,000  
Retained Earnings (Accumulated Deficit), Ending Balance     $ (210,470,000) $ (189,741,000)
Preferred Stock, Par or Stated Value Per Share     $ 0.001 $ 0.001
Series A Convertible Preferred Stock [Member]        
Stock Issued During Period, Shares, New Issues 22,000      
Payments of Stock Issuance Costs $ 500,000      
Preferred Stock, Par or Stated Value Per Share $ 0.001   $ 0.001 $ 0.001
Shares Issued, Price Per Share $ 1,000      
Proceeds from Issuance of Convertible Preferred Stock $ 22,000,000      
Common Stock [Member]        
Stock Issued During Period, Shares, New Issues     1,494,579 896,811
Sales Agreement [Member] | Cowen and Company, LLC [Member] | Common Stock [Member]        
Sale of Stock, Maximum Value of Stock Offered   $ 40,000,000    
Sale of Stock, Commission to Sales Agent, Percentage     3.00%  
Stock Issued During Period, Shares, New Issues       896,811
Proceeds from Issuance of Common Stock       $ 1,400,000
Payments of Stock Issuance Costs       $ 100,000
Sales Agreement [Member] | New Leaf Venture Partners LLC [Member] | Common Stock [Member]        
Stock Issued During Period, Shares, New Issues     1,494,579  
Proceeds from Issuance of Common Stock     $ 3,000,000  
Payments of Stock Issuance Costs     $ 46,000