Metabolism of Equilenin in MCF-7 and MDA-MB-231 Human Breast Cancer Cells

Sulfate conjugates of the B-ring unsaturated estrogens, equilin, equilenin, and 8-dehydroestrone, and their 17α- and 17β-dihydro analogues, constitute about 54% of Premarin (Wyeth-Ayerst), the most commonly prescribed estrogen formulation in estrogen replacement therapy. Despite the wide clinical use of Premarin, there have been very few studies on the metabolism of the B-ring unsaturated estrogens in humans and there is no information regarding the fate of these compounds in breast tissue or tumors. In this study, we investigated the metabolism of equilenin in two lines of human breast-cancer cells, MCF-7 and MDA-MB-231. MCF-7 cells respond to treatment with Ah-receptor agonists with induction of cytochromes P450 1A1 and 1B1, whereas in MDA-MB-231 cells P450 1B1 is predominantly induced. Metabolites of equilenin were identified and quantified by GC/MS utilizing a series of synthetic metabolite standards and deuterium-labeled analogues as internal standards. In the two cell lines, the same pathways of equ...