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

6 Months Ended

Jun. 30, 2023

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Pre-funded Warrants for Common Stock Issued and Outstanding

As of June 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	$	0.0001		—
April 30, 2020	None	$	0.0001		—
May 20, 2022	None	$	0.0001		13,281,250
Total pre-funded warrants					13,281,250

Schedule of Company's 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 June 30,
	2023				2022
	Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value
Stock options		63,417,415	$	0.30		429,000	$	0.96

Summary of Stock-Based Compensation Expense

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

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2023		2022		2023		2022
Research and development	$	394	$	691	$	1,171	$	1,392
General and administrative		1,517		1,327		2,449		2,726
Total stock-based compensation expense	$	1,911	$	2,018	$	3,620	$	4,118

Schedule of Estimated 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						Six Months Ended
	June 30,						June 30,
	2023			2022			2023			2022
2016 Plan
Expected term (in years)		6.03			5.98			6.03			6.01
Expected volatility		117	%		82	%		115	%		82	%
Risk-free interest		3.98	%		2.96	%		3.99	%		2.07	%
Dividend yield		—			—			—			—
2016 ESPP
Expected term (in years)		—			—			0.49			0.49
Expected volatility		—			—			181	%		76	%
Risk-free interest		—			—			4.99	%		1.09	%
Dividend yield		—			—			—			—