Nonsteroidal bivalent estrogen ligands: an application of the bivalent concept to the estrogen receptor

The estrogen receptor (ER) is a hormone-regulated transcription factor that binds, as a dimer, to estrogens and to specific DNA sequences. To explore at a fundamental level the geometric and topological features of bivalent-ligand binding to the ER dimer, dimeric ER crystal structures were used to rationally design nonsteroidal bivalent estrogen ligands. Guided by this structure-based ligand design, we prepared two series of bivalent ligands (agonists and antagonists) tethered by flexible spacers of varying lengths (7-47 Å) and evaluated their ER-binding affinities for the two ER subtypes and their biological activities in cell lines. Bivalent ligands based on the agonist diethylstilbestrol (DES) proved to be poor candidates, but bivalent ligands based on the antagonist hydroxytamoxifen (OHT) were well suited for intensive study. Binding affinities of the OHT-based bivalent ligands were related to spacer length in a distinctive fashion, reaching two maximum values at 14 and 29 Å in both ER subtypes. These results demonstrate that the bivalent concept can operate in determining ER-ligand binding affinity and suggest that two distinct modes operate for the binding of bivalent estrogen ligands to the ER dimers, an intermolecular as well as an intramolecular mode. Our insights, particularly the possibility of intramolecular bivalent binding on a single ER monomer, may provide an alternative strategy for preparing more selective and active ER antagonists for endocrine therapy of breast cancer.

Synthesis and characterization of fluorescent 4-hydroxytamoxifen conjugates with unique antiestrogenic properties

Membrane receptors for steroid hormones are currently a subject of considerable debate. One approach to selectively target these putative receptors has been to couple ligands to substances that restrict cell permeability. Using this approach, an analogue of the estrogen receptor ligand 4-hydroxytamoxifen was attached to fluorescent dyes with differing degrees of predicted cell permeability. The conjugates bound to estrogen receptor in vitro, but all three conjugates, including one predicted to be cell-impermeable, inhibited estradiol-induced transcriptional activation. Fluorescence microscopy revealed cytoplasmic localization for all three conjugates. We further characterized a 4-hydroxytamoxifen analogue conjugated to a BODIPY fluorophore in breast cancer cell lines. Those experiments suggested a similar, but not identical, mode of action to 4-hydroxytamoxifen, as the fluorescent conjugate was equally effective at inhibiting proliferation of both tamoxifen-sensitive and tamoxifen-resistant breast cancer cell lines. While these findings point to significant complicating factors in designing steroid hormone mimics targeted to the plasma membrane, the results also reveal a possible new direction for designing estrogen receptor modulators.