Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Summary of Stock Option Activity

The following table summarizes stock option activity for the 2002 Plan and 2018 Plan for the year ended December 31, 2020:

	Total Options	Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at Ended December 31, 2018	1,545,403	$	2.48	7.6	$	1,593,487
Granted	1,260,540	$	4.19
Exercised	(143,654)	$	1.71
Cancelled or expired	(84,571)	$	4.65
Outstanding at December 31, 2019	2,577,718	$	3.29	7.9	$	7,914,459
Granted	1,227,159	$	5.09
Exercised	(51,572)	$	1.85
Cancelled or expired	(155,145)	$	4.89
Outstanding at December 31, 2020	3,598,160	$	3.85	7.5	$	1,737,432
Vested and expected to vest at December 31, 2020	3,598,160	$	3.85	7.5	$	1,737,432
Vested and exercisable at December 31, 2020	1,864,186	$	3.26	6.5	$	1,559,887

Estimated Fair Value of Option on Grant Date using Weighted Average Assumptions

The fair value of each option was estimated on the date of grant using the weighted-average assumptions in the table below:

	Year Ended December 31,
	2020			2019
Risk-free interest rate	1.37		%	2.36		%
Expected term (in years)	6.0			5.9
Expected volatility	71		%	69		%
Expected dividend yield	—			—
Fair value of common stock	$	5.09		$	4.19

Stock-based Compensation Expense

Stock-based compensation expense has been reported in the Company’s statements of operations for the years ended December 31, 2020 and 2019 as follows (in thousands):

	Year Ended December 31,
	2020		2019
General and administrative	$	1,594	$	1,135
Research and development	643		286
Total stock-based compensation	$	2,237	$	1,421