Summary of Significant Accounting Policies	6 Months Ended
Jun. 30, 2026
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying condensed financial statements have been prepared in accordance with United States (“U.S.”) generally accepted accounting principles ("U.S. GAAP") and the rules and regulations of the SEC. Certain information and note disclosures normally included in annual financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to those rules and regulations. In February 2026, the Company became a standalone publicly traded company, and its financial statements are now presented on a standalone basis. Prior to the Separation in February 2026, the Company’s historical financial statements were derived from the Former Parent’s financial statements and accounting records and were presented on a combined basis. The financial statements for all periods presented, including the historical results of the Company prior to February 27, 2026, are now referred to as “Condensed Financial Statements.” For periods prior to the Separation, the accompanying condensed financial statements present, on a historical basis, the assets, liabilities, expenses and cash flows directly attributable to Atrium, which have been prepared from the Former Parent’s consolidated financial statements and accounting records, and are presented on a stand-alone basis as if the operations had been conducted independently from the Former Parent. Certain information and footnote disclosures typically included in annual financial statements prepared in accordance with U.S. GAAP have been condensed or omitted. Accordingly, these unaudited condensed financial statements should be read in conjunction with the Company’s audited annual combined financial statements and the notes thereto for the year ended December 31, 2025, which are included in Exhibit 99.1 to the Company's Registration Statement on Form 10-12B/A, as amended (File No. 001-43008), which was filed with the SEC and became effective on February 26, 2026 (the “Form 10”). In the Quarterly Report on Form 10-Q for the period ended March 31, 2026, the Company's condensed Statement of Changes in Stockholders' Equity/Former Parent's Deficit for the period ended March 31, 2026 incorrectly included 100,000 unvested awards under the 2026 Incentive Award Plan as issued and outstanding common shares. Having concluded the incorrect inclusion was not material to the current or previously issued financial statements, either quantitatively or qualitatively, the Company has revised the statement to exclude these unvested awards as they did not represent issued and outstanding common shares as of that date, with no effect on total stockholders' equity, net loss, or net loss per share. The error had no impact on any of the other Condensed Financial Statements. Cash, Cash Equivalents and Restricted Cash The Company considers cash equivalents to be only those investments which are highly liquid, readily convertible to cash and have an original maturity of three months or less at the date of purchase. The Company issued letters of credit primarily under certain lease agreements that have been collateralized by cash deposits for an equal amount and are recorded within restricted cash on the condensed balance sheets based on the term of the underlying lease. The Company invests excess cash in money market funds. Money market funds are highly liquid investments with original maturities of three months or less and are classified as cash equivalents. These investments are carried at fair value. Short-term Investments The Company’s short-term investments consist of excess cash that has been invested in marketable securities with high credit ratings. These securities consist of U.S. Treasury securities and are, classified as available-for-sale. The Company has classified all of its available-for-sale marketable securities, including those with maturity dates beyond one year, as current assets on the condensed balance sheet as it may sell these securities at any time for use in current operations even if they have not yet reached maturity. The Company carries these securities at fair value and reports unrealized gains and losses as a separate component of accumulated other comprehensive loss. Amortization and accretion of any purchase premiums or discounts of debt securities are included in interest income in the condensed statements of operations and comprehensive loss. Gains and losses on sales are recorded based on the trade date and determined using the specific identification method. The Company periodically assesses its available-for-sale marketable securities for impairment. For debt securities in an unrealized loss position, this assessment first takes into account the Company's intent to sale, or whether it is more likely than not that the Company will be required to sell the security before recovery of its amortized cost basis. If either of these criteria are met, the debt security’s amortized cost basis is written down to fair value through interest and other, net. For debt securities in an unrealized loss position that do not meet the aforementioned criteria, the Company assesses whether the decline in fair value has resulted from credit losses or other factors. In making this assessment, the Company considers the extent to which fair value is less than amortized cost, any changes to the rating of the security by a rating agency, and any adverse conditions specifically related to the security, among other factors. If this assessment indicates that a credit loss may exist, the present value of cash flows expected to be collected from the security are compared to the amortized cost basis of the security. If the present value of cash flows expected to be collected is less than the amortized cost basis, a credit loss exists and an allowance for credit losses will be recorded in other income (expense), net, limited by the amount that the fair value is less than the amortized cost basis. Any additional impairment not recorded through an allowance for credit losses is recognized in other comprehensive loss. Changes in the allowance for credit losses are recorded as provision for (or reversal of) credit loss expense. Losses are charged against the allowance when management believes the uncollectability of an available-for-sale security is confirmed or when either of the criteria regarding intent or requirement to sell is met. These changes are recorded in other income (expense), net. Concentration of Risk Financial instruments that potentially subject the Company to concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash, and short-term investments. The Company maintains accounts in federally insured financial institutions in excess of federally insured limits. The Company also holds money market funds that are not federally insured. However, management believes the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which these deposits are held and of the money market funds and other entities in which these investments are made. The Company has contractual payment terms with each of its collaborators, and the Company monitors their financial performance and credit worthiness so that it can properly assess and respond to any changes in their credit profile. To date, there have been no write-offs or allowances of collaboration receivables. Fair Value of Measurements 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 at the measurement date. Assets and liabilities that are measured at fair value are reported using a three-level fair value hierarchy that prioritizes the inputs used to measure fair value. This hierarchy maximizes the use of observable inputs and minimizes the use of unobservable inputs and is as follows: Level 1—Quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. Level 2—Inputs other than quoted prices in active markets that are observable for the asset or liability, either directly or indirectly. Level 3—Inputs that are significant to the fair value measurement and are unobservable (i.e. supported by little market activity), which requires the reporting entity to develop its own valuation techniques and assumptions. The carrying values of collaboration receivable, prepaid assets, accounts payable, and accrued liabilities approximate their fair values due to the short-term nature of these assets and liabilities. Stock-Based Compensation Stock-based compensation expense represents the cost of the grant date fair value of employee stock option grants recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis. Stock-based compensation expense for stock option grants is determined using the Black-Scholes option pricing model and is recorded at the estimated fair value of the award as of the grant date and recognized as expense on a straight-line basis over the requisite service period (usually the vesting period) of the stock-based award. Stock-based compensation expense for restricted stock units is recorded at the market price of a share of Company's stock on the date of grant and is recognized as expense on a straight-line basis over the service period. The estimation of fair value for stock-based compensation requires management to make estimates and judgments about, among other things, the estimated life of options and volatility of the Company’s common stock. These judgments directly affect the amount of compensation expense that will be recognized. Forfeitures are accounted for as incurred. The Black-Scholes option-pricing model requires the use of subjective assumptions, including the risk-free interest rate, the expected stock price volatility, the expected term of stock options, and the expected dividend yield. The Company has limited historical stock option activity and therefore estimates the expected term of stock options granted using the simplified method, which represents the average of the contractual term of the stock option and its weighted-average vesting period. As there is limited trading history for the Company’s common stock, the Company has determined expected volatility based on the average historical stock price volatility of comparable publicly-traded 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 comparable companies are chosen based on their similar size, stage in the life cycle or area of therapeutic focus. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The risk-free interest rate assumption was based on 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. The expected dividend yield assumption is zero as the Company has never paid and has no plans to pay dividends on its common stock in the foreseeable future. The fair value of the underlying common stock used within the Black-Scholes option-pricing model is based on the closing price of common stock on the date of grant. Stock-Based Compensation for Periods Prior to the Spin-Off Prior to the Spin-Off, certain of the Company’s employees have historically participated in the Former Parent’s non-cash stock-based compensation plans. Non-cash stock-based compensation expense for periods prior to the Spin-Off has been allocated to the Company based on a proportionate cost allocation method primarily based on a percentage of the operating expense. Net Loss Per Share Basic net loss per share of common stock is calculated by dividing net loss by the weighted-average shares outstanding during the period. For purposes of the diluted net loss per share calculation, potential shares of common stock, consisting of outstanding stock options and restricted stock units, have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented. Net loss per share for periods prior to the Spin-Off was calculated based on the approximately 15.5 million shares of common stock distributed to the Former Parent's stockholders at the time of the Spin-Off, in addition to approximately 1.6 million of Make Whole Awards granted at the time of the Spin-Off that will be settled in common stock based only on the passage of time. The common stock and Make Whole Awards are assumed to be outstanding throughout all periods presented up to and including February 27, 2026. The same number of shares is being utilized for the calculation of basic and diluted loss per share for all periods presented prior to the Spin-Off. Recently Issued Accounting Pronouncements Not Yet Adopted Disaggregation of Income Statement Expenses In November 2024, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2024-03, Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40), to improve the disclosures of expenses by requiring public business entities to provide further disaggregation of relevant expense captions (i.e., employee compensation, depreciation, intangible asset amortization) in a separate note to the financial statements, a qualitative description of the amounts remaining in relevant expense captions that are not separately disaggregated quantitatively, and the total amount of selling expenses and, in an annual reporting period, an entity’s definition of selling expenses. The transition method is prospective with the retrospective method permitted, and ASU 2024-03 will be effective for the Company for its annual period ending December 31, 2027 and interim periods for the interim period beginning January 1, 2028. The Company is currently evaluating the impact on its disclosures. Intangibles-Internal-Use Software In September 2025, the FASB issued ASU 2025-06, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40), which eliminates the project stage model and introduces a probable-to-complete recognition threshold for the capitalization of software development costs. The amendment in the update requires an assessment of uncertainty associated with software development activities, additional disclosures for capitalized internal-use software costs and consideration of website development costs. ASU 2025-06 is effective for annual periods beginning after December 15, 2027. ASU 2025-06 can be applied prospectively, through a modified transition approach, or retrospectively, and early adoption is permitted. The Company is currently evaluating the impact that this guidance will have on the presentation of its condensed financial statements and accompanying notes.

Basis of Presentation

The accompanying condensed financial statements have been prepared in accordance with United States (“U.S.”) generally accepted accounting principles ("U.S. GAAP") and the rules and regulations of the SEC. Certain information and note disclosures normally included in annual financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to those rules and regulations.

In February 2026, the Company became a standalone publicly traded company, and its financial statements are now presented on a standalone basis. Prior to the Separation in February 2026, the Company’s historical financial statements were derived from the Former Parent’s financial statements and accounting records and were presented on a combined basis. The financial statements for all periods presented, including the historical results of the Company prior to February 27, 2026, are now referred to as “Condensed Financial Statements.”

For periods prior to the Separation, the accompanying condensed financial statements present, on a historical basis, the assets, liabilities, expenses and cash flows directly attributable to Atrium, which have been prepared from the Former Parent’s consolidated financial statements and accounting records, and are presented on a stand-alone basis as if the operations had been conducted independently from the Former Parent.

Certain information and footnote disclosures typically included in annual financial statements prepared in accordance with U.S. GAAP have been condensed or omitted. Accordingly, these unaudited condensed financial statements should be read in conjunction with the Company’s audited annual combined financial statements and the notes thereto for the year ended December 31, 2025, which are included in Exhibit 99.1 to the Company's Registration Statement on Form 10-12B/A, as amended (File No. 001-43008), which was filed with the SEC and became effective on February 26, 2026 (the “Form 10”).

In the Quarterly Report on Form 10-Q for the period ended March 31, 2026, the Company's condensed Statement of Changes in Stockholders' Equity/Former Parent's Deficit for the period ended March 31, 2026 incorrectly included 100,000 unvested awards under the 2026 Incentive Award Plan as issued and outstanding common shares. Having concluded the incorrect inclusion was not material to the current or previously issued financial statements, either quantitatively or qualitatively, the Company has revised the statement to exclude these unvested awards as they did not represent issued and outstanding common shares as of that date, with no effect on total stockholders' equity, net loss, or net loss per share. The error had no impact on any of the other Condensed Financial Statements.

Cash, Cash Equivalents and Restricted Cash

The Company considers cash equivalents to be only those investments which are highly liquid, readily convertible to cash and have an original maturity of three months or less at the date of purchase.

The Company issued letters of credit primarily under certain lease agreements that have been collateralized by cash deposits for an equal amount and are recorded within restricted cash on the condensed balance sheets based on the term of the underlying lease.

The Company invests excess cash in money market funds. Money market funds are highly liquid investments with original maturities of three months or less and are classified as cash equivalents. These investments are carried at fair value.

Short-term Investments

The Company’s short-term investments consist of excess cash that has been invested in marketable securities with high credit ratings. These securities consist of U.S. Treasury securities and are, classified as available-for-sale. The Company has classified all of its available-for-sale marketable securities, including those with maturity dates beyond one year, as current assets on the condensed balance sheet as it may sell these securities at any time for use in current operations even if they have not yet reached maturity. The Company carries these securities at fair value and reports unrealized gains and losses as a separate component of accumulated other comprehensive loss. Amortization and accretion of any purchase premiums or discounts of debt securities are included in interest income in the condensed statements of operations and comprehensive loss. Gains and losses on sales are recorded based on the trade date and determined using the specific identification method.

The Company periodically assesses its available-for-sale marketable securities for impairment. For debt securities in an unrealized loss position, this assessment first takes into account the Company's intent to sale, or whether it is more likely than not that the Company will be required to sell the security before recovery of its amortized cost basis. If either of these criteria are met, the debt security’s amortized cost basis is written down to fair value through interest and other, net. For debt securities in an unrealized loss position that do not meet the aforementioned criteria, the Company assesses whether the decline in fair value has resulted from credit losses or other factors. In making this assessment, the Company considers the extent to which fair value is less than amortized cost, any changes to the rating of the security by a rating agency, and any adverse conditions specifically related to the security, among other factors. If this assessment indicates that a credit loss may exist, the present value of cash flows expected to be collected from the security are compared to the amortized cost basis of the security. If the present value of cash flows expected to be collected is less than the amortized cost basis, a credit loss exists and an allowance for credit losses will be recorded in other income (expense), net, limited by the amount that the fair value is less than the amortized cost basis. Any additional impairment not recorded through an allowance for credit losses is recognized in other comprehensive loss. Changes in the allowance for credit losses are recorded as provision for (or reversal of) credit loss expense. Losses are charged against the allowance when management believes the uncollectability of an available-for-sale security is confirmed or when either of the criteria regarding intent or requirement to sell is met. These changes are recorded in other income (expense), net.

Concentration of Risk

Financial instruments that potentially subject the Company to concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash, and short-term investments. The Company maintains accounts in federally insured financial institutions in excess of federally insured limits. The Company also holds money market funds that are not federally insured. However, management believes the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which these deposits are held and of the money market funds and other entities in which these investments are made.

The Company has contractual payment terms with each of its collaborators, and the Company monitors their financial performance and credit worthiness so that it can properly assess and respond to any changes in their credit profile. To date, there have been no write-offs or allowances of collaboration receivables.

value are reported using a three-level fair value hierarchy that prioritizes the inputs used to measure fair value. This hierarchy maximizes the use of observable inputs and minimizes the use of unobservable inputs and is as follows:

Level 1—Quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2—Inputs other than quoted prices in active markets that are observable for the asset or liability, either directly or indirectly.

Level 3—Inputs that are significant to the fair value measurement and are unobservable (i.e. supported by little market activity), which requires the reporting entity to develop its own valuation techniques and assumptions.

The carrying values of collaboration receivable, prepaid assets, accounts payable, and accrued liabilities approximate their fair values due to the short-term nature of these assets and liabilities.

Stock-Based Compensation

Stock-based compensation expense represents the cost of the grant date fair value of employee stock option grants recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis.

Stock-based compensation expense for stock option grants is determined using the Black-Scholes option pricing model and is recorded at the estimated fair value of the award as of the grant date and recognized as expense on a straight-line basis over the requisite service period (usually the vesting period) of the stock-based award. Stock-based compensation expense for restricted stock units is recorded at the market price of a share of Company's stock on the date of grant and is recognized as expense on a straight-line basis over the service period. The estimation of fair value for stock-based compensation requires management to make estimates and judgments about, among other things, the estimated life of options and volatility of the Company’s common stock. These judgments directly affect the amount of compensation expense that will be recognized. Forfeitures are accounted for as incurred.

The Black-Scholes option-pricing model requires the use of subjective assumptions, including the risk-free interest rate, the expected stock price volatility, the expected term of stock options, and the expected dividend yield. The Company has limited historical stock option activity and therefore estimates the expected term of stock options granted using the simplified method, which represents the average of the contractual term of the stock option and its weighted-average vesting period. As there is limited trading history for the Company’s common stock, the Company has determined expected volatility based on the average historical stock price volatility of comparable publicly-traded 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 comparable companies are chosen based on their similar size, stage in the life cycle or area of therapeutic focus. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The risk-free interest rate assumption was based on 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. The expected dividend yield assumption is zero as the Company has never paid and has no plans to pay dividends on its common stock in the foreseeable future. The fair value of the underlying common stock used within the Black-Scholes option-pricing model is based on the closing price of common stock on the date of grant.

Stock-Based Compensation for Periods Prior to the Spin-Off

Prior to the Spin-Off, certain of the Company’s employees have historically participated in the Former Parent’s non-cash stock-based compensation plans. Non-cash stock-based compensation expense for periods prior to the Spin-Off has been allocated to the Company based on a proportionate cost allocation method primarily based on a percentage of the operating expense.

Net Loss Per Share

Basic net loss per share of common stock is calculated by dividing net loss by the weighted-average shares outstanding during the period. For purposes of the diluted net loss per share calculation, potential shares of common stock, consisting of outstanding stock options and restricted stock units, have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented.

Net loss per share for periods prior to the Spin-Off was calculated based on the approximately 15.5 million shares of common stock distributed to the Former Parent's stockholders at the time of the Spin-Off, in addition to approximately 1.6 million of Make Whole Awards granted at the time of the Spin-Off that will be settled in common stock based only on the passage of time. The common stock and Make Whole Awards are assumed to be outstanding throughout all periods presented up to and including February 27, 2026. The same number of shares is being utilized for the calculation of basic and diluted loss per share for all periods presented prior to the Spin-Off.

Recently Issued Accounting Pronouncements Not Yet Adopted

Disaggregation of Income Statement Expenses

In November 2024, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2024-03, Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40), to improve the disclosures of expenses by requiring public business entities to provide further disaggregation of relevant expense captions (i.e., employee compensation, depreciation, intangible asset amortization) in a separate note to the financial statements, a qualitative description of the amounts remaining in relevant expense captions that are not separately disaggregated quantitatively, and the total amount of selling expenses and, in an annual reporting period, an entity’s definition of selling expenses. The transition method is prospective with the retrospective method permitted, and ASU 2024-03 will be effective for the Company for its annual period ending December 31, 2027 and interim periods for the interim period beginning January 1, 2028. The Company is currently evaluating the impact on its disclosures.

Intangibles-Internal-Use Software

In September 2025, the FASB issued ASU 2025-06, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40), which eliminates the project stage model and introduces a probable-to-complete recognition threshold for the capitalization of software development costs. The amendment in the update requires an assessment of uncertainty associated with software development activities, additional disclosures for capitalized internal-use software costs and consideration of website development costs. ASU 2025-06 is effective for annual periods beginning after December 15, 2027. ASU 2025-06 can be applied prospectively, through a modified transition approach, or retrospectively, and early adoption is permitted. The Company is currently evaluating the impact that this guidance will have on the presentation of its condensed financial statements and accompanying notes.