Novel Selective Estrogen Receptor Ligand Conjugates Incorporating Endoxifen-Combretastatin and Cyclofenil-Combretastatin Hybrid Scaffolds: Synthesis and Biochemical Evaluation.

Nuclear receptors such as the estrogen receptors (ERα and ERß) modulate the effects of the estrogen hormones and are important targets for design of innovative chemotherapeutic agents for diseases such as breast cancer and osteoporosis. Conjugate and bifunctional compounds which incorporate an ER ligand offer a useful method of delivering cytotoxic drugs to tissue sites such as breast cancers which express ERs. A series of novel conjugate molecules incorporating both the ER ligands endoxifen and cyclofenil-endoxifen hybrids covalently linked to the antimitotic and tubulin targeting agent combretastatin A-4 were synthesised and evaluated as ER ligands. A number of these compounds demonstrated pro-apoptotic effects, with potent antiproliferative activity in ER-positive MCF-7 breast cancer cell lines and low cytotoxicity. These conjugates displayed binding affinity towards ERα and ERß isoforms at nanomolar concentrations e.g., the cyclofenil-amide compound 13e is a promising lead compound of a clinically relevant ER conjugate with IC50 in MCF-7 cells of 187 nM, and binding affinity to ERα (IC50 = 19 nM) and ERß (IC50 = 229 nM) while the endoxifen conjugate 16b demonstrates antiproliferative activity in MCF-7 cells (IC50 = 5.7 nM) and binding affinity to ERα (IC50 = 15 nM) and ERß (IC50 = 115 nM). The ER binding effects are rationalised in a molecular modelling study in which the disruption of the ER helix-12 in the presence of compounds 11e, 13e and 16b is presented These conjugate compounds have potential application for further development as antineoplastic agents in the treatment of ER positive breast cancers.

Design, Synthesis and Biochemical Evaluation of Novel Selective Estrogen Receptor Ligand Conjugates Incorporating an Endoxifen-Combretastatin Hybrid Scaffold.

Nuclear-receptors are often overexpressed in tumours and can thereby be used as targets when designing novel selective chemotherapeutic agents. To date, many conjugates incorporating an estrogen receptor (ER) ligand have been synthesised in order to direct chemical agents to tissue sites containing ERs. A series of ER ligand conjugates were synthesised incorporating an antagonistic ER ligand scaffold based on endoxifen, covalently-bound via an amide linkage to a variety of combretastatin-based analogues, which may act as antimitotic agents. These novel endoxifen-combretastatin hybrid scaffold analogues were biochemically evaluated in order to determine their antiproliferative and cytotoxicity effects in both the ER-positive MCF-7 and the ER-negative MDA-MB-231 human breast cancer cell lines. ER competitive binding assays were carried out to assess the binding affinity of the lead conjugate 28 towards both the ERα and ERß isoforms. In results from the NCI 60-cell line screen, the lead conjugate 28 displayed potent and highly selective antiproliferative activity towards the MCF-7 human cancer cell line (IC50 = 5 nM). In the ER-binding assays, the lead conjugate 28 demonstrated potent ER competitive binding in ERα (IC50 value: 0.9 nM) and ERß (IC50 value: 4.7 nM). Preliminary biochemical results also demonstrate that the lead conjugate 28 may exhibit pure antagonism. This series makes an important addition to the class of ER antagonists and may have potential applications in anticancer therapy.