Tert-butylphenoxyethyl diethylamine (BPEA), a compound synthesized by us, was designed to incorporate features important in binding to antiestrogen-binding sites (AEBS) while lacking features important in binding to the estrogen receptor (ER). With this compound, we have addressed the question of the role of AEBS in mediating the growth modulatory effects of antiestrogens. BPEA has an affinity for AEBS 6% that of tamoxifen and an affinity for ER less than 0.0003% that of estradiol. BPEA (10−11–10−6 M) had no effect on the growth of MCF-7 breast cancer cells and no effect on inhibition of the growth of MCF-7 cells by different concentrations of the antiestrogen tamoxifen. In addition, BPEA (even at doses of 1 mg/day·50 g rat) exhibited no uterotropic or antiuterotropic activity in immature rats and had no influence on the agonistic or antagonistic activity of varying concentrations of tamoxifen on uterine weight. Hence, we conclude that occupancy of AEBS, at least by BPEA, does not modulate growth of the uterus or breast cancer cells and does not influence the potency of tamoxifen as an antiestrogen. These findings raise serious doubts about the role of the AEBS in mediating directly the growth modulatory effects of antiestrogens.
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