Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Stock Option Activity

A summary of the status of the Company’s stock option activity for the year ended December 31, 2018 is presented in the table and narrative below:

	2018
	Options			Weighted Average Exercise Price
Outstanding at January 1, 2018		12,160		$	6.37
Granted		6,592			3.03
Exercised		(764	)		2.69
Forfeited		(1,564	)		4.24
Cancelled		(1,474	)		8.08
Outstanding at December 31, 2018		14,950		$	5.14
Options exercisable at December 31, 2018		8,396		$	6.50
Options vested and expected to vest at December 31, 2018		14,194		$	5.23

Summary of Stock Options Outstanding

The following table summarizes information about stock options outstanding at December 31, 2018:

	Options Outstanding						Options Vested
Range of Exercise Prices	Number Outstanding		Weighted Average Remaining Contractual Life (Years)		Weighted Average Exercise Price		Number Vested		Weighted Average Exercise Price
$0.00 – $2.00		—		—	$	—		—	$	—
$2.01 – $4.00		8,287		7.2		3.05		2,805		3.07
$4.01 – $6.00		1,816		7.5		4.12		1,040		4.12
$6.01 – $8.00		2,746		5.1		7.47		2,497		7.47
$8.01 – $10.00		944		3.9		8.71		916		8.72
$10.01 –$12.00		84		6.2		10.41		65		10.42
$12.01 – $14.00		1,073		5.3		13.41		1,073		13.41
		14,950		6.5	$	5.14		8,396	$	6.50

Assumptions Used to Value Options Granted

The assumptions used to value options granted are as follows:

	For the Years Ended December 31,
	2018		2017		2016
Expected term of option		6.0 years		6.0 years		6.0 – 6.25 years
Expected volatility		79% – 81%		82% – 83%		81% – 84%
Risk free interest rate		2.55 – 2.98%		2.04 – 2.27%		1.15 – 2.09%
Expected dividend yield		0%		0%		0%

Compensation Expense Related to Option Grants Made to Employees and Consultants

Compensation expense related to option grants and the 2006 ESPP is included in research and development and general and administrative expense as follows:

	For the Years Ended December 31,
	2018		2017		2016
Research and development	$	2,679	$	4,490	$	4,695
General and administrative		6,701		6,083		6,311
Total	$	9,380	$	10,573	$	11,006

Assumptions Used to Value Options Granted Under 2006 ESPP Plan

The assumptions used to value options granted under the 2006 ESPP are as follows:

	For the Years Ended December 31,
	2018		2017		2016
Expected term of option		6 months		6 months		6 months
Expected volatility		47% – 60%		58% – 73%		55% – 66%
Risk free interest rate		1.30 – 2.1%		0.05 – 1.3%		0.05 – 0.06%
Expected dividend yield		0%		0%		0%