Atypical Lone Pair-π Interaction with Quinone Methides in a Series of Imido-Ferrociphenol Anticancer Drug Candidates.

Ferrociphenols, especially those possessing a heterocycle at the terminus of an aliphatic chain, display strong anticancer activity through a novel redox mechanism that generates active metabolites such as quinone methides (QMs). X-ray crystallography and UV/Vis spectroscopy reveal that the specific lone pair (lp)-π interaction between a carbonyl group of the imide and the quinone motif of the QM plays an important role in the exceptional cytotoxic behaviour of their imido-ferrociphenol precursors. This intramolecular lp-π interaction markedly enhanced the stability of the QMs and lowered the pKa values of the corresponding phenol/phenolate couples. As the first example of such a non-covalent interaction that stabilizes QMs remotely, it not only expands the scope of the lp-π interaction in supramolecular chemistry, but also represents a new mode of stabilization of a QM. This unprecedented application of lp-π interactions in imido-ferrociphenol anticancer drug candidates may also have great potential in drug discovery and organocatalyst design.

Intracellular Localization of an Osmocenyl-Tamoxifen Derivative in Breast Cancer Cells Revealed by Synchrotron Radiation X-ray Fluorescence Nanoimaging.

A series of tamoxifen-like metallocifens of the group-8 metals (Fe, Ru, and Os) has strong antiproliferative activity on the triple-negative breast cancer cells (MDA-MB-231). To shed light on the mechanism of action of these molecules, synchrotron radiation X-ray fluorescence nanoimaging studies were performed on cells exposed to osmocenyl-tamoxifen (Oc-OH-Tam) to disclose its intracellular distribution. High-resolution mapping of the lipophilic Oc-OH-Tam in cells revealed its preferential accumulation in the endomembrane system. This is consistent with the ability of the amino nitrogen chain of the compounds to be protonated at physiological pH and responsible for electrostatic interactions between Oc-OH-Tam and membranes. A comprehensive scenario is proposed that provides new insight into the cellular behavior and activation of Oc-OH-Tam and advances the understanding of its mechanism of action.

Ferrocifen type anti cancer drugs.

Despite current developments in therapeutics focusing on biotechnologically-oriented species, the unflagging utility of small molecules or peptides in medicine is still producing strong results. In 2014 for example, of the 41 new medicines authorized for sale, 33 belonged to the category of small molecules, while in 2013 they represented 24 of 27, according to the FDA. This can be explained as the result of recent forays into new or long-neglected areas of chemistry. Medicinal organometallic chemistry can provide us with an antimalarial against resistant parasitic strains, as attested by the phase II clinical development of ferroquine, with a new framework for conceptual advances based on three-dimensional space-filling, and with redox or indeed catalytic intracellular properties. In this context, bioferrocene species with antiproliferative potential have for several years been the subject of sustained effort, based on some initial successes and on the nature of ferrocene as a stable aromatic, with low toxicity, low cost, and possessing reversible redox properties. We show here the different antitumoral approaches offered by ferrocifen derivatives, originally simple derivatives of tamoxifen, which over the course of their development have proved to possess remarkable structural and mechanistic diversity. These entities act via various targets, some of which have been identified, that are triggered according to the concentration of the products. They also act according to the nature of the cancer cells and their functionality, by mechanistic pathways that can operate either synergistically or not, in successive, concomitant or sequential ways, depending for example on newly identified signaling pathways inducing senescence or apoptosis. Here we present a first attempt to rationalize the behavior of these entities with various anticancer targets.

Ferrocenyl Quinone Methide-Thiol Adducts as New Antiproliferative Agents: Synthesis, Metabolic Formation from Ferrociphenols, and Oxidative Transformation.

Ferrociphenols (FCs) and their oxidized, electrophilic quinone methide metabolites (FC-QMs) are organometallic compounds related to tamoxifen that exhibit strong antiproliferative properties. To evaluate the reactivity of FC-QMs toward cellular nucleophiles, we studied their reaction with selected thiols. A series of new compounds resulting from the addition of these nucleophiles, the FC-SR adducts, were thus synthesized and completely characterized. Such conjugates are formed upon metabolism of FCs by liver microsomes in the presence of NADPH and thiols. Some of the FC-SR adducts exhibit antiproliferative properties comparable to those of their FC precursors. Under oxidizing conditions they either revert to their FC-QM precursors or transform into new quinone methides (QMs) containing the SR moiety, FC-SR-QM. These results provide interesting data about the reactivity and mechanism of antiproliferative effects of FCs, and also open the way to a new series of organometallic antitumor compounds.

Antiplasmodial activity of iron(II) and ruthenium(II) organometallic complexes against Plasmodium falciparum blood parasites.

This work reports the in vitro activity against Plasmodium falciparum blood forms (W2 clone, chloroquine-resistant) of tamoxifen-based compounds and their ferrocenyl (ferrocifens) and ruthenocenyl (ruthenocifens) derivatives, as well as their cytotoxicity against HepG2 human hepatoma cells. Surprisingly with these series, results indicate that the biological activity of ruthenocifens is better than that of ferrocifens and other tamoxifen-like compounds. The synthesis of a new metal-based compound is also described. It was shown, for the first time, that ruthenocifens are good antiplasmodial prototypes. Further studies will be conducted aiming at a better understanding of their mechanism of action and at obtaining new compounds with better therapeutic profile.

Organometallic Antitumor Compounds: Ferrocifens as Precursors to Quinone Methides.

The synthesis and chemical oxidation profile of a new generation of ferrocifen derivatives with strong antiproliferative behavior in vitro is reported. In particular, the hydroxypropyl derivative HO(CH2 )3 C(Fc)=C(C6 H4 OH)2 (3 b) exhibited exceptional antiproliferative activity against the cancer cell lines HepG2 and MDA-MB-231 TNBC, with IC50 values of 0.07 and 0.11 µM, respectively. Chemical oxidation of 3 b yielded an unprecedented tetrahydrofuran-substituted quinone methide (QM) via internal cyclization of the hydroxyalkyl chain, whereas the corresponding alkyl analogue CH3 CH2 -C(Fc)=C(C6 H4 OH)2 merely formed a vinyl QM. The ferrocenyl group in 3 b plays a key role, not only as an intramolecular reversible redox "antenna", but also as a stabilized carbenium ion "modulator". The presence of the oxygen heterocycle in 3 b-QM enhances its stability and leads to a unique chemical oxidation profile, thus revealing crucial clues for deciphering its mechanism of action in vivo.

Atypical McMurry cross-coupling reactions leading to a new series of potent antiproliferative compounds bearing the key [ferrocenyl-ene-phenol] motif.

In the course of the preparation of a series of ferrocenyl derivatives of diethylstilbestrol (DES), in which one of the 4-hydroxyphenyl moieties was replaced by a ferrocenyl group, the McMurry reaction of chloropropionylferrocene with a number of mono-aryl ketones unexpectedly yielded the hydroxylated ferrocenyl DES derivatives, 5a-c, in poor yields (10%-16%). These compounds showed high activity on the hormone-independent breast cancer cell line MDA-MB-231 with IC50 values ranging from 0.14 to 0.36 µM. Surprisingly, non-hydroxylated ferrocenyl DES, 4, showed only an IC50 value of 1.14 µM, illustrating the importance of the hydroxyethyl function in this promising new series. For comparison, McMurry reactions of the shorter chain analogue chloroacetylferrocene were carried out to see the difference in behaviour with mono-aryl ketones versus a diaryl ketone. The effect of changing the length of the alkyl chain adjacent to the phenolic substituent of the hydroxylated ferrocenyl DES was studied, a mechanistic rationale to account for the unexpected products is proposed, and the antiproliferative activities of all of these compounds on MDA-MB-231 cells lines were measured and compared. X-ray crystal structures of cross-coupled products and of pinacol-pinacolone rearrangements are reported.

Deciphering the activation sequence of ferrociphenol anticancer drug candidates.

The complete oxidation sequence of a model for ferrociphenols, a new class of anticancer drug candidate, is reported. Cyclic voltammetry was used to monitor the formation of oxidation intermediates on different timescales, thereby allowing the electrochemical characterization of both the short-lived and stable species obtained from the successive electron-transfer and deprotonation steps. The electrochemical preparation of the ferrocenium intermediate enabled a stepwise voltammetric determination of the stable oxidation compounds obtained upon addition of a base as well as the electron stoichiometry observed for the overall oxidation process. A mechanism has been established from the electrochemical data, which involves a base-promoted intramolecular electron transfer between the phenol and the ferrocenium cation. The resulting species is further oxidized then deprotonated to yield a stable quinone methide. To further characterize the transient species successively formed during the two-electron oxidation of the ferrociphenol to its quinone methide, EPR was used to monitor the fate of the paramagnetic species generated upon addition of imidazole to the electrogenerated ferrocenium. The study revealed the passage from an iron-centered to a carbon-centered radical, which is then oxidized to yield the quinone methide, namely, the species that interacts with proteins and so forth under biological conditions.

Synthetic and mechanistic pathways of cis and trans-hydroxytamoxifen drug derivatives reacting with Cp*Rh complexes that involve η1-N, η2-N,O, η1-O, and η6 bonding modes, via a novel N-π rearrangement; relative binding affinities and computer docking studies of cis and trans-η6-Cp*Rh-hydroxytamoxifen complexes at the estrogen, ERα and ERß receptors, and growth inhibition to breast cancer cells.

The reactions of the breast cancer drug metabolite derivatives of tamoxifen, cis and trans-hydroxytamoxifen, cis-1 and trans-2, with [Cp*Rh(L)(3)](2+) complexes (L = H(2)O or MeOH), in CH(2)Cl(2) and CH(3)OH solvents, initially provided the kinetic η(1)-N complexes, cis-4 (OTf(-), CH(3)OH) and trans-5 (OTf(-), CH(3)OH), which underwent a novel, regioselective, intramolecular N-π rearrangement to give the cis and trans-η(6)-phenol substituted complexes, cis-6 and trans-7, via η(2)-N,O, η(1)-O, and ether aromatic ring η(6) intermediates. Recent density functional theory (DFT) calculations showed a preferred ground state for η(1)-N; η(2)-N,O; η(1)-O; and the η(6) complexes, including the prominent roles of the triflate anion (OTf(-)), and solvent molecules (CH(2)Cl(2) and CH(3)OH), and provided further steric, electronic, and thermodynamic data on the mechanism of the N-π rearrangement. The η(6) complex, cis-6, was shown to be an antagonist for ERα estrogen receptor binding, in a competition experiment with the female hormone, estradiol; therefore, computer docking studies of this biologically active complex at the estrogen receptors, ERα and ERß, also provided information on the binding modes and thermodynamic parameters, while bioassay results provided growth inhibition data on both hormone dependent and independent breast cancer cell lines.

Antimicrobial, Antitumor and Side Effects Assessment of a Newly Synthesized Tamoxifen Analog.

BACKGROUND: Tamoxifen citrate is a very prevalent drug marketed under several trade names like Apo-Tamox, Nolvadex, Tamec, Tamizam, and Tamoplex. This molecule is approved by the FDA for breast cancer treatment. Some studies have shown that tamoxifen has anti-tuberculosis and antiparasitic activities. Like any drug, tamoxifen possesses side effects, more or less dangerous. AIMS: Basically, this work is a comparative study that aims to: primarily compare the antimicrobial and antitumor activities of tamoxifen and a newly synthesized tamoxifen analog; and to determine the molecule with lesser side effects. METHODS: Three groups of mice were injected with tamoxifen citrate and compound 2(1,1-bis[4-(3- dimethylaminopropoxy)phenyl]-2-phenyl-but-1-ene dihydrochloride) at doses corresponding to C1 (1/10), C2 (1/50), and C3 (1/100) to compound 2 lethal dose (LD50 = 75 mg/kg) administered to adult mice. A group of noninjected mice served as a study control. RESULTS: Experimental results suggest that compound 2 has better antitumor and antimicrobial activity than tamoxifen citrate besides its lower toxicity effects. CONCLUSION: The results obtained from the present study confirmed the antitumor and antimicrobial effect of tamoxifen citrate and its hematological side effects. Compound 2 seems to be more effective than tamoxifen citrate for antitumor and antimicrobial treatment while having less hematological side effects and less disruption of the blood biochemical parameters. These findings encourage us to perform further studies on compound 2 and test it for other therapeutic uses for which tamoxifen was found effective.

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 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(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.

Small Structural Differences between Two Ferrocenyl Diphenols Determine Large Discrepancies of Reactivity and Biological Effects.

The ferrocenyl diphenol complexes 1,1-bis(4'-hydroxyphenyl)-2-ferrocenyl-but-1-ene (1) and 1,2-bis(4'-hydroxyphenyl)-1-ferrocenyl-but-1-ene [(Z)-2], which differ by the relative position of the two phenolic substituents, display dramatically different antiproliferative activities on cancer cells (1 is far more cytotoxic than 2). In this study, our goal was to discover the origin of this difference by comparing their reactivity and biological behaviour. In terms of common behaviour, we found that 1 and 2 are both efficient inhibitors of thioredoxin reductase (TrxR) in vitro after oxidation by a horseradish peroxidase/H2 O2 system. However, as 1 is only a moderate inhibitor of TrxR in MDA-MB-231 cells, TrxR is probably not the major target responsible for the cytotoxicity of 1. In terms of differences, we noted that 1 induced a significant redox imbalance characterised by lipid peroxidation and thiol oxidation, and a moderate decrease of the mitochondrial membrane potential in breast cancer cells, whereas 2 has almost no effect. These results underline the importance of the trans configuration in the ferrocenyl-double bond-phenol motif, which is present in 1 but is cis in (Z)-2.

Synthesis and structure-activity relationships of the first ferrocenyl-aryl-hydantoin derivatives of the nonsteroidal antiandrogen nilutamide.

We present here the first synthesis of organometallic complexes derived from the nonsteroidal antiandrogen nilutamide, bearing a ferrocenyl substituent at position N(1) or at C(5) of the hydantoin ring; for comparison, we also describe the C(5) p-anisyl organic analogue. All of these complexes retain a modest affinity for the androgen receptor. The N-substituted complexes show a weak or moderate antiproliferative effect (IC 50 around 68 microM) on hormone-dependent and -independent prostate cancer cells, while the C(5)-substituted compounds exhibit toxicity levels 10 times higher (IC 50 around 5.4 microM). This strong antiproliferative effect is probably due to a structural effect linked to the aromatic character of the ferrocene rather than to its organometallic feature. In addition, it seems connected to a cytotoxic effect rather than an antihormonal one. These results open the way toward a new family of molecules that are active against both hormone-dependent and hormone-independent prostate cancer cells.

Nanoparticles loaded with ferrocenyl tamoxifen derivatives for breast cancer treatment.

For the first time, two organometallic triphenylethylene compounds (Fc-diOH and DFO), with strong antiproliferative activity in breast cancer cells, but insoluble in biological fluids, were incorporated in two types of stealth nanoparticles (NP): PEG/PLA nanospheres (NS) and nanocapsules (NC). Their physicochemical parameters were measured (size, zeta potential, encapsulation and loading efficiency), and their biological activity was assessed. In vitro drug release after high dilution of loaded NPs was measured by estradiol binding competition in MELN cells. The influence of the encapsulated drugs on the cell cycle and apoptosis was studied by flow cytometry analyses. Notwithstanding potential drug adsorption at the NP surface, Fc-diOH and DFO were incorporated efficiently in NC and NS, which slowly released both compounds. They arrested the cell cycle in the S-phase and induced apoptosis, whose activity is increased by loaded NS. A decrease in their antiproliferative activity by the antioxidant alpha-tocopherol indicated that reactive oxygen species (ROS) may be involved. Therefore, nanosystems, containing for the first time a high load of anticancer organometallic triphenylethylenes, have been developed. Their small size and delayed drug release, combined with their enhanced apoptotic potential, are compatible with an increased persistence in the blood and a promising antitumour activity.

A new generation of ferrociphenols leads to a great diversity of reactive metabolites, and exhibits remarkable antiproliferative properties.

Organometallic compounds bearing the redox motif [ferrocenyl-ene-phenol] have very promising antiproliferative properties which have been further improved by incorporating pertinent substituents able to engender new mechanisms. Here we show that novel ferrociphenols bearing a hydroxypropyl chain exhibit strong antiproliferative effects, in most cases much better than those of cisplatin, tamoxifen, or of previously described ferrociphenols devoid of this terminal OH. This is illustrated, in the case of one of these compounds, by its IC50 values of 110 nM for MDA-MB-231 triple negative breast cancer cells and of 300 nM for cisplatin-resistant A2780cisR human ovarian cancer cells, and by its GI50 values lower than 100 nM towards a series of melanoma and renal cancer cell lines of the NCI-60 panel. Interestingly, oxidative metabolism of these hydroxypropyl-ferrociphenols yields two kinds of quinone methides (QMs) that readily react with various nucleophiles, such as glutathione, to give 1,6- and 1,8-adducts. Protonation of these quinone methides generates numerous reactive metabolites leading eventually to many rearrangement and cleavage products. This unprecedented and fully characterized metabolic profile involving a wide range of electrophilic metabolites that should react with cell macromolecules may be linked to the remarkable profile of antiproliferative activities of this new series. Indeed, the great diversity of unexpected reactive metabolites found upon oxidation will allow them to adapt to various situations present in the cancer cell. These data initiate a novel strategy for the rational design of anticancer molecules, thus opening the way to new organometallic potent anticancer drug candidates for the treatment of chemoresistant cancers.

Aryl Butenes Active against K562 Cells and Lacking Tyrosinase Inhibitory Activity as New Leads in the Treatment of Leukemia.

BACKGROUND & OBJECTIVE: The inhibitory effects of four series of aryl butene derivatives, active against breast cancer, on the monophenolase activity of tyrosinase, in melanin-free ink from Sepia officinalis, have been studied. Hydroxytamoxifen 1, ferrociphenol 17 and several aryl butene analogs have shown strong antiproliferative activity on hormone-dependent and hormone-independent breast cancer cells and were evaluated in leukemia K562 cell proliferation. Their potential to induce skin depigmentation by evaluating their anti-tyrosinase activity was also estimated. In order to better rationalize the tyrosinase inhibitory action and the binding mode of the compounds, docking studies were carried out. CONCLUSION: Hydroxytamoxifen and some aryl butenes showed strong antiproliferative effects against K562 cells at 1 µM without showing tyrosinase inhibition. If phenolic compounds 16 and 17 showed the best antiproliferative activity on K562, Hydroxytamoxifen and compounds 5, 10, 20 and 21 have been identified as candidates for further development against chronic myeloid leukemia (CML), and are predicted to not induce depigmentation of the skin, a side effect encountered with imatinib, conventionally used for the treatment of CML.

Ferrocifens labelled with an infrared rhenium tricarbonyl tag: synthesis, antiproliferative activity, quantification and nano IR mapping in cancer cells.

Antiproliferative activities of several members of the ferrocifen family, both in vitro and in vivo, are well documented although their precise location in cancer cells has not yet been elucidated. However, two different infrared imaging techniques have been used to map the non-cytotoxic cyrhetrenyl analogue of ferrociphenol in a single cell. This observation prompted us to tag two ferrocifens with a cyrhetrenyl unit [CpRe(CO)3; Cp = η5-cyclopentadienyl] by grafting it, via an ester bond, either to one of the phenols (4, 5) or to the hydroxypropyl chain (6). Complexes 4-6 retained a high cytotoxicity on breast cancer cells (MDA-MB-231) with IC50 values in the range 0.32-2.5 µM. Transmission IR spectroscopy was used to quantify the amount of cyrhetrenyl tag present in cells incubated with 5 or 6. The results show that after a 1-hour incubation of cells at 37 °C, complexes 5 and 6 are mainly present within cells while only a limited percentage, quantified by ICP-OES, remained in the incubation medium. AFM-IR spectroscopy, a technique coupling infrared irradiation with near-field AFM detection, was used to map the cyrhetrenyl unit in a single MDA-MB-231 cell, incubated at 37 °C for 1 hour with 10 µM of 6. The results show that signal distribution of the characteristic band of the Re(CO)3 entity at 1950 cm-1 matched those of amide and phosphate, thus indicating a location of the complex mainly in the cell nucleus.

A New Series of Succinimido-ferrociphenols and Related Heterocyclic Species Induce Strong Antiproliferative Effects, Especially against Ovarian Cancer Cells Resistant to Cisplatin.

Ferrociphenols are known to display anticancer properties by original mechanisms dependent on redox properties and generation of active metabolites such as quinone methides. Recent studies have highlighted the positive impact of oxidative stress on chemosensitivity and prognosis of ovarian cancer patients. Ovarian adenocarcinomas are shown to be an excellent model for defining the impact of selected ferrociphenols as new therapeutic drugs for such cancers. This work describes the syntheses and preliminary mechanistic research of unprecedented multitargeting heterocyclic ferrociphenols bearing either a succinimidyl or phthalimidyl group that show exceptional antiproliferative behavior against epithelial ovarian cancer cells resistant to cisplatin. Owing to the failure of the present pharmaceutical options, such as carboplatin a metallodrug based on Pt coordination chemistry, these species may help to overcome the problem of lethal resistance. Currently, ferrociphenolic entities generally operate via apoptotic and senescence pathways. We present here our first results in this new cyclic-imide series.

Tamoxifen-like metallocifens target the thioredoxin system determining mitochondrial impairment leading to apoptosis in Jurkat cells.

Tamoxifen-like metallocifens (TLMs) of the group-8 metals (Fe, Ru, and Os) show strong anti-proliferative activity on cancer cell lines resistant to apoptosis, owing to their unique redox properties. In contrast, the thioredoxin system, which is involved in cellular redox balance, is often overexpressed in cancer cells, especially in tumour types resistant to standard chemotherapies. Therefore, we investigated the effect of these three TLMs on the thioredoxin system and evaluated the input of the metallocene unit in comparison with structurally related organic tamoxifens. In vitro, all three TLMs became strong inhibitors of the cytosolic (TrxR1) and mitochondrial (TrxR2) isoforms of thioredoxin reductase after enzymatic oxidation with HRP/H2O2 while none of the organic analogues was effective. In Jurkat cells, TLMs inhibited mainly TrxR2, resulting in the accumulation of oxidized thioredoxin 2 and cell redox imbalance. Overproduction of ROS resulted in a strong decrease in the mitochondrial membrane potential, translocation of cytochrome c to the cytosol and activation of caspase 3, thus leading to apoptosis. None of these events occurred with organic tamoxifens. The mitochondrial fraction of cells exposed to TLMs contained a high amount of the corresponding metal, as quantified by ICP-OES. The lipophilic and cationic character associated with the singular redox properties of the TLMs could explain why they alter the mitochondrial function. These results provide new insights into the mechanism of action of tamoxifen-like metallocifens, underlying their prodrug behaviour and the pivotal role played by the metallocenic entity in their cytotoxic activity associated with the induction of apoptosis.

The length of the bridging chain in ansa-metallocenes influences their antiproliferative activity against triple negative breast cancer cells (TNBC).

In order to examine whether the length of the bridging chain in ansa-ferrocenes affects their antiproliferative activity against MDA-MB-231 triple negative breast cancer cell lines (TNBC), we synthesized derivatives of the type 1-[bis-(4-hydroxyphenyl)]methylidene-[n]ferrocenophane and 1-[(4-hydroxyphenyl)-phenyl]methylidene-[n]ferrocenophane with n = 3, 4, 5. We found that the derivatives of [3]ferrocenophane, the compounds with the shortest bridging chains, are the most active. IC50 values were 0.09 ± 0.01, 2.41 ± 0.10, and 1.85 ± 0.25 µM for the dihydroxyphenyl derivatives, with n = 3, 4, 5, respectively. These differences can be explained in terms of modification of the key metabolites (radical versus quinone methides) within the ansa series depending on the length of the bridging chain. The derivative of [5]ferrocenophane, possessing two -[bis-(4-hydroxyphenyl)]methylidene groups, was also prepared. Surprisingly, this relatively large molecule is also active (IC50 = 2.7 ± 0.3 µM). Two ruthenocenophane analogs were also synthesized. These ruthenium compounds are practically inactive against MDA-MB-231 cells. The unusual chemistry of these different compounds is discussed in terms of elucidating the mechanism underlying their diverse antiproliferative activity, and their specific advantages are evaluated.