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 March 31, 2018, there were 0.5 million shares remaining available for grant under the 2011 Plan.

During the three months ended March 31, 2018 and 2017, the Company issued options to purchase 0.6 million and 0.0 million shares of common stock, respectively. These options generally vest over a three-year period for employees. Options granted to directors generally vest immediately.

The fair value of stock options granted to employees during the three months ended March 31, 2018 and 2017 was estimated at the date of grant using the following assumptions:

	Three Months Ended March 31,
	2018	2017
Risk-free interest rate	2.3 – 2.7%		2.1	%
Expected dividend yield	0%	0%
Expected term	5.8 years	5.8 years
Expected volatility	62 – 63%	67 – 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 estimates expected forfeitures based on historical experience and recognizes compensation costs only for those equity awards expected to vest.

The Company recognized stock-based compensation expense during the three months ended March 31, 2018 and 2017 as follows:

	Three Months Ended March 31,
(in thousands)	2018		2017
Employee awards:
Research and development expense	$	334	$	468
General and administrative expense		430		328
Total stock-based compensation expense	$	764	$	796

The following table summarizes stock option activity during the three months ended March 31, 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		566		$	10.74
Exercised		—		$	—
Forfeited		(145	)	$	21.04
Outstanding at March 31, 2018		2,037		$	19.00		7.22	$	—
Vested and expected to vest at March 31, 2018		2,036		$	19.00		7.22	$	—
Exercisable at March 31, 2018		1,032		$	25.17		4.97	$	—

The weighted-average grant date fair value per share of stock options granted during the three months ended March 31, 2018 and 2017 was $6.26 and $24.30, 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 months ended March 31, 2017 was $1.4 million. There were no options exercised during the three months ended March 31, 2018.

As of March 31, 2018, there was $7.2 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 2.58 years.