Note 9 - Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 2017

Notes Tables

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Research and development	$	14,333	$	174,932	$	81,737	$	601,260
General and administrative		278,168		215,425		892,213		520,730
Total stock-based compensation expense	$	292,501	$	390,357	$	973,950	$	1,121,990

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		340,041			2.49
Expired		(1,461	)		15.00
Outstanding at September 30, 2017		2,545,989		$	7.34		7.37
Exercisable at September 30, 2017		1,746,809		$	8.51		6.62

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

Expected term (in years)	6.03
Risk-free interest rate	2.0	%
Expected volatility	114.9	%
Dividend yield	—	%