Note 11 - Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 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		1,511		$	6.91
Exercised		-			N/A
Cancelled		(270	)	$	7.77
Expired		(3	)	$	18.03
Outstanding at June 30, 2018		6,773		$	7.15	6.34
Exercisable at June 30, 2018		4,546		$	6.96	5.00
Vested and expected to vest at June 30, 2018		6,250		$	7.12	6.09

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

	Three Months Ended						Six Months Ended
	June 30,						June 30,
	2018			2017			2018			2017
Risk-free interest rate		2.91	%		1.95	%		2.71	%		2.17	%
Expected life (in years)		7.15			7.04			6.72			6.78
Expected volatility		70	%		72	%		69	%		72	%
Expected dividend yield		--			--			--			--

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

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2018		2017		2018		2017
Research and development expenses	$	452	$	326	$	977	$	561
General and administrative expenses		1,678		1,260		2,201		1,628
Total stock-based compensation expense	$	2,130	$	1,586	$	3,178	$	2,189