Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2023

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

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

As of March 31, 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		3,750,000
April 30, 2020	None	$	0.0001		11,860,328
May 20, 2022	None	$	0.0001		13,281,250
Total pre-funded warrants					28,891,578

Schedule of Company's Stock Awards Granted

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

	Three Months Ended March 31,
	2023				2022
	Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value
Stock options		4,440,500	$	0.44		2,143,000	$	3.18

Summary of Stock-Based Compensation Expense

Total stock-based compensation expense related to the 2016 Plan and 2016 ESPP was as follows (in thousands):

	Three Months Ended
	March 31,
	2023		2022
Research and development	$	777	$	701
General and administrative		932		1,400
Total stock-based compensation expense	$	1,709	$	2,101

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
	March 31,
	2023			2022
2016 Plan
Expected term (in years)		6.02			6.02
Expected volatility		99	%		82	%
Risk-free interest		4.06	%		0.19	%
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		—			—