Note 9 - Stock-based Compensation (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Notes Tables

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

	Year Ended December 31,
	2018		2017		2016
Research and development	$	1,142	$	1,109	$	1,114
General and administrative		2,287		2,118		2,229
Total	$	3,429	$	3,227	$	3,343

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

	Year Ended December 31,
	2018			2017			2016
Weighted average expected volatility		93.5	%		94.5	%		84.4	%
Expected term (in years)		6.07			6.06			6.05
Risk free interest rate		2.55	%		2.09	%		1.45	%
Expected dividend yield		0	%		0	%		0	%

Share-based Payment Arrangement, Option, Activity [Table Text Block]

	Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2018		4,597,226		$	5.12		7.4	$	404
Granted		1,182,500		$	2.66
Exercised		-
Forfeited (1)		(2,606,289	)	$	2.79
Expired (1)		(2,400,590	)	$	6.51
Outstanding at September 30, 2019		772,847		$	4.89		7.3	$	-
Exercisable at September 30, 2019		476,390		$	6.13		6.4	$	-
Vested or expected to vest at September 30, 2019 (2)		772,847		$	4.89		7.3	$	-

	Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2017		2,681,072		$	7.18		6.8	$	121
Granted		2,041,600		$	2.61
Exercised		-
Forfeited		(83,433	)	$	5.82
Expired		(42,013	)	$	13.30
Outstanding at December 31, 2018		4,597,226		$	5.12		7.4	$	404
Exercisable at December 31, 2018		2,067,356		$	7.57		5.7	$	241
Vested or expected to vest at December 31, 2018 (1)		4,597,226		$	5.12		7.4	$	404