Atossa Therapeutics Updates (Z)-Endoxifen Clinical Trials at SABCS

Atossa Therapeutics presented four clinical trial updates on (Z)-endoxifen at the San Antonio Breast Cancer Symposium, SABCS, held December 9-12, 2025, in San Antonio, TX. Presentation Highlights: Initial results from RECAST DCIS: Multicenter platform trial testing active surveillance and novel endocrine therapy agents for DCIS management: Early RECAST findings suggest that short-term endocrine therapy combined with MRI response assessment may identify patients with low-risk DCIS who can avoid surgery and pursue active surveillance, offering a potential pathway to reduce overtreatment while personalizing care. Low dose (Z)-endoxifen in the I-SPY2 Endocrine Optimization Pilot: The excellent tolerability and biologic activity of low-dose (Z)-endoxifen, demonstrated by reductions in Ki-67, MRI tumor volume, and ctDNA, support its potential as an effective neoadjuvant endocrine therapy for HR+/HER2- early breast cancer and justifies dose escalation and combination strategies to further improve patient outcomes. (Z)-Endoxifen Maintains ERalpha Antagonist Function Against ESR1 Mutants via Inactive Conformation Stabilization and Reversal of Mutant ESR1-Associated Transcriptional Signatures: Findings demonstrate (Z)-endoxifen as a promising therapeutic option for patients with ER+/ESR1 mutant breast cancer, a population with limited effective endocrine treatments and high unmet need. A Randomized Phase 2 Non-Inferiority Trial of (Z)-Endoxifen + Goserelin vs Exemestane + Goserelin as a Neoadjuvant Treatment for Premenopausal Women with ER+/HER2- Breast Cancer : (Z)-endoxifen with Ovarian Function Suppression (OFS) may offer a more tolerable and biologically potent alternative to aromatase-inhibitor (AI)-based regimens for premenopausal ER+/HER2- patients, potentially expanding endocrine therapy options and improving adherence while maintaining robust neoadjuvant efficacy