Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2021

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 and the 2014 Plan as of and for the year ended December 31, 2021 is as follows:

		Weighted		Aggregate
	Number of	Average Exercise		Intrinsic
	Shares	Price		Value
Outstanding, December 31, 2020	5,469,393	$	15.02
Granted	1,422,750		17.82
Exercised	(136,787)		11.98
Forfeited	(239,457)		18.01
Expired	(3,619)		24.93
Outstanding, December 31, 2021	6,512,280	$	15.58	$	3,537
Weighted average remaining contractual life as of December 31, 2021 (in years)	6.88
Options exercisable, December 31, 2021	4,120,917	$	14.73	$	3,525
Weighted average remaining contractual life as of December 31, 2021 (in years)	5.79
Options vested and expected to vest as of December 31, 2021	6,449,787	$	15.39	$	3,537
Weighted average remaining contractual life as of December 31, 2021 (in years)	6.06

Summary of Assumptions Used in Black-Scholes Option Pricing Model

During the years ended December 31, 2021, 2020 and 2019, the Company granted 1,422,750, 1,377,850 and 1,324,000 stock options, respectively, to employees and non-employee members of the Board of Directors. There were no options granted to nonemployee consultants during the years ended December 31, 2021, 2020 and 2019. 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,
	2021		2020		2019
Risk-free interest rate	0.66% - 1.41	%	0.35% - 1.57	%	1.55% - 2.62	%
Expected volatility	71.6% - 83.5	%	71.8% - 74.8	%	71.1% - 75.2	%
Expected dividend yield	0	%	0	%	0	%
Expected life of employee and Board options (in years)	6.25		6.25		6.25

Summary of Compensation Expense Relating to Stock Options

	Year Ended December 31,
	2021		2020		2019
Research and development	$	7,126	$	6,765	$	5,206
General and administrative		9,569		4,943		5,094
Total stock option expense	$	16,695	$	11,708	$	10,300

Employees And Non-Employee Members Of Board Of Directors [Member]

Summary of Restricted Stock Unit Activity

A summary of restricted stock unit activity related to employees and non-employee members of the Company’s Board of Directors as of and for the year ended December 31, 2021 is as follows:

		Weighted
	Number of	Average Grant
	Units	Date Fair Value
Outstanding, December 31, 2020	235,250	$	16.25
Awarded	530,715		17.95
Vested and released	(189,421)		17.20
Outstanding, December 31, 2021	576,544	$	17.50
Restricted stock units exercisable (vested and deferred), December 31, 2021	—