Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2023

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		264,303		$	189.00		7.90	$	328
Granted		235,199			45.00
Exercised		—			—
Forfeited		(94,421	)		153.50
Outstanding as of December 31, 2022		405,081		$	113.75		6.72	$	2
Options vested and expected to vest as of December 31, 2022		397,772		$	112.50		6.78	$	2
Options exercisable as of December 31, 2022		163,450		$	171.00		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		7,600		$	203.25
Granted		—			—
Vested		—			—
Forfeited		(1,940	)		203.25
Unvested restricted stock units as of December 31, 2022		5,660		$	203.25

Stock-Based Compensation Expense

Total stock-based compensation expense related to all plans was as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Research and development (1)	$	2,965	$	639	$	4,136	$	2,031
General and administrative		1,820		926		4,269		3,653
Total stock-based compensation expense	$	4,785	$	1,565	$	8,405	$	5,684

(1)  For the three and nine months ended September 30, 2023, $2.7 million and $2.9 million, respectively, was recognized as stock compensation expense related to the Parapyre Option Obligation. There were no such expenses for the three and nine months ended September 30, 2022.

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

Assumptions used to Estimate the Fair Value of Stock Options Granted

The following table summarizes the weighted-average Black-Scholes option pricing model assumptions used to estimate the fair value of stock options granted under the Company’s 2016 Plan, and the shares purchasable under the 2016 ESPP during the periods presented:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2023			2022			2023			2022
2016 Plan
Expected term (in years)		6.08			6.02			6.04			5.96
Expected volatility		101	%		85	%		111	%		83	%
Risk-free interest		4.28	%		3.16	%		4.07	%		2.43	%
Dividend yield		—			—			—			—
2016 ESPP
Expected term (in years)		0.50			0.50			0.49			0.49
Expected volatility		222	%		95	%		181	%		84	%
Risk-free interest		5.29			3.26			4.99	%		1.95	%
Dividend yield		—			—			—			—

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	%

Pre-funded Warrants for Common Stock Issued and Outstanding

As of September 30, 2023, the following pre-funded warrants for common stock were issued and outstanding:

Issue Date	Expiration Date		Exercise Price		Number of Warrants Outstanding
February 8, 2019		None	$	0.0025		—
April 30, 2020		None	$	0.0025		—
May 20, 2022		None	$	0.0025		250,000
Total pre-funded warrants						250,000

As of December 31, 2022, the following pre-funded warrants to purchase common stock were issued and outstanding:

Issue Date	Expiration Date		Exercise Price		Number of Warrants Outstanding
February 2019		None	$	0.0025		150,000
April 2020		None	$	0.0025		474,413
May 2022		None	$	0.0025		531,250
Total pre-funded warrants						1,155,663

Stock Awards Granted

The following table summarizes the Company’s stock awards granted under all plans for each of the periods indicated:

	Three Months Ended September 30,								Nine Months Ended September 30,
	2023				2022				2023				2022
	Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value
Stock options		1,044,667	$	14.50		50,806	$	16.75		3,867,366	$	9.65		153,686	$	52.50