In March 2017, the Company’s Board of Directors adopted, and the stockholders approved, the 2017 Stock Option and Incentive Plan (the “2017 Plan”), that became effective in April 2017. The 2017 Plan provides for the issuance of incentive awards up to 4,600,000 shares of common stock to officers, employees, consultants and directors, less the number of shares subject to issued and outstanding awards under the 2011 Plan that were assumed in the Merger. The 2017 Plan also provides that the number of shares reserved for issuance thereunder will be increased annually on the first day of each year beginning in 2018 by four percent (4%) of the shares of our common stock outstanding on the last day of the immediately preceding year or such smaller increase as determined by our Board of Directors. In March 2019, the Company’s Board of Directors approved a 4% increase, adding 1,623,520 shares to the 2017 Plan, which was effective as of January 1, 2019.

The options granted generally vest over 48 months. Under the 2017 Plan, options vest in installments of 25% at the one-year anniversary and thereafter in 36 equal monthly installments beginning on the 1^st of the month after the one-year anniversary date, subject to the employee’s continuous service with the Company. In May 2019, the Company issued a special retention grant of options to purchase an aggregate of 2,419,050 shares of common stock which vest in installments of 50% at June 30, 2020 and 50% at June 30, 2021, subject to the employee’s continuous service with the Company. The options generally expire ten years after the date of grant. The fair value of the options at the date of grant is recognized as an expense over the requisite service period. During the three months ended June 30, 2019 and 2018, option awards to purchase an aggregate of 2,801,600 and 1,515,200 shares of common stock were granted, respectively, and option awards to purchase an aggregate of 4,622,428 and 1,515,200 shares of common stock were granted in the six months ended June 30, 2019 and 2018, respectively.

As of June 30, 2019 and December 31, 2018, 969,529 and 2,959,562 shares are reserved for issuance under the 2017 Plan, respectively.

The following table summarizes the stock option activity during the six months ended June 30, 2019:

	Stock Option Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in Years)		Aggregate Intrinsic Value
Outstanding, January 1, 2019		3,351,132		$	3.73		8.24	$	142,788
Granted		4,622,428			1.86
Exercised		—			—
Expired/cancelled		(1,201,753	)		2.80
Outstanding, June 30, 2019		6,771,807		$	2.62		8.65	$	17,133
Expected to vest, June 30, 2019		5,183,231		$	2.37	9.35		$	—
Options exercisable, June 30, 2019		1,588,576		$	3.43		6.37	$	17,133

Aggregate intrinsic value represents the estimated fair value of the Company’s common stock at in excess of the weighted average exercise price multiplied by the number of options outstanding or exercisable.

Compensation expense for stock options was $604,661 and $822,850 for the three months ended June 30, 2019 and 2018, respectively, and $1,226,346 and $1,554,223 for the six months ended June 30, 2019 and 2018, respectively. As of June 30, 2019, there was $5,956,838 of unrecognized compensation cost related to stock options, which is expected to be recognized over a weighted average period of 2.60.

The Company recognized compensation expense for restricted stock of $0 and $54,907 for the three months ended June 30, 2019 and 2018, respectively, and $0 and $403,255 for the six months ended June 30, 2019 and 2018. All restricted stock vested in October 2018.

The Company recognized the following compensation cost related to employee and non-employee stock-based compensation activity:

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	73,463	$	114,949	$	232,794	$	174,013
General and administrative		531,198		762,808		993,552		1,783,465
Total	$	604,661	$	877,757	$	1,226,346	$	1,957,478

The Company uses the Black-Scholes option pricing model to determine the estimated fair value for stock-based awards. Option pricing and models require the input of various assumptions, including the option’s expected life, expected dividend yield, price volatility and risk-free interest rate of the underlying stock. Accordingly, the weighted-average fair value of the options granted during the three months ended June 30, 2019 was $0.62 per share, and $1.11 per share for the six months ended June 30, 2019. The calculation was based on the following assumptions.

	Three Months Ended	Six Months Ended
	June 30, 2019	June 30, 2019
Expected term (years)	5.82	5.77
Risk-free interest rate	2.21%	2.32%
Expected volatility	65.97%	65.83%
Expected dividend yield	0.00%	0.00%