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Stock-based Compensation
 9 Months Ended
Sep. 30, 2025
Share-Based Payment Arrangement [Abstract]  
Stock-based Compensation
8. Stock-based Compensation
2017 Stock Incentive Plan
In December 2017, we adopted the 2017 Stock Incentive Plan (the “2017 Plan”), as amended and restated, under which we could grant incentive stock options (“ISOs”), non-qualified stock options, restricted stock awards (“RSAs”), restricted stock units (“RSUs”), stock appreciation rights and other stock-based awards to eligible employees, officers, directors and consultants. The terms of stock options and RSAs, including vesting requirements, were determined by our board of directors, subject to the provisions of the 2017 Plan.
2021 Stock Incentive Plan
In April 2021, our board of directors adopted and our stockholders approved the 2021 Stock Incentive Plan (the “2021 Plan”), which became effective immediately prior to the effectiveness of our initial public offering (the “IPO”). As a result of the adoption of the 2021 Plan, no further awards will be made under the 2017 Plan.
The 2021 Plan provides for the grant of ISOs, non-qualified stock options, RSAs, RSUs, stock appreciation rights and other stock-based awards. Our employees, officers, directors, consultants and advisors are eligible to receive awards under the 2021 Plan. The terms of awards, including vesting requirements, are determined by our board of directors, subject to the provisions of the 2021 Plan.
We initially registered 3,352,725 shares of common stock under the 2021 Plan, pursuant to a Registration Statement on Form S-8 filed with the SEC on April 30, 2021, which was comprised of (i) 2,843,116 shares of common stock reserved for issuance under the 2021 Plan, (ii) 31,884 shares of common stock originally reserved for issuance under the 2017 Plan that became available for issuance under the 2021 Plan upon the completion of the IPO, and (iii) 477,725 shares of unvested restricted stock subject to repurchase by us that may become issuable under the 2021 Plan following such repurchase. The 2021 Plan also provides that an additional number of shares will be added annually to the shares authorized for issuance under the 2021 Plan on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2022 and continuing until, and including, the fiscal year ending December 31, 2031. The number of shares added each year will be equal to the lesser of (i) 5% of the number of outstanding common stock on such date and (ii) such amount as determined by our board of directors. As of September 30, 2025, a cumulative total of 7,152,859 additional shares have been added to the total shares authorized for issuance under the 2021 Plan in accordance with these terms.
2021 Employee Stock Purchase Plan
The 2021 Employee Stock Purchase Plan (“2021 ESPP”) permits eligible employees to purchase shares of our common stock at a discount and consists of consecutive six-month offering periods, each containing a single six-month purchase period. On the first day of each offering period, each employee who is enrolled in the 2021 ESPP will automatically receive an option to purchase up to a whole number of shares of our common stock. The purchase price of each of the shares purchased, in a given purchase period, will be equal to 85% of the lesser of the closing price of a share of our common stock on (i) the first day of the offering period, or (ii) the last day of the offering period. During the nine months ended September 30, 2025 and 2024, 39,853 shares and 35,180 shares of our common stock, respectively, were purchased by participants of the 2021 ESPP.
Inducement Stock Option Awards
We may grant inducement equity awards in the form of non-qualified stock options to purchase shares of our common stock to newly hired employees pursuant to Nasdaq Listing Rule 5635(c)(4) (“Inducement Awards”). During the nine months ended
September 30, 2025 we granted 201,720 Inducement Awards. No Inducement Awards were granted during the nine months ended September 30, 2024. The valuation assumptions and activity associated with Inducement Awards are included in the stock option activity described below.
Stock-Based Compensation Expense
Total stock-based compensation expense recognized in the condensed consolidated statements of operations was as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2025202420252024
(in thousands)
Research and development
$585 $976 $2,613 $3,674 
General and administrative
648 1,035 2,419 3,244 
Total stock-based compensation
$1,233 $2,011 $5,032 $6,918 
RSU Activity
We have granted RSUs to our employees under the 2021 Plan. As of December 31, 2024, all RSUs granted to employees or non-employees had become fully vested or had been previously forfeited. No RSUs were granted during the nine months ended September 30, 2025. Accordingly, we had no unrecognized stock-based compensation expense related to unvested RSUs as of September 30, 2025. The aggregate fair value of RSUs that vested during the nine months ended September 30, 2024, was $0.7 million based upon the fair value of the stock underlying the RSUs on the day of vesting. No RSUs vested during the three months ended September 30, 2024.
Stock Option Activity
The fair value of stock options granted during the three and nine months ended September 30, 2025 and 2024 was calculated on the date of grant using the following weighted-average assumptions:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2025202420252024
Risk-free interest rate
3.9 %4.3 %4.4 %4.0 %
Expected term (in years)
5.56.05.96.0
Expected annual dividend yield
— %— %— %— %
Expected volatility
96.9 %91.9 %95.4 %92.6 %
Using the Black-Scholes option pricing model, the weighted-average grant date fair value of stock options granted during the three months ended September 30, 2025 and 2024 was $0.81 and $1.76 per share, respectively. Using the Black-Scholes option pricing model, the weighted-average grant date fair value of stock options granted during the nine months ended September 30, 2025 and 2024 was $1.16 and $3.57 per share, respectively.
The following table summarizes stock option activity during the nine months ended September 30, 2025: 
Options Outstanding
Number of OptionsWeighted-Average Exercise Price per ShareWeighted-Average Remaining
Contractual Life
(in years)		Aggregate Intrinsic Value (in millions)
Outstanding at December 31, 20247,634,937 $6.24 
Granted
3,390,691 $1.49 
Exercised
— $— 
Cancelled
(834,315)$4.49 
Outstanding at September 30, 202510,191,313 $4.80 7.31$1.7 
Exercisable at September 30, 20255,948,176 $6.55 6.20$0.3 
No stock options were exercised during the three and nine months ended September 30, 2025. The aggregate intrinsic fair value of stock options exercised during the three and nine months ended September 30, 2024 was nominal for each period.
As of September 30, 2025, we had unrecognized stock-based compensation expense related to unvested stock options of $6.6 million, which we expect to recognize over a weighted-average period of approximately 2.0 years.