Tamoxifen and Metabolites in MCF₇ Cells: Correlation between Binding to Estrogen Receptor and Inhibition of Cell Growth¹ Free

The binding of [³H]tamoxifen ([³H]Tam), a nonsteroidal antiestrogen, and of 4-[³H]hydroxytamoxifen ([³H]OH-Tam), a metabolite accumulated in vivo in target cell nuclei, was characterized in soluble extracts of human breast cancer MCF₇ cells growing in a medium depleted in estrogens. Saturation analysis indicated a much higher affinity for OH-Tam (K_d = 0.15 nM) than for Tam (K_d = 4.8 nM). The binding of [³H]Tam and [³H]estradiol was competitive and mutually exclusive, and the binding site concentration (0.16 to 0.47 pmol/mg total protein) was similar for both ligands, strongly suggesting that antiestrogens were binding to the estrogen receptor (ER) in these cells.

The ability of Tam and of some of its metabolites or derivatives to prevent the MCF₇ cell growth was found to be correlated with their affinity for ER as determined by direct interaction or by binding competition with [³]estradiol on the uterine and MCF₇ cytosol ER. OH-Tam was the highest-affinity compound and was 100-fold more active than Tam. The inhibitions observed were actually due to Tam and OH-Tam, respectively, since we did not detect any significant metabolism of these two labeled compounds by the MCF₇ cells. N-desmethyltamoxifen, the other Tam metabolite found in high concentration in human plasma, was as effective as Tam while cis-tamoxifen appeared less effective. Compound E, which has no lateral chain, was the only tested compound having a good affinity for ER and a poor efficiency in preventing cell growth. These results support the hypothesis that antiestrogens control the growth of breast cancer by acting directly on the ER located in cancer cells.