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 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 March 31, 2019, the Company had options outstanding to purchase 1,367,077 shares of its common stock, which includes options outstanding under its 2006 Stock Plan that was terminated in June 2015. As of March 31, 2019, 958,345 shares were available for future issuance.

The following table summarizes stock option activity during the three months ended March 31, 2019:


		Weighted
		average
		exercise
	Shares	price
Outstanding at January 1, 2019	1,132,618	$	48.52
Options granted	242,500		126.45
Options exercised	(8,041)		10.36
Options cancelled	—		—
Outstanding at March 31, 2019	1,367,077	$	62.57
Exercisable at March 31, 2019	673,303	$	20.31

The total cash received by the Company as a result of stock option exercises was $0.1 million and $0.3 million, respectively, for the three months ended March 31, 2019 and 2018. The weighted-average grant date fair values, based on the Black-Scholes option model, of options granted during the three months ended March 31, 2019 and 2018 were $101.08 and $96.57, 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 52,063 unvested restricted shares outstanding at a weighted average grant date fair value of $9.45 at March 31, 2019 and December 31, 2018.

Stock-Based Compensation Expense

Stock-based compensation expense during the three months ended March 31, 2019 and 2018 was as follows (in thousands):


	Three Months Ended March 31,
	2019		2018
Stock-based compensation expense by type of award:
Stock options	$	5,901	$	1,046
Restricted stock		121		121
Total stock-based compensation expense	$	6,022	$	1,167

Effect of stock-based compensation expense by line item:
Research and development	$	1,877	$	376
General and administrative		4,145		791
Total stock-based compensation expense included in net loss	$	6,022	$	1,167

Unrecognized stock-based compensation expense as of March 31, 2019 was as follows (in thousands):


			Weighted
	Unrecognized		average
	stock		remaining
	compensation		period
	expense		(in years)
Stock options	$	40,286	3.16
Restricted stock		151	0.31
Total	$	40,437	3.14