Atossa Therapeutics Announces Positive (Z)-Endoxifen Data at ASCO 2026, Highlighting Potential in ESR1-Mutated Breast Cancer

Clinical-stage biopharmaceutical company Atossa Therapeutics, Inc. (NASDAQ: ATOS) announced today that two abstracts concerning its investigational drug (Z)-Endoxifen have been accepted for presentation at the 2026 American Society of Clinical Oncology (ASCO) Annual Meeting, scheduled for May 29 to June 2 in Chicago. The abstracts highlight the drug's inhibitory activity in breast cancer models harboring ESR1 mutations and provide updates on the ongoing Phase 2 EVANGELINE clinical trial.

The first abstract presents preclinical data demonstrating that (Z)-Endoxifen can inhibit estrogen receptor signaling in breast cancer models with ESR1 mutations. In MCF-7 breast cancer cells, the drug reduced estrogen receptor activity to between 16% and 26% of control group levels. The data indicate that the compound maintained sustained inhibitory effects against the Y537N, Y537S, and D538G mutations. Compared to the oral selective estrogen receptor degraders elacestrant and imlunestrant, (Z)-Endoxifen showed superior inhibitory efficacy in the ESR1-mutated setting.

The second abstract details the EVANGELINE study, an ongoing Phase 2 clinical trial evaluating (Z)-Endoxifen at a daily dose of 40 mg in combination with goserelin as a neoadjuvant therapy for premenopausal women with estrogen receptor-positive/HER2-negative breast cancer. The study's primary objective is to determine the proportion of patients with a baseline Ki-67 above 10% who achieve a reduction to 10% or below after four weeks of treatment. The trial, which employs a Simon two-stage design, began patient enrollment in May 2025.

Dr. Steven Quay, President and CEO of Atossa, stated, "Our preclinical data show that (Z)-Endoxifen achieves robust estrogen receptor inhibition against clinically relevant ESR1 mutations associated with endocrine resistance, while the EVANGELINE trial is assessing its potential in combination with ovarian function suppression for neoadjuvant treatment in premenopausal women. Together, these abstracts reinforce our belief that (Z)-Endoxifen has the potential to address significant unmet medical needs across multiple treatment settings for ER-positive breast cancer."