Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Stock-Based Compensation Expense Related to Common Stock Options, Restricted Common Stock and Common Stock Warrants

Following is the stock-based compensation expense related to common stock options, restricted common stock and common stock warrants:

	Year Ended December 31,
	2019		2018
Research and development	$	318	$	1,944
General and administrative		1,365		2,501
Total stock-based compensation expense	$	1,683	$	4,445

Weighted Average Assumptions Used to Determine Fair Value of Options Granted

The fair value of the options granted is determined using the Black-Scholes option-pricing model. The following weighted average assumptions were used:

	2018			2018
Risk-free interest rate		2.68	%		2.47	%
Expected life of the options		6.0 years			5.7 years
Expected volatility of the underlying stock		104	%		104	%
Expected dividend rate		0	%		0	%

Summary of Stock Option Activity

The following table summarizes the stock option activity in the stock-based compensation plans:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2017		5,155,263		$	4.11
Granted		1,011,875			5.01
Forfeited/Cancelled		(1,354,330	)		7.31
Exercised		(2,098,829	)		2.00
Outstanding, December 31, 2018		2,713,979		$	4.67
Granted		530,000			4.72
Forfeited/Cancelled		(92,730	)		2.91
Exercised		(150,993	)		1.83
Outstanding, December 31, 2019		3,000,256		$	4.88		6.22	$	705
Exercisable, December 31, 2019		2,592,756		$	4.90		5.78	$	705

Summary of Additional Information Regarding Outstanding and Exercisable Options under Stock Based Compensation Plans

The following table summarizes additional information regarding outstanding and exercisable options under our stock-based compensation plans at December 31, 2019:

	Options Outstanding						Options Exercisable
Exercise Price (Range)	Number of Shares		Weighted Average Remaining Contractual Life		Weighted Average Exercise Price		Number of Shares		Weighted Average Exercise Price
			(in years)
$0.87 – 1.00		190,500		6.95	$	0.88		190,500	$	0.88
$1.01 – 3.00		677,103		6.02		2.37		677,103		2.37
$3.01 – 5.00		1,071,678		8.19		4.36		664,178		4.13
$5.01 – 8.00		878,475		4.27		6.55		878,475		6.55
$8.01 – 13.38		182,500		4.06		13.38		182,500		13.38
		3,000,256		6.22	$	4.88		2,592,756	$	4.90