Vitamin E increases biomarkers of estrogen stimulation when taken with tamoxifen

Vitamin E (alpha-tocopherol acetate, AT) diminishes the antiproliferative effect of tamoxifen on breast cancer cells in vitro.

A prospective study of seven women taking tamoxifen for adjuvant therapy of breast cancer. Four who were already taking AT supplements had random core biopsies of the normal breast and again 30 days after discontinuing AT. Three who were not on AT had biopsies before and after adding AT 400 mg for 30 days. Biopsies were stained for estrogen receptor (ER) and the mitogen-activated protein kinase p-ERK. Tissue extracts were assayed for p-ERK by enzyme-linked immunosorbent assay. Serum levels of alpha-tocopherol and tamoxifen were measured.

Five out of seven patients had lower tamoxifen levels while taking AT, four of these went to subtherapeutic levels. Biopsies showed 23% of ductal cells were ER positive when patients were off AT and 70% on AT (P = 0.02). P-ERK staining was 21% off AT and 82% on AT. Five of seven patients had significantly higher tissue p-ERK when on AT.

Biomarkers of estrogen-stimulation (ER, progesterone receptor, and p-ERK) were higher in breast biopsies of women taking vitamin E supplements while taking tamoxifen. Findings suggest that vitamin E supplements may interfere with the therapeutic effects of tamoxifen.