Michel Huché

A series of unconjugated ferrocenyl phenols: prospects as anticancer agents.

We recently reported that a ferrocenyl diphenol butene derivative showed a very strong cytotoxic effect on both hormone‐dependent and ‐independent breast cancer cell lines. In order to obtain more information about the structure–activity relationship in the cytotoxicity of small ferrocene compounds, we have prepared a series of simple unconjugated ferrocenyl diphenol complexes (ortho,para; meta,para; para,para). These compounds retain a reasonable to good affinity for both estrogen receptor types, with higher values for the β form, and superior binding for the para,para diphenol complex (RBA=28 %). In vitro these complexes exhibit significant cytotoxic effects on hormone‐independent prostate (PC3) and breast cancer cell lines (MDA‐MB231), with IC50 values between 2.5 and 4.1 μM. This effect is more marked with PC3, the ortho,para diphenol complex proving the most effective. On the hormone‐dependent MCF7 breast cancer cell line, the observed effect seems to be the result of two components, one cytotoxic (antiproliferative), the other estrogenic (proliferative). Electrochemical studies show that the cytotoxic effect of the complexes correlates with the ease of oxidation of the ferrocene group. All these complexes are much less cytotoxic than the ferrocenyl diphenol butene derivative.

Synthesis, Cytotoxicity, and COMPARE Analysis of Ferrocene and [3]Ferrocenophane Tetrasubstituted Olefin Derivatives against Human Cancer Cells

Herein we report the antiproliferative effects of a series of 28 compounds against the MDA‐MB‐231 breast cancer cell line, including the synthesis of seven new [3]ferrocenophanyl and four new ferrocenyl compounds. For each p‐R‐phenyl substitution pattern investigated, the [3]ferrocenophanyl derivatives were more cytotoxic than the corresponding ferrocenyl derivative, with the highest activity found for compounds with protic substituents. Theoretical calculations of the HOMO–LUMO gap for the molecules in the Fe3+ oxidation state suggest a higher reactivity for the [3]ferrocenophanyl derivatives. A lead compound from each series, a [3]ferrocenophanyl and a ferrocenyl compound, possessing two phenol groups, were screened against the NCI/DTP 60‐cell‐line panel. The mean activity over all cell lines was better than cisplatin for both compounds, and both compounds showed subpanel selectivity for leukemia, CNS cancer, and renal cancer. Low systemic toxicity and lack of interaction with DNA (when in the reduced form), suggest that the compounds may act as prodrugs.

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(CH(2))(2)N(CH(3))(2) side chain, responsible for the drugs 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(CH(2))(2)C(O)[(eta(5)-C(5)H(4))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(CH(2))(n)[(eta(5)-C(5)H(4))FeCp], n = 1-4) and which show a decreasing affinity for ERalpha (ER = estrogen receptor) with increasing chain length, act as estrogens on MCF-7 cells, and mild cytotoxics on PC-3 prostate cancer cells, with IC(50) values around 10 microM. The two monophenolic derivatives of 2, 2 a and 2 b, which show a reduced affinity for ERalpha 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-ERalpha 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.

Selective estrogen-receptor modulators (SERMs) in the cyclopentadienylrhenium tricarbonyl series: Synthesis and biological behaviour

A series of organometallic antiestrogens based on the OH‐tamoxifen (OH‐Tam) skeleton and bearing the (η5‐C5H4)ReI(CO)3 unit has been prepared by using McMurry coupling for the purpose of studying their biological behaviour. The cyclopentadienylrhenium tricarbonyl moiety is indeed stable in biological media, compact, lipophilic and easy to handle. Furthermore, this study allowed us to select the best candidates for subsequent use as radiopharmaceuticals either for imaging or therapy by using appropriate radionucleides, namely 99mTc and 188Re. In these molecules the β‐phenyl group of OH‐Tam has been replaced by the (η5‐C5H4)Re(CO)3 moiety, and the length of the dimethylamino side chain O(CH2)nN(CH3)2 was varied (n=2, 3, 4, 5 and 8). The compounds 7 a–7 e were obtained as mixtures of their Z and E isomers, which could be separated by semipreparative HPLC. Unlike their ferrocene homologues, the compounds do not isomerise in solution. Structural identification was carried out with NMR spectroscopy by using the HMBC and NOE techniques and was confirmed by the X‐ray structural determination of (E)‐7 a (n=2). These molecules were more lipophilic than OH‐Tam (log Po/w=4.5–6.3) and they were all reasonably well recognized by the two forms of the estrogen receptor (ERα and ERβ). For example, (Z)‐7 b (n=3) has high relative binding affinity (RBA) values of 31 % for ERα and 16.8 % for ERβ. The antiproliferative effects of two pairs of isomers, (Z)‐ and (E)‐7 b (n=3) and (Z)‐ and (E)‐7 d (n=5), were studied at a molarity of 1 μM on two breast‐cancer cell lines, MCF7 (ERα positive) and MDA‐MB231 (ERα negative). These molecules had an antiproliferative effect on MCF7 cells slightly higher than that of OH‐Tam and no effect on MDA‐MB231 cells. Thus, the antiproliferative effect observed on the MCF7 cells seemed essentially to be linked to an antiestrogenic effect. Molecular modelling studies have allowed us to rationalise these effects and select the best compounds for future development of a radioactive series.

Novel Estradiol Derivatives Labeled with Ru, W, and Co Complexes. Influence on Hormone‐Receptor Affinity of Several Organometallic Groups at the 17α Position

In order to elucidate the extent to which recognition of the estrogen receptor is influenced by addition of an organometallic substituent at the 17 alpha position, modification of 17 beta-estradiol at this position was carried out by using the organometallic groups -C identical to C(eta 5-C5H4)RuCp, CH2-(eta 5-C5H4)RuCp, -C identical to C-(eta 5-C5H4)-W(CO)3(Me), -(C identical to CCHO)Co2(CO)6, and -(C identical to CCH2OH)Co2(CO)6. The relative binding affinity (RBA) values for estradiol receptor alpha showed that recognition was good (RBA between 20 and 13.5%) when the organometallic moiety was attached at the end of a rigid alkyne spacer. However, the affinity of the modified hormone for the receptor was severely reduced (RBA = 1%) for a substituent such as -CH2-(eta 5-C5H4)RuCP, in which the spacer is reduced to a single flexible sp3 carbon atom, allowing the organometallic moiety greater freedom of movement around the attachment point. The RBA values found were in agreement with results obtained from a molecular-modeling study in which 5, an organometallic hormone with a rigid spacer, or 7, a molecule with a flexible spacer, was inserted into the cavity of the recently characterized Ligand-Binding Domain of estrogen receptor alpha.

The Presence of a Ferrocenyl Unit on an Estrogenic Molecule is Not Always Sufficient to Generate in vitro Cytotoxicity

We recently reported the dual (antihormonal and cytotoxic) functionality of ferrocifens, which are organometallic complexes derived from hydroxytamoxifen, the standard molecule in the treatment of hormone‐dependent breast cancers. To test the hypothesis that the presence of a ferrocenyl substituent on molecules with an affinity for the estrogen receptor is sufficient to give them cytotoxic properties in vitro, we prepared complexes derived from estradiol with a ferrocenyl substituent at positions 7α and 17α. The complexes thus obtained retain a satisfactory level of affinity for the estrogen receptor (RBA values higher than 12 %). At low concentrations (0.1–1 μM) the complexes show an estrogenic effect in vitro equivalent to that of estradiol on hormone‐dependent (MCF‐7) breast cancer cells, and no cytotoxic effect on hormone‐independent (MDA‐MB‐231) breast cancer cells. At high concentrations (up to 50 μM) the 17α‐ethynylferrocenyl estradiol and 7α‐ferrocenylmethylthio estradiol become cytotoxic (IC50=13.2 μM and 18.8 μm, respectively) while the 17α‐ferrocenylestradiol remains non toxic. The low toxicity of these compounds support our hypothesis that electronic communication between the ferrocenyl and phenol moieties in the hydroxyferrocifens series is a key parameter in the generation of cytotoxic effects at submicromolar concentrations.

Synthesis, receptor binding, molecular modeling, and proliferative assays of a series of 17α-arylestradiols

A series of new derivatives of estradiol substituted at position 17α by various aryls has been synthesized. This was made possible by efficient activation methods for the addition of aryllithiums to the carbonyl group at position 17 of estrone by using tetramethylethylenediamine (TMEDA) or BF3⋅OEt2. Their relative binding affinity (RBA) for the α and the β forms of the estrogen receptor (ER) have been measured. All except one of the compounds synthesized had an RBA value of around 10 % which indicates a level of tolerance towards the bulky substituent at position 17. The lipophilicity values measured for these compounds are higher than that found for estradiol (E2). A study of their proliferative/antiproliferative effects was carried out on hormone‐dependent (MCF7) and hormone‐independent (MDA‐MB231) breast cancer cell lines. It is interesting to note that all the compounds are estrogenic. The possibility of easily attaching an iodine at the end of a phenyl spacer opens up a route to new radiopharmaceuticals for use in radioimaging.