6. Stock-based Compensation

In June 2015, upon obtaining stockholder approval at its annual shareholder meeting, the Company implemented its new 2015 Stock Plan. The 2015 Stock Plan replaced the 2006 Stock Plan, which was terminated upon adoption of the 2015 Stock Plan. Shares of common stock reserved for outstanding awards under the 2006 Stock Plan that lapse or are canceled will be added back to the share reserve available for future awards under the 2015 Stock Plan. The 2015 Stock Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock and other stock-based compensation awards to employees, officers, directors, and consultants of the Company. The administration of the 2015 Stock Plan is under the general supervision of the compensation committee of the board of directors. The exercise price of the stock options is determined by the compensation committee of the board of directors, provided that incentive stock options are granted with an exercise price not less than fair market value of the common stock on the date of grant and expire no later than ten years from the date the option is granted. As of June 30, 2018, the Company had options outstanding to purchase 1,137,507 shares of its common stock, which includes options outstanding under its 2006 Stock Plan that was terminated in June 2015. As of June 30, 2018, 1,239,595 shares were available for future issuance.

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


		Weighted
		average
		exercise
	Shares	price
Outstanding at January 1, 2018	976,777	$	11.97
Options granted	218,900		164.00
Options exercised	(58,170)		24.90
Options canceled	—		—
Outstanding at June 30, 2018	1,137,507	$	40.57
Exercisable at June 30, 2018	478,898	$	11.19

The total cash received by the Company as a result of stock option exercises was $1.4 million and $0, respectively, for the six months ended June 30, 2018 and 2017. The weighted-average grant date fair values, based on the Black-Scholes option model, of options granted during the six months ended June 30, 2018 and 2017 were $129.03 and $12.68, respectively.

Restricted Common Stock

The Company's share-based compensation plan provides for awards of restricted shares of common stock to employees, officers, directors and consultants to the Company. Restricted stock awards are subject to forfeiture if employment or service terminates during the prescribed retention period. Restricted shares vest over the service period. There were 104,127 unvested restricted shares outstanding at a weighted average grant date fair value of $9.45 at June 30, 2018 and December 31, 2017.

Stock-Based Compensation Expense

Stock-based compensation expense during the three and six months ended June 2018 and 2017 was as follows (in thousands):


	Three Months				Six Months
	Ended June 30,				Ended June 30,
	2018		2017		2018		2017
Stock-based compensation expense by type of award:
Stock options	$	2,104	$	527	$	3,150	$	981
Restricted stock		123		149		244		297
Total stock-based compensation expense	$	2,227	$	676	$	3,394	$	1,278

Effect of stock-based compensation expense by line item:
Research and development	$	732	$	168	$	1,108	$	305
General and administrative		1,495		508		2,286		973
Total stock-based compensation expense included in net loss	$	2,227	$	676	$	3,394	$	1,278

Unrecognized stock-based compensation expense as of June 30, 2018 was as follows (in thousands):


			Weighted
	Unrecognized		average
	stock		remaining
	compensation		period
	expense		(in years)
Stock options	$	26,400	2.38
Restricted stock		520	1.06
Total	$	26,920	2.35