Note 8 - Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 2017

Notes Tables

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Research and development	$	24,072	$	184,051	$	67,404	$	426,328
General and administrative		290,994		154,111		614,045		305,305
Total stock-based compensation expense	$	315,066	$	338,162	$	681,449	$	731,633

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

	Number of Options			Weighted average exercise price per share		Weighted average remaining contractual life (in years)
Balance at January 1, 2017		2,207,409		$	8.09
Granted		320,041			2.54
Expired		(1,461	)		15.00
Outstanding at June 30, 2017		2,525,989		$	7.38		7.60
Exercisable at June 30, 2017		1,636,919		$	8.80		6.74

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

Expected term (in years)	5.68
Risk-free interest rate	2.0	%
Expected volatility	114.9	%
Dividend yield	—	%