Structural requirements for the pharmacological activity of nonsteroidal antiestrogens in vitro

The structure-activity relationships of a tamoxifen (TAM) (Z-1-(4β-dimethylaminoethoxyphenyl)1,2-diphenylbut-1-ene) series have been investigated. The tamoxifen derivatives were assayed in vitro by their modulation of estradiol (E₂)-stimulated prolactin synthesis in primary cultures of dispersed rat pituitary gland cells. Monohydroxylation of TAM in position 4 of the stilbene ring system was found to be the optimal substitution for binding to the estrogen receptor [relative binding affinity (RBA) = 234] and to inhibit E₂ (1 nM)-stimulated prolactin synthesis (IC₅₀ 7 nM) by pituitary cells in primary culture. Substitution in positions 3 and 4 to form a catechol did not decrease affinity for the estrogen receptor (RBA = 252), and potency as an antiestrogen was maintained in the prolactin assay (IC₅₀ 20 nM) as long as oxidation of the catechol was prevented. All of the hydroxylated derivatives of tamoxifen tested were estrogen antagonists; however, removal of the alkylaminoethoxy side chain from TAM produced a full estrogen agonist with low potency (20 nM). In contrast, removal of the side chain from 4-hydroxytamoxifen (4-OH TAM) produced a partial agonist. A structural analogue of 4-OH TAM, 3-[β-dimethylaminoethoxy]-11-ethyl-12-(4-hydroxyphenyl)5,6-dihydrodibenzo [a,e]-cyclooctene (7c) had a decreased potency (IC₅₀ 16 nM) compared with 4-OH TAM (IC₅₀ 4 nM in the same experiment) as an estrogen antagonist. If the side chain was changed from a dimethylaminoethoxy to glyceryl, antagonist activity was reduced (IC₅₀ 0.8 μM). An allyl side chain produced a compound with no antiestrogenic activity at concentrations up to 1 μM. An adaptation of Belleau's macromolecular perturbation theory is suggested to explain the interaction of agonists, antagonists, and partial agonists at the ligand binding site of the estrogen receptor.