The U.S. Food and Drug Administration (FDA) has issued a “Study May Proceed” letter to Atossa Therapeutics for their trial of (Z)-endoxifen in patients with metastatic breast cancer, the company announced in a press release.
“This letter marks an important regulatory milestone for Atossa,” shared president and chief executive officer Dr. Steven Quay. “We appreciate the FDA’s review and look forward to advancing this clinical investigation.”
In late 2025, Atossa submitted an investigational new drug application (IND) for (Z)-endoxifen in metastatic breast cancer. The “Study May Proceed” letter indicates that the FDA now authorizes the company to begin clinical testing of (Z)-endoxifen in this indication.
The planned study will evaluate the safety and efficacy of (Z)-endoxifen across multiple dose levels in patients with estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer.
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Atossa is currently conducting several clinical trials of (Z)-endoxifen in different patient populations. The I-SPY2 trial found that 10 mg (Z)-endoxifen daily was well-tolerated in patients with early ER-positive HER2-negative breast cancer. Tumor volume and levels of Ki-67, a tumor marker, were also significantly reduced in patients receiving the drug.
In addition, the EVANGELINE clinical trial demonstrated that 86% of premenopausal patients with ER-positive HER2-negative breast cancer who received (Z)-endoxifen experienced significant reductions in Ki-67 levels at four weeks.
(Z)-endoxifen is an oral medication that inhibits and degrades estrogen receptors on tumor cells. By blocking hormone signaling, the drug aims to repress tumor growth. Atossa is also assessing (Z)-endoxifen in patients with Duchenne muscular dystrophy.
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