6. Stock-Based Compensation

The Company’s 2011 Stock Incentive Plan (the “2011 Plan”) is administered by the Company’s Board of Directors 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.

At June 30, 2018, there were 0.6 million shares remaining available for grant under the 2011 Plan.

During the six months ended June 30, 2018 and 2017, the Company issued options to purchase 0.6 million and 1.9 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 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.

The fair value of stock options granted to employees during the six months ended June 30, 2018 and 2017 were estimated at the date of grant using the following assumptions:

	Six Months Ended June 30,
	2018	2017
Risk-free interest rate	2.3 – 2.9%	1.7 – 2.1%
Expected dividend yield	0%	0%
Expected term	5.3 – 5.8 years	5.7 – 6.1 years
Expected volatility	62 – 63%	65 – 68%

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 expected term. 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. Management’s assumptions do not include an estimated forfeiture rate.

The Company recognized stock-based compensation expense during the three and six months ended June 30, 2018 and 2017 as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2018		2017		2018		2017
Employee awards:
Research and development expense	$	280	$	4,916	$	614	$	5,383
General and administrative expense		500		3,818		930		4,146
Total stock-based compensation expense	$	780	$	8,734	$	1,544	$	9,529

The following table summarizes stock option activity during the six months ended June 30, 2018:

						Weighted-Average Remaining		Aggregate
(in thousands, except per share amounts)	Options			Weighted-Average Exercise Price		Contractual Term (in years)		Intrinsic Value
Outstanding at December 31, 2017		1,616		$	22.07		6.65	$	60,031
Granted		625		$	10.64
Exercised		—		$	—
Forfeited		(298	)	$	22.04
Outstanding at June 30, 2018		1,943		$	18.40		6.98	$	—
Vested and expected to vest at June 30, 2018		1,943		$	18.40		6.98	$	—
Exercisable at June 30, 2018		1,028		$	23.81		4.92	$	—

The weighted-average grant date fair value per share of stock options granted during the six months ended June 30, 2018 and 2017 was $6.19 and $3.80, respectively.

The aggregate intrinsic value is 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 stock options exercised during the three and six months ended June 30, 2017 was $0.9 million and $2.3 million, respectively. There were no options exercised during the three and six months ended June 30, 2018.

As of June 30, 2018, there was $6.4 million of total unrecognized stock-based compensation expense related to unvested employee stock awards. The Company expects to recognize this expense over a weighted-average period of approximately 2.31 years.