Investigations of new lead structures for the design of selective estrogen receptor modulators

Heterocyclic derivatives of (R,S)/(S,R)-1-(2-chloro-4-hydroxyphenyl)-2-(2,6-dichloro-4-hydroxyphenyl)ethylenediamine (L1) were synthesized and tested for estrogen receptor binding. The selection of the heterocycles was based on theoretical consideration. (2R,3S)/(2S,3R)-2-(2-Chloro-4-hydroxyphenyl)-3-(2,6-dichloro-4-hydroxyphenyl)piperazine 2, (4R,5S)/(4S,5R)-4-(2-chloro-4-hydroxyphenyl)-5-(2,6-dichloro-4-hydroxyphenyl)-2-imidazoline 3, and 4-(2-chloro-4-hydroxyphenyl)-5-(2,6-dichloro-4-hydroxyphenyl)imidazole 4 possess a spatial structure with neighboring aromatic rings as is realized in hormonally active [1,2-diphenylethylenediamine]platinum(II) complexes. The 1,2-diphenylethane pharmacophor, however, cannot adapt an antiperiplanar conformation to interact with the estrogen receptor (ER) comparable to synthetic (e.g., diethylstilbestrol (DES)) or steroidal (e.g., estradiol (E2)) estrogens. Due to the different spatial structures, the heterocycles cause only a marginal displacement of E2 from its binding site (relative binding affinity (RBA) < 0.1%). Nevertheless, unequivocally ER mediated gene activation was verified on the MCF-7-2a cell line. Imidazoline 3 as the most active compound reached the maximum effect of E2 (100% activation) in a concentration of 5 x 10(-7) M, while piperazine 2 and imidazole 4 activate luciferase expression only in a small but significant amount of 20% and 27%, respectively. We therefore assigned these heterocyclic compounds to a second class of hormones (type-II-estrogens), which are attached at the ER at different amino acids than DES or E2 (type-I-estrogens).