Note 11 - Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 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		941		$	11.30
Exercised		(35	)	$	6.62
Cancelled		(46	)	$	6.36
Expired		(28	)	$	26.66
Outstanding at March 31, 2017		5,719		$	8.11		6.29
Exercisable at March 31, 2017		3,889		$	7.96		4.86
Vested and expected to vest at March 31, 2017		5,285		$	8.04		6.03

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

	Three Months Ended
	March 31,
	2017			2016
Risk-free interest rate		2.22	%		1.55	%
Expected life (in years)		6.73			6.71
Expected volatility		72	%		75	%
Expected dividend yield		--			--

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

	Three Months Ended
	March 31,
	2017		2016
Research and development expenses	$	235	$	275
General and administrative expenses		368		301
Total stock-based compensation expense	$	603	$	576