Tamoxifen-based probes for the study of estrogen receptor-mediated transcription

Prodigiosenes conjugated to tamoxifen and estradiol

We report the synthesis of the first click-appended prodigiosene conjugates. Four prodigiosene conjugates of estradiol functionalised at the 7α-position were prepared, as were three prodigiosene conjugates of tamoxifen. The coupling between a prodigiosene and an 11-hydroxy estradiol derivative via an ether linkage was investigated, as was the 11- and 7-functionalisation of the estradiol core. The robustness of estradiol protecting groups was severely challenged by reactions typically used to equip such frameworks for 11- and 7-functionalisation. Specifically, and important to synthesis involving estradiol, TBS, TMS and THP are not useful protecting groups for the functionalisation of this core. When the chemical features of the therapeutic agent limit the choice of protecting group (in this case, prodigiosenes bearing aryl, NH, alkenyl and ester groups), click chemistry becomes an attractive synthetic strategy. The anti-cancer activity of the seven click prodigiosene conjugates was evaluated.

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.

Synthesis and characterization of bioactive tamoxifen-conjugated polymers

Macromolecular conjugates of tamoxifen could perhaps be used to circumvent some of the limitations of the extensively used breast cancer drug. To test the feasibility of these conjugates, a 4-hydroxytamoxifen analogue was conjugated to a diaminoalkyl linker and then conjugated to activated esters of a poly(methacrylic acid) polymer synthesized by atom transfer radical polymerization. A polymer conjugated to the 4-hydroxytamoxifen analogue with a six-carbon linker showed high affinity for both estrogen receptor alpha and estrogen receptor beta and potent antagonism of the estrogen receptor in cell-based transcriptional reporter assays. These results suggest that the conjugation of 4-hydroxytamoxifen to a polymer results in a macromolecular conjugate that can display ligand in a manner that can be recognized by estrogen receptor and still act as a potent antiestrogen in cells.