Atossa Therapeutics to Host Virtual KOL Event Featuring Dr. Laura Esserman to Discuss Development of (Z)-Endoxifen in ER-Positive Breast Cancer

Event to feature Dr. Laura Esserman, internationally recognized surgical oncologist, breast cancer expert, and founder and principal investigator of the I-SPY Trials

SEATTLE,May 14, 2026/PRNewswire/ -- Atossa Therapeutics, Inc. (Nasdaq:ATOS) ("Atossa" or the "Company"), a clinical-stage biopharmaceutical company developing novel therapies in oncology and other areas of significant unmet medical need, today announced that it will host a virtual key opinion leader event focused on the evolving clinical and translational data supporting the development of (Z)-endoxifen, the Company's investigational selective estrogen receptor modulator/degrader, or SERM/D, as a potential next-generation endocrine therapy backbone across multiple estrogen receptor ("ER") -positive breast cancer settings.

The event will feature internationally recognized surgical oncologist and breast cancer expert Dr. Laura Esserman. Dr. Esserman serves as Director of the University of California San Francisco Breast Care Center and is the founder and principal investigator of the I-SPY Trials, an innovative platform designed to accelerate the development of personalized breast cancer therapies.

"Dr. Esserman has been at the forefront of innovation in breast cancer clinical research, adaptive trial design, and precision oncology for decades," said Dr. Steven Quay, M.D., Ph.D., Chairman and Chief Executive Officer of Atossa Therapeutics. "We believe (Z)-endoxifen has the potential to become more than a single-indication therapy; it may serve as an endocrine therapy platform across multiple ER-positive breast cancer settings. Dr. Esserman's leadership in translational breast cancer research makes her an ideal expert to discuss the emerging clinical and biomarker data supporting this opportunity."

The discussion is expected to address several topics of interest to clinicians, investors, and potential strategic partners, including:

• Recent data from the I-SPY2 Endocrine Optimization Pilot study evaluating (Z)-endoxifen as monotherapy in newly diagnosed ER-positive breast cancer patients

• Translational biomarker data, including Ki-67 reduction, MRI functional tumor volume changes, and circulating tumor DNA dynamics

• Data supporting (Z)-endoxifen activity across clinically relevant ESR1 mutations, including Y537N, Y537S, and D538G

• The rationale for (Z)-endoxifen as a potential next-generation endocrine therapy backbone

• Combination strategies, including CDK4/6 inhibitor combinations

• Development opportunities in premenopausal breast cancer and endocrine therapy optimization

Date:Tuesday, May 19, 2026Time:1:00 to 2:00 p.m. PTModerator:Michael King, Managing Director at Rodman & RenshawRegistration:https://zoom.us/webinar/register/WN_fd6jdLUvQPC-3J84CUEnMA

Attendees may submit questions to[email protected]up to 24 hours prior to the event.

A replay of the webcast will be available on the Company's "Investors" portion of its website for at least 60 days following the live event.

(Z)-endoxifen is a potent Selective Estrogen Receptor Modulator/Degrader, or SERM/D, with demonstrated activity across multiple mechanisms of action. Atossa is evaluating its potential applications in oncology and rare diseases. The Company's proprietary oral formulation has shown a favorable safety profile and pharmacology distinct from tamoxifen, including ER-targeted effects and PKC inhibition. Atossa's (Z)-endoxifen is not approved for any indication.

Atossa Therapeutics, Inc. (Nasdaq:ATOS) is a clinical-stage biopharmaceutical company developing innovative medicines in oncology and other areas of significant unmet need. The Company's lead product candidate, (Z)-endoxifen, is currently in development across several clinical settings.

Atossa's (Z)-endoxifen program is supported by a growing global intellectual property portfolio, including multiple recently issued U.S. patents and numerous pending applications worldwide. More information is available athttps://atossatherapeutics.com.

This press release contains certain "forward-looking statements" within the meaning of applicable securities laws, including but not limited to statements regarding the Company's development and regulatory strategy and related milestones; the potential indications that the Company may pursue for (Z)-endoxifen; the potential role of (Z)-endoxifen as an endocrine therapy platform; the potential for (Z)-endoxifen to receive regulatory approval and the timing thereof; expectations regarding the design, enrollment, data, timing, results and outcomes of the Company's clinical studies; and the potential market and growth opportunities for the Company. Words such as "expect," "potential," "continue," "may," "will," "should," "could," "would," "seek," "intend," "plan," "estimate," "anticipate," "believe," "design," "predict," "future," and similar expressions are intended to identify forward-looking statements.

Forward-looking statements in this press release are subject to risks and uncertainties that may cause actual results, outcomes, or the timing of actual results or outcomes to differ materially from those projected or anticipated, including, without limitation, risks and uncertainties associated with: the Company's ability to successfully execute its strategy to shorten clinical development timelines for its lead program, (Z)-endoxifen; the expected timing, completion and results of preclinical studies, clinical trials, and research and development programs; the unpredictable relationship between preclinical study results and clinical study results; the timing or likelihood of regulatory filings and approvals; the outcome or timing of necessary regulatory approvals; the Company's ability to maintain compliance with Nasdaq listing requirements; the Company's ability to establish and maintain intellectual property rights covering its products; the impact of general macroeconomic conditions on the Company's business; the Company's ability to raise capital; and other risks and uncertainties detailed from time to time in Atossa's filings with the SEC, including, without limitation, its Annual Reports on Form 10-K and Quarterly Reports on Form 10-Q.

Forward-looking statements are presented as of the date of this press release. Except as required by law, the Company does not intend to update any forward-looking statements.