In December 2013, the Company adopted the 2013 Equity Incentive Plan (as subsequently amended and restated, the “Plan”), which provides for the issuance of options, stock appreciation rights, stock awards and stock units. On January 1, 2018, in accordance with the terms of the Plan, the total shares authorized for issuance under the plan increased by 750,000 to 6,531,333. This increase represents the lesser of 750,000 shares or 4% of the total shares outstanding calculated as of the end of the most recent fiscal year. The exercise price per share shall not be less than the fair value of the Company’s underlying common stock on the grant date and no option may have a term in excess of ten years. In December 2017, the Company issued inducement awards in the form of an option to purchase 775,000 shares of the Company’s common stock with an exercise price of $6.05 per share and a restricted stock unit award to acquire 40,000 shares of the Company’s common stock, to Rick Russell in connection with his appointment as the Company’s President. These inducement awards are outside of, but subject to the terms generally consistent with, the Plan, as a material inducement to Mr. Russell’s acceptance of employment with the Company in accordance with Nasdaq Listing Rule 5635(c)(4). In June 2018, the Company increased the aggregate number of shares of common stock authorized for issuance under the Plan by 2,500,000 shares. Stock option activity for employees and non-employees for the three months ended March 31, 2019 is as follows:

	Shares Issuable Pursuant to Stock Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Total Intrinsic Value (in thousands)
Outstanding January 1, 2019		8,498,047		$	6.99		8.1	$	5,214

Granted		—		$	—
Exercised		(87,500	)	$	6.00
Forfeited		(31,875	)	$	6.51
Outstanding March 31, 2019		8,378,672		$	7.00		7.8	$	11,212
Exercisable March 31, 2019		4,078,271		$	6.46		6.8	$	7,635
Available for future grant		801,654

The weighted average grant-date fair value of stock options outstanding on March 31, 2019 was $5.17 per share. Total unrecognized compensation costs related to non-vested stock options at March 31, 2019 was approximately $21.2 million and is expected to be recognized within future operating results over a weighted-average period of 3.0 years. The total intrinsic value of the options exercised during the three months ended March 31, 2019 was approximately $0.2 million. No options were exercised during the three months ended March 31, 2018.

The expected term of the employee-related options was estimated using the “simplified” method as defined by the SEC’s Staff Accounting Bulletin No. 107, Share-Based Payment. The volatility assumption was determined by examining the historical volatilities for industry peer companies, as the Company did not have sufficient trading history for its common stock. The risk-free interest rate assumption is based on the U.S. Treasury instruments whose term was consistent with the expected term of the options. The dividend assumption is based on the Company’s history and expectation of dividend payouts. The Company has never paid dividends on its common stock and does not anticipate paying dividends on its common stock in the foreseeable future. Accordingly, the Company has assumed no dividend yield for purposes of estimating the fair value of the options.

The Company uses the Black-Scholes model to estimate the fair value of stock options granted. For stock options granted to employees during the three months ended March 31, 2019 and 2018, the Company utilized the following assumptions:

	March 31,
	2019⁽¹⁾		2018
Expected term (years)		—		6.25
Risk free interest rate		—		2.33	%
Volatility		—		83	%
Dividend yield		—		0	%
Weighted average grant date fair value per share of common stock		—	$	4.47

(1)	There were no stock options granted to employees during the three months ended March 31, 2019.

The Company from time to time grants options to purchase common stock to non-employees for services rendered and records expense ratably over the vesting period of each award. The Company estimates the fair value of the stock options using the Black-Scholes valuation model at each reporting date. The Company granted zero stock options to non-employees and recorded stock-based compensation expense of $0.2 million during the three months ended March 31, 2019. The Company granted 40,000 stock options to non-employees and recorded stock-based compensation expense of $0.2 million during the three months ended March 31, 2018.

For stock options granted to non-employees, the Company utilized the following assumptions:

	March 31,
	2019⁽¹⁾		2018
Expected term (years)		—	8.4-9.8
Risk free interest rate		—	2.71-2.74%
Volatility		—	85-113%
Dividend yield		—		0	%
Weighted average reporting date fair value per share of common stock		—	$	5.56

(1)	There were no stock options granted to non-employees during the three months ended March 31, 2019.

RSU activity under the Plan for the three months ended March 31, 2019 is as follows:

			Weighted- Average
			Grant Date
	RSUs		Fair Value
Unvested January 1, 2019		127,300	$	11.71
Granted		—	$	—
Vested		—	$	—
Forfeited		—	$	—
Unvested March 31, 2019		127,300	$	11.71

RSUs awarded to employees generally vest one-fourth per year over four years from the anniversary of the date of grant, provided the employee remains continuously employed with the Company. Shares of the Company’s stock are delivered to the employee upon vesting, subject to payment of applicable withholding taxes. The fair value of RSUs is equal to the closing price of the Company’s common stock on the date of grant. Total unrecognized compensation costs related to non-vested RSUs at March 31, 2019 was approximately $1.3 million and is expected to be recognized within future operating results over a period of 1.8 years. The following table presents stock-based compensation expense included in the Company’s consolidated statements of operations:

	Three Months Ended
	March 31,
	2019		2018
Research and development	$	700,263	$	524,146
General and administrative		1,761,436		1,589,790
Total	$	2,461,699	$	2,113,936