8. STOCK-BASED COMPENSATION

Stock option activity under the Company’s 2012 Equity Incentive Plan (the “2012 Plan”) and 2013 Equity Incentive Plan (the “2013 Plan”) for the three months ended March 31, 2019 is summarized as follows:


	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2018		3,339,471		$	1.03		8.1	$	—
Cancelled		(1,536,304	)		0.57		—		—

Outstanding at March 31, 2019		1,803,167		$	1.03		7.2	$	—

Vested and expected to vest at March 31, 2019		1,766,323		$	1.41		7.2	$	—

Exercisable at March 31, 2019		1,723,797		$	1.37		7.1	$	—

As of March 31, 2019, the weighted average grant date fair value of vested options and options outstanding was $1.82.

Stock-Based Compensation Expense

The Company did not grant any stock options to employees during the three ended March 31, 2019 but did grant stock options to employees during the three months ended March 31, 2018. The Company estimates the fair value of stock options as of the date of grant using the Black-Scholes option pricing model and restricted stock based on the stock price, with the exception of those stock options that included a market condition. The Company estimates the fair value of stock options that include a market condition using a Monte-Carlo model. Stock options and restricted stock issued to non-board member, non-employees are accounted for using the fair value approach and are subject to periodic revaluation over their vesting terms.

Stock-based compensation expense amounted to $0.1 million and $0.4 million for the three months ended March 31, 2019 and 2018, respectively.

The allocation of stock-based compensation for all options granted and restricted stock awards is as follows:


	Three Months Ended March 31,
	2019			2018
	(in thousands)
Research and development	$	(38	)	$	96
General and administrative		145			307

Total stock-based compensation expense	$	107		$	403

The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants and non-employee stock options grants for the three months ended March 31, 2018 were as follows:


	Three Months Ended March 31, 2018
	Employees			Non-employees
Risk-free interest rate		2.70	%		1.97	%
Expected volatility		88.2	%		74.0	%
Expected term (in years)		6.08			6.08
Expected dividend yield		0.0	%		0.0	%