Atossa Therapeutics (NASDAQ: ATOS) announced the publication of a peer-reviewed preclinical study in npj Breast Cancer evaluating five novel chemical entities structurally related to (Z)-endoxifen, conducted in collaboration with Mayo Clinic.

The study, published July 20, 2026, assessed compounds AT416E, AT416Z, AT402E, AT402Z, and AT300 across a range of laboratory assays, including two- and three-dimensional tumor-cell growth, apoptosis, cell-cycle progression, migration, invasion, and estrogen receptor transcriptional activity.

Several compounds showed anti-estrogenic effects in estrogen receptor-positive breast cancer models, including models with activating ESR1 mutations, which are associated with endocrine resistance in advanced disease. In certain experimental conditions, selected compounds combined with the CDK4/6 inhibitor abemaciclib produced additive to synergistic activity comparable to or greater than abemaciclib combined with (Z)-endoxifen.

RNA-sequencing analyses identified both shared anti-estrogenic effects and distinct compound-specific transcriptional activity. The study authors concluded that select compounds warrant further in vivo safety and efficacy evaluation, including as potential second- or third-line approaches for recurrent disease. The 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, President and Chief Executive Officer of Atossa Therapeutics. "While these results are early and preclinical, we believe they provide a strong rationale for further evaluation of selected candidates."

Atossa's (Z)-endoxifen is not approved for any indication. The company is a clinical-stage biopharmaceutical firm based in Seattle.