Comparative analyses of mechanistic differences among antiestrogens

Antiestrogens such as tamoxifen are one of the most effective methods of tr eating estrogen receptor (ER alpha) positive breast cancers; however, the e ffectiveness of this therapy is limited by the almost universal development of resistance to the drug. If antiestrogens are recognized differently by the cell as it has been suggested, then in disease conditions where tamoxif en fails to function effectively, a mechanistically different antiestrogen might yield successful results. Although many antiestrogens have been devel oped, a direct comparison of their mechanisms of action is lacking, thus li miting their utility. Therefore, to determine if there are mechanistic diff erences among available antiestrogens, we have carried out a comprehensive analysis of the molecular mechanisms of action of 4-hydroxy-tamoxifen (4OHT ), idoxifene, raloxifene, GW7604, and ICI 182,780. Using a novel set of pep tides that recognize different surfaces on ER alpha, we have found that fol lowing binding to ER alpha, each ligand induces a distinct ER alpha-ligand conformation. Furthermore, transcriptional assays indicate that each ER alp ha-ligand complex is recognized distinctly by the transcription machinery, and consequently, antiestrogens vary in their ability to inhibit estradiol- and 4OHT-mediated activities. Relative binding assays have shown that the affinity of these ligands for ER alpha is not always representative of thei r inhibitory activity. Using this assay, we have also shown that the pharma cology of each antiestrogen is influenced differently by hormone binding pr oteins. Furthermore, GW7604, like ICI 182, 780, but unlike the other anties trogens evaluated, decreases the stability of the receptor. Overall, our re sults indicate that there are clear mechanistic distinctions among each of the antiestrogens studied. However, GW7604 and ICI 182,780 differ more sign ificantly from tamoxifen than idoxifene and raloxifene. These data, which r eveal differences among antiestrogens, should assist in the selection of co mpounds for the clinical regulation of ER alpha function.