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Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2021

A summary of the Company’s unvested shares is as follows:

Balance at December 31, 2020	2,746,759
Share vesting	(1,379,360	)
Balance at September 30, 2021	1,367,399

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance consists of the following:

	September 30, 2021
Common stock warrants		7,679,228
Stock options and performance-based units outstanding		4,434,366
Shares available for issuance under the 2019 Incentive Plan		1,931,919
Shares available for issuance under the ESPP Plan		842,036
Balance at September 30, 2021		14,887,549

Summary of PSU Activity Under the 2019 Incentive Award Plan

The following table summarizes PSU activity under the 2019 Incentive Award Plan during the nine months ended September 30, 2021.

	Number of Stock Units			Weighted- Average Grant Date Fair Value Per Share
Unvested balance at December 31, 2020		220,000		$	32.48
Granted		130,050		$	37.65
Vested		—		$	—
Forfeited		(10,000	)	$	29.43
Unvested balance at September 30, 2021		340,050		$	34.55

Summary of Weighted-Average Assumptions Used to Estimate Fair Value of Stock Options Granted and ESPP Awards

The weighted-average assumptions used to estimate the fair value of stock options using the Black-Scholes option valuation model were as follows:

	Nine Months Ended September 30,
	2021			2020
Assumptions:
Expected term (in years)		6.00			5.23
Expected volatility		67.37	%		56.24	%
Risk free interest rate		0.65	%		0.36	%
Dividend yield		—			—

Summary of Stock Option Activity

A summary of the Company’s stock option activity and related information is as follows:

	Options Outstanding			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2020		2,728,742		$	21.36		9.10	$	34,432
Options granted		1,568,250			38.61
Options exercised and shares vested		(95,071	)		18.73
Options cancelled		(107,605	)		34.07
Balance at September 30, 2021		4,094,316		$	27.70		8.65	$	30,591
Options exercisable as of September 30, 2021		941,254		$	18.73		7.95	$	13,122

Summary of Stock-Based Compensation Expense

Stock-based compensation expense recognized for all equity awards, including founder stock, has been reported in the statements of operations as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development expense	$	1,024	$	604	$	2,910	$	842
General and administrative expense		3,395		1,466		9,564		2,619
Total	$	4,419	$	2,070	$	12,474	$	3,461

Employee Stock Purchase Plan

Summary of Weighted-Average Assumptions Used to Estimate Fair Value of Stock Options Granted and ESPP Awards

The weighted-average assumptions used to estimate the fair value of ESPP awards using the Black-Scholes option valuation model were as follows:

	Nine Months Ended September 30,
	2021			2020
Assumptions:
Expected term (in years)		0.69			1.00
Expected volatility		76.25	%		76.25	%
Risk free interest rate		0.09	%		0.15	%
Dividend yield		—			—