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

Abstract

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 ERβ 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 ERβ (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.

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

“…Additionally, the secondary amine group present on the basic side chain can undergo amide type coupling reactions to synthesise the prototype conjugated compounds. The OTBDMS protected endoxifen ligand 2a was prepared in a multistep route as we previously reported via the McMurry reaction which is a low valent titanium mediated crossed coupling of substituted benzophenones and ketones [ 31 ].…”

“…The use of the endoxifen scaffold in the novel conjugate design should allow the selectivity to deliver a cytotoxic agent directly to the tumour site. Previous studies by Burke [ 30 ] and Keely [ 31 ] have demonstrated the use of the antiestrogen endoxifen for the conjugation of estradiol and cytotoxic drugs such as doxorubicin. The high affinity nonsteroidal ER ligand cyclofenil diphenol (F6060) [ 32 , 33 ] is also selected in the present work as an ER targeting core ligand for development of a novel conjugate design.…”

“…ER ligand conjugates of cytotoxic agents, photodynamic therapeutic agents and radioligands which deliver cytotoxic agents have been reported [ 31 , 37 ]. We have previously reported stable conjugates of endoxifen (a tamoxifen metabolite and ER antagonist) with DNA alkylating agents, aromatase inhibitors, COX2 inhibitors and antitubulin compounds which demonstrate antiproliferative and ER binding effects [ 38 ].…”

Drug conjugates—an emerging approach to treat breast cancer

“…Additionally, this conjugate displayed high affinity toward the estrogen receptor α and was superior to cisplatin in inducing tumor regression in the MCF‐7 human breast cancer tumors in a mouse model. Recently, conjugates of selective estrogen receptor modulators (ie, endoxifen or cyclofenil) linked to combretastatin, an antimitotic agent that binds to the beta subunit of tubulin, demonstrated potent (nmol/L) antiproliferative activity against MCF‐7 cells (Table ) …”

“…Recently, conjugates of selective estrogen receptor modulators (ie, endoxifen or cyclofenil) linked to combretastatin, an antimitotic agent that binds to the beta subunit of tubulin, demonstrated potent (nmol/L) antiproliferative activity against MCF-7 cells (Table 4). 56,58 An estrogen receptor antagonist, oxabicycloheptene sulfonate, linked to the histone deacetylase (HDAC) inhibitor vorinostat (suberanilohydroxamic acid) showed higher antitumor potency in MCF-7 cells compared to the oxabicycloheptene sulfonate alone, indicating that parallel inhibition of HDAC and the estrogen receptor is beneficial for anticancer action. 107 DNA-alkylating agents linked to estrogen receptor ligands is yet another approach to developing novel hybrid drugs targeting specific phenotypes of breast cancer.…”