0001193125-26-126188 | 10-K | XBRL Viewer

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Employee Benefit Plan - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Defined Contribution Plan [Abstract]
Employer benefit plan contribution	$ 0.4	$ 0.2

Fair Value Measurements - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Apr. 30, 2024	Dec. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-Sale [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset,Transfers, Net		$ 0	$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net, Total		$ 0	$ 0
Preferred stock, par value		$ 0.001	$ 0.001
Series B Preferred Stock
Debt Securities, Available-for-Sale [Line Items]
Sale of stock, number of shares issued	8,823,523
Preferred stock, par value	$ 17.00	$ 17.002

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
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-Name Accounting Standards Codification
-Section 50
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Cybersecurity Risk Management, Strategy, and Governance	12 Months Ended
Cybersecurity Risk Management, Strategy, and Governance	Dec. 31, 2025
Cybersecurity Risk Management, Strategy, and Governance [Line Items]
Cybersecurity Risk Management Processes for Assessing, Identifying, and Managing Threats [Text Block]	Item 1C. Cybersecurity. Cyber Risk Management and Strategy We have adopted processes for assessing, identifying, and managing cybersecurity risks, that are integrated into our overall enterprise management framework, built into our information technology function and are designed to help protect our information assets and operations from internal and external cybersecurity threats, protect employee and clinical trial information from unauthorized access or attack, as well as secure our networks and systems. Such processes include physical, procedural, and technical safeguards, and periodic review of our procedures in an effort to identify risks and refine our practices. To support our internal resources, we leverage external tools and resources, including a managed service provider that provides ongoing support for the protection of our information technology infrastructure. We have an employee security awareness program, required upon onboarding and on an annual basis thereafter, that is designed to raise awareness of cybersecurity threats across functions. As part of this employee training program, we periodically conduct phishing tests. We have also implemented a process to assess and review the cybersecurity practices of certain third-party vendors and service providers that may be critical to the operations of our business and who have access to our information systems including, as appropriate, through the inclusion of cybersecurity requirements in our contracts. We have not identified any cybersecurity incidents or threats that have materially affected us or are reasonably likely to materially affect us, including our business strategy, results of operations or financial condition; however, like other companies in our industry, we and our third-party vendors may, from time to time, experience threats and security incidents relating to our and our third-party vendors’ information systems. For more information, see “Risk Factors–Risks related to our dependence on third parties–Our information technology systems, or those used by our CROs or other contractors or consultants, may fail or suffer security breaches, which could adversely affect our business.” Governance Related to Cybersecurity Risks Our audit committee of the board of directors, (the “Audit Committee”), is responsible for overseeing cybersecurity risk, pursuant to the Audit Committee charter, and periodically updates our board of directors on such matters. The Audit Committee receives periodic updates from management regarding cybersecurity matters, and we have a process for the Audit Committee to be notified between such updates in the event of any significant new cybersecurity threats or incidents. Management is responsible for the operational oversight of company-wide cybersecurity strategy, policy, and standards across relevant departments to assess and help prepare us to address cybersecurity risks. Our Senior Manager of Information Technology reports to our Chief Financial and Operating Officer and oversees the day-to-day implementation and management of our cybersecurity program. Our Senior Manager of Information Technology has approximately 20 years of experience in information technology and regularly reports to executive management, the company’s disclosure committee, and the Audit Committee on cyber matters, as appropriate.
Cybersecurity Risk Third Party Oversight and Identification Processes [Flag]	true
Cybersecurity Risk Materially Affected or Reasonably Likely to Materially Affect Registrant [Flag]	false
Cybersecurity Risk Board of Directors Oversight [Text Block]	Our audit committee of the board of directors, (the “Audit Committee”), is responsible for overseeing cybersecurity risk, pursuant to the Audit Committee charter, and periodically updates our board of directors on such matters. The Audit Committee receives periodic updates from management regarding cybersecurity matters, and we have a process for the Audit Committee to be notified between such updates in the event of any significant new cybersecurity threats or incidents.
Cybersecurity Risk Board Committee or Subcommittee Responsible for Oversight [Text Block]	audit committee of the board of directors, (the “Audit Committee”), is responsible for overseeing cybersecurity risk,
Cybersecurity Risk Process for Informing Board Committee or Subcommittee Responsible for Oversight [Text Block]	Our audit committee of the board of directors, (the “Audit Committee”), is responsible for overseeing cybersecurity risk, pursuant to the Audit Committee charter, and periodically updates our board of directors on such matters
Cybersecurity Risk Role of Management [Text Block]	Management is responsible for the operational oversight of company-wide cybersecurity strategy, policy, and standards across relevant departments to assess and help prepare us to address cybersecurity risks. Our Senior Manager of Information Technology reports to our Chief Financial and Operating Officer and oversees the day-to-day implementation and management of our cybersecurity program. Our Senior Manager of Information Technology has approximately 20 years of experience in information technology and regularly reports to executive management, the company’s disclosure committee, and the Audit Committee on cyber matters, as appropriate.
Cybersecurity Risk Management Positions or Committees Responsible [Flag]	true
Cybersecurity Risk Management Positions or Committees Responsible [Text Block]	Our Senior Manager of Information Technology reports to our Chief Financial and Operating Officer and oversees the day-to-day implementation and management of our cybersecurity program.
Cybersecurity Risk Management Expertise of Management Responsible [Text Block]	Our Senior Manager of Information Technology has approximately 20 years of experience in information technology and regularly reports to executive management, the company’s disclosure committee, and the Audit Committee on cyber matters, as appropriate.
Cybersecurity Risk Process for Informing Management or Committees Responsible [Text Block]	Management is responsible for the operational oversight of company-wide cybersecurity strategy, policy, and standards across relevant departments to assess and help prepare us to address cybersecurity risks
Cybersecurity Risk Management Positions or Committees Responsible Report to Board [Flag]	true

Consolidated statements of cash flows (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Proceeds from the issuance of preferred stock	$ 0	$ 149,924

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Effective Income Tax Rate Reconciliation [Line Items]
Income tax expense or benefit	$ 0	$ 0
Valuation allowance	103,632,000	60,664,000	$ 39,695,000
Uncertain tax positions	0
Estimated interest or penalties recognized on uncertain tax positions	0	0
Federal
Effective Income Tax Rate Reconciliation [Line Items]
Net operating loss carryforwards	$ 66,900,000	37,600,000
Percentage of limited utilization of taxable income	80.00%
Tax credit	$ 5,700,000
Expiration years	2043
State
Effective Income Tax Rate Reconciliation [Line Items]
Net operating loss carryforwards	$ 92,900,000	$ 49,100,000
Net operating loss caryforwards expire year	2041
Tax credit	$ 1,200,000
Expiration years	2037

License Agreements - Additional Information (Details) - Lonza License Agreement - USD ($)	1 Months Ended	12 Months Ended
	Oct. 31, 2021	Dec. 31, 2025	Dec. 31, 2024
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Royalty payment term upon first commercial sale	10 years
License agreement, royalty payments		$ 0	$ 0
License agreement, annual payment		$ 500,000	$ 500,000

Fair Value Measurements - Roll-forward of Fair Value of the Series A and Series B Preferred Stock Tranche Right Liability Using Level 3 Inputs (Details) - Series B Preferred Stock [Member] - Level 3 $ in Thousands	12 Months Ended
	Dec. 31, 2024 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value at beginning balance	$ 2,874
Change in fair value of Series B preferred stock tranche right liability	(2,859)
Final settlement of Series B preferred stock tranche right liability	(15)
Fair value at ending balance	$ 0

Fair Value Measurements

12 Months Ended

Dec. 31, 2025

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair value measurements

The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy used to determine such fair values (in thousands):


	Fair Value Measurements at December 31, 2025
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents:
Money market funds	$	85,866	$	—	$	—	$	85,866
U.S. treasury securities		—		14,991		—		14,991
Short-term investments:
U.S. treasury securities		—		159,455		—		159,455
Corporate debt securities		—		75,216		—		75,216
Government agency bonds		—		5,260		—		5,260
	$	85,866	$	254,922	$	—	$	340,788


	Fair Value Measurements at December 31, 2024
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents:
Money market funds	$	321,047	$	—	$	—	$	321,047
U.S. treasury bills		4,173		—		—		4,173
Short-term investments:
U.S. treasury bills		—		77,165		—		77,165
U.S. government agency bonds		—		67,394		—		67,394
	$	325,220	$	144,559	$	—	$	469,779

There were no transfers between Level 1, Level 2 and Level 3 during the years ended December 31, 2025 and 2024.

The Company classifies its U.S. treasury securities, corporate debt securities and government agency bonds as short-term based on each instrument’s availability for use in current operations. The fair value of the Company’s U.S. treasury securities, corporate debt securities and government agency bonds are classified as Level 2 because they are valued using observable inputs to quoted market prices, benchmark yields, reported trades, broker/dealer quotes or alternative pricing sources with reasonable levels of price transparency.

Short-term investments consisted of the following (in thousands):


	December 31, 2025
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
U.S. treasury securities	$	159,033	$	422	$	—		$	159,455
Corporate debt securities		75,117		102		(3	)		75,216
Government agency bonds		5,253		7		—			5,260
Total short-term investments:	$	239,403	$	531	$	(3	)	$	239,931


	December 31, 2024
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
U.S. treasury bills	$	77,142	$	37	$	(14	)	$	77,165
U.S. government agency bonds		67,442		34		(82	)		67,394
Total short-term investments:	$	144,584	$	71	$	(96	)	$	144,559

The contractual maturities of the Company’s short-term investments in available-for-sale securities held were as follows (in thousands):


	December 31, 2025		December 31, 2024
Due within one year	$	221,506	$	109,943
Due after one year through two years		18,425		34,616
Total available-for-sale securities	$	239,931	$	144,559

Valuation of preferred stock tranche right liabilities

The fair value of the preferred stock tranche right liability in the table below is composed of the fair value of the obligation to issue Series B redeemable convertible preferred stock (“Series B Preferred Stock”) (Note 8). The fair value of the preferred stock tranche right liability was based on significant inputs not observable in the market, which represented a Level 3 measurement within the fair value hierarchy.

The fair value of the Series B Preferred Stock tranche right liability was determined using an option pricing model as it represents an option for the Series B Option Shares (as defined in Note 8). The valuation considered as inputs the estimated fair value of the Series B Preferred Stock as of each valuation date, the risk-free interest rate, volatility, expected dividends, and estimated time to the tranche closing.

The most significant assumption in the valuation model impacting the fair value of the preferred stock tranche right liability is the fair value of the Company’s Series B Preferred Stock as of each measurement date. The Company determined the fair value per share of the underlying Series B Preferred Stock by taking into consideration the most recent sales of its Series B Preferred Stock, results obtained from third-party valuations and additional factors the Company deemed relevant. In April 2024, upon satisfaction of certain conditions, the Company issued and sold 8,823,523 shares of Series B Preferred Stock at a price of $17.00 per share, which resulted in the settlement of the associated Series B preferred stock tranche right liability. The fair value of Series B Preferred Stock was $17.002 per share upon the closing.

The following table presents a roll-forward of the fair value of the Series B preferred stock tranche right liability during the year ended December 31, 2024, for which fair value was determined using Level 3 inputs (in thousands):


	Series B Preferred Stock Tranche Right Liability

Fair value at December 31, 2023	$	2,874
Change in fair value of Series B preferred stock tranche right liability		(2,859	)
Final settlement of Series B preferred stock tranche right liability		(15	)
Fair value at December 31, 2024	$	—

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References

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+ Details

License Agreements

12 Months Ended

Dec. 31, 2025

Organization, Consolidation and Presentation of Financial Statements [Abstract]

License Agreements

14. License agreements

License agreement with Lonza

In October 2021, in connection with an asset purchase agreement entered into with Astellas Pharma, Inc. (“Astellas”), the Company and Lonza Sales AG (“Lonza”) entered into a license agreement (as amended, the “Lonza License Agreement”). Pursuant to the Lonza License Agreement, the Company obtained a worldwide, non-exclusive, sublicensable (subject to Lonza’s right of pre-approval with respect to any sublicense of manufacturing activities) license to certain intellectual property rights owned by Lonza. Lonza was the originator of the master cell bank for verekitug (formerly referred to as ASP7266 and UPB-101, collectively referred to as “the Compound”) developed by Astellas.

As consideration for the rights and licenses granted to the Company under the Lonza License Agreement, the Company agreed to pay Lonza certain royalties and annual payments, both payable in Swiss francs, in respect of the manufacturing and sale of the Compound, such amounts to be determined by the party manufacturing the Compound, and range from no annual payment to up to a mid-six-figure annual payment, and a less-than-one percent to a low-single-digit percentage royalty on net sales of the Compound. In accordance with the Lonza License Agreement, the Company entered into a sublicense with Wuxi Biologics (Hong Kong) Limited to manufacture the Compound, requiring the Company to pay a mid-six-figure annual fee to Lonza pursuant to this provision.

Any royalties due under the Lonza License Agreement are payable on a country-by-country basis until ten years from the first commercial sale of the Compound in that particular country.

During the years ended December 31, 2025 and 2024, the Company did not make any royalty payments to Lonza under the Lonza License Agreement. The Lonza agreement continues for an indefinite period of time unless otherwise terminated. The Company has the right to terminate the Lonza License Agreement at any time by providing prior written notice to Lonza.

During each of the years ended December 31, 2025 and 2024, the Company made an annual payment in the amount of $0.5 million to Lonza pursuant to the Lonza License Agreement and recognized it as research and development expense in the consolidated statements of operations and comprehensive loss.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2025

Accounting Policies [Abstract]

Use of Estimates

Use of estimates

The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenue and expenses, and the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. The Company assesses estimates on an ongoing basis; however, actual results could materially differ from those estimates. Significant estimates and assumptions reflected within these consolidated financial statements include, but are not limited to, prepaid and accrued research and development expenses, including those related to contract research organizations (“CROs”), contract manufacturing organizations (“CMOs”) and other third-party vendors, the valuation of the Company’s common stock prior to the Company’s IPO in October 2024 and stock-based awards and the valuation of the preferred stock tranche right liabilities. Changes in estimates are recorded in the period in which they become known.

Concentration of Credit Risk and of Significant Suppliers

Concentration of credit risk and of significant suppliers

Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash, cash equivalents and short-term investments. The Company deposits its cash and cash equivalents in financial institutions in amounts that may exceed federally insured limits, and has not experienced any losses on such accounts and does not believe it is exposed to any unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company’s short-term investments consist of U.S. treasury securities, government agency bonds and corporate debt securities which the Company believes represent minimal credit risk.

The Company is dependent on third-party manufacturers to supply products for research and development activities related to verekitug, including preclinical and clinical studies and testing. In particular, the Company relies and expects to continue to rely on a small number of manufacturers for the supply of verekitug. The Company’s preclinical and clinical studies and testing could be adversely affected by a significant interruption in the supply.

Foreign Currency Gains and Losses

Foreign currency gains and losses

The functional currency and the reporting currency of the Company is the U.S. dollar. Adjustments that arise from exchange rate changes on transactions denominated in a currency other than the functional currency are included as foreign exchange gains and

losses in other expense, net in the consolidated statements of operations and comprehensive loss. The Company has not recognized material foreign currency transaction gains or losses during the years ended December 31, 2025 and 2024.

Cash and Cash Equivalents

Cash and cash equivalents

The Company considers all short-term, highly liquid investments, with an original maturity of three months or less, to be cash equivalents, and as of December 31, 2025 and 2024, includes amounts held in money market funds in the amount of $86.0 million and $321.0 million, respectively and in U.S. treasury securities of $15.0 million and $4.2 million, respectively.

Restricted Cash

Restricted cash

Restricted cash consisted of a letter of credit totaling $0.2 million as of December 31, 2025 and 2024, that is required to be maintained in connection with the Company's lease arrangements. The letter of credit is in the name of the Company's landlord and is required to fulfill lease requirements in the event the Company should default on its lease obligations. As of December 31, 2025 and 2024, the Company classified its restricted cash as non-current assets on the consolidated balance sheets based on the release date of the restriction.

Short-Term Investments

Short-term investments

Available-for-sale securities consist of investments with original maturities greater than 90 days at acquisition date. The Company classifies any investments with maturities beyond one year as short term, based on their highly liquid nature and because such available-for-sale securities represent the investment of cash that is available for current operations.

The Company’s debt security investments are classified as available-for-sale and are carried at fair value, with the unrealized gains and losses reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. Realized gains and losses and declines in fair value due to credit-related factors are based on the specific identification method and are included as other expense, net in the consolidated statements of operations and comprehensive loss. The Company recorded interest income on available-for-sale investments of $16.9 million and $12.1 million during the years ended December 31, 2025 and 2024, respectively, which is classified as interest income in the consolidated statements of operations and comprehensive loss.

At each balance sheet date, the Company assesses available-for-sale debt securities in an unrealized loss position to determine whether the unrealized loss or any potential credit losses should be recognized in other expense, net. The Company evaluates whether it intends to sell, or it is more likely than not that it will be required to sell, the security before recovery of its amortized cost basis. The Company also evaluates whether the decline in fair value has resulted from credit losses or other factors. In making this assessment, the Company considers the severity of the impairment, any changes in interest rates, changes to the underlying credit ratings and forecasted recovery, among other factors. The credit-related portion of unrealized losses, and any subsequent improvements, are recorded in other expense, net. The portion that is not credit-related is treated in accordance with other unrealized losses as a component of accumulated other comprehensive income (loss) in stockholders’ equity. There have been no impairment or credit losses recognized during any of the periods presented.

Fair Value Measurements

Fair value measurements

Certain assets and liabilities of the Company are carried at fair value under U.S. GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

Level 1 Quoted prices in active markets for identical assets or liabilities.

Level 2 Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

Level 3 Unobservable inputs that are supported by little or no market activity that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

The carrying values of the Company’s accounts receivables, prepaid expenses and other current assets, accounts payable and accrued expenses and other current liabilities approximate their fair values due to the short-term nature of these assets and liabilities. The Company’s cash equivalents, short-term investments and the preferred stock tranche right liability are carried at fair value (Note 3).

Property and Equipment

Property and equipment

The Company records property and equipment at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset, as follows:


		Estimated Useful Life
Lab equipment		5 years
Computer equipment		3 years
Office equipment		5 years
Leasehold improvements		Shorter of remaining lease term or estimated useful life

Estimated useful lives are periodically assessed to determine if changes are appropriate. Leasehold improvements are amortized using the straight-line method over the lesser of the lease term or its estimated economic useful life. Lease terms are based upon the initial lease agreement and do not consider potential renewals or extensions until such time that the renewals or extensions are contracted. Expenditures for maintenance and repairs that do not improve or extend the life of the respective assets are expensed as incurred. When assets are retired or otherwise disposed of, the cost of these assets and related accumulated depreciation or amortization are eliminated from the consolidated balance sheets and any resulting gains or losses are included in the consolidated statements of operations and comprehensive loss in the period of disposal. Costs for capital assets not yet placed into service are capitalized as construction-in-progress and depreciated once placed into service.

Impairment of Long-lived Assets

Impairment of long-lived assets

The Company evaluates its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of such assets may not be recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value. For the years ended December 31, 2025 and 2024, the Company did not record any impairment losses on long-lived assets.

Operating Leases

Operating leases

The Company determines if an arrangement is or contains a lease, as defined by ASU 2016-02, Leases (Topic 842) (“ASC 842”), at the lease inception date by evaluating whether the arrangement conveys the right to use an identified asset and whether the Company obtains substantially all of the economic benefits from and has the ability to direct the use of the asset. If an arrangement is determined to be or contain a lease, the lease is assessed for classification as either an operating or finance lease at the lease commencement date, defined as the date on which the leased asset is made available for use by the Company, based on the economic characteristics of the lease.

ASC 842 includes certain practical expedients that can be elected for new leases that are executed after the adoption of the new requirements. The Company elected the practical expedient to not separate lease and non-lease components. The Company also elected to apply the short-term lease recognition exemption which eliminates the requirement to present on the consolidated balance sheets leases with a term of 12 months or less. These two practical expedients were elected for all classes of underlying assets.

At the lease commencement date, the Company recognizes a lease liability and a right-of-use (“ROU”) asset representing its right to use the underlying asset over the lease term. The initial measurement of the lease liability is calculated as the present value of the future lease payments in the contract and the ROU asset is measured as the lease liability plus initial direct costs and prepaid lease payments, less lease incentives granted by the lessor. The subsequent measurement of a lease is dependent on whether the lease is classified as an operating lease or a finance lease. Operating lease cost is recognized on a straight-line basis over the lease term in the consolidated statements of operations and comprehensive loss.

The Company’s leases require other payments such as costs related to taxes, insurance, maintenance, and other expenses. These costs are generally variable in nature and based on the actual costs incurred and required by the lease. As the Company has elected to not separate lease and non-lease components for all classes of underlying asset, all variable costs associated with the lease are expensed in the period incurred and presented and disclosed as variable lease costs. The Company’s lease agreements do not contain any material residual value guarantees or material restrictive financial covenants.

ASC 842 requires that a lessee use the rate implicit in the lease when measuring the lease liability and ROU asset. If the rate implicit in the lease is not readily determinable, the Company is permitted to use its incremental borrowing rate, which is defined as the rate of interest that the Company would have to pay to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. Since the rate implicit in the lease is not readily determinable, the Company uses its incremental borrowing rate when measuring its leases. The incremental borrowing rate is calculated by considering the Company’s credit standing, the lease term and the impact of collateral.

Most leases include options to renew and, or, terminate the lease, which can impact the lease term. The exercise of these options is at the Company’s discretion. Periods covered by an option to extend a lease are not included in the lease term as the Company is not reasonably certain it will exercise this option. Additionally, periods covered by an option to terminate the lease are included in the lease term as it is reasonably certain that the Company will not exercise this option.

Classification and Accretion of Redeemable Convertible Preferred Stock

Classification and accretion of redeemable convertible preferred stock

The holders of Preferred Stock (as defined in Note 8) have certain redemption rights in the event of a deemed liquidation event that, in certain situations, are not solely within the control of the Company and would call for the redemption of the then outstanding Preferred Stock (Note 8). Therefore, the Preferred Stock is classified as mezzanine equity outside of stockholders’ equity on the consolidated balance sheets. The Company recorded the Preferred Stock at fair value upon issuance, net of tranche right liabilities (Note 8) and associated issuance costs. The net carrying value of redeemable convertible preferred stock were accreted to their redemption values through a charge to additional paid-in capital or accumulated deficit over the period from date of issuance to the earliest date on which the holders could, at their option, elect to redeem their shares. In connection with the IPO, all outstanding shares of convertible preferred stock converted into an aggregate of 33,321,149 shares of the Company’s common stock.

Preferred Stock Tranche Right Liabilities

Preferred stock tranche right liabilities

The purchase agreements for the Company’s Preferred Stock provide the Company an obligation to issue additional Preferred Stock in subsequent closings upon the satisfaction of certain conditions (the “preferred stock tranche rights”) (Note 8).

The Company classified such preferred stock tranche rights as liabilities on its consolidated balance sheets (the “preferred stock tranche right liabilities”) as each preferred stock tranche right was determined to be a freestanding financial instrument that may require the Company to transfer assets to settle its obligation upon events outside of its control. The preferred stock tranche right liabilities were initially recorded at fair value upon the issuance date and are subsequently remeasured to fair value at each reporting date and immediately prior to being settled. Changes in fair value of the preferred stock tranche right liabilities are recognized as a component of other income, net in the consolidated statements of operations and comprehensive loss. Upon settlement of the tranche rights, the Company derecognized the related liability, and stopped recognizing changes in the fair value of the preferred stock tranche right liability. Any issuance costs allocated to the preferred stock tranche right liabilities were immediately expensed.

Revenue Recognition

Revenue recognition

The Company enters into license arrangements, pursuant to which it may provide research and development services for third parties.

To determine revenue recognition for arrangements that the Company determines are within the scope of ASC 606, Revenue from Contracts with Customers, (“ASC 606”), the Company performs the following five steps: (i) identify the promised goods or services in the contract; (ii) determine whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measure the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations; and (v) recognize revenue when, or as, the Company satisfies each performance obligation. At contract inception, the Company assesses whether the goods or services promised within each contract are a separate performance obligation. Goods and services that are determined not to be distinct are combined with other promised goods and services until a distinct bundle is identified. The Company then allocates the transaction price, the amount of consideration the Company expects to be entitled to from a customer in exchange for the promised goods or services, to each performance obligation and recognizes the associated revenue when each performance obligation is satisfied.

In determining the appropriate amount of revenue to be recognized, the Company uses judgment to determine: (a) the number of performance obligations; (b) the transaction price; (c) the stand-alone selling price for each performance obligation identified in the contract; and (d) the contract term and pattern of satisfaction of the performance obligations. The Company uses judgment to determine whether milestones or other variable consideration should be included in the transaction price. The transaction price is allocated to the identified performance obligations on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. At contract inception, the Company estimates the total costs required to satisfy the performance obligation and subsequently updates the estimate at each reporting period. Accordingly, the Company’s estimates may change in the future and those changes could result in a change in amounts of revenue recognized and could be material.

During the years ended December 31, 2025 and 2024, the Company generated revenue from a research and development arrangement with Maruho Co., Ltd (“Maruho”), which is accounted for under ASC 606. Pursuant to the agreement, the Company provides to Maruho research and development services related to verekitug in Japan, and Maruho reimburses the Company for these costs incurred in performing the research and development services (Note 15).

The Company records accounts receivable when its right to receive consideration is solely based on the passage of time. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s consolidated balance sheets. Amounts expected to be recognized as revenue within one year following the balance sheet date are classified as current deferred revenue. Amounts not expected to be recognized as revenue within one year following the balance sheet date are classified as deferred revenue, net of current portion. Payment terms and conditions generally require payment within 60 days of invoicing.

Segment Information

Segment information

Operating segments are defined as components of an enterprise for which separate discrete financial information is available for evaluation by the chief operating decision maker (“CODM”) in deciding how to allocate resources and assess performance. The Company’s CODM, its Chief Executive Officer, views the Company’s operations and manages its business on a consolidated basis as a single operating segment, which is the business of developing treatments for inflammatory diseases, with an initial focus on severe respiratory disorders. Revenue is generated exclusively from certain transactions with Maruho located in Japan and all assets are held in the United States (Note 18).

Research and Development Expenses

Research and development expenses

Research and development expenses are expensed as incurred. Research and development expenses include salaries and benefits, stock-based compensation expense, licensed technology, external costs of third-party vendors that conduct research and development activity on behalf of the Company, and other operational costs related to the Company’s research and development activities including costs related to a research and development arrangement with Maruho.

Prepaid and Accrued Research and Development Expenses

Prepaid and accrued research and development expenses

The Company recognizes research and development expense and records accruals for estimated costs of research and development activities conducted by third-party service providers, which include CROs that conduct research, preclinical studies and clinical trials on the Company’s behalf, including in connection with the Company’s research and development arrangement, and CMOs that manufacture the Company’s product candidate for use in preclinical and clinical trials. The majority of the Company’s service providers invoice in arrears for services performed, on a pre-determined schedule or when contractual milestones are met; however, some require advanced payments. The Company makes estimates of the accrued expenses and includes these costs in accrued liabilities in the consolidated balance sheets and within research and development expense in the

consolidated statements of operations and comprehensive loss based on facts and circumstances known to the Company at that time. These costs are a significant component of the Company’s research and development expenses.

The Company accrues for these costs based on factors such as estimates of the amount of work completed through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and in accordance with agreements established with its third-party service providers for such services. The Company makes significant judgments and estimates in determining the accrued research and development liabilities balance at each reporting period. As actual costs become known, the Company adjusts its accrued estimates. To date, there have been no material adjustments to the Company’s estimates of accrued research and development expenses. The Company records advance payments to service providers as prepaid expenses and other current assets, which are expensed as the contracted services are performed. If the actual timing of the performance of services varies from the estimate, then the Company adjusts the amount of the accrued expense or the prepaid expense accordingly.

General and Administrative Expenses

General and administrative expenses

General and administrative expenses consist primarily of salaries and benefits, including stock-based compensation expense; professional fees for legal, accounting, auditing, tax and consulting services; travel expenses; and facility-related expenses, which include expenses for rent and maintenance of facilities and other operating costs. The Company expenses all general and administrative expenses as incurred.

Patent and Trademarks

Patent and trademarks

Costs to secure, defend and maintain patents, including those in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.

Stock-based Compensation Expense

Stock-based compensation expense

The Company measures all stock-based awards granted to employees, directors, and non-employee service providers based on fair value on the date of the grant, and recognizes the resulting fair value over the requisite service period. The Company uses the Black-Scholes option-pricing model to estimate the fair value of stock options granted. The Company has elected to recognize stock-based compensation expense for service-based stock options with graded vesting on a straight-line basis over the requisite service period, which is generally the vesting period. The Company recognizes expense related to stock options that contain performance conditions only when it is considered probable that the performance condition will be achieved. Stock-based compensation expense for stock options with performance conditions is recognized using graded vesting. The Company accounts for forfeitures as they occur.

The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.

Comprehensive Loss

Comprehensive loss

Comprehensive loss is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from nonowner sources, including unrealized gains and losses on marketable securities held as available for sale. For the years ended December 31, 2025 and 2024, comprehensive loss includes net loss and unrealized gains (losses) on short-term investments.

Net Loss Per Share

Net loss per share

The Company calculated basic and diluted net loss per share attributable to common stockholders in conformity with the two-class method required for companies with participating securities. The Company’s Preferred Stock is considered to be a participating security as the holders are entitled to receive dividends at a dividend rate payable in preference and priority to the holders of common stock. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period has been distributed. There is no

allocation required under the two-class method during periods of loss since the participating securities do not have a contractual obligation to share in the losses of the Company.

Under the two-class method, basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share attributable to common stockholders is computed by (i) adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities and (ii) dividing the diluted net loss attributable to common stockholders by the weighted-average number of common shares outstanding for the period, including potential dilutive common shares. For purposes of this calculation, Preferred Stock and stock options to purchase common stock are considered potential dilutive common shares.

The Company has generated a net loss for each of the periods presented. Accordingly, basic and diluted net loss per share attributable to common stockholders are the same because the inclusion of the potentially dilutive securities would be anti-dilutive.

Income Taxes

Income taxes

The Company accounts for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are determined based on differences between financial reporting and tax reporting bases of assets and liabilities and are measured using enacted tax rates and laws that are expected to be in effect when the differences are expected to reverse. Realization of deferred tax assets is dependent upon future earnings, the timing and amount of which are uncertain. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.

The Company utilizes a two-step approach to recognize and measure uncertain tax positions. The first step is to evaluate the tax position for recognition by determining if the weight of available evidence indicates that it is more likely than not that the position will be sustained upon tax authority examination, including resolution of related appeals or litigation processes, if any. The second step is to measure the tax benefit as the largest amount that is more than 50% likely of being realized upon ultimate settlement.

Recently Adopted/Issued Accounting Pronouncements Not Yet Adopted

Recently adopted accounting pronouncements

In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures, which focuses on the rate reconciliation and income taxes paid. ASU No. 2023-09 requires a public business entity (“PBE”) to disclose, on an annual basis, a tabular rate reconciliation using both percentages and currency amounts, broken out into specified categories with certain reconciling items further broken out by nature and jurisdiction to the extent those items exceed a specified threshold. In addition, all entities are required to disclose income taxes paid, net of refunds received disaggregated by federal, state/local, and foreign and by jurisdiction if the amount is at least 5% of total income tax payments, net of refunds received. For PBEs, the new standard is effective for annual periods beginning after December 15, 2024, with early adoption permitted. An entity has the option to adopt this ASU prospectively or retrospectively. As of December 31, 2025, the Company adopted this new ASU retrospectively and it only impacts the Company's income tax disclosures with no impact to its operations, cash flows, or financial condition.

Recently issued accounting pronouncements not yet adopted

From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. The Company qualifies as an ‘‘emerging growth company,’’ as defined in the Jumpstart Our Business Startups Act of 2012, and has elected not to ‘‘opt out’’ of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and non-public companies, the Company can adopt the new or revised standard at the time non-public companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to ‘‘opt out’’ of such extended transition period or (ii) no longer qualifies as an emerging growth company. The Company may choose to early adopt any new or revised accounting standards whenever such early adoption is permitted for non-public companies.

In November 2024, the FASB issued ASU 2024-03, Disaggregation of Income Statement Expenses, which requires more detailed information about specified categories of expenses (purchases of inventory, employee compensation, depreciation, amortization, and depletion) included in certain expense captions presented on the face of the income statement. This ASU is effective for fiscal years beginning after December 15, 2026, and for interim periods within fiscal years beginning after December 15, 2027. Early adoption is permitted. The amendments may be applied either (1) prospectively to financial statements issued for reporting periods after the effective date of this ASU or (2) retrospectively to all prior periods presented in the financial statements. The Company is currently evaluating the impact of adopting this ASU on its consolidated financial statements and related disclosures.

Summary of Significant Accounting Policies - Schedule of Estimated Useful Life of Each Asset (Details)	Dec. 31, 2025
Lab Equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Life	5 years
Computer Equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Life	3 years
Office Equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Life	5 years
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Estimated Useful Life, Term, Description	Shorter of remaining lease term or estimated useful life

Accrued expenses and other current liabilities (Tables)

12 Months Ended

Dec. 31, 2025

Payables and Accruals [Abstract]

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	December 31,		December 31,
	2025		2024
Accrued external research and development expenses	$	5,057	$	2,807
Accrued employee compensation and benefits		4,192		2,697
Accrued consultant and professional fees		757		488
	$	10,006	$	5,992

Segment Reporting (Tables)

12 Months Ended

Dec. 31, 2025

Segment Reporting [Abstract]

Schedule of Segment Net Loss, Including Disaggregated Components of Operating Expenses

The Company's consolidated and segment net loss, including disaggregated components of operating expenses is as follows (in thousands):


	Year Ended December 31,
	2025			2024
Collaboration revenue	$	2,854		$	2,370
Operating expenses:
Research and development:
Verekitug program:
Asthma indication		47,694			28,069
COPD indication		34,562			3,115
CRSwNP indication		7,564			10,524
Manufacturing costs		23,013			6,580
Personnel expenses		18,343			9,911
Professional fees and other		5,630			4,767
Total research and development expenses		136,806			62,966
General and administrative:
Personnel expenses		15,188			10,842
Professional fees		6,718			4,349
Other		4,503			1,977
Total general and administrative expenses		26,409			17,168
Total operating expenses		163,215			80,134
Loss from operations		(160,361	)		(77,764	)
Other income (expense):
Change in fair value of preferred stock tranche right liability		—			2,859
Interest income		16,933			12,123
Other expense, net		(15	)		(24	)
Total other income, net		16,918			14,958
Segment and consolidated net loss	$	(143,443	)	$	(62,806	)

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2025

Share-Based Payment Arrangement [Abstract]

Schedule of Assumptions Used in Black-Scholes Option-pricing Model to Determine the Fair Value of Stock Options Granted

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:


	Year Ended December 31,
	2025			2024
Expected volatility		92.4	%		79.1	%
Expected dividends		0	%		0	%
Expected term (in years)		6.0			6.2
Risk-free rate		4.24	%		3.41	%

Summary of Stock Options Activity

The following table summarizes the activity of stock options with service-based and performance-based vesting conditions during the year ended December 31, 2025:


	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Intrinsic Value (in thousands)
Outstanding as of December 31, 2024		6,490,571		$	5.54		7.9	$	71,266
Granted		2,222,826			9.45
Exercised		(608,155	)		3.95
Forfeited		(52,151	)		7.37
Cancelled		(92,363	)		3.57
Outstanding as of December 31, 2025		7,960,728		$	6.77		8.0	$	162,277
Options exercisable December 31, 2025		3,619,126		$	5.50		7.2	$	78,346
Vested and expected to vest December 31, 2025		7,960,728		$	6.77		8.0	$	162,277

Schedule of Classification of Stock-based Compensation in Condensed Consolidated Statements of Operations and Comprehensive Loss

The following table illustrates the classification of stock-based compensation in the consolidated statements of operations and comprehensive loss (in thousands):


	Year Ended December 31,
	2025		2024
General and administrative	$	7,478	$	4,842
Research and development		2,854		1,162
	$	10,332	$	6,004

Leases (Tables)

12 Months Ended

Dec. 31, 2025

Leases [Abstract]

Summary of Components of Lease Cost

The components of lease cost, which are included in the consolidated statements of operations and comprehensive loss, were as follows (in thousands):


	December 31,
	2025		2024
Lease Cost:
Operating lease cost	$	739	$	256
Short-term lease cost		—		202
Variable lease cost		—		19
Total lease cost	$	739	$	477

Schedule of Supplemental Disclosure Cash Flow Information Related to Leases

Supplemental disclosure of cash flow information related to leases were as follows (in thousands):


	December 31,
	2025		2024
Cash paid for amounts included in the measurement of lease liabilities	$	742	$	207

Schedule of Weighted Discount Rate and Remaining Lease Term

The weighted-average discount rate and remaining lease term were as follows:


	December 31,
	2025			2024
Weighted-average discount rate — operating leases		11.7	%		11.7	%
Weighted-average remaining lease term — operating leases		1.8			2.8

Schedule of Maturities of Operating Lease Liabilities

The maturities of operating lease liabilities were as follows (in thousands):


Year Ended December 31,	Amount
2026		759
2027		644
Total lease payments		1,403
Less: imputed interest		(134	)
Present value of lease liabilities		1,269
Less: operating lease liabilities, current portion		(720	)
Operating lease liabilities, net of current portion	$	549

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Pay vs Performance Disclosure
Net Income (Loss)	$ (143,443)	$ (62,806)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 946
-SubTopic 830
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479168/946-830-55-10

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476153/220-40-55-4

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 21
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476153/220-40-55-21

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 220
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147476153/220-40-55-14

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(i))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(k)(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
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+ Details

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2025

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Schedule of Financial Assets and Liabilities Measured at Fair Value


	Fair Value Measurements at December 31, 2025
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents:
Money market funds	$	85,866	$	—	$	—	$	85,866
U.S. treasury securities		—		14,991		—		14,991
Short-term investments:
U.S. treasury securities		—		159,455		—		159,455
Corporate debt securities		—		75,216		—		75,216
Government agency bonds		—		5,260		—		5,260
	$	85,866	$	254,922	$	—	$	340,788


	Fair Value Measurements at December 31, 2024
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents:
Money market funds	$	321,047	$	—	$	—	$	321,047
U.S. treasury bills		4,173		—		—		4,173
Short-term investments:
U.S. treasury bills		—		77,165		—		77,165
U.S. government agency bonds		—		67,394		—		67,394
	$	325,220	$	144,559	$	—	$	469,779

Schedule of Short-Term Investments

Short-term investments consisted of the following (in thousands):


	December 31, 2025
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
U.S. treasury securities	$	159,033	$	422	$	—		$	159,455
Corporate debt securities		75,117		102		(3	)		75,216
Government agency bonds		5,253		7		—			5,260
Total short-term investments:	$	239,403	$	531	$	(3	)	$	239,931


	December 31, 2024
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
U.S. treasury bills	$	77,142	$	37	$	(14	)	$	77,165
U.S. government agency bonds		67,442		34		(82	)		67,394
Total short-term investments:	$	144,584	$	71	$	(96	)	$	144,559

Schedule of Contractual Maturities of Short-term Investments

The contractual maturities of the Company’s short-term investments in available-for-sale securities held were as follows (in thousands):


	December 31, 2025		December 31, 2024
Due within one year	$	221,506	$	109,943
Due after one year through two years		18,425		34,616
Total available-for-sale securities	$	239,931	$	144,559

Schedule of Assumptions Used in Option-Pricing Model to Determine Fair Value of Series B Preferred Stock Tranche Right Liability

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:


	Year Ended December 31,
	2025			2024
Expected volatility		92.4	%		79.1	%
Expected dividends		0	%		0	%
Expected term (in years)		6.0			6.2
Risk-free rate		4.24	%		3.41	%

Roll-forward of Fair Value of the Series A and Series B Preferred Stock Tranche Right Liability Using Level 3 Inputs


	Series B Preferred Stock Tranche Right Liability

Fair value at December 31, 2023	$	2,874
Change in fair value of Series B preferred stock tranche right liability		(2,859	)
Final settlement of Series B preferred stock tranche right liability		(15	)
Fair value at December 31, 2024	$	—

- Definition

Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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+ Details

Nature of the Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Oct. 04, 2024	Oct. 31, 2024 USD ($) $ / shares shares	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Product Information [Line Items]
Stock split ratio	1.049
Net Income (Loss)			$ (143,443)	$ (62,806)
Accumulated deficit			$ (334,223)	$ (190,780)
IPO
Product Information [Line Items]
Sale of stock, number of shares issued \| shares		17,250,000
Sale of stock, price per share \| $ / shares		$ 17.00
Proceeds from IPO		$ 268,800
Underwriting discounts and commissions		20,500
Other offering costs		$ 3,900
Over Allotment Option
Product Information [Line Items]
Sale of stock, number of shares issued \| shares		2,250,000

Fair Value Measurements - Schedule of Contractual Maturities of Short-term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Fair Value Disclosures [Abstract]
Due within one year	$ 221,506	$ 109,943
Due after one year through two years	18,425	34,616
Total available-for-sale securities	$ 239,931	$ 144,559

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Paragraph 1
-Subparagraph (b)
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+ Details

Segment Reporting

12 Months Ended

Dec. 31, 2025

Segment Reporting [Abstract]

Segment Reporting

18. Segment reporting

The Company currently has a single reportable operating segment and revenue generated exclusively from the Maruho Agreement. The Company’s chief executive officer, who is the CODM, manages the Company on a consolidated basis and utilizes consolidated net loss as a basis for resource allocation and decision making. The CODM considers budget-to-actual variances for each of the disaggregated components of operating expenses when making decisions about allocating resources and evaluating performance. The measure of segment assets is reported on the consolidated balance sheet as total consolidated assets. In addition, the CODM is regularly provided information on total cash, which is inclusive of cash, cash equivalents and short-term investments, as a measure of segment assets. As of December 31, 2025, the Company’s cash, cash equivalents and short-term investments were $341.5 million.

The Company's consolidated and segment net loss, including disaggregated components of operating expenses is as follows (in thousands):


	Year Ended December 31,
	2025			2024
Collaboration revenue	$	2,854		$	2,370
Operating expenses:
Research and development:
Verekitug program:
Asthma indication		47,694			28,069
COPD indication		34,562			3,115
CRSwNP indication		7,564			10,524
Manufacturing costs		23,013			6,580
Personnel expenses		18,343			9,911
Professional fees and other		5,630			4,767
Total research and development expenses		136,806			62,966
General and administrative:
Personnel expenses		15,188			10,842
Professional fees		6,718			4,349
Other		4,503			1,977
Total general and administrative expenses		26,409			17,168
Total operating expenses		163,215			80,134
Loss from operations		(160,361	)		(77,764	)
Other income (expense):
Change in fair value of preferred stock tranche right liability		—			2,859
Interest income		16,933			12,123
Other expense, net		(15	)		(24	)
Total other income, net		16,918			14,958
Segment and consolidated net loss	$	(143,443	)	$	(62,806	)

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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Reference 14: http://www.xbrl.org/2003/role/exampleRef
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Reference 19: http://www.xbrl.org/2003/role/disclosureRef
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Reference 20: http://www.xbrl.org/2003/role/disclosureRef
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Reference 22: http://www.xbrl.org/2003/role/disclosureRef
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Reference 23: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Stock-based compensation - Schedule of Classification of Stock-based Compensation in Condensed Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation	$ 10,332	$ 6,004
General and Administrative
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation	7,478	4,842
Research and Development
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation	$ 2,854	$ 1,162

Income Taxes - Summary of Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Income Tax Disclosure [Abstract]
Valuation allowance at beginning of year	$ (60,664)	$ (39,695)
Increases recorded to income tax provision	(42,968)	(20,969)
Valuation allowance at beginning of year	$ (103,632)	$ (60,664)

Commitments and contingencies

12 Months Ended

Dec. 31, 2025

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

13. Commitments and contingencies

Legal matters

The Company is subject to contingent liabilities, such as legal proceedings and claims, that arise in the ordinary course of business activities. The Company accrues for loss contingencies when losses become probable and are reasonably estimable. If the reasonable estimate of the loss is a range and no amount within the range is a better estimate, the minimum amount of the range is recorded as a liability on the consolidated balance sheets. The Company does not accrue for contingent losses that, in its judgment, are considered to be reasonably possible, but not probable; however, it discloses the range of reasonably possible losses. As of December 31, 2025 and 2024, the Company was not a party to any material legal proceedings or claims and no liabilities were recorded for loss contingencies.

Contracts

The Company enters into contracts in the normal course of business with various third parties for preclinical research studies, clinical trials, testing, manufacturing, and other services. These contracts generally provide for termination upon notice and are cancellable without significant penalty or payment, and do not contain any minimum purchase commitments.

Guarantees and indemnifications

In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with all board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company is not aware of any claims under indemnification arrangements that could have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2025 and 2024.

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2025

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders

Basic and diluted net loss per share attributable to common stockholders was calculated as follows (dollar amounts in thousands):


	Year Ended December 31,
	2025			2024
Numerator:
Net loss	$	(143,443	)	$	(62,806	)
Preferred Stock cumulative dividends		—			(13,589	)
Net loss attributable to common stockholders	$	(143,443	)	$	(76,395	)
Denominator:
Weighted-average common shares outstanding, basic and diluted		53,852,752			13,682,326
Net loss per share attributable to common stockholders, basic and diluted	$	(2.66	)	$	(5.58	)

Schedule of Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share

The following potentially dilutive securities have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect:


	As of December 31,
	2025		2024
Stock options to purchase common stock		7,960,728		6,490,571

Income Taxes - Summary of Effective Income Tax Rate Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Income Tax Expense (Benefit), Effective Income Tax Rate Reconciliation, Amount [Abstract]
U.S. federal statutory income tax rate	$ (30,123,000)	$ (13,189,000)
State and local income taxes, net of federal income tax effect	0	0
Effect of changes in tax laws or rates enacted in the current period	0	0
Research and development tax credits	(2,136,000)	(2,897,000)
Change in valuation allowance	32,342,000	16,270,000
Nontaxable or nondeductible items. Other	(83,000)	(184,000)
Income Tax Expense (Benefit), Total	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
U.S. federal statutory income tax rate	21.00%	21.00%
State and local income taxes, net of federal income tax effect	(0.00%)	0.00%
Effect of changes in tax laws or rates enacted in the current period	0.00%	0.00%
Research and development tax credits	1.50%	4.60%
Change in valuation allowance	(22.50%)	(25.90%)
Nontaxable or nondeductible items, Other	0.00%	0.30%
Effective income tax rate	0.00%	0.00%

Property and Equipment, Net - Schedule of Property Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 903	$ 738
Less: Accumulated depreciation and amortization	(344)	(156)
Property and equipment, net	559	582
Office Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	559	465
Computer Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	202	202
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	87	$ 71
Lab Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 55

Accrued expenses and other current liabilities

12 Months Ended

Dec. 31, 2025

Payables and Accruals [Abstract]

Accrued expenses and other current liabilities

6. Accrued expenses and other current liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	December 31,		December 31,
	2025		2024
Accrued external research and development expenses	$	5,057	$	2,807
Accrued employee compensation and benefits		4,192		2,697
Accrued consultant and professional fees		757		488
	$	10,006	$	5,992

Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shor-term investments	$ 239,931	$ 144,559
Fair Value on Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	340,788	469,779
Fair Value on Recurring Basis \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	85,866	325,220
Fair Value on Recurring Basis \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	254,922	144,559
Fair Value on Recurring Basis \| Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	85,866	321,047
Fair Value on Recurring Basis \| Money Market Funds \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	85,866	321,047
Fair Value on Recurring Basis \| US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	14,991	4,173
Shor-term investments	159,455	77,165
Fair Value on Recurring Basis \| US Treasury Securities \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		4,173
Fair Value on Recurring Basis \| US Treasury Securities \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	14,991
Shor-term investments	159,455	77,165
Fair Value on Recurring Basis \| Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shor-term investments	75,216
Fair Value on Recurring Basis \| Corporate Debt Securities \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shor-term investments	75,216
Fair Value on Recurring Basis \| U.S. Government Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shor-term investments	5,260	67,394
Fair Value on Recurring Basis \| U.S. Government Agency Bonds \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shor-term investments	$ 5,260	$ 67,394

Stock-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		12 Months Ended
	Aug. 19, 2024	Mar. 31, 2024	Feb. 29, 2024	Dec. 31, 2025	Dec. 31, 2024	Jan. 01, 2026
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock options, granted				2,222,826
Aggregate intrinsic value of stock options exercised				$ 7.8	$ 0.3
Weighted-average grant-date fair value of options granted				$ 7.32	$ 5.10
Unrecognized compensation cost related to unvested stock options				$ 21.4
Incremental stock-based compensation				$ 0.7
2024 Stock Option and Incentive Plan and 2021 Stock Option and Grant Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock reserved for future issuance				11,483,191
2024 Stock Option and Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock reserved for future issuance				3,522,463
Increase in percentage of outstanding number of common stock				5.00%
2024 Stock Option and Incentive Plan \| Subsequent Events
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock reserved for future issuance						2,711,887
2024 Employee Stock Purchase Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock reserved for future issuance	488,467
Shares issued				26,197
Increase in number of shares available for issuance	976,934
Increase in percentage of outstanding number of common stock				1.00%
2024 Employee Stock Purchase Plan \| Subsequent Events
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock reserved for future issuance						0
Stock Options to Purchase Common Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Weighted average period in years of recognition for unvested stock options				2 years 4 months 24 days
Performance-based Stock Options
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Stock options, granted				1,206,249
Chief Operating Officer
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares held			142,935
Exercisable, termination term			1 year
Chief Executive Officer
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares held		532,553
Stock options vested		38,245
Exercisable, termination term		2 years

Leases - Summary of Components of Lease Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Lease, Cost [Abstract]
Operating lease cost	$ 739	$ 256
Short-term lease cost	0	202
Variable lease cost	0	19
Total lease cost	$ 739	$ 477

Redeemable Convertible Preferred Stock - Additional Information (Details)	1 Months Ended			6 Months Ended	12 Months Ended
	Apr. 30, 2024 USD ($) $ / shares shares	Mar. 31, 2024 USD ($) Director $ / shares	Jun. 30, 2023 $ / shares shares	Jun. 30, 2023 USD ($) $ / shares shares	Dec. 31, 2024 USD ($) $ / shares shares	Dec. 31, 2025 $ / shares shares	Dec. 31, 2023 shares
Conversion of Stock [Line Items]
Convertible preferred stock converted to common stock						33,321,149
Preferred stock, par value \| $ / shares					$ 0.001	$ 0.001
Preferred stock, shares authorized					10,000,000	10,000,000
Preferred stock, shares outstanding					0	0
Preferred stock, shares issued					0	0
Issuance costs \| $					$ 3,900,000
Series B Preferred Stock [Member]
Conversion of Stock [Line Items]
Preferred stock, par value \| $ / shares	$ 17.00					$ 17.002
Sale of stock, number of shares issued	8,823,523
Number of directors approval by stock holders \| Director		1
Redeemable Convertible Preferred Stock
Conversion of Stock [Line Items]
Redeemable convertible preferred stock, shares outstanding							22,941,170
Series B Redeemable Convertible Preferred Stock
Conversion of Stock [Line Items]
Sale of stock, number of shares issued	8,823,523			2,941,170
Sale of stock, price per share \| $ / shares	$ 17	$ 17.00	$ 17	$ 17
Redeemable convertible preferred stock, shares issued					0	0
Redeemable convertible preferred stock, shares outstanding					0	0
Convertible preferred stock approval by number of directors \| Director		6
Fair value of convertible preferred stock \| $	$ 150,000,000
Gross cash proceeds \| $	150,000,000	$ 50,000,000		$ 50,000,000
Issuance costs \| $				600,000
Preferred stock tranche right liability settled \| $		$ 11,800,000		$ 100,000
Series B Redeemable Convertible Preferred Stock \| Maximum
Conversion of Stock [Line Items]
Sale of stock, number of shares issued			11,764,693
Issuance costs \| $	100,000
Preferred stock tranche right liability settled \| $	$ 100,000
Series B Redeemable Convertible Preferred Stock \| Milestone Shares
Conversion of Stock [Line Items]
Sale of stock, number of shares issued				8,823,523

Accrued expenses and other current liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 5,057	$ 2,807
Accrued employee compensation and benefits	4,192	2,697
Accrued consultant and professional fees	757	488
Accrued expenses and other current liabilities	$ 10,006	$ 5,992

Property and Equipment, Net

12 Months Ended

Dec. 31, 2025

Property, Plant and Equipment [Abstract]

Property and Equipment, Net

5. Property and equipment, net

Property and equipment, net consisted of the following (in thousands):


	December 31,			December 31,
	2025			2024
Office equipment	$	559		$	465
Computer equipment		202			202
Leasehold improvements		87			71
Lab Equipment		55			—
		903			738
Less: Accumulated depreciation and amortization		(344	)		(156	)
Property and equipment, net	$	559		$	582

Depreciation and amortization expense related to property and equipment, net was $0.2 million and less than $0.1 million for of the years ended December 31, 2025 and 2024, respectively.

Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2025	Mar. 20, 2026	Jun. 30, 2025
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2025
Document Fiscal Year Focus	2025
Document Annual Report	true
Document Fiscal Period Focus	FY
Entity Registrant Name	Upstream Bio, Inc.
Trading Symbol	UPB
Entity Central Index Key	0002022626
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction	false
Entity Common Stock, Shares Outstanding		54,419,986
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Public Float			$ 467,859,469
Entity File Number	001-42366
Entity Tax Identification Number	38-4187694
Entity Address, Address Line One	890 Winter Street
Entity Address, Address Line Two	Suite 200
Entity Address, City or Town	Waltham
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02451
City Area Code	781
Local Phone Number	208-2466
Entity Incorporation, State or Country Code	DE
Document Transition Report	false
Title of 12(b) Security	Common Stock, par value $0.001 per share
Security Exchange Name	NASDAQ
Documents Incorporated by Reference	Portions of the Registrant’s definitive proxy statement for its 2026 Annual Meeting of Stockholders to be filed pursuant to Regulation 14A within 120 days of the end of the Registrant’s fiscal year ended December 31, 2025 are incorporated by reference in Part III of this Annual Report on Form 10-K to the extent stated herein.
Auditor Name	PricewaterhouseCoopers LLP
Auditor Firm ID	238
Auditor Location	Boston, Massachusetts
Auditor Opinion [Text Block]	Opinion on the Financial Statements We have audited the accompanying consolidated balance sheets of Upstream Bio, Inc. and its subsidiary (the "Company") as of December 31, 2025 and 2024, and the related consolidated statements of operations and comprehensive loss, of redeemable convertible preferred stock and stockholder's equity (deficit) and of cash flows for the years then ended, including the related notes (collectively referred to as the "consolidated financial statements"). In our opinion, the consolidated financial statements present fairly, in all material respects, the financial position of the Company as of December 31, 2025 and 2024, and the results of its operations and its cash flows for the years then ended in conformity with accounting principles generally accepted in the United States of America.

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid research and development expense	$ 6,204	$ 6,125
Interest receivable	1,653	567
Prepaid insurance	881	852
Prepaid employee-related costs	155	58
Other	727	494
Prepaid expenses and other current assets	$ 9,620	$ 8,096

Redeemable Convertible Preferred Stock

12 Months Ended

Dec. 31, 2025

Equity [Abstract]

Redeemable Convertible Preferred Stock

8. Redeemable convertible preferred stock

The Company has issued Series A Preferred Stock (“Series A Preferred Stock”) and Series B Preferred Stock, which are collectively referred to as the Preferred Stock.

Immediately prior to the closing of the Company’s IPO on October 15, 2024, pursuant to the stock split and a proportional adjustment to the existing conversion ratios of each series of the Company’s Preferred Stock as discussed further below, all of the Company’s outstanding shares of convertible preferred stock were converted into an aggregate of 33,321,149 shares of common stock.

Issuance and sale of Series B redeemable convertible preferred stock

In June 2023, the Company executed the Series B Stock Preferred Purchase Agreement (the “Series B Agreement”) to issue and sell up to 11,764,693 shares of Series B Preferred Stock at a price of $17.00 per share. In the initial closing in June 2023, the Company issued 2,941,170 shares of Series B Preferred Stock resulting in gross cash proceeds of $50.0 million and incurred $0.6 million of issuance costs, of which $0.1 million was allocated to the preferred stock tranche right liability and recognized in the consolidated statement of operations and comprehensive loss as general and administrative expense. Pursuant to the Series B Agreement, the Company has the right (“Series B Option”) to issue and sell an additional 8,823,523 shares of Series B Preferred Stock (“Series B Option Shares”) at the same price of $17.00 per share after the initial closing but prior to March 31, 2024 upon approval of at least six (6) board of directors of which at least one (1) has to be appointed by the holders of Series B Preferred Stock. If the Company does not exercise the Series B Option prior or at a date which would occur at the earlier of (i) March 31, 2024 or (ii) the closing of an acquisition agreement signed prior to March 31, 2024, the holders of Series B Preferred Stock will have the right but not obligation to require the Company to issue and sell the Series B Option Shares at the same purchase price of $17.00 per share (the “Series B preferred stock tranche right”). Upon the initial closing of the Series B Preferred Stock, the Company recorded a preferred stock tranche right liability of $11.8 million and a corresponding reduction to the carrying value of the Series B Preferred Stock. The fair value of the Series B preferred stock tranche right was allocated from the gross cash proceeds of $50.0 million of the Series B Preferred Stock issuance, and the residual value was then allocated to the Series B Preferred Stock.

In April 2024, pursuant to the satisfaction of the Series B Option contemplated in the Series B Agreement, the Company issued and sold 8,823,523 shares of Series B Preferred Stock at a price of $17.00 per share, which resulted in gross cash proceeds of $150.0 million. As a result of this issuance, the Series B preferred stock tranche right liability of less than $0.1 million was settled and the Series B Preferred Stock was recorded at its fair value of $150.0 million. The Company incurred less than $0.1 million of issuance costs in connection with the Series B Option closing.

Upon issuance of the Preferred Stock, the Company assessed the embedded conversion and liquidation features of the securities and determined that such features did not require the Company to separately account for these features.

As of December 31, 2025 and 2024, there were no shares of redeemable convertible preferred stock issued or outstanding.

The Company’s third amended and restated certificate of incorporation authorized the issuance of preferred stock with a par value of $0.001 per share. The number of shares of preferred stock authorized to be issued is 10,000,000 shares as of December 31, 2025. The shares of preferred stock are currently undesignated and no shares are issued or outstanding

Property and Equipment, Net - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 188,000	$ 89,000
Maximum
Property, Plant and Equipment [Line Items]
Depreciation and amortization		$ 100,000

- Definition

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

+ References + Details

Net loss per share

12 Months Ended

Dec. 31, 2025

Earnings Per Share [Abstract]

Net Loss per Share

12. Net loss per share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows (dollar amounts in thousands):


	Year Ended December 31,
	2025			2024
Numerator:
Net loss	$	(143,443	)	$	(62,806	)
Preferred Stock cumulative dividends		—			(13,589	)
Net loss attributable to common stockholders	$	(143,443	)	$	(76,395	)
Denominator:
Weighted-average common shares outstanding, basic and diluted		53,852,752			13,682,326
Net loss per share attributable to common stockholders, basic and diluted	$	(2.66	)	$	(5.58	)

Prior to June 2023, the Company’s Series A Preferred Stockholders were not entitled to cumulative dividends. In connection with the Series B Agreement in June 2023, the Company modified the dividend rights for its Series A Preferred Stockholders such that they became entitled to cumulative dividends based on the original issuance dates of the respective Series A Preferred Stock. As such, for the year ended December 31, 2024, the Company calculated its net loss attributable to common stockholders by adjusting its net loss for the aggregate cumulative dividends that had accrued since the original issuances dates in the period in which the Preferred Stockholders became legally entitled to such dividends.

The Company’s potentially dilutive securities, which include stock options to purchase common stock and Preferred Stock, have

been excluded from the computation of diluted net loss per share as the effect would be anti-dilutive. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same.

The following potentially dilutive securities have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect:


	As of December 31,
	2025		2024
Stock options to purchase common stock		7,960,728		6,490,571

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2025 USD ($) Segment shares	Dec. 31, 2024 USD ($)
Change in Accounting Estimate [Line Items]
Cash and cash equivalents	$ 86,000,000	$ 321,000,000
Letter of credit	200,000	200,000
Interest income	16,900,000	12,100,000
Impairment of long-lived assets	$ 0	0
Number of operating segments \| Segment	1
Segment Reporting, CODM, Individual Title and Position or Group Name [Extensible Enumeration]	Chief Executive Officer
Convertible preferred stock converted to common stock \| shares	33,321,149
Payment term of invoicing	60 days
Change in accounting principle accounting standards update adopted [true false]	true
Change in accounting principle accounting standards update immaterial effect [true false]	true
Accounting Standards Update [Extensible Enumeration]	Accounting Standards Update 2023-09 [Member]
Change in accounting principle accounting standards update adoption date	Dec. 31, 2025
US Treasury Securities
Change in Accounting Estimate [Line Items]
Cash and cash equivalents	$ 15,000,000	$ 4,200,000

Fair Value Measurements - Schedule of Short-Term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 239,403	$ 144,584
Gross Unrealized Gains	531	71
Gross Unrealized Losses	(3)	(96)
Fair Value	239,931	144,559
US Treasury Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	159,033	77,142
Gross Unrealized Gains	422	37
Gross Unrealized Losses		(14)
Fair Value	159,455	77,165
Corporate Debt Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	75,117
Gross Unrealized Gains	102
Gross Unrealized Losses	(3)
Fair Value	75,216
U.S. Government Agency Bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	5,253	67,442
Gross Unrealized Gains	7	34
Gross Unrealized Losses	0	(82)
Fair Value	$ 5,260	$ 67,394

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

+ Details

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Common Stock

12 Months Ended

Dec. 31, 2025

Equity [Abstract]

Common Stock

9. Common stock

Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any, subject to the preferential dividend rights of the Preferred Stock.

As of December 31, 2025 and 2024, the Company’s third amended and restated certificate of incorporation authorized the issuance of 500,000,000 shares of common stock, par value $0.001 per share. As of December 31, 2025 and 2024, there were 54,237,750 shares and 53,603,398 shares of common stock issued and outstanding, respectively.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2025

Accounting Policies [Abstract]

Schedule of Estimated Useful Life of Each Asset

Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset, as follows:


		Estimated Useful Life
Lab equipment		5 years
Computer equipment		3 years
Office equipment		5 years
Leasehold improvements		Shorter of remaining lease term or estimated useful life

Nature of the Business and Basis of Presentation

12 Months Ended

Dec. 31, 2025

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Nature of the Business and Basis of Presentation

1. Nature of the business and basis of presentation

Upstream Bio, Inc. was incorporated in April 2021, under the laws of the State of Delaware, and along with its consolidated subsidiary (collectively, the “Company” or “Upstream”), is focused on developing treatments for inflammatory diseases, with an initial focus on severe respiratory disorders. Since its inception, the Company has devoted substantially all of its efforts to raising capital and incurring research and development expenses related to advancing verekitug, a clinical-stage monoclonal antibody that targets and inhibits the Thymic Stromal Lymphopoietin receptor.

Risks and uncertainties

The Company is subject to risks and uncertainties common to early-stage companies in the biopharmaceutical industry, including, but not limited to, the successful development of verekitug, the development of new technological innovations by competitors, dependence on key personnel, the ability to attract and retain qualified employees, protection of proprietary technology, compliance with governmental regulations and the ability to secure additional capital to fund operations and commercial success of verekitug. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be maintained, that any therapeutic products developed will obtain required regulatory approval or that any approved or consumer products will be commercially viable. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will generate significant product sales.

Stock split

On October 4, 2024, the Company effected a 1.049-for-one stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios of each series of the Company’s preferred stock (Note 8). Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this stock split and adjustment of the preferred stock conversion ratios.

Liquidity

The Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the issuance of these consolidated financial statements.

The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. The Company has historically financed its operations principally through the issuance and sale of redeemable convertible preferred stock and the proceeds from its initial public offering (“IPO”), which was completed in October 2024. In connection with its IPO, the Company issued and sold 17,250,000 shares of common stock, including 2,250,000 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, at a price to the public of $17.00 per share. As a result of the IPO, the Company received $268.8 million in net proceeds, after deducting $20.5 million in underwriting discounts and commissions, and $3.9 million in other offering costs. The Company has incurred recurring losses and negative cash flows from operations since its inception and expects to continue to incur losses and negative cash flows for the foreseeable future as it continues the research and development of verekitug. The Company incurred net losses of $143.4 million and $62.8 million for the years ended December 31, 2025 and 2024, respectively. As of December 31, 2025, the Company had an accumulated deficit of $334.2 million.

The Company expects its cash, cash equivalents and short-term investments will be sufficient to fund its operating expenses and capital expenditure requirements for at least the next twelve months from the date of issuance of these consolidated financial statements. The Company will need additional financing to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant product revenue, if ever, the Company expects to fund its operations through equity offerings or debt financings, credit or loan facilities, potentially other capital resources, or a combination of one or more of these funding sources. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. The Company’s failure to raise capital as and when needed could have a negative impact on its financial condition and its ability to pursue its business strategies. If adequate funds are not available to the Company, the Company may be required to delay, reduce or eliminate clinical programs, obtain funds through arrangements with collaborators on terms unfavorable to the Company or pursue merger or acquisition strategies. There can be no assurances the Company will be able to obtain additional funding. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all.

Basis of presentation

The accompanying consolidated financial statements reflect the operations of the Company. Intercompany balances and transactions have been eliminated in consolidation. The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Income Taxes - Summary of Components of Deferred Tax Asset and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Deferred tax assets:
Net operating loss carryforwards	$ 19,917	$ 10,994
Research and development tax credit carryforward	6,616	3,754
Operating lease liability	347	501
Accrued expenses	1,118	705
Stock-based compensation	3,828	2,261
Capitalized research and development expense	56,357	25,525
Amortization of acquired IPR&D	15,928	17,412
Total deferred tax assets before valuation allowance	104,111	61,152
Valuation allowance	(103,632)	(60,664)	$ (39,695)
Total deferred tax assets - net of valuation allowance	479	488
Deferred tax liabilities:
ROU asset	(334)	(488)
Other	(145)
Total deferred tax liabilities	$ (479)	$ (488)

Leases - Additional Information (Details) $ in Thousands	12 Months Ended
Leases - Additional Information (Details) $ in Thousands	Dec. 31, 2025 USD ($) ft²
Lessee, Lease, Description [Line Items]
Rent payment, year two	$ 759
Rent payment, year three	$ 644
890 Winter Street in Waltham, Massachusetts
Lessee, Lease, Description [Line Items]
Lease description	In July 2024, the Company entered into an operating lease agreement for office space located at 890 Winter Street in Waltham, Massachusetts. The lease commenced in September 2024 and the Company began paying monthly rent starting one month after the lease commenced.
Area of real estate property \| ft²	16,801
Operating lease, term of contract	3 years
Rent payment, year one	$ 700
Rent payment, year two	800
Rent payment, year three	$ 800
460 Totten Pond Road, Waltham, Massachusetts
Lessee, Lease, Description [Line Items]
Operating lease effective date	Oct. 09, 2024
Lease expired date	Jun. 30, 2024

Net Loss per Share - Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Numerator:
Net loss	$ (143,443)	$ (62,806)
Preferred stock cumulative dividends	0	(13,589)
Net loss attributable to common stockholders	$ (143,443)	$ (76,395)
Denominator:
Weighted-average common shares outstanding, basic	53,852,752	13,682,326
Weighted-average common shares outstanding, diluted	53,852,752	13,682,326
Net loss per share attributable to common stockholders, basic	$ (2.66)	$ (5.58)
Net loss per share attributable to common stockholders, diluted	$ (2.66)	$ (5.58)

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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+ Details

- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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-URI https://asc.fasb.org/1943274/2147483443/250-10-50-4

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 105
-SubTopic 10
-Name Accounting Standards Codification
-Section 65
-Paragraph 9
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479343/105-10-65-9

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 65
-Paragraph 17
-Subparagraph (d)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480336/718-10-65-17

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
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-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483443/250-10-50-6

Reference 8: http://www.xbrl.org/2003/role/exampleRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 52
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482635/260-10-55-52

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section 65
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-Publisher FASB
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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-Paragraph 2
-Subparagraph (g)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478666/740-323-65-2

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
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-Name Accounting Standards Codification
-Section 50
-Paragraph 3
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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
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-Name Accounting Standards Codification
-Section 55
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-Publisher FASB
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section 65
-Paragraph 1
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Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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Reference 15: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org/1943274/2147483443/250-10-50-11

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
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Reference 17: http://www.xbrl.org/2003/role/disclosureRef
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Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
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-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482689/260-10-45-2

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 60B
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482689/260-10-45-60B

Reference 20: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(25))
-Publisher FASB
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Reference 21: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Section S99
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Reference 22: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-Subparagraph (SX 210.7-04(23))
-Publisher FASB
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Reference 23: http://www.xbrl.org/2003/role/disclosureRef
-Topic 260
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-URI https://asc.fasb.org/1943274/2147482689/260-10-45-7

Reference 24: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 65
-Paragraph 4
-Subparagraph (f)(4)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481538/470-20-65-4

+ Details

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

+ Details

- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

+ References

+ Details

Leases - Schedule of Supplemental Disclosure Cash Flow Information Related to Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Leases [Abstract]
Cash paid for amounts included in the measurement of lease liabilities	$ 742	$ 207

Leases - Schedule of Weighted Discount Rate and Remaining Lease Term (Details)	Dec. 31, 2025	Dec. 31, 2024
Leases [Abstract]
Weighted-average discount rate - operating leases	11.70%	11.70%
Weighted-average remaining lease term - operating leases	1 year 9 months 18 days	2 years 9 months 18 days

Consolidated balance sheets - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Current assets:
Cash and cash equivalents	$ 101,578	$ 325,892
Short-term investments	239,931	144,559
Accounts receivable	668	613
Prepaid expenses and other current assets	9,620	8,096
Total current assets	351,797	479,160
Property and equipment, net	559	582
Operating lease right-of-use assets	1,222	1,783
Restricted cash	194	194
Total assets	353,772	481,719
Current liabilities:
Accounts payable	2,726	4,041
Accrued expenses and other current liabilities	10,006	5,992
Operating lease liabilities, current portion	720	704
Total current liabilities	13,452	10,737
Operating lease liabilities, net of current portion	549	1,130
Total liabilities	14,001	11,867
Commitments and contingencies (Note 13)
Stockholders' equity (deficit):
Preferred stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2025 and December 31, 2024; no shares issued and outstanding at December 31, 2025 and December 31, 2024
Common stock, $0.001 par value; 500,000,000 shares authorized at December 31, 2025 and December 31, 2024; 54,237,750 and 53,603,398 shares issued and outstanding at December 31, 2025 and December 31, 2024, respectively	54	53
Additional paid-in capital	673,410	660,604
Accumulated other comprehensive income (loss)	530	(25)
Accumulated deficit	(334,223)	(190,780)
Total stockholders' equity	339,771	469,852
Total liabilities and stockholders' equity	$ 353,772	$ 481,719

Common Stock - Additional Information (Details) - $ / shares	Dec. 31, 2025	Dec. 31, 2024
Class of Stock [Line Items]
Common stock, shares authorized	500,000,000	500,000,000
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares issued	54,237,750	53,603,398
Common stock, shares outstanding	54,237,750	53,603,398

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-8A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-2

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 480
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-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479857/480-10-S50-1

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Topic 505
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-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 6: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Topic 272
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483014/272-10-45-3

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 272
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482987/272-10-50-1

Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(d))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-URI https://asc.fasb.org/1943274/2147481112/505-10-50-13

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
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-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-14

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-18

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
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-Publisher FASB
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Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
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-Name Accounting Standards Codification
-Section S99
-Paragraph 1
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Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
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-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(i)(2))
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Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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Reference 20: http://www.xbrl.org/2003/role/disclosureRef
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479886/946-10-S99-3

Reference 21: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Subparagraph (SX 210.6-03(i)(2)(ii))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479886/946-10-S99-3

+ Details

Commitments and Contingencies - Additional Information (Details) - USD ($)	Dec. 31, 2025	Dec. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
Liabilities recorded for loss contingencies	$ 0	$ 0

Consolidated statements of operations and comprehensive loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Income Statement [Abstract]
Collaboration revenue	$ 2,854	$ 2,370
Operating expenses:
Research and development	136,806	62,966
General and administrative	26,409	17,168
Total operating expenses	163,215	80,134
Loss from operations	(160,361)	(77,764)
Other income (expense):
Change in fair value of preferred stock tranche right liability	0	2,859
Interest income	16,933	12,123
Other expense, net	(15)	(24)
Total other income, net	16,918	14,958
Net loss	(143,443)	(62,806)
Redeemable convertible preferred stock cumulative dividends	0	(13,589)
Net loss attributable to common stockholders	$ (143,443)	$ (76,395)
Net loss per share attributable to common stockholders, basic	$ (2.66)	$ (5.58)
Net loss per share attributable to common stockholders, diluted	$ (2.66)	$ (5.58)
Weighted-average common shares outstanding, basic	53,852,752	13,682,326
Weighted-average common shares outstanding, diluted	53,852,752	13,682,326
Comprehensive loss:
Net loss	$ (143,443)	$ (62,806)
Unrealized gain (loss) on investments, net of tax	555	(46)
Total other comprehensive income (loss)	555	(46)
Comprehensive loss	$ (142,888)	$ (62,852)

Prepaid Expenses and Other Current Assets

12 Months Ended

Dec. 31, 2025

Prepaid Expense and Other Assets, Current [Abstract]

Prepaid Expenses and Other Current Assets

4. Prepaid expenses and other current assets

Prepaid expenses and other current assets consisted of the following (in thousands):


	December 31,		December 31,
	2025		2024
Prepaid research and development expense	$	6,204	$	6,125
Interest receivable		1,653		567
Prepaid insurance		881		852
Prepaid employee-related costs		155		58
Other		727		494
	$	9,620	$	8,096

Segment Reporting - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2025 USD ($) Segment
Segment Reporting [Abstract]
Number of operating segments	1
Number of reportable segments	1
Segment reporting, expense information used by codm, description	The Company’s chief executive officer, who is the CODM, manages the Company on a consolidated basis and utilizes consolidated net loss as a basis for resource allocation and decision making.
Segment Reporting, CODM, Individual Title and Position or Group Name [Extensible Enumeration]	Chief Executive Officer
Cash, cash equivalents and short-term investments \| $	$ 341.5

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2025

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Use of estimates

Concentration of credit risk and of significant suppliers

Foreign currency gains and losses

Cash and cash equivalents

Restricted cash

Short-term investments

Fair value measurements

Level 1 Quoted prices in active markets for identical assets or liabilities.

Property and equipment


		Estimated Useful Life
Lab equipment		5 years
Computer equipment		3 years
Office equipment		5 years
Leasehold improvements		Shorter of remaining lease term or estimated useful life

Impairment of long-lived assets

Operating leases

Classification and accretion of redeemable convertible preferred stock

Preferred stock tranche right liabilities

Revenue recognition

The Company enters into license arrangements, pursuant to which it may provide research and development services for third parties.

Segment information

Research and development expenses

Prepaid and accrued research and development expenses

General and administrative expenses

Patent and trademarks

Stock-based compensation expense

Comprehensive loss

Net loss per share

allocation required under the two-class method during periods of loss since the participating securities do not have a contractual obligation to share in the losses of the Company.

Income taxes

Recently adopted accounting pronouncements

Recently issued accounting pronouncements not yet adopted

Income Taxes

12 Months Ended

Dec. 31, 2025

Income Tax Disclosure [Abstract]

Income Taxes

11. Income Taxes

The Company's entire pretax loss for the year ended December 31, 2025 and 2024 was from its U.S. domestic operations. During the years ended December 31, 2025 and 2024, the Company did not record a provision for income taxes because it has incurred net operating losses since inception and maintains a full valuation allowance against its deferred tax assets.

A reconciliation of the U.S. federal statutory income tax rate to the Company's effective income tax rate is as follows (amount in thousands):


	Year Ended December 31, 2025
	Amount			Rate
U.S. federal statutory income tax rate	$	(30,123	)		21.0	%
State and local income taxes, net of federal income tax effect		—			0.0
Effect of changes in tax laws or rates enacted in the current period		—			0.0
Tax Credits
Research and development tax credits		(2,136	)		1.5
Change in valuation allowance		32,342			(22.5	)
Nontaxable or nondeductible items
Other		(83	)		0.0
Effective income tax rate	$	—			(0.0	)%


	Year Ended December 31, 2024
	Amount			Rate
U.S. federal statutory income tax rate	$	(13,189	)		21.0	%
State and local income taxes, net of federal income tax effect		—			0.0
Effect of changes in tax laws or rates enacted in the current period		—			0.0
Tax Credits
Research and development tax credits		(2,897	)		4.6
Change in valuation allowance		16,270			(25.9	)
Nontaxable or nondeductible items
Other		(184	)		0.3
Effective income tax rate	$	—			(0.0	)%

The Company's effective tax rate does not include any impact from state and local income taxes as the Company has been in losses since inception.

The significant components of the Company’s deferred tax assets and liabilities are summarized as follows (in thousands):


	Year Ended December 31,
	2025			2024
Deferred tax assets:
Net operating loss carryforwards	$	19,917		$	10,994
Research and development tax credit carryforward		6,616			3,754
Operating lease liability		347			501
Accrued expenses		1,118			705
Stock-based compensation		3,828			2,261
Capitalized research and development expense		56,357			25,525
Amortization of acquired IPR&D		15,928			17,412
Total deferred tax assets before valuation allowance		104,111			61,152
Valuation allowance		(103,632	)		(60,664	)
Total deferred tax assets - net of valuation allowance	$	479		$	488
Deferred tax liabilities:
ROU asset		(334	)		(488	)
Other		(145	)		—
Total deferred tax liabilities	$	(479	)	$	(488	)
Net deferred tax asset (liability)	$	—		$	—

As of December 31, 2025, the Company had federal and state net operating loss (“NOLs”) carryforwards of $66.9 million and $92.9 million, respectively. As of December 31, 2024, the Company had federal and state NOLs carryforwards of $37.6 million and $49.1 million, respectively. The federal NOLs are not subject to expiration and are limited in utilization to 80% of taxable income and the state NOLs begin to expire in 2041. The Company also has federal and state research and development credits of $5.7 million and $1.2 million, respectively, which will, if not utilized, begin to expire in 2043 and 2037, respectively.

Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Under the applicable accounting standards, management has considered the Company’s history of losses and concluded that it is more likely than not that the Company will not recognize the benefits of its federal and state deferred tax assets. Accordingly, a full valuation allowance of $103.6 million and $60.7 million has been established as of December 31, 2025 and 2024, respectively.

Changes in valuation allowance for deferred tax assets during the years ended December 31, 2025 and 2024 related primarily to the increase in NOL carryforwards and research and development tax credit carryforwards, offset by amortization of acquired IPR&D in 2025 and were as follows (in thousands):


	Year Ended December 31,
	2025			2024
Valuation allowance at beginning of year	$	(60,664	)	$	(39,695	)
Increases recorded to income tax provision		(42,968	)		(20,969	)
Valuation allowance at end of year	$	(103,632	)	$	(60,664	)

Utilization of the NOL and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership change limitations that have occurred previously or that could occur in the future. These ownership changes may limit the amount of NOL and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. The Company has not completed a study to assess whether a change of ownership has occurred, or whether there have been multiple ownership changes since its formation, due to the significant cost and complexity associated with a study. There could also be additional ownership changes in the future which may result in additional limitations on the utilization of NOL carryforwards and credits.

As of December 31, 2025, the Company has not recorded any amounts for uncertain tax positions. The Company’s policy is to recognize interest and penalties accrued on any uncertain tax positions as a component of income tax expense, if any, in its consolidated statements of operations and comprehensive loss. For the years ended December 31, 2025 and 2024, no estimated interest or penalties were recognized on uncertain tax positions.

The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. The Company’s tax years are still open under statute from December 31, 2021, to the present. Earlier years may be examined to the extent that tax credit or net operating loss carryforwards are used in future periods. The resolution of tax matters is not expected to have a material effect on the Company's consolidated financial statements.

The Company does not pay income taxes at the federal or state level as it has been in losses since inception.

- Definition

The entire disclosure for income tax.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 740
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477617/942-740-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 740
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
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+ Details

Stock-based Compensation - Summary of Stock Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Share-Based Payment Arrangement [Abstract]
Number of Options, Outstanding as of December 31, 2024	6,490,571
Number of Options, Granted	2,222,826
Number of Options, Exercised	(608,155)
Number Of Options, Forfeited	(52,151)
Number of Options, Cancelled	(92,363)
Number of Options, Outstanding as of December 31, 2025	7,960,728	6,490,571
Number of Options, Options exercisable December 31, 2025	3,619,126
Number of Options, Vested and expected to vest December 31, 2025	7,960,728
Weighted Average Exercise Price, Outstanding as of December 31, 2024	$ 5.54
Weighted Average Exercise Price, Granted	9.45
Weighted Average Exercise Price, Exercised	3.95
Weighted Average Exercise Price, Forfeited	7.37
Weighted Average Exercise Price, Cancelled	3.57
Weighted Average Exercise Price, Outstanding as of December 31, 2025	6.77	$ 5.54
Weighted Average Exercise Price, Options exercisable December 31, 2025	5.5
Weighted Average Exercise Price, Vested and expected to vest December 31, 2025	$ 6.77
Weighted-Average Remaining Contractual Term, Outstanding	8 years	7 years 10 months 24 days
Weighted-Average Remaining Contractual Term, Options exercisable December 31, 2025	7 years 2 months 12 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest December 31, 2025	8 years
Intrinsic Value, Outstanding	$ 162,277	$ 71,266
Intrinsic Value, Options exercisable December 31, 2025	78,346
Intrinsic Value, Vested and expected to vest December 31, 2025	$ 162,277

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2025

Income Tax Disclosure [Abstract]

Summary of Effective Income Tax Rate Reconciliation

A reconciliation of the U.S. federal statutory income tax rate to the Company's effective income tax rate is as follows (amount in thousands):


	Year Ended December 31, 2025
	Amount			Rate
U.S. federal statutory income tax rate	$	(30,123	)		21.0	%
State and local income taxes, net of federal income tax effect		—			0.0
Effect of changes in tax laws or rates enacted in the current period		—			0.0
Tax Credits
Research and development tax credits		(2,136	)		1.5
Change in valuation allowance		32,342			(22.5	)
Nontaxable or nondeductible items
Other		(83	)		0.0
Effective income tax rate	$	—			(0.0	)%


	Year Ended December 31, 2024
	Amount			Rate
U.S. federal statutory income tax rate	$	(13,189	)		21.0	%
State and local income taxes, net of federal income tax effect		—			0.0
Effect of changes in tax laws or rates enacted in the current period		—			0.0
Tax Credits
Research and development tax credits		(2,897	)		4.6
Change in valuation allowance		16,270			(25.9	)
Nontaxable or nondeductible items
Other		(184	)		0.3
Effective income tax rate	$	—			(0.0	)%

Summary of Components of Deferred Tax Asset and Liabilities

The significant components of the Company’s deferred tax assets and liabilities are summarized as follows (in thousands):


	Year Ended December 31,
	2025			2024
Deferred tax assets:
Net operating loss carryforwards	$	19,917		$	10,994
Research and development tax credit carryforward		6,616			3,754
Operating lease liability		347			501
Accrued expenses		1,118			705
Stock-based compensation		3,828			2,261
Capitalized research and development expense		56,357			25,525
Amortization of acquired IPR&D		15,928			17,412
Total deferred tax assets before valuation allowance		104,111			61,152
Valuation allowance		(103,632	)		(60,664	)
Total deferred tax assets - net of valuation allowance	$	479		$	488
Deferred tax liabilities:
ROU asset		(334	)		(488	)
Other		(145	)		—
Total deferred tax liabilities	$	(479	)	$	(488	)
Net deferred tax asset (liability)	$	—		$	—

Summary of Valuation Allowance


	Year Ended December 31,
	2025			2024
Valuation allowance at beginning of year	$	(60,664	)	$	(39,695	)
Increases recorded to income tax provision		(42,968	)		(20,969	)
Valuation allowance at end of year	$	(103,632	)	$	(60,664	)

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2025

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	December 31,			December 31,
	2025			2024
Office equipment	$	559		$	465
Computer equipment		202			202
Leasehold improvements		87			71
Lab Equipment		55			—
		903			738
Less: Accumulated depreciation and amortization		(344	)		(156	)
Property and equipment, net	$	559		$	582

Segment Reporting - Schedule of Segment Net Loss, Including Disaggregated Components of Operating Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Collaboration revenue	$ 2,854	$ 2,370
Research and development:
Manufacturing costs	23,013	6,580
Personnel expenses	18,343	9,911
Professional fees and other	5,630	4,767
Total research and development expenses	136,806	62,966
General and administrative:
Personnel expenses	15,188	10,842
Professional fees	6,718	4,349
Other	4,503	1,977
Total general and administrative expenses	26,409	17,168
Total operating expenses	163,215	80,134
Loss from operations	(160,361)	(77,764)
Other income (expense):
Change in fair value of preferred stock tranche right liability	0	2,859
Interest income	16,933	12,123
Other expense, net	(15)	(24)
Total other income, net	16,918	14,958
Net loss	(143,443)	(62,806)
Asthma Indication
Research and development:
Verekitug program	47,694	28,069
CRSwNP Indication
Research and development:
Verekitug program	7,564	10,524
COPD Indication
Research and development:
Verekitug program	$ 34,562	$ 3,115

Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Option-pricing Model to Determine the Fair Value of Stock Options Granted (Details)	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Share-Based Payment Arrangement [Abstract]
Expected volatility	92.40%	79.10%
Expected dividends	0.00%	0.00%
Expected term (in years)	6 years	6 years 2 months 12 days
Risk-free rate	4.24%	3.41%

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Dec. 31, 2025
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Consolidated statements of cash flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Cash flows from operating activities:
Net loss	$ (143,443)	$ (62,806)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	188	89
Stock-based compensation expense	10,332	6,004
Change in fair value of preferred stock tranche right liability	0	(2,859)
Net amortization of premiums and accretion of discounts on short-term investments	(1,502)	(1,655)
Non-cash lease expense	561	182
Changes in operating assets and liabilities:
Accounts receivable	(55)	(515)
Prepaid expenses and other assets	(1,524)	(1,008)
Accounts payable	(1,281)	2,017
Accrued expenses and other current liabilities	4,014	1,512
Operating lease liabilities	(565)	(133)
Net cash used in operating activities	(133,275)	(59,172)
Cash flows from investing activities:
Purchases of short-term investments	(385,878)	(290,609)
Maturities of short-term investments	292,563	231,635
Purchases of equipment	(165)	(511)
Net cash used in investing activities	(93,480)	(59,485)
Cash flows from financing activities:
Proceeds from the issuance of Series B redeemable convertible preferred stock, net of issuance costs paid	0	149,924
Proceeds from initial public offering, net of underwriters discounts and commissions	0	272,723
Payments of initial public offering costs	(34)	(3,896)
Proceeds from exercises of stock options	2,279	159
Proceeds from issuance of common stock under employee stock purchase plan	196	0
Net cash provided by financing activities	2,441	418,910
Net increase (decrease) in cash, cash equivalents and restricted cash	(224,314)	300,253
Cash, cash equivalents and restricted cash at beginning of period	326,086	25,833
Cash, cash equivalents and restricted cash at end of period	101,772	326,086
Cash, cash equivalents and restricted cash at end of period:
Cash and cash equivalents	101,578	325,892
Restricted cash	194	194
Total cash, cash equivalents and restricted cash at end of period	101,772	326,086
Supplemental cash flow information:
Right-of-use asset obtained in exchange for operating lease liability	0	1,922
Supplemental disclosure of non-cash investing and financing activities:
Initial public offering costs included in accounts payable	0	34
Series B Preferred Stock
Supplemental disclosure of non-cash investing and financing activities:
Settlement of preferred stock tranche right liability	$ 0	$ 15

Consolidated statements of redeemable convertible preferred stock and stockholders' equity (deficit) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2024 USD ($)
Redeemable convertible preferred stock issuance costs	$ 75
Issuance costs	3,900
Initial public offering, underwriting fee	$ 20,500

Related Parties - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Related Party Transaction [Line Items]
Collaboration revenue	$ 2,854	$ 2,370
Related party accounts receivable	668	613
Stock-based compensation expense	10,332	6,004
Maruho \| Maruho Agreement
Related Party Transaction [Line Items]
Cost reimbursements received	2,800	1,900
Collaboration revenue	2,900	2,400
Related party accounts receivable	$ 700	$ 600

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

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+ Details

Consolidated balance sheets (Parenthetical) - $ / shares	Dec. 31, 2025	Dec. 31, 2024
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	54,237,750	53,603,398
Common stock, shares outstanding	54,237,750	53,603,398

Employee Benefit Plan

12 Months Ended

Dec. 31, 2025

Postemployment Benefits [Abstract]

Employee Benefit Plan

17. Employee benefit plan

The Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Matching contributions to the plan may be made at the discretion of the Company’s board of directors. The Company made $0.4 million and $0.2 million in contributions to the plan during the years ended December 31, 2025 and 2024, respectively.

Revenue

12 Months Ended

Dec. 31, 2025

Revenue from Contract with Customer [Abstract]

Revenue

15. Revenue

Maruho agreement

In October 2021, in connection with an asset purchase agreement entered into with Astellas, the Company entered into an agreement (as amended, the “Maruho Agreement”), under which it granted Maruho an exclusive, irrevocable, perpetual, royalty-free, sublicensable (subject to its right of first negotiation) license. Pursuant to the Maruho Agreement, the Company maintains its responsibility for and controls the global research and development of the Maruho license product, including in Japan. The Company will conduct specified clinical trial activities for Japan as part of its global research and development plan. Maruho will reimburse the Company for the costs of these research and development activities, including the cost of drug supply. Maruho has the right to terminate the Maruho Agreement at any time by providing 60 days prior written notice to the Company with no substantial penalty.

The Company concluded that Maruho is a customer under the Maruho Agreement, and as such, the Maruho Agreement falls within the scope of ASC 606. The Company identified one performance obligation under the Maruho Agreement related to the performance of research and development services, which are an output of the Company’s ordinary activities, in Japan. The Company determined that the transaction price of the Maruho Agreement as of December 31, 2025 consisted solely of variable consideration. The variable consideration was estimated using the expected value method based on the Company’s experience and best judgment of the total reimbursable costs expected to be incurred through the period of performance.

The transaction price is being recognized as revenue over time using the cost-to-cost input method, which the Company believes best depicts the transfer of control to the customer. Under the cost-to-cost input method, the extent of progress towards completion is measured based on the ratio of actual costs incurred in Japan to the total estimated costs expected to satisfy the performance obligation. The calculation of the total estimated costs to fulfill the performance obligation includes costs associated with employees, clinical and development, manufacturing, and out-of-pocket costs expected to be paid to third parties. The estimate of the Company’s measure of progress and estimate of variable consideration to be included in the transaction price will be updated at each reporting period as a change in estimate. The Company excludes disclosures related to the aggregate amount of the transaction price allocated to the performance obligation that are unsatisfied as of the end of the reporting period because the contract has an initial expected term of one year or less. The Company currently expects to continue providing research and development services to Maruho under the Maruho Agreement through the completion of its Phase 2 clinical trials, and if successful, through any Phase 3 clinical trials.

- Definition

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
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+ Details

Leases

12 Months Ended

Dec. 31, 2025

Leases [Abstract]

Leases

7. Leases

As of December 31, 2025, the Company was a party to a lease related to commercial real estate under a non-cancelable lease term and a short-term lease related to commercial real estate.

In July 2024, the Company entered into an operating lease agreement for office space located at 890 Winter Street in Waltham, Massachusetts. The lease commenced in September 2024 and the Company began paying monthly rent starting one month after the lease commenced. The Company occupies approximately 16,801 square feet of space under a three-year agreement expiring in October 2027. Initial base rent was approximately $0.7 million for the first year and approximately $0.8 million for the second and third year.

During the year ended December 31, 2024, the Company had an operating lease for office space at 460 Totten Pond Road, Waltham, Massachusetts. In July 2024, the Company provided notice of termination. This notice became effective on October 9, 2024, after which the Company’s rights and obligations under this lease ceased. The lease expired on June 30, 2024, after which the Company continued to pay rent on a month-to-month basis until October 9, 2024. Under its lease, the Company pays a proportional share of operating expenses. Such operating expenses are subject to annual adjustment and are accounted for as variable payments in the period in which they are incurred.

The components of lease cost, which are included in the consolidated statements of operations and comprehensive loss, were as follows (in thousands):


	December 31,
	2025		2024
Lease Cost:
Operating lease cost	$	739	$	256
Short-term lease cost		—		202
Variable lease cost		—		19
Total lease cost	$	739	$	477

Supplemental disclosure of cash flow information related to leases were as follows (in thousands):


	December 31,
	2025		2024
Cash paid for amounts included in the measurement of lease liabilities	$	742	$	207

The weighted-average discount rate and remaining lease term were as follows:


	December 31,
	2025			2024
Weighted-average discount rate — operating leases		11.7	%		11.7	%
Weighted-average remaining lease term — operating leases		1.8			2.8

The maturities of operating lease liabilities were as follows (in thousands):


Year Ended December 31,	Amount
2026		759
2027		644
Total lease payments		1,403
Less: imputed interest		(134	)
Present value of lease liabilities		1,269
Less: operating lease liabilities, current portion		(720	)
Operating lease liabilities, net of current portion	$	549

Related Parties

12 Months Ended

Dec. 31, 2025

Related Parties

16. Related parties

In October 2021, the Company entered into the Maruho Agreement (Note 15). Through the date of the IPO, Maruho was considered to be a related party because it was one of the co-founders of the Company and had representation on the Company’s board of directors. Since the closing of the IPO in October 2024, Maruho is no longer considered a related party as they no longer have representation on the Company’s board of directors. During the years ended December 31, 2025 and 2024, the Company

received payments of $2.8 million and $1.9 million, respectively, in cost reimbursements from Maruho. The Company recorded collaboration revenue of $2.9 million and $2.4 million during the years ended December 31, 2025 and 2024, respectively. As of December 31, 2025 and 2024, there was $0.7 million and $0.6 million in accounts receivable, respectively, representing amounts due for qualifying reimbursable expenses related to the Maruho Agreement.

- Definition

The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.

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+ Details

Consolidated statements of redeemable convertible preferred stock and stockholders' equity (deficit) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Temporary equity, beginning balance at Dec. 31, 2023		$ 230,935
Temporary equity, beginning balance, shares at Dec. 31, 2023		22,941,170
Beginning balance at Dec. 31, 2023	$ (123,126)		$ 3	$ 4,824	$ (127,974)	$ 21
Beginning balance, shares at Dec. 31, 2023			2,992,479
Issuance of Series B redeemable convertible preferred stock in connection with the settlement of the tranche right liability, net of issuance costs		$ 149,939
Issuance of Series B redeemable convertible preferred stock in connection with the settlement of the tranche right liability, net of issuance costs, shares		8,823,523
Conversion of convertible preferred stock to common stock upon closing of initial public offering		$ (380,874)
Conversion of convertible preferred stock to common stock upon closing of initial public offering, shares		(31,764,693)
Conversion of convertible preferred stock to common stock upon closing of initial public offering	380,874		$ 33	380,841
Conversion of convertible preferred stock to common stock upon closing of initial public offering, shares			33,321,149
Issuance of common stock from initial public offering, net of issuance costs	268,793		$ 17	268,776
Issuance of common stock from initial public offering, net of issuance costs, shares			17,250,000
Exercise of stock options, net of tax withholding	159			159
Exercise of stock options, net of tax withholding, shares			39,770
Stock-based compensation expense	6,004			6,004
Unrealized gain (loss) on available-for-sale securities, net of tax	(46)					(46)
Net loss	(62,806)				(62,806)
Ending balance at Dec. 31, 2024	469,852		$ 53	660,604	(190,780)	(25)
Ending balance, shares at Dec. 31, 2024			53,603,398
Exercise of stock options, net of tax withholding	2,279		$ 1	2,278
Exercise of stock options, net of tax withholding, shares			608,155
Stock-based compensation expense	10,332			10,332
Issuance of common stock under employee stock purchase plan	196			196
Issuance of common stock under employee stock purchase plan, shares			26,197
Unrealized gain (loss) on available-for-sale securities, net of tax	555					555
Net loss	(143,443)				(143,443)
Ending balance at Dec. 31, 2025	$ 339,771		$ 54	$ 673,410	$ (334,223)	$ 530
Ending balance, shares at Dec. 31, 2025			54,237,750

Prepaid Expenses and Other Current Assets (Tables)

12 Months Ended

Dec. 31, 2025

Prepaid Expense and Other Assets, Current [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following (in thousands):


	December 31,		December 31,
	2025		2024
Prepaid research and development expense	$	6,204	$	6,125
Interest receivable		1,653		567
Prepaid insurance		881		852
Prepaid employee-related costs		155		58
Other		727		494
	$	9,620	$	8,096

Revenue - Additional Information (Details)	1 Months Ended
Revenue - Additional Information (Details)	Oct. 31, 2021
Maruho Agreement
Disaggregation of Revenue [Line Items]
License agreement, termination written notice period	60 days

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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+ Details

Leases - Schedule of Maturities of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Lessee, Operating Lease, Liability, to be Paid, Fiscal Year Maturity [Abstract]
2026	$ 759
2027	644
Total lease payments	1,403
Less: imputed interest	(134)
Present value of lease liabilities	1,269
Less: operating lease liabilities, current portion	(720)	$ (704)
Operating lease liabilities, net of current portion	$ 549	$ 1,130

Net Loss per Share - Schedule of Dilutive Securities Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Stock Options to Purchase Common Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive securities excluded from calculation of diluted net loss per share	7,960,728	6,490,571

Stock-based Compensation

12 Months Ended

Dec. 31, 2025

Share-Based Payment Arrangement [Abstract]

Stock-based Compensation

10. Stock-based compensation

2021 Stock incentive plan

The Company’s 2021 Stock Option and Grant Plan (the “2021 Plan”) provided for the Company to grant incentive stock options, non-qualified stock options, restricted stock awards, unrestricted stock awards and restricted stock units (collectively, the “Awards”) to among others, members of the board of directors, employees, consultants and other key persons to the Company and its affiliates. The 2021 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board.

In October 2024, the Company completed its IPO, and in connection with the closing, the board of directors determined that no further awards would be granted under the 2021 Plan and any remaining options available for grant would cease to be available. Awards outstanding under the 2021 Plan will continue to be governed by their existing terms. Shares of unused common stock underlying any awards that are forfeited, canceled or reacquired by the Company prior to vesting will again be available for the grant of awards under the 2024 Plan.

2024 Stock option and incentive plan

On August 19, 2024, the Company’s board of directors adopted, and on October 4, 2024 its stockholders approved, the 2024 Stock Option and Incentive Plan (the “2024 Plan”), which became effective upon the date immediately preceding the date on which the IPO registration statement was declared effective by the SEC. The 2024 Plan allows the Company to make equity-based and cash-based incentive awards to its officers, employees, directors, and consultants. The 2024 Plan provides for the grant of incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock units, restricted shares of common stock and other stock-based awards. The number of shares reserved under the 2024 Plan is subject to adjustment in the event of a stock split, stock dividend or other change in the Company’s capitalization. In addition, the number of shares reserved and available for issuance under the 2024 Plan will automatically increase on January 1, 2025 and each January 1 thereafter, by five percent of the outstanding number of shares of its common stock on the immediately preceding December 31 or such lesser number of shares as determined by the Company’s compensation committee. On January 1, 2026, the number of shares of common stock that may be issued under the 2024 Plan increased by 2,711,887 shares of common stock.

The shares of common stock underlying any awards under the 2024 Plan and the 2021 Plan that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expire or are otherwise terminated (other than by exercise) will be added back to the shares of common stock available for issuance under the 2024 Plan.

As of December 31, 2025, the Company had a total of 11,483,191 shares of common stock reserved under the 2024 Plan and the 2021 Plan, and 3,522,463 shares available for future issuance under the 2024 Plan.

2024 Employee stock purchase plan

On August 19, 2024, the Company’s board of directors adopted, and on October 4, 2024 its stockholders approved, the 2024 Employee Stock Purchase Plan (the “2024 ESPP”), which became effective on the date immediately preceding the date on which the IPO registration statement was declared effective by the SEC. A total of 488,467 shares of common stock were initially reserved for issuance under this plan. The 2024 ESPP provides that the number of shares reserved and available for issuance will automatically increase on the first day of each fiscal year, beginning with the fiscal year commencing on January 1, 2025 and continuing each January 1 thereafter through January 1, 2034, by the least of (i) 976,934 shares of common stock, (ii) one percent of the outstanding number of shares of common stock on the immediately preceding December 31, or (iii) such lesser number of shares of common stock as determined by the administrator of the 2024 ESPP. The number of shares reserved under the 2024 ESPP is subject to adjustment in the event of a stock split, stock dividend or other change in the Company’s capitalization. On January 1, 2026, there was no increase to the number of shares of common stock that may be issued under the 2024 ESPP Plan.

As of December 31, 2025, 26,197 shares have been issued under the 2024 ESPP.

Fair value inputs

The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected option term is calculated based on the simplified method for awards with service-based conditions, which uses the midpoint between the vesting date and the contractual term, as the Company does not have sufficient historical data to develop an estimate based on participant behavior. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

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:


	Year Ended December 31,
	2025			2024
Expected volatility		92.4	%		79.1	%
Expected dividends		0	%		0	%
Expected term (in years)		6.0			6.2
Risk-free rate		4.24	%		3.41	%

Stock options

The Company generally grants stock-based awards with service-based vesting. During the year ended December 31, 2024, the Company granted performance-based stock options to certain employees and directors for the purchase of an aggregate 1,206,249 shares of common stock with a vesting commencement date contingent upon the achievement of the Series B Option closing, which was achieved in April 2024. The Company determined that it met all the conditions to establish a grant date for these performance-based stock options at the original issuance date and that the performance condition was deemed probable of achievement, as the board of directors had approved the Series B Option closing prior to the grant date. The vesting of the performance-based stock options is also subject to the grantees’ continued service until the fourth anniversary of the Series B Option closing.

The following table summarizes the activity of stock options with service-based and performance-based vesting conditions during the year ended December 31, 2025:


	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Intrinsic Value (in thousands)
Outstanding as of December 31, 2024		6,490,571		$	5.54		7.9	$	71,266
Granted		2,222,826			9.45
Exercised		(608,155	)		3.95
Forfeited		(52,151	)		7.37
Cancelled		(92,363	)		3.57
Outstanding as of December 31, 2025		7,960,728		$	6.77		8.0	$	162,277
Options exercisable December 31, 2025		3,619,126		$	5.50		7.2	$	78,346
Vested and expected to vest December 31, 2025		7,960,728		$	6.77		8.0	$	162,277

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock for those stock options that had exercise prices lower than the estimated fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2025 and 2024 was $7.8 million and $0.3 million, respectively.

The weighted-average grant-date fair value of options granted during the years ended December 31, 2025 and 2024 was $7.32 and $5.10, respectively.

As of December 31, 2025, there was $21.4 million of total unrecognized compensation cost related to unvested stock options, which is expected to be recognized over a weighted-average period of 2.4 years.

Modification of certain stock-based compensation awards

In February 2024, the Company entered into a separation agreement with the Company’s former Chief Operating Officer (“COO”), effective March 2024. Under the terms of the separation agreement, stock options for the purchase of 142,935 shares of common stock, representing all of the vested options held by the former COO as of the date of her termination, became exercisable for one year following her termination.

In March 2024, the Company entered into a separation agreement with the Company’s former Chief Executive Officer (“CEO”), effective March 2024. Under the terms of the separation agreement, vesting of options for the purchase of 38,245 shares of common stock held by the former CEO were accelerated with no change to the exercise price of such options. In addition, stock options for the purchase of 532,553 shares of common stock, representing all of the vested options held by the former CEO as of the date of her termination, became exercisable for two years following her termination.

As a result of these modifications, the Company recognized $0.7 million of incremental stock-based compensation during the year ended December 31, 2024.

The following table illustrates the classification of stock-based compensation in the consolidated statements of operations and comprehensive loss (in thousands):


	Year Ended December 31,
	2025		2024
General and administrative	$	7,478	$	4,842
Research and development		2,854		1,162
	$	10,332	$	6,004