Atossa Therapeutics Announces Publication of Novel (Z)-Endoxifen-Related Compounds Demonstrating Potent Anti-Cancer Activity in ER-Positive Breast Cancer

Peer-reviewed preclinical study published in npj Breast Cancer identifies new chemical entities with activity in ESR1-mutant models

Findings suggest potential for combination with abemaciclib

SEATTLE, July 28, 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 clinical need, today announced the publication of a peer-reviewed preclinical study in npj Breast Cancer evaluating five novel chemical entities structurally related to (Z)-endoxifen. The study titled, "Novel (Z)-endoxifen-related new chemical entities exhibit potent anti-cancer activity in ERα+ breast cancer," reported anti-estrogenic and anti-cancer activity across multiple estrogen receptor-positive breast cancer models, including models harboring clinically relevant activating mutations in ESR1.

Journal: npj Breast Cancer Publication Date: July 20, 2026 Article Title and Link: Novel (Z)-endoxifen-related new chemical entities exhibit potent anti-cancer activity in ERα+ breast cancer Research Collaboration: Mayo Clinic and Atossa Therapeutics, Inc.

• The investigators evaluated five previously uncharacterized compounds generated during the synthesis of (Z)-endoxifen - AT416E, AT416Z, AT402E, AT402Z and AT300 - alongside (Z)-endoxifen in a broad panel of laboratory assays. The studies assessed two- and three-dimensional tumor-cell growth, apoptosis, cell-cycle progression, migration, invasion, estrogen receptor transcriptional activity, gene-expression changes and activity in combination with the CDK4/6 inhibitor abemaciclib.

• Several compounds demonstrated potent anti-estrogenic effects, and affected multiple anti-cancer processes including apoptosis, cell-cycle progression, migration, invasion and estrogen receptor-driven transcription.

• In certain experimental settings and models, selected compounds combined with abemaciclib demonstrated additive to synergistic activity that was comparable to or greater than the activity observed with abemaciclib plus (Z)-endoxifen.

• The compounds also showed activity in models containing activating ESR1 mutations, which are associated with endocrine resistance and recurrent or metastatic estrogen receptor-positive breast cancer.

• RNA-sequencing analyses identified shared anti-estrogenic effects as well as distinct compound-specific transcriptional programs that may help differentiate the candidates.

• The authors concluded that select compounds warrant further in vivo safety evaluation, as well as efficacy studies, including as potential second- or third-line approaches for recurrent disease. These findings are preclinical and do not establish safety or efficacy in patients.

"This publication expands the scientific foundation of our endoxifen platform and identifies additional compounds with compelling activity across difficult-to-treat estrogen receptor-positive breast cancer models," said Dr. Steven C. Quay, M.D., Ph.D., President and Chief Executive Officer of Atossa Therapeutics. "Of particular interest is the activity observed in ESR1-mutant models and in combination with a CDK4/6 inhibitor. While these results are early and preclinical, we believe they provide a strong rationale for further evaluation of selected candidates as we continue to explore opportunities to address endocrine resistance and recurrent disease."

Estrogen receptor-positive breast cancer is the most common molecular subtype of breast cancer. Although endocrine therapies are effective for many patients, recurrence and late relapse remain important clinical challenges. Activating mutations in ESR1 can allow estrogen receptor signaling to continue despite estrogen deprivation and are a recognized mechanism of acquired resistance in advanced disease. New therapies capable of inhibiting estrogen receptor signaling in ESR1-mutant tumors, alone or in rational combinations, may help address this unmet need.

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.

(Z)-endoxifen is a potent Selective Estrogen Receptor Modulator/Degrader (SERM/D) with demonstrated activity across multiple mechanisms of interest. 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'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 at https://atossatherapeutics.com.

This press release contains certain "forward-looking statements" within the meaning of applicable securities laws, including, but not limited to, our expectations regarding the Company's development and regulatory strategy and related milestones, including the potential indications that the Company may pursue for (Z)-endoxifen, the potential role of (Z)-endoxifen and (Z)-endoxifen-related compounds in endocrine therapies, 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, the potential clinical significance of preclinical data, 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," or other similar expressions or statements regarding intent, belief or current expectations, are 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: our ability to successfully execute our strategy to shorten our clinical development timelines for our lead program, (Z)-endoxifen; expected timing, completion and results of our 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; our ability to maintain compliance with Nasdaq listing requirements; our ability to establish and maintain intellectual property rights covering our products; the impact of general macroeconomic conditions on our business; our 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, we do not intend to update any forward-looking statements.