Note 11 - Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

Notes Tables

Share-based Compensation, Stock Options, Activity [Table Text Block]

						Weighted
				Weighted		Average
	Number			Average		Remaining
	of Shares			Exercise Price		Contractual Life
Outstanding at January 1, 2018		5,535		$	7.25		6.04
Granted		1,313		$	6.61
Exercised		-			N/A
Cancelled		(24	)	$	8.78
Expired		(3	)	$	18.03
Outstanding at March 31, 2018		6,821		$	7.12		6.57
Exercisable at March 31, 2018		4,349		$	6.88		5.04
Vested and expected to vest at March 31, 2018		6,181		$	7.08		6.28

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

	Three Months Ended
	March 31,
	2018		2017
Risk-free interest rate		2.68%		2.22%
Expected life (in years)		6.66		6.73
Expected volatility		69%		72%
Expected dividend yield		--		--

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

	Three Months Ended
	March 31,
	2018		2017
Research and development expenses	$	525	$	235
General and administrative expenses		523		368
Total stock-based compensation expense	$	1,048	$	603