Note 10 - Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Notes Tables

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

	Year Ended December 31,
	2017		2016		2015
Research and development	$	1,109	$	1,114	$	650
General and administrative		2,118		2,229		1,514
Total	$	3,227	$	3,343	$	2,164

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

	Year Ended December 31,
	2017			2016			2015
Weighted average expected volatility		94.5	%		84.4	%		79.8	%
Expected term (in years)		6.06			6.05			6.11
Risk free interest rate		2.09	%		1.45	%		1.76	%
Expected dividend yield		0	%		0	%		0	%

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

	Options			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2016		2,166,254		$	8.55		7.1	$	124
Granted		719,337		$	1.99
Exercised		(74,001	)	$	1.45
Forfeited		(30,272	)	$	10.69
Expired		(100,246	)	$	2.60
Outstanding at December 31, 2017		2,681,072		$	7.18		6.8	$	121
Exercisable at December 31, 2017		1,495,789		$	8.18		5.9	$	85
Vested or expected to vest at December 31, 2017 (1)		2,681,072		$	7.18		6.8	$	121