Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

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,
	2019		2018		2017
Risk-free interest rate	1.55% - 2.62	%	2.51% - 3.09	%	1.85% - 2.57	%
Expected volatility	71.1% - 75.2	%	82.6% - 92.8	%	75.3% - 84.5	%
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

Summary of Compensation Expense Relating to Stock Options

During the years ended December 31, 2019, 2018 and 2017, the Company recognized compensation expense relating to stock options as follows:

	Year Ended December 31,
	2019		2018		2017
Research and development	$	5,206	$	3,919	$	2,433
General and administrative		5,094		4,482		3,897
Total stock option expense	$	10,300	$	8,401	$	6,330

Summary of Stock Option Activity

A summary of the Company’s stock option activity related to employees, non-employee members of the Board of Directors and non-employee consultants for the 2019 Plan, the 2014 Plan and the 2004 Plan as of and for the year ended December 31, 2019 is as follows:

		Weighted		Aggregate
	Number of	Average Exercise		Intrinsic
	Shares	Price		Value
Outstanding, December 31, 2018	4,004,422	$	13.34
Granted	1,324,000		17.44
Exercised	(555,847)		10.98
Expired	—		—
Forfeited	(322,058)		15.12
Outstanding, December 31, 2019	4,450,517	$	14.73	$	10,350
Weighted average remaining contractual life as of December 31, 2019 (in years)	7.35
Options exercisable, December 31, 2019	2,407,027	$	12.97	$	8,733
Weighted average remaining contractual life as of December 31, 2019 (in years)	6.18
Options vested and expected to vest as of December 31, 2019	4,450,517	$	14.73	$	10,350
Weighted average remaining contractual life as of December 31, 2019 (in years)	7.35

Share-based Payment Arrangement, Nonemployee [Member] | Consultants [Member] | Re-Measurement [Member]

Summary of Assumptions Used in Black-Scholes Option Pricing Model

On January 1, 2019, the Company used the Black Scholes option valuation model to remeasure the fair value of all outstanding unvested options that had been granted to non-employee consultants in accordance with ASU 2018-07. At the end of each fiscal quarter during the years ended December 31, 2018 and 2017, the Company used the Black-Scholes option valuation model 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. The range of assumptions used by the Company on January 1, 2019 and during the years ended December 31, 2018 and 2017 are as follows:

	January 1,	December 31,
	2019	2018	2017
Risk-free interest rate	2.59% - 2.62%	1.82% - 3.02%	1.28% - 2.39%
Expected volatility	58.9% - 84.6%	58.2% - 101.0%	74.6% - 87.3%
Expected dividend yield	0%	0%	0%
Expected life of non-employee options (in years)	0.81 - 8.19	0.25 - 8.94	0.62 - 9.94