Atossa Therapeutics Announces Acceptance of Four Abstracts Highlighting (Z)-Endoxifen Research for Presentation at the 2025 San Antonio Breast Cancer Symposium

Studies highlight findings in the use of (Z)-endoxifen to advance breast cancer treatment and prevention

Atossa Therapeutics, Inc. (Nasdaq: ATOS) (“Atossa” or the “Company”), a clinical-stage biopharmaceutical company developing innovative medicines in oncology, announces that four abstracts featuring data on (Z)-endoxifen have been accepted for presentation at the San Antonio Breast Cancer Symposium (SABCS), being held on December 9-12, 2025, in San Antonio, TX.

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“We continue to add to our body of clinical evidence. AtSABCS 2025, we look forward to four poster presentations highlighting findings from studies evaluating the use of (Z)-endoxifen to advance breast cancer treatment and prevention,” said Dr. Steven Quay, Atossa Therapeutics President and Chief Executive Officer.

Poster Presentation Details:Title: Initial results from RECASTDCIS: Multicenter platform trial testing active surveillance and novel endocrine therapy agents forDCIS managementDate/Time: Thursday, December 11, 2025, 12:30pm – 2:00pm CTTitle: Low dose (Z)-endoxifen in the I-SPY2 Endocrine Optimization PilotDate/Time: Thursday, December 11, 2025, 12:30pm – 2:00pm CTTitle: (Z)-Endoxifen MaintainsERa Antagonist Function Against ESR1 Mutants via Inactive Conformation Stabilization and Reversal of Mutant ESR1-Associated Transcriptional SignaturesDate/Time: Friday, December 12, 2025, 7:00am – 8:30am CTTitle: A Randomized Phase 2 Non-Inferiority Trial of (Z)-Endoxifen +Goserelin vs Exemestane + Goserelin as Neoadjuvant Treatment for Premenopausal Women with ER+/HER2- Breast Cancer (EVANGELINE)Date/Time: Friday, December 12, 2025, 12:30pm – 2:00pm CT

AboutAtossa Therapeutics

Atossa Therapeutics, Inc. (Nasdaq: ATOS) is a clinical-stage biopharmaceutical company developing innovative therapies for significant unmet needs in breast cancer. Atossa's strategy emphasizes disciplined capital allocation, focusing resources on programs and data packages that can enable future regulatory submissions and potential commercialization. For more information, visit www.atossatherapeutics.com and refer to Atossa's filings with the U.S. Securities and Exchange Commission (SEC).

Forward-Looking StatementsThis press release contains forward-looking statements, including statements regardingAtossa's commercial readiness, financing strategy, operational plans, and the development and potential commercialization of (Z)-endoxifen. These statements are subject to risks and uncertainties that could cause actual results to differ materially, including market conditions, regulatory outcomes, clinical results, manufacturing and supply, intellectual-property challenges, and the Company's ability to secure sufficient funding on acceptable terms. For a discussion of risks and uncertainties, please refer to Atossa's filings with the U.S. Securities and Exchange Commission. Atossa undertakes no obligation to update forward-looking statements, except as required by law.

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