Note 2 - Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Notes Tables

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]

	Year Ended December 31,
	2018		2017		2016
Research and development	$	696	$	413	$	2,095
General and administrative		964		2,063		3,478
Total share-based compensation expense	$	1,660	$	2,476	$	5,573

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

	Year Ended December  31,
	2018			2017			2016
Weighted-average grant date fair value per share		$4.32			$4.85			$7.29
Expected volatility	67%	-	80%	65%	-	67%	65%	-	70%
Risk-free interest rate	2.38%	-	3.12%	1.83%	-	2.41%	1.09%	-	2.44%
Expected life (in years)	1	–	10	5.38	–	10	5	–	10
Dividend yield		–			–			–