Stock Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2021

Summary of Stock-based Compensation Expense

Stock-based compensation expense included in the Company’s statements of operations and comprehensive loss is as follows (in thousands):

	Years Ended December 31,
	2021		2020		2019
Research and development	$	1,128	$	1,286	$	1,175
General and administrative		2,543		2,765		1,814
Total	$	3,671	$	4,051	$	2,989

Summary of Stock Option Activity

A summary of the stock option activity under the 2021, 2017 and 2011 Plans is as follows:

	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2020		4,110,691		$	3.89		7.3	$	124
Granted		3,341,350			1.87
Exercised		—			—
Cancelled		(686,204	)		4.72
Outstanding at December 31, 2021		6,765,837		$	2.81		7.6	$	3
Exercisable at December 31, 2021		3,282,830		$	3.52		6.2	$	3

Restricted Stock Units (RSUs)

Summary of Nonvested RSUs

A summary of the status of nonvested RSUs as of December 31, 2021 and the changes during the year then ended are presented below (in thousands, except fair values):

	Shares			Weighted-Average Grant Date Fair Value
Nonvested at December 31, 2020		620,277		$	1.29
Granted		—			—
Vested		(620,277	)		1.29
Forfeited		—			—
Nonvested at December 31, 2021		—			—

Employees and Directors

Schedule of Assumptions Used in Black-Scholes Option Pricing Model to Estimate Fair Value of Stock Options

The fair value of each stock option granted to employees and directors was estimated on the date of grant using the Black-Scholes option-pricing model, with the following range of assumptions for the years ended December 31, 2021, 2020 and 2019:

	Years Ended December 31,
	2021	2020	2019
Risk-free interest rate	0.4% – 1.4%	0.4% – 0.5%	1.4% – 2.6%
Expected dividend yield	—%	—%	—%
Expected term (in years)	5.0 – 7.0	5.5 – 6.1	5.5 – 6.8
Expected volatility	92% – 95%	94% – 95%	80% – 83%