Share-Based Payments

Pursuant to the terms of the Company's 2014 Stock Incentive Plan (the "2014 Plan"), the number of shares authorized for issuance automatically increases on the first day of each fiscal year. On January 1, 2018, the number of shares reserved for issuance under the 2014 Plan increased by 1,102,362 shares. As of June 30, 2018, the total number of shares of common stock available for issuance under the 2014 Plan was 873,812.

The Company also maintains the Company's 2009 Stock Incentive Plan, as amended and restated, and the Company's 2014 Employee Stock Purchase Plan ("2014 ESPP").

Stock-Based Compensation Expense

Stock-based compensation expense by award type was as follows (in thousands):



	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Stock options	$	283	$	268	$	659	$	455
Restricted stock	—		51		$	22	102
Restricted stock units	—		—		$	—	3
Employee stock purchase plan	2		3		5		6
	$	285	$	322	$	686	$	566

The Company allocated stock-based compensation expense as follows in the condensed consolidated statements of operations and comprehensive loss (in thousands):



	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development expense	$	123	$	83	$	288	$	123
General and administrative expense	162		239		$	398	$	443
	$	285	$	322	$	686	$	566

At June 30, 2018, there was $2.7 million of total unrecognized compensation expense related to unvested stock options and shares issued pursuant to the 2014 ESPP. This unrecognized compensation expense is expected to be recognized over a weighted-average period of 2.63 years.

Stock Options

A summary of the stock option activity is presented below:



	Shares		Weighted-Average Exercise Price
Outstanding at December 31, 2017	2,695,796		$	3.16
Granted	1,643,400		1.82
Exercised	(55,259	)	0.53
Cancelled or forfeited	(256,111	)	2.95
Outstanding at June 30, 2018	4,027,826		$	2.66
Exercisable at June 30, 2018	1,738,331		$	3.53
Vested and expected to vest at June 30, 2018⁽¹⁾	3,977,226		$	2.67


⁽¹⁾	Represents the number of vested stock options, plus the number of unvested stock options expected to vest. The Company adopted ASU 2016-09 as of January 1, 2017 and records forfeitures as they occur.

During the fourth quarter of 2017, the Company issued stock option awards to certain employees which contained performance vesting conditions. Certain of the vesting milestones were achieved during the fourth quarter 2017 and the expense related to these awards was recognized in full during the period. Certain other performance conditions had previously not been considered probable of vesting and therefore no expense was recognized. During the first and second quarter of 2018, these conditions were deemed probable of occurring and the expense for these awards has been recognized over the estimated vesting period.

Restricted Stock

From time to time, upon approval by the Board, certain employees, directors and advisors have been granted restricted shares of common stock. A summary of the restricted stock is presented below:

Employee Stock Purchase Plan

On March 14, 2018, the Company issued and sold 9,565 shares of its common stock pursuant to the 2014 ESPP at a purchase price of $0.98 per share. During the three months ended June 30, 2018, the Company did not issue or sell any shares of its common stock pursuant to the 2014 ESPP. The Company has estimated the number of shares to be issued at the end of the current offering period and recognizes expense over the requisite service period.

Restricted
Stock

Weighted-Average
Grant Date
Fair Value

Unvested at December 31, 2017

Unvested at June 30, 2018