Benzestrol Isomer Stereochemistry Determines the Distinct Estrogenic Activities and Conformations of the Eight Isomers When Bound to Estrogen Receptor α.

The nonsteroidal estrogen, benzestrol, has potent estrogenic activity, and through a recent stereocontrolled synthesis, we have obtained all eight of its constituent stereoisomers. We find that only one of them, RSS benzestrol, has very high binding affinity for estrogen receptor alpha (ERα); the other seven isomers have 60 to 600-fold lower affinity. We now show that the potencies of the isomers in two cell activity assays, proliferation of ER-positive breast cancer cells and stimulation of estrogenic gene activity, reflect their varying binding affinities for ERα. The crystal structure of the RSS isomer itself is consistent with its presumed absolute configuration and also reveals its conformational flexibility within the solid crystal lattice. We modeled each of the 8 benzestrol stereoisomers bound to ERα. Their calculated binding energies and internal torsional energies grouped with their experimentally measured binding affinities and biological activities, and the conformation for the highest affinity RSS isomer bound to ERα maps closely onto the conformation of the ERα-bound potent nonsteroidal estrogen, trans-diethylstilbestrol. Hence, we now provide a structural context for this congeneric series of benzestrol stereoisomers by proposing energy-based conformations they adopt when bound to ERα that underlie their effectiveness as estrogens.