The U.S. Food and Drug Administration issued a ‘Study May Proceed’ letter on January 6 2026, clearing Atossa Therapeutics to begin a pivotal clinical study of its next‑generation selective estrogen receptor modulator, (Z)-Endoxifen, in patients with metastatic estrogen‑positive, HER2‑negative breast cancer.
(Z)-Endoxifen blocks estrogen‑receptor signaling while promoting receptor degradation and simultaneously targets the protein kinase C beta‑1 pathway, a driver of endocrine resistance. The oral formulation bypasses first‑pass metabolism, delivering more predictable exposure than tamoxifen and potentially improving efficacy in tumors that have become refractory to standard endocrine therapies.
Financially, Atossa reported a net loss of $8.7 million for the third quarter of 2025, up from a $7.2 million loss in Q3 2024, while operating expenses rose to $9.3 million from $6.4 million. Cash and cash equivalents stood at $51.8 million as of September 30 2025, providing a runway to fund the upcoming study and further development.
CEO Dr. Steven Quay emphasized that the FDA letter marks an “important regulatory milestone” and signals that the company is moving (Z)-Endoxifen toward its final Phase 3 studies. He noted that the drug’s dual mechanism could reshape the standard of care for patients whose tumors have escaped existing endocrine options.
The study’s approval positions Atossa to generate critical safety, efficacy, and dosing data that could accelerate the drug’s path to market. With a growing intellectual‑property portfolio and a strategic focus on metastatic disease, the milestone strengthens the company’s competitive positioning and supports its long‑term growth strategy.
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