Atrium Therapeutics Earns $15 Million Milestone Payment from Bristol Myers Squibb Under Global Cardiovascular Collaboration

SAN DIEGO, April 23, 2026 /PRNewswire/ -- Atrium Therapeutics, Inc. (NASDAQ:RNA) (the "Company"), a biopharmaceutical company dedicated to delivering RNA therapeutics directly to the heart, announced today it has earned a $15 million development milestone payment from Bristol Myers Squibb (NYSE:BMY). The milestone was achieved upon the successful delivery of a development candidate for the first licensed compound targeting a cardiology indication under the Company's ongoing collaboration.

"This milestone marks a meaningful step forward for Atrium, further expanding our RNA delivery platform and our ability to generate high quality cardiology development candidates," said Kathleen Gallagher, President and Chief Executive Officer of Atrium Therapeutics. "The successful advancement of this first development candidate reflects the strength of our science, the productivity of our collaboration with Bristol Myers Squibb, and our shared commitment to deliver transformative therapies for patients with cardiac disease."

The payment is pursuant to Atrium's global licensing and research collaboration with Bristol Myers Squibb focused on the discovery, development and commercialization of innovative RNA-based therapies for multiple cardiovascular indications.

Under the terms of the agreement, Atrium is eligible to receive up to approximately $1.35 billion in research and development milestone payments, up to approximately $825 million in commercial milestone payments, and tiered royalties up to low double-digits on net sales. Bristol Myers Squibb will fund all future clinical development, regulatory and commercialization activities coming from the collaboration.

About Atrium Therapeutics

Atrium Therapeutics, Inc. (NASDAQ:RNA) is pioneering targeted delivery of ribonucleic acid (RNA) therapeutics to the heart to transform the standard of care for people living with cardiomyopathies. The Company's proprietary technology - designed at Avidity Biosciences, Inc. - combines the tissue selectivity of monoclonal antibodies (mAbs) and other targeted delivery ligands with the precision of oligonucleotides. Atrium Therapeutics' platform is designed to selectively target the underlying drivers of genetically driven cardiac diseases through targeted, non-viral delivery of small interfering RNA (siRNA). This approach builds upon learnings from delivery to the skeletal muscle and applies it for efficient delivery to the heart with the potential to overcome challenges associated with non-specific tissue delivery. The Company's pipeline consists of two precision cardiology candidates, ATR 1072 for PRKAG2 (Protein Kinase AMP-activated non-catalytic subunit Gamma 2) syndrome and ATR 1086 for PLN (phospholamban) cardiomyopathy, and two undisclosed research targets in rare cardiomyopathies.

For more information about our RNA delivery platform, development pipeline and people, please visit https://atriumtherapeutics.com/ and engage with us on LinkedIn.

Availability of Other Information About Atrium Therapeutics

Investors and others should note that Atrium Therapeutics communicates with its investors and the public using its website https://atriumtherapeutics.com/, including, but not limited to, Atrium Therapeutics' disclosures, investor presentations and FAQs, Securities and Exchange Commission ("SEC") filings, press releases, public conference call transcripts and webcast transcripts, as well as on X (formerly Twitter) and LinkedIn. The information that Atrium Therapeutics posts on its website or on X or LinkedIn could be deemed to be material information. As a result, Atrium Therapeutics encourages investors, the media and others interested to review the information that it posts there on a regular basis. The contents of Atrium Therapeutics' website or social media shall not be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended.

About PRKAG2 Syndrome

PRKAG2 syndrome is a rare, autosomal dominant, early-onset cardiomyopathy caused by mutations in the PRKAG2 gene, which encodes the Gamma 2 regulatory subunit of AMPK. Mutations enhance AMPK activity leading to abnormal glycogen accumulation in heart muscle cells leading to thickened heart muscles, electrical conduction problems, and arrhythmias. There are 1,000 – 2,000 people with PRKAG2 syndrome in the US. Current management is limited to symptomatic treatment; no approved therapies exist to address the underlying genetic driver of disease.

About PLN Cardiomyopathy

PLN (phospholamban) cardiomyopathy is a rare autosomal dominant, progressive cardiac disease caused by mutations in PLN, a key regulator of SERCA2a calcium pump. PLN mutations produce protein aggregates that disrupt endoplasmic reticulum processes and lead to dilated, arrhythmogenic, or hypertrophic cardiomyopathies and a significantly increased risk of heart failure and sudden cardiac death. There are 2,000 – 4,000 people with pathogenic PLN variants in the US. No approved therapies target the underlying molecular cause of the disease.

Forward-Looking Statements

This press release contains forward-looking statements within the meaning of the United States Private Securities Litigation Reform Act of 1995. Forward-looking statements can generally be identified by words such as "potential," "can," "will," "plan," "may," "could," "would," "expect," "anticipate," "look forward," "believe," "committed," "investigational," "pipeline," "launch," or similar terms, or by express or implied discussions regarding Atrium Therapeutics' ("Atrium's" or "our") future results of operations and financial condition; research and development plans; anticipated timing, design and conduct of ongoing and planned preclinical studies and clinical trials for product candidates; our expectations regarding our RNA delivery platform and ability to generate high quality cardiology development candidates, the timing and likelihood of regulatory filings and approvals for product candidates; the potential safety and therapeutic benefits of our product candidates; the timing and likelihood of success; plans and objectives of management for future operations; and future results of anticipated product development efforts. You should not place undue reliance on these statements. Such forward-looking statements are based on our current beliefs and expectations regarding future events, and are subject to significant known and unknown risks and uncertainties. Particular areas where risks or uncertainties could cause Atrium's actual results to be materially different than those expressed in Atrium's forward-looking statements include but are not limited to: the initiation, timing, progress, potential registrational quality, and results of our research and development programs, preclinical studies, any clinical trials, and other regulatory submissions; the beneficial characteristics, including potential safety, efficacy and therapeutic effects of our product candidates and the potential advantages of our product candidates compared to alternative therapies; the success and capabilities of the RNA delivery platform; the prevalence of certain diseases and conditions we intend to treat and our estimates of the potential market opportunity for our product candidates; the timing of and costs involved in obtaining and maintaining regulatory approval of our current and any future product candidates; our ability to develop our current and future product candidates; the implementation of our strategic plans for our business, product candidates, research programs and technologies; developments related to our competitors and our industry; our competitive position and the success of competing therapies that are or may become available; our ability to maintain our current license agreements and collaborations and identify and enter into future license agreements and collaborations; the expected potential benefits of strategic collaborations with third parties and our ability to attract collaborators in the future; our reliance on third parties for manufacturing and to conduct preclinical studies and clinical trials of our product candidates; our ability to efficiently and cost-effectively conduct our current and future clinical trials; the costs of operating as a public company; the accuracy of our estimates regarding future expenses, future revenue, capital requirements and the need for additional financing; the period over which we estimate our existing cash and cash equivalents will be sufficient to fund our future operating expenses and capital expenditure requirements; and other factors specified in Atrium's Registration Statement on Form 10, initially publicly filed by Atrium with the Securities and Exchange Commission (the "SEC") on December 10, 2025 and in other filings and furnishings made by Atrium with the SEC from time to time. Atrium is providing the information in this communication as of this date and does not undertake any obligation to update any forward-looking statements contained in this communication as a result of new information, future events or otherwise, except as required by law.

Investor and Media Contact:

Stephanie Kenney, Chief Corporate Affairs Officer, [email protected]

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