Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Employee and Non-Employee Stock Option Activity

The following table summarizes employee and non-employee stock option activity for the year ended December 31, 2023:

	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, 2022		405,082		$	113.75		6.72	$	2
Granted		8,776,245			9.67
Exercised		(46,246	)		8.22
Forfeited		(637,686	)		43.00
Outstanding as of December 31, 2023		8,497,395		$	12.13		8.40	$	98,928
Options vested and expected to vest as of December 31, 2023		8,497,395		$	12.13		8.40	$	98,928
Options exercisable as of December 31, 2023		1,065,700		$	24.72		5.62	$	13,328

Employee Restricted Stock Activity

The following table summarizes employee restricted stock activity for the year ended December 31, 2023:

	Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2022		5,660		$	203.25
Granted		153,865			18.17
Vested		—			—
Forfeited		(5,660	)		203.25
Unvested restricted stock units as of December 31, 2023		153,865		$	18.17

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, 2023, 2022, and 2021 was as follows (in thousands):

	Year Ended December 31,
	2023				2022				2021
	Employees		Non- Employees		Employees		Non- Employees		Employees		Non- Employees
Research and development	$	2,910	$	11,328	$	2,591	$	—	$	2,723	$	—
General and administrative		11,327		109		4,520		—		5,315		—
Total stock-based compensation expense	$	14,237	$	11,437	$	7,111	$	—	$	8,038	$	—

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,
	2023			2022			2021
Stock Options Granted
Expected term (in years)		5.88			6.00			5.99
Expected volatility		107	%		84	%		83	%
Risk-free interest		4.37	%		2.93	%		0.88	%
Dividend yield		0	%		0	%		0	%
2016 ESPP
Expected term (in years)		0.49			0.49			0.50
Expected volatility		181	%		84	%		86	%
Risk-free interest		4.99	%		1.95	%		0.08	%
Dividend yield		0	%		0	%		0	%