Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2022

Share-Based Payment Arrangement [Abstract]

Summary of Employee and Non-Employee Stock Option Activity

The following table summarizes employee and non-employee stock option activity for the year ended December 31, 2022:

	Shares Issuable Under Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)		(in thousands)
Outstanding as of December 31, 2021		6,607,587		$	7.56		7.90	$	328
Granted		5,879,991			1.80
Exercised		—			—
Forfeited		(2,360,531	)		6.14
Outstanding as of December 31, 2022		10,127,047		$	4.55		6.72	$	2
Options vested and expected to vest as of    December 31, 2022		9,944,309		$	4.50		6.78	$	2
Options exercisable as of December 31, 2022		4,086,256		$	6.84		4.93	$	—

Summary of Employee Restricted Stock Activity

The following table summarizes employee restricted stock activity for the year ended December 31, 2022:

	Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2021		190,000		$	8.13
Granted		—			—
Vested		—			—
Forfeited		(48,500	)		8.13
Unvested restricted stock units as of December 31, 2022		141,500		$	8.13

Summary of Stock-Based Compensation Expense

Total stock-based compensation expense recognized from the Company’s equity incentive plans, 2018 Plan, and the 2016 ESPP for the years ended December 31, 2022, 2021, and 2020 was as follows (in thousands):

	Year Ended December 31,
	2022				2021				2020
	Employees		Non- Employees		Employees		Non- Employees		Employees		Non- Employees
Research and development	$	2,591	$	—	$	2,723	$	—	$	2,168	$	36
General and administrative		4,520		—		5,315		—		4,052		—
Total stock-based    compensation expense	$	7,111	$	—	$	8,038	$	—	$	6,220	$	36

Summary of Weighted-Average Assumptions Used in Calculating Fair Value of Awards

The following table summarizes the weighted-average assumptions used in calculating the fair value of the awards:

	Year Ended December 31,
	2022			2021			2020
Stock Options Granted
Expected term (in years)		6.00			5.99			6.10
Expected volatility		84	%		83	%		76	%
Risk-free interest		2.93	%		0.88	%		1.06	%
Dividend yield		0	%		0	%		0	%
2016 ESPP
Expected term (in years)		0.49			0.50			0.50
Expected volatility		84	%		86	%		76	%
Risk-free interest		1.95	%		0.08	%		0.75	%
Dividend yield		0	%		0	%		0	%