Note 6 - Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

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,
	2018		2017		2018		2017
Research and development	$	16,274	$	24,072	$	32,646	$	67,404
General and administrative		303,495		290,994		611,790		614,045
Total stock-based compensation expense	$	319,769	$	315,066	$	644,436	$	681,449

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, 2018		2,555,989		$	7.32
Granted		660,000			1.02
Expired		(2,192	)		17.10
Outstanding at June 30, 2018		3,213,797		$	6.02		7.25
Exercisable at June 30, 2018		2,185,235		$	7.46		6.41

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

	2018			2017
Expected term (in years)		5.57			5.68
Risk-free interest rate		2.4	%		2.0	%
Expected volatility		114.7	%		114.9	%
Dividend yield		—	%		—	%