In 2008, the Company adopted the 2008 Stock Incentive Plan (as amended, the “2008 Plan”) for employees, officers, directors, consultants and advisors. The 2011 Stock Incentive Plan (the “2011 Plan”) became effective upon closing of the Company’s initial public offering in April 2012. Upon effectiveness of the 2011 Plan, no further awards were available to be issued under the 2008 Plan. The 2011 Plan is administered by the Board and permits the Company to grant incentive and non-qualified stock options, stock appreciation rights, restricted stock, restricted stock units and other stock-based awards. Additional shares also become available for grant by reason of the forfeiture, cancellation, expiration or termination of existing awards. The Company registered 0.5 million, 0.5 million, 0.4 million, 0.4 million, 0.4 million and 0.3 million additional shares of common stock related to the 2011 Plan in March 2019, March 2018, February 2016, February 2015, March 2014 and February 2013, respectively. As of December 31, 2019, there were 1.0 million shares remaining available for grant under the 2011 Plan.

The Board authorized and declared a special cash dividend of $20.0 million to holders of the Company's common stock, which was payable on September 5, 2019 to stockholders of record as of the close of business on August 28, 2019 (the “September Special Dividend”). The Board authorized and declared a special cash dividend of $6.7 million on the Company's common stock, which was payable on December 23, 2019 to stockholders of record as of the close of business on December 16, 2019 (the “December Special Dividend”). The Board determined, in accordance with the adjustment provision of the 2011 Plan, that the special cash dividends were unusual and non-recurring and that appropriate adjustment to the stock options to purchase shares of the Company’s common stock outstanding under the 2011 Plan was required. The Company treated those adjustments as a modification to the original stock option grant because the terms of the agreements were modified in order to preserve the value of the option awards after a large non-recurring cash dividend. These options were amended to decrease the exercise price and increase the shares subject to the stock option.

The calculation of the incremental compensation expense is based on the excess of the fair value of the award measured immediately before and after the modification. As a result, the Company recognized an incremental compensation expense of $0.1 million associated with the modification arising from the December Special Dividend for the year ended December 31, 2019. However, as the fair value of the underlying stock decreased more than the exercise price upon modification arising from the September Special Dividend, and the result was a decrease in the fair value of such options. Accordingly, no incremental value was provided and no additional compensation cost was recorded by the Company for the September Special Dividend.

During the years ended December 31, 2019 and 2018, the Company issued options to purchase 3.9 million and 1.2 million shares of common stock, respectively. Stock options granted to employees vest over a three-year period. Stock options granted to non-employee directors prior to 2018 generally vested immediately. Stock options granted to non-employee directors in 2019 and 2018 vest over a one-year period, ending on the earlier of the one-year anniversary of the grant date or the day prior to the Company’s next annual meeting of stockholders after the grant date. Stock options granted to non-employee consultants in 2019 vest over a two-year period.

The fair value of stock options granted to employees during the years ended December 31, 2019 and 2018 was estimated at the date of grant using the following assumptions:

	Years Ended December 31,
	2019	2018
Risk-free interest rate	1.6 – 1.8%	2.3 – 2.9%
Expected dividend yield	0%	0%
Expected term	5.3 – 5.4 years	5.3 – 5.8 years
Expected volatility	67%	62 – 64%

The Company uses the simplified method to calculate the expected term, as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term of its stock options. Under this approach, the expected term is calculated to be the average of the ten-year contractual term of the option and the weighted-average vesting term of the option, taking into consideration multiple vesting tranches. The computation of expected volatility is based on the historical volatility of comparable companies from a representative peer group selected based on industry and market capitalization. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. Stock-based compensation expense related to employee stock options is measured using the fair value of the award at the grant date and is adjusted quarterly to reflect actual forfeitures. Stock-based compensation expense is then recognized on a straight-line basis over the vesting period, which is also the requisite service period.

The Company recognized stock-based compensation expense during the years ended December 31, 2019 and 2018 as follows:

	Years Ended December 31,
(in thousands)	2019		2018
Research and development expense	$	223	$	1,092
General and administrative expense		2,148		1,958
Total stock-based compensation expense	$	2,371	$	3,050

The following table summarizes stock option activity during the year ended December 31, 2019:

(in thousands, except per share amounts)	Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2018		2,233		$	14.97		7.66	$	—
Granted		3,910		$	12.67
Exercised		(37	)	$	3.90
Forfeited		(4,182	)	$	14.32
Outstanding at December 31, 2019		1,924		$	11.93		6.71	$	12
Vested and expected to vest at December 31, 2019		1,924		$	11.93		6.71	$	12
Exercisable at December 31, 2019		1,360		$	14.14		5.97	$	2

The weighted-average grant date fair value of stock options granted during the years ended December 31, 2019 and 2018 was $8.41 and $4.47, respectively.

The aggregate intrinsic value was calculated as the difference between the exercise price of the stock options and the fair value of the underlying common stock. The aggregate intrinsic value of options exercised during the year ended December 31, 2019 was $0.1 million. There were no option exercises during the year ended December 31, 2018.

As of December 31, 2019, there was $2.0 million of total unrecognized stock-based compensation expense related to unvested employee stock options. The Company expects to recognize this expense over a weighted-average period of approximately 1.25 years.