Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2023

Stock-Based Compensation [Abstract]

Components of Stock-Based Compensation Expense

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

	Year Ended December 31,
	2023		2022
Research and development	$	829	$	810
General and administrative		1,432		2,057
Total stock-based compensation expense	$	2,261	$	2,867

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:

	2023			2022
Risk-free interest rate		3.84	%		1.85	%
Expected life of the options	5.6 years			5.7 years
Expected volatility of the underlying stock		85.6	%		93.7	%
Expected dividend rate		0	%		0	%

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, 2021		4,895,561		$	3.14
Granted		1,070,000			1.75
Forfeited/Cancelled		(220,000	)		2.68
Exercised		—			—
Outstanding, December 31, 2022		5,745,561		$	2.90
Granted		1,025,000			1.18
Forfeited/Cancelled		(436,720	)		2.23
Exercised		—			—
Outstanding, December 31, 2023		6,333,841		$	2.66		6.77	$	632
Exercisable, December 31, 2023		4,541,761		$	2.79		6.54	$	447

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, 2023:

	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		120,500		2.96	$	0.89		120,500	$	0.89
$1.01 – 3.00		5,046,604		7.45		2.01		3,302,857		2.09
$3.01 – 5.00		809,237		4.98		4.28		760,904		4.71
$5.01 – 8.00		220,000		4.11		5.92		220,000		5.92
$8.01 – 13.38		137,500		0.06		13.38		137,500		13.38
		6,333,841		6.77	$	2.66		4,541,761	$	2.79