Note 6 - Stock-based Compensation (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Notes Tables

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

Common Stock Reserved for Future Issuance [Table Text Block]

	September 30,		December 31,
	2019		2018
Conversion of Series A Preferred Stock		21,771,032		22,112,775
Stock-based compensation awards		6,818,214		5,163,957
Employee Stock Purchase Plan		118,120		118,120
Total		28,707,366		27,394,852

	December 31,		December 31,
	2018		2017
Conversion of Series A Preferred Stock		22,112,775		22,112,775
Stock-based compensation awards		5,163,957		3,572,457
Employee Stock Purchase Plan		118,120		192,463
Total		27,394,852		25,877,695