Equity

A summary of the stock option activity under the Company’s plans for the six months ended June 30, 2018 is as follows:



	Number of shares	Weighted average price
Options outstanding at December 31, 2017	6,342,526	$	6.51
Options granted	2,168,602	$	8.34
Less:
Options exercised	1,030,576	$	5.53
Options canceled or expired	25,498	$	8.72
Options outstanding at June 30, 2018	7,455,054	$	7.18
Exercisable at June 30, 2018	2,958,222	$	6.14

Of the 7,455,054 outstanding options at June 30, 2018, 840,001 were stock options issued to non-employees of the Company of which 416,663 options were vested and 423,338 options were unvested as of June 30, 2018.

The fair value of each stock option award granted during the six months ended June 30, 2018 was estimated as of the grant date using a trinomial lattice model with the following weighted average assumptions:



	Six Months Ended June 30, 2018
Expected term (in years)	2.0 - 4.0
Risk-free interest rate (%)	2.4%
Expected volatility (%)	35.6% - 45.5%
Dividend yield (%)	—
Weighted average fair value/share at grant date	$2.61

As of June 30, 2018, there was approximately $4.5 million of unrecognized share based compensation expense related to stock options that will be recognized over a weighted-average period of approximately 0.8 years.

A summary of the restricted stock activity under the Company’s plans for the six months ended June 30, 2018 is as follows:



	Number of shares		Weighted average price
Nonvested at December 31, 2017	327,211		$	7.27
Granted	41,382		$	11.60
Vested	(119,180	)	7.27
Nonvested at June 30, 2018	249,413		$	7.99

Stock based compensation expense recognized for stock options and restricted stock and included in the consolidated statements of operations was allocated as follows (in thousands):



	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development expense	$	251	$	285	$	269	$	327
General and administrative expense	2,082		1,637		3,688		2,725
Total stock based compensation expense	$	2,333	$	1,922	$	3,957	$	3,052

Stock based compensation recorded in research and development relates to unvested options granted to a non-employee.

Employee Stock Purchase Plan

We offer an employee stock purchase plan (“ESPP”) through which eligible employees may purchase shares of our common stock at a discount. On May 25, 2017, the Company amended the ESPP, increasing the discount from 5% to 15% of the fair market value of the Company’s common stock. As a result of this change, we began recording stock based compensation expense related to the ESPP during the quarter ended September 31, 2017.

During the three months ended June 30, 2018 and 2017, employees purchased 29,266 and 26,569 shares, respectively under the ESPP. The expense recorded for these periods was approximately $0.1 million and $0, respectively. During the six months ended June 30, 2018 and 2017, employees purchased 66,188 and 50,932 shares, respectively under the ESPP. The expense recorded for these periods was approximately $0.1 million and $0, respectively.