Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Summary of Assumptions Used in Black-Scholes Option Pricing Model

The fair values of stock options granted during the three and six months ended June 30, 2018 and 2017 were estimated as of the dates of grant using the Black-Scholes option pricing model with the following assumptions:

	Three Months Ended June 30,		Six Months Ended June 30,
	2018	2017	2018	2017
Risk-free interest rate	2.85% - 2.94%	1.85% - 1.88%	2.51% - 2.94%	1.85% - 2.57%
Expected volatility	85.7% - 92.8%	83.3%	85.7% - 92.8%	75.3% - 83.3%
Expected dividend yield	0%	0%	0%	0%
Expected life of employee options (in years)	6.25	6.25	6.25	6.25
Expected life of non-employee options    (in years)	NA	10	NA	10

Summary of Compensation Expense Relating to Stock Options

During the three and six months ended June 30, 2018 and 2017, the Company recognized compensation expense in the accompanying Condensed Statements of Comprehensive Loss relating to stock options, as follows:  

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development	$	852	$	603	$	1,500		1,166
General and administrative		1,217		715		2,440		1,260
Total stock option expense	$	2,069	$	1,318	$	3,940	$	2,426

Summary of Stock Option Activity

A summary of stock option award activity related to employees, non-employee members of the Company’s Board of Directors and non-employee consultants as of and for the six months ended June 30, 2018 is presented below:

	Number of Shares			Weighted Average Exercise Price
Outstanding, December 31, 2017		3,492,141		$	11.75
Granted		732,500			14.13
Exercised		(222,132	)		7.87
Expired		(6,562	)		17.41
Forfeited		(124,753	)		10.95
Outstanding, June 30, 2018		3,871,194		$	12.44
Options exercisable, June 30, 2018		1,746,033

Non-employee Consultants [Member]

Summary of Compensation Expense Relating to Stock Options

Included in the table above are the following amounts of compensation expense recognized with regard to stock options that were granted to non-employee consultants, including the effect of re-measurement of the fair values of those options, as described above:

	Three Months Ended June 30,					Six Months Ended June 30,
	2018		2017			2018		2017
Research and development	$	93	$	3		$	121	$	146
General and administrative		40		(7	)		179		32
Total stock option expense	$	133	$	(4	)	$	300	$	179

Re-Measurement [Member]

Summary of Assumptions Used in Black-Scholes Option Pricing Model

As of June 30, 2018 and 2017, the Company used the Black-Scholes option valuation model with the following ranges of assumptions to re-measure the fair value of all outstanding options that had been granted to non-employee consultants during the vesting period of each tranche in accordance with ASC 505-50:  

	June 30,
	2018	2017
Risk-free interest rate	1.92% - 2.82%	2.02% - 2.28%
Expected volatility	60.4% - 96.2%	76.4% - 81.3%
Expected dividend yield	0%	0%
Expected life of non-employee options (in years)	0.25 - 8.69	6.58 - 9.69