Acyclic amides as estrogen receptor ligands: synthesis, binding, activity and receptor interaction

We have prepared a series of bisphenolic amides that mimic bibenzyl and homobibenzyl motifs commonly found as substructures in ligands for the estrogen receptor (ER). Representative members were prepared from three classes: N-phenyl benzamides, N-phenyl acetamides, and N-benzyl benzamides; in some cases the corresponding thiocarboxamides and sulfonamides were also prepared. Of these three classes, the N-phenyl benzamides had the highest affinity for ER, the N-phenyl acetamides had lower, and the N-benzyl benzamides were prone to fragmentation via a quinone methide intermediate. In the N-phenyl benzamide series, the highest affinity analogues had bulky N-substituents; a CF3 group, in particular, conferred high affinity. The thiocarboxamides bound better than the corresponding carboxamides and these bound better than the corresponding sulfonamides. Binding affinity comparisons suggest that the p-hydroxy group on the benzoate ring, which contributes most to the binding, is playing the role of the phenolic hydroxyl of estradiol. Computational studies and NMR and X-ray crystallographic analysis indicate that the two anilide systems studied have a strong preference for the s-cis or exo amide conformation, which places the two aromatic rings in a syn orientation. We used this structural template, together with the X-ray structure of the ER ligand binding domain, to elaborate an additional hydrogen bonding site on a benzamide system that elevated receptor binding further. When assayed on the individual ER subtypes, ERalpha and ERbeta, these compounds show modest binding affinity preference for ERalpha. In a reporter gene transfection assay of transcriptional activity, the amides generally have full to nearly full agonist character on ERalpha, but have moderate to full antagonist character on ERbeta. One high affinity carboxamide is 500-fold more potent as an agonist on ERalpha than on ERbeta. This work illustrates that ER ligands having simple amide core structures can be readily prepared, but that high affinity binding requires an appropriate distribution of bulk, polarity, and functionality. The strong conformational preference of the core anilide function in all of these ligands defines a rather rigid geometry for further structural and functional expansion of these series.