Synthesis and evaluation of 11β-(4-Substituted phenyl) estradiol analogs: Transition from estrogen receptor agonists to antagonists

Abstract

Introduction As part of our program to develop estrogen receptor (ER) targeted imaging and therapeutic agents we chose to evaluate 11β-substituted estradiol analogs as a representative scaffold. Previous synthetic studies provided an entry into this class of compounds and other work indicated that 11β-(substituted aryl) estradiol analogs were potent antagonists of the ER. Little information existed about the specific structural features involved in the transition from agonism to antagonism for the 11β-aryl estradiol analogs or their potential as scaffolds for drug conjugation. Methods We prepared and characterized a series of 11β-(4-Substituted phenyl) estradiol analogs using modifications of existing synthetic methods. The new compounds, as well as standard steroidal agonists and antagonists, were evaluated as competitive ligands for the ERβ-LBD. Functional assays used the induction of alkaline phosphatase in Ishikawa cells to determine potency of the compounds as ER agonists or antagonists. Results The synthetic strategy successfully generated a series of compounds in which the 4-substituent was sequentially modified from hydroxyl to methoxy to azidoethoxy/N,N-dimethylaminoethoxy and eventually to a prototypical 1,4-naphthoquinone-containing moiety. The new compounds all retained high relative binding affinity (RBA) for the ERα-LBD, ranging from 13–83% that of estradiol. No subtype selectivity was observed. More importantly, the transition from agonist to antagonist activity occurs at the 4-methoxy stage where the compound is a mixed antagonist. More notably, antagonism appeared to be more dependent upon the size of the 11β-substituent than upon the nature of the terminal group Conclusions We have developed a synthetic strategy that provides facile access to potent 11β-(4-substituted phenyl) estradiol analogs. The resultant compounds retain high affinity for the ERα-LBD and, more importantly, demonstrate potent antagonist activity in cells. Large functionalities distal to the 11β-phenyl ring had little additional effect on either affinity or efficacy, suggesting the incorporation of diverse imaging or biologically active groups can be attached without significantly compromising the ER-binding capacity. Future studies are in progress to exploit the 11β-aryl estradiol analogs as potential drug delivery systems and imaging agents.

Synthesis and preliminary evaluation steroidal antiestrogen–geldanamycin conjugates

“…9,10 Deltenone 3-ethylene ketal 1 was converted initially to the 11β-(4-hydroxyphenyl) estra-4,9-diene-3,17-dione 2 . This compound then served as the intermediate for the preparation of the requisite 11β-(4-azidoethoxyphenyl)estradiol 4a and 11β-(4-N-propargyl-N-methylaminoethoxyphenyl) estradiol components 4b .…”

“…Having demonstrated that additional substituents distal to the nitrogen in the side chain did not adversely affect either binding or efficacy, we felt that the steroidal components were appropriate substrates for subsequent ligation reactions. 10 …”

“…The ER-targeting component was developed in our initial work with the 11β-(4-substituted-oxyphenyl) estradiols. 9,10 Based on the affinity of the steroids for the ER and their antiestrogenic activity, we prepared a steroidal antiestrogen-mitomycin C conjugate to test our concept. 11 Although the compound retained high ER affinity and antiestrogenic properties, it was no more active than mitomycin C and displayed no selectivity toward ER-expressing breast cancer cells.…”

Selective Human Estrogen Receptor Partial Agonists (ShERPAs) for Tamoxifen-Resistant Breast Cancer

“…A similar shift at helix 11 by placing a 4-methoxy phenyl group at the 11β position of E 2 was also proposed as a potential mechanism for inducing ligand transition from an agonist to a partial agonist. 53 In the case of ERα/BPAF crystal structures, two conformations were revealed in the same crystal structure, with one monomer of the homodimer occupying an “antagonist” conformation and the other an “agonist” conformation. This emphasizes that ligand:ER complexes can adopt multiple stable binding modes and that these might correspond to agonist, antagonist or partial agonist conformations (Fig.…”

Convergent synthesis of a steroidal antiestrogen-mitomycin C hybrid using “click” chemistry

“…42,43 Of equal importance for drug delivery, the 11β-(4-substituted-oxyphenyl) estradiols express high ER affinity and are potent antiestrogens. 44 As such, they would not elicit a proliferative effect in breast cancer cells.…”