Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

The following table summarizes stock option activity during the three months ended March 31, 2019:

	Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term		Aggregate Intrinsic Value
Outstanding at January 1, 2019		9,583,349		$	2.28
Granted		712,500		$	0.60
Exercised		—		$	—
Forfeited		(106,030	)	$	2.28
Outstanding at March 31, 2019		10,189,819		$	2.17		7.37	$	524,000
Exercisable at March 31, 2019		5,926,360		$	2.21		6.54	$	228,000

Assumptions used in Black-Scholes Pricing Model for New Grants

The fair value of stock options subject only to service or performance conditions that are granted to employees is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions noted the following table:

	Three Months Ended March 31,
	2019		2018
Volatility factor	88.27% - 90.49%		80.18% - 83.40%
Expected term (in years)	5.50 - 6.25		5.50 - 6.25
Risk-free interest rates	2.24% - 2.55%		2.64%
Dividend yield		—		—