Atossa Therapeutics cleared for metastatic breast cancer study

Atossa Therapeutics (NASDAQ:ATOS) reached a significant regulatory milestone on January 6, 2026, receiving a "Study May Proceed" letter from the U.S. FDA for its Investigational New Drug (IND) application.

This clearance allows the company to initiate a clinical study of its lead candidate, (Z)-endoxifen, specifically for patients with metastatic ER+/HER2- breast cancer.

The study is designed to evaluate the drug's efficacy in patients whose tumors have developed resistance to standard endocrine therapies, such as aromatase inhibitors or fulvestrant.

(Z)-endoxifen is a potent selective estrogen receptor modulator/degrader (SERM/D) that acts as the most active metabolite of tamoxifen, but is delivered directly to bypass the need for liver metabolism.

Beyond traditional estrogen receptor blockade, (Z)-endoxifen targets the PKCβ1 (Protein Kinase C beta 1) oncogenic signaling pathway.

This dual-mechanism approach is intended to overcome "endocrine resistance"—a common failure point in metastatic breast cancer where the cancer learns to grow despite low estrogen levels.

While no new clinical data were released with this announcement, the company previously presented data at SABCS 2025 showing a 72% median MRI tumor volume reduction in early-stage trials.