Synthesis and Structure–Activity Relationships of Ferrocenyl Tamoxifen Derivatives with Modified Side Chains

We report here the synthesis and cell‐proliferation properties of derivatives of the breast cancer drug tamoxifen, in which the O(CH2)2N(CH3)2 side chain, responsible for the drug's antiestrogenic properties, has been modified by a ferrocenyl moiety. We recently reported the diphenol compound 5, in which this amino chain had been replaced with an acyl‐ferrocenyl (O(CH2)2C(O)[(η5‐C5H4)FeCp]) group, and which showed antiproliferative effects against both the hormone‐dependent MCF‐7 and ‐independent MDA‐MB‐231 breast cancer cell lines. We now report the results of a structure–activity relationship (SAR) study, in which the lateral chain length has been varied, the ketone group has been omitted, and the number of phenol groups has been varied. Compounds 1–4, with a side chain lacking the carbonyl function (O(CH2)n[(η5‐C5H4)FeCp], n=1–4) and which show a decreasing affinity for ERα (ER=estrogen receptor) with increasing chain length, act as estrogens on MCF‐7 cells, and mild cytotoxics on PC‐3 prostate cancer cells, with IC50 values around 10 μM. The two monophenolic derivatives of 2, 2 a and 2 b, which show a reduced affinity for ERα compared to 2, are also estrogenic, but are only slightly cytotoxic. Finally, we have reexamined compound 5 and discovered that its antiproliferative effect against the MCF‐7 cell line does not arise from antiestrogenicity as we had originally suspected, but by means of a cytotoxic pathway. This compound is also sensitive to the number of phenol groups as cell death is diminished when one of the hydroxyl groups is omitted (5 a and 5 b). Molecular modeling studies of the ligand–ERα binding stability are broadly consistent with the experimental binding affinity results for compounds 2, 2 a, 2 b, 5, 5 a, and 5 b. Electrochemical experiments show that 1–4, 2 a, and 2 b are stable to oxidation on the electrochemical timescale, unlike 5, 5 a, and 5 b, and that cytotoxicity is related to less positive phenol oxidation potentials. The SAR study shows that the presence of a ketone group and two phenol groups is necessary for strong receptor binding and cytotoxic effects, and that all compounds are estrogenic, despite the presence of a bulky side chain. Go bio! We report here the synthesis and cell‐proliferation studies of the first derivatives of the breast cancer drug tamoxifen in which an organometallic moiety replaces the antiestrogenic side chain (see graphic). Structure–activity relationship studies show that the compound possessin