Stock-Based Compensation

9 Months Ended

Sep. 30, 2023

Share-Based Payment Arrangement [Abstract]

2021 Plan

The total remaining shares of common stock authorized for issuance under the 2021 Stock Option and Incentive Plan increased from 5,262,917 as of December 31, 2022 to 6,368,021 as of September 30, 2023 primarily due to the automatic annual increase provision.

2016 Plan

The total remaining shares of common stock authorized for issuance under the 2016 Stock Incentive Plan as of September 30, 2023 and December 31, 2022 were 2,081,561 shares and 2,206,655 shares, respectively.

2021 Employee Stock Purchase Plan

The total remaining shares of common stock authorized for issuance under the 2021 Employee Stock Purchase Plan (2021 ESPP) increased from 563,115 as of December 31, 2022 to 861,286 as of September 30, 2023 due to the automatic annual increase provision within the 2021 ESPP.

Stock-Based Compensation

Stock-based compensation expense recorded in the condensed consolidated statements of operations and comprehensive loss is as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Research and development expenses	$	1,601	$	1,158	$	4,317	$	2,836
General and administrative expenses	1,795		1,549		4,953		4,177
Total	$	3,396	$	2,707	$	9,270	$	7,013

Stock Option Valuation

The following table presents, on a weighted-average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted during the nine months ended September 30, 2023 and 2022:


	September 30, 2023		September 30, 2022
Risk‑free interest rate	4.09	%	2.11	%
Expected volatility	72	%	71	%
Expected dividend yield	—		—
Expected term (in years)	6.02		6.04

Early Exercise of Unvested Stock Options

Shares purchased by employees pursuant to the early exercise of stock options are not deemed, for accounting purposes, to be outstanding shares until those shares vest according to their respective vesting schedules. Cash received from employee exercises of unvested options is included in current liabilities on the balance sheet. Amounts recorded are reclassified to common stock and additional paid-in capital as the shares vest. Vesting can occur in the year of exercise and thereafter. There were 21,149 and 53,741 unvested shares related to early exercises of stock options as of September 30, 2023 and December 31, 2022, respectively. As of September 30, 2023 and December 31, 2022, the liability associated with the unvested early exercise of stock options was $0.1 million and $0.2 million, respectively.

Stock Options

The following table summarizes the Company’s stock option activity during the nine months ended September 30, 2023:


	Number of Shares	Weighted‑ Average Exercise Price

Outstanding as of December 31, 2022	5,028,850	$	10.95
Granted	799,144	13.79
Exercised	(145,149)	5.20
Forfeited	(120,298)	15.16
Outstanding as of September 30, 2023	5,562,547	$	11.40
Exercisable as of September 30, 2023(1)	3,232,149	$	9.20

(1)This represents the number of vested and unvested options exercisable as of September 30, 2023.

The weighted-average grant-date fair value of stock options granted during the nine months ended September 30, 2023 and 2022 was $9.19 per share and $7.26 per share, respectively. As of September 30, 2023, there was $23.8 million of unrecognized compensation cost related to unvested stock options, which is expected to be recognized over a weighted-average period of 2.40 years.

Restricted Stock Units

During the nine months ended September 30, 2023, restricted stock units (RSUs) were granted to employees with vesting conditions based on continued service over time. Accordingly, stock-based compensation expense for such awards is recognized using a straight-line attribution model over the vesting term of each RSU. The fair value of each RSU is based on the closing price of the Company's common stock on the date of grant.

A summary of restricted stock activity during the nine months ended September 30, 2023 is as follows:


	Shares	Weighted‑ Average Grant‑Date Fair Value
Unvested as of December 31, 2022	463,964	$	12.26
Granted	890,647	$	14.22
Vested	(138,361)	$	12.30
Forfeited	(36,945)	$	12.27
Unvested as of September 30, 2023	1,179,305	$	13.74

As of September 30, 2023, there was $14.9 million of unrecognized stock-based compensation expense related to restricted stock that is expected to vest. These costs are expected to be recognized over a weighted-average remaining vesting period of 3.3 years.