Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

A summary of the Company’s stock option activity related to employees, non-employee members of the Board of Directors and non-employee consultants as of and for the year ended December 31, 2018 is as follows:

				Weighted-		Aggregate
	Number of			Average		Intrinsic
	Options			Exercise Price		Value
Outstanding at December 31, 2017		3,492,141		$	11.75
Granted		1,197,500			16.15
Exercised		(451,685	)		9.29
Expired		(60,111	)		10.79
Forfeited		(173,423	)		12.12
Outstanding at December 31, 2018		4,004,422		$	13.34	$	6,627
Weighted average remaining contractual life as of December 31, 2018 (in years)		7.92
Options exercisable at December 31, 2018		1,974,979		$	11.70	$	4,985
Weighted average remaining contractual life as of December 31, 2018 (in years)		7.00
Options vested and expected to vest as of December 31, 2018		4,004,422		$	13.34	$	6,627
Weighted average remaining contractual life as of December 31, 2018 (in years)		7.92

Summary of Assumptions Used in Black-Scholes Option Pricing Model

The fair values of the stock options granted to those groups were estimated using the Black-Scholes option valuation model with the following ranges of assumptions (see Note 2, Summary of Significant Accounting Policies - Stock-Based Compensation):

	Year Ended December 31,
	2018		2017	2016
Risk-free interest rate	2.51% - 3.09%		1.85% - 2.57%	1.19% - 1.93%
Expected volatility	82.6% - 92.8%		75.3% - 84.5%	67.8% - 77.8%
Expected dividend yield	0%		0%	0%
Expected life of employee and Board of Directors' options (in years)	6.25		6.25	6.25
Expected life of non-employee options (in years)		—	10	10

Summary of Compensation Expense Relating to Stock Options

During the years ended December 31, 2018, 2017 and 2016, the Company recognized compensation expense relating to stock options (excluding compensation expense related to the vesting of restricted stock units of $476 in R&D and $1,217 in G&A in 2018), as follows:

	Year Ended December 31,
	2018		2017		2016
Research and development	$	3,919	$	2,433	$	1,301
General and administrative		4,482		3,897		1,499
Total stock option expense	$	8,401	$	6,330	$	2,800

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:

	Year Ended December 31,
	2018		2017		2016
Research and development	$	195	$	170	$	(79	)
General and administrative		192		200		(20	)
Total stock option expense	$	387	$	370	$	(99	)

Re-Measurement [Member]

Summary of Assumptions Used in Black-Scholes Option Pricing Model

At the end of each fiscal quarter during the years ended December 31, 2018, 2017 and 2016, 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:

	Year Ended December 31,
	2018	2017	2016
Risk-free interest rate	1.82% - 3.02%	1.28% - 2.39%	1.35% - 2.38%
Expected volatility	58.2% - 101.0%	74.6% - 87.3%	70.8% - 75.5%
Expected dividend yield	0%	0%	0%
Expected life of non-employee options (in years)	0.25 - 8.94	0.62 - 9.94	7.08 - 9.60