Results of a phase I study in women showed that the treatment was safe and reduced tumors.

Z-endoxifen, a potent derivative of the drug tamoxifen, could be a new treatment for the most common form of breast cancer in women with metastatic disease.

This finding was reported from a clinical trial conducted by researchers at Mayo Clinic and the National Cancer Institute. Results of the Z-endoxifen trial were published Aug. 30, 2017, in the Journal of Clinical Oncology.

The final results of a first-in-human phase I study of Z-endoxifen in women with estrogen receptor positive metastatic breast cancer showed that the treatment was safe and resulted in tumor shrinkage in women whose tumors had progressed using standard anti-estrogen therapies, including tamoxifen.

"Tamoxifen is converted into endoxifen in the liver by an enzyme called CYP2D6. Our previous research found that tamoxifen may be less effective in women with poor CYP2D6 metabolism," said Matthew P. Goetz, M.D., the study's lead author and an oncologist at Mayo Clinic in Rochester, Minnesota.