Note 11 - Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 2017

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, 2017		4,887		$	7.57		5.81
Granted		1,134		$	10.57
Exercised		(80	)	$	5.89
Cancelled		(151	)	$	8.94
Expired		(288	)	$	22.48
Outstanding at September 30, 2017		5,502		$	7.40		6.16
Exercisable at September 30, 2017		3,757		$	6.94		4.91
Vested and expected to vest at September 30, 2017		5,166		$	7.30		5.97

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

	Three Months Ended						Nine Months Ended
	September 30,						September 30,
	2017			2016			2017			2016
Risk-free interest rate		1.82	%		1.17	%		2.17	%		1.53	%
Expected life (in years)		5.19			5.20			6.76			6.77
Expected volatility		74	%		77	%		72	%		74	%
Expected dividend yield		--			--			--			--

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

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2017		2016		2017		2016
Research and development expenses	$	380	$	186	$	941	$	635
General and administrative expenses		555		236		2,183		1,323
Total stock-based compensation expense	$	935	$	422	$	3,124	$	1,958