The interaction of raloxifene and the active metabolite of the antiestrogen EM-800 (SC 5705) with the human estrogen receptor

A naturally occurring mutation at amino acid 351 (D351Y) in the human estro gen receptor (ER) can change the pharmacology of antiestrogens. Raloxifene is converted from an antiestrogen to an estrogen, whereas the biological pr operties of the steroidal pure antiestrogen ICI 182,780 are not affected by the D351Y ER (Levenson, A. S., and Jordan, V. C. Cancer Res., 58: 1872-187 5, 1998). We propose an assay system that can be used to classify antiestro gens by determining their ability to up-regulate transforming growth factor alpha (TGF-alpha) mRNA in MDA-MB-231 cells stably transfected with either wild-type or D351Y ER. The novel compound EM-800 and its active metabolite, EM-652, have been reported to be p.o. active nonsteroidal pure antiestroge ns. Using the D351Y cell line, EM-652 is able to up-regulate TGF-alpha mRNA in a dose-dependent manner and to a similar extent as estradiol, whereas i n the wild-type cell line, it acts as an antiestrogen. In addition, the pur e antiestrogen ICI 182,780 is capable of inhibiting EM-652-induced TGF-alph a mRNA expression at the D351Y ER. In MCF-7 cells expressing wild-type ER, it has previously been shown that ICI 182,780 decreases ER only at the prot ein level. EM-652 treatment does not decrease ER protein levels to a simila r extent as ICI 182,780 treatment, and, in addition, EM-652 has no effect o n ER mRNA levels. In proliferation assays, EM-652 is as effective as raloxi fene in inhibiting cell growth. From these studies, we conclude that the re ason the pharmacology of EM-652 is similar to that of raloxifene is because they both fit the ER in the same manner, and their biology depends on an i nteraction of the antiestrogenic side chain with amino acid 351.