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Abstract
Estrogen Receptors (ERs) regulate gene transcription by at least two distinct nuclear pathways: (i) directly binding of liganded ERs to estrogen response elements (EREs), the “ER/ERE pathway,” and (ii) indirect tethering of liganded ERs to other DNA transcription factors, such as AP1 (Activator Protein 1), the “ER/AP1 pathway.” Two clinically important ER targeting drugs Raloxifene and Tamoxifen function as antagonists of genes bearing EREs but as agonists/partial agonists on AP1 response elements. Using a ligand receptor design strategy that was used previously to generate the orthogonal ligand-receptor pair ES8/ERβ(E305A), a Raloxifene based analog DRL527 was developed. DRL527 has limited selectivity as an antagonist but has an unprecendented ERE/AP1 response profile.
Where as traditional ER antagonists, Raloxifene and Tamoxifen target the ER ligand –binding pocket, a few groups have tried to develop antagonists that target the coactivator binding pocket which recognizes the signature leucine-rich, alphahelical motif “LXXLL” found on most of the coactivators. As this motif is targeted by most nuclear receptors, it is uncertain if such a strategy would provide adequate selectivity. Computer-aided molecular design was used to develop a series of small molecules called coactivator-binding inhibitors (or CBIs) that target the coactivator binding groove and are tethered to a Raloxifene scaffold, which binds with very high affinity to the hormone binding site of ER. It was envisioned that a “chelate effect” would provide a greater binding affinity and Raloxifene would provide the specificity for the CBIs. A series of Raloxifene tethered CBIs were synthesized and evaluated. The nanomolar potent inhibitors did not show an increase in potency relative to Raloxifene suggesting that while it is quite possible to use the ligand-binding pocket to increase the selectivity of the CBIs, there is an inherent caveat in the approach as the analogs potentially could compete with the intramolecular binding of helix-12 itself.
