Share-Based Compensation

Pursuant to the evergreen provisions of the Company's 2013 stock incentive plan (the "2013 Plan"), annual increases have resulted in the addition of an aggregate of approximately 8,554,000 additional shares to the 2013 Plan, including for 2019, an increase of approximately 1,656,000 shares, or approximately 4% of the total number of shares of the Company's common stock outstanding as of January 1, 2019. As of June 30, 2019, the Company had approximately 2,704,000 shares available for grant under the 2013 Plan.

Share-based compensation expense, net of estimated forfeitures, includes expenses related to stock options and RSUs granted to employees, non-employee directors and consultants, as well as options granted to employees to purchase shares under the ESPP. Stock-based compensation by award type was as follows:



	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Stock options	$	1,488	$	1,836	$	3,110	$	3,968
Restricted stock units	709		819		1,530		1,765
Employee stock purchase plan	10		7		37		11
Total	$	2,207	$	2,662	$	4,677	$	5,744

The Company allocated stock-based compensation expense in the Company’s Consolidated Statements of Operations and Comprehensive Loss as follows:



	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	989	$	1,106	$	2,159	$	2,546
General and administrative	1,218		1,556		2,518		3,198
Total	$	2,207	$	2,662	$	4,677	$	5,744

Stock Options

A summary of the stock option activity, weighted average exercise prices, options outstanding, exercisable and expected to vest as of June 30, 2019 is as follows (in thousands except weighted average exercise price):



	Number of Shares Underlying Options		Weighted Average Exercise Price
Outstanding, December 31, 2018	5,903		$	13.72
Granted	137		$	1.35
Forfeited	(261	)	$	26.18
Expired	(89	)	$	12.72
Outstanding, June 30, 2019	5,690		$	12.89
Vested and exercisable, June 30, 2019	2,850		$	21.83
Vested and expected to vest, June 30, 2019	5,463		$	13.26

As of June 30, 2019, there were approximately $6.4 million of unrecognized compensation costs, net of estimated forfeitures, related to stock option awards grants, which are expected to be recognized over a remaining weighted average period of 1.8 years.

RSUs

The following table presents a summary of the Company's outstanding RSU awards granted as of June 30, 2019 (in thousands except weighted average grant-date fair value):



	Restricted Stock Units		Weighted Average Grant-Date Fair Value
Outstanding, December 31, 2018	679		$	15.61
Awarded	10		$	1.38
Vested	(49	)	$	52.83
Forfeited	(5	)	$	38.98
Outstanding, June 30, 2019	635		$	12.40
Outstanding, Expected to vest	518		$	6.58

As of June 30, 2019, there were approximately $2.1 million of unrecognized compensation costs, net of estimated forfeitures, related to RSUs grants, which are expected to be recognized over a remaining weighted average period of 1.5 years.

ESPP

As of June 30, 2019, there were 920,707 shares available for future purchases under the ESPP. There were no shares of common stock issued under the ESPP during the three months ended June 30, 2019 and 2018. There were 31,522 shares of common stock issued under the ESPP during the six months ended June 30, 2019. Cash proceeds from ESPP purchases were $41 thousand during the six months ended June 30, 2019. There were 12,229 shares of common stock issued under the ESPP during the six months ended June 30, 2018. Cash proceeds from ESPP purchases were $27 thousand during the six months ended June 30, 2018.