Description of Business

Atrium Therapeutics, Inc. (“the Company,” “Atrium” or “we”) is a clinical-stage biopharmaceutical company focused on developing the delivery of ribonucleic acid (“RNA”) therapeutics to the heart muscle for people living with cardiomyopathies. Atrium’s proprietary technology leverages the RNA delivery platform initially developed at Avidity Biosciences, Inc. (“Former Parent” or “Avidity”), which combines the tissue selectivity of monoclonal antibodies (“mAbs”) and other targeted delivery ligands with the precision of oligonucleotides. The Company was incorporated in Delaware in September 2025. The Company’s headquarters and operations are located in San Diego, California and it operates in one segment.

Agreement and Plan of Merger & Separation and Distribution Agreement

In October 2025, Avidity entered into an Agreement and Plan of Merger with Novartis AG (“Novartis”) and Ajax Acquisition Sub, Inc., a wholly owned subsidiary of Novartis (“Merger Sub”), pursuant to which Merger Sub merged with and into Avidity (the “Merger”), with Avidity surviving as an indirect wholly owned subsidiary of Novartis.

In connection with the Merger, Avidity entered into a Separation and Distribution Agreement (the “Separation Agreement”) with Atrium, a newly formed Delaware corporation and wholly owned subsidiary of Avidity, and Novartis. Pursuant to the Separation Agreement, Avidity undertook a pre-closing reorganization (the “Separation”) to transfer to the Company all assets and liabilities related to its early-stage precision cardiology programs and certain collaboration, license and research agreements. Avidity retained all other assets and liabilities.

Following the Separation, on February 26, 2026, Avidity distributed all the outstanding shares of the Company’s common stock, par value $0.001 per share (“common stock”) to Avidity’s stockholders on a pro rata basis (the “Spin-Off”). Each holder of the Former Parent’s common stock, par value $0.0001 per share (“Former Parent Common Stock”) received 1 share of common stock for every 10 shares of the Former Parent's Common Stock held of record as of the close of business, Eastern Time, on February 12, 2026. In the Spin-Off, 15,514,966 shares of common stock were distributed to holders of the Former Parent's Common Stock.

Additionally, in the Spin-Off, holders of options to purchase shares of the Former Parent's Common Stock (the “Former Parent Stock Options”) and holders of restricted stock units denominated in shares of the Former Parent's Common Stock, whether subject to time-based or performance-based vesting, that were granted under any equity plans, agreements or arrangements of the Former Parent (“Former Parent RSUs” and together with the Former Parent Stock Options, the “Former Parent Equity Awards”) received a non-transferable Make Whole Award that will be settled in shares of common stock at a ratio of 1 share of common stock for every 10 shares of the Former Parent Common Stock underlying each such Former Parent Equity Award, as required by the terms of the Separation Agreement and as permitted by the Securities and Exchange Commission’s (the “SEC”) Staff Legal Bulletin No. 4. The Company granted Make Whole Awards to certain holders of Former Parent Equity Awards for an aggregate of 1,590,677 shares of its common stock, which awards were outstanding as of June 30, 2026. These awards are required to be settled as soon as administratively practicable after the effective time of the Spin-Off, but in no event after March 15, 2027. See Note 7.

From and after the completion of the Spin-Off, the Company continues to operate as an independent, publicly traded company. The Company was capitalized with $270.0 million in cash, less the sum of the amount of marketable securities and cash, cash equivalents and restricted cash contained in any accounts owned by the Company as of the close of business on the day prior to the date of the Spin-Off. The Company is led by a dedicated management team and board of directors. Avidity has no continuing ownership interest in the Company following the Spin-Off.

In connection with the Spin-Off, the Company entered into various agreements relating to transition services, licenses and certain other matters with the Former Parent. For additional information regarding these agreements, see Note 8.

Liquidity

The Company has devoted substantially all its efforts to therapeutic drug discovery and development. The Company has historically been dependent upon Avidity for all its working capital and financing requirements as Avidity used a centralized approach to cash management and financing of its operations prior to the Spin-Off. As a result, the Company did not maintain its own bank accounts and had no cash, cash equivalents, and short-term investments prior to the establishment of its independent cash management structure. The Company no longer participates in Avidity’s centralized treasury system and independently manages its own liquidity. The Company has incurred recurring losses since its inception and expects to continue to generate operating losses for the foreseeable future. As of June 30, 2026, the Company had an accumulated deficit of $8.1 million, cash and cash equivalents of $72.3 million and short-term investments of $191.6 million.

The Company believes that existing cash, cash equivalents, and short-term investments will be sufficient to fund the Company’s operations for at least 12 months from the date of the filing of this Quarterly Report on Form 10-Q. The Company’s operations are subject to risks and uncertainties common to early-stage biotechnology companies, including, but not limited to, the ability to progress product candidates through clinical development, the development of new technological innovations by competitors, reliance on key personnel, protection of proprietary technology, compliance with regulatory requirements, and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development, including preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts may require substantial capital, personnel, and infrastructure. Even if development efforts are successful, there is no assurance that the Company will generate significant revenue from product sales.

The Company plans to finance its future cash needs primarily to support its longer‑term development activities through equity offerings, debt financings, or other capital sources, including potential collaborations, licenses, and other similar arrangements. The timing and terms of any such transactions, if pursued, will depend on market conditions and other factors.