The first titanocenyl dichloride moiety vectorised by a selective estrogen receptor modulator (SERM). Synthesis and preliminary biochemical behaviour

Metallodrugs against Breast Cancer: Combining the Tamoxifen Vector with Platinum(II) and Palladium(II) Complexes

The luminal A-subtype of breast cancer, where the oestrogen receptor α (ERα) is overexpressed, is the most frequent one. The prodrug tamoxifen (1) is the clinically used agent, inhibiting the ERα activity via the formation of several active metabolites, such as 4-hydroxytamoxifen (2) or 4,4'-dihydroxytamoxifen (3). In this study, we present the tamoxifen derivative 4-[1,1-bis(4-methoxyphenyl)but-1-en-2-yl]-2,2'-bipyridine (4), which was combined with platinum or palladium dichloride, the former a well-known scaffold in anticancer treatment, to give [PtCl₂(4-κ²N,N')] (5) or [PdCl₂(4-κ²N,N'] (6). To prevent fast exchange of weakly coordinating chlorido ligands in aqueous solution, a bulky, highly stable and hydrophobic nido-carborate(-2) ([C₂B₉H₁₁]^2-) was incorporated. The resulting complexes [3-(4-κ²N,N')-3,1,2-PtC₂B₉H₁₁] (7) and [3-(4-κ²N,N')-3,1,2-PdC₂B₉H₁₁] (8) exhibit a dramatic change in electronic and biological properties compared to 5 and 6. Thus, 8 is highly selective for triple-negative MDA-MB-231 cells (IC₅₀ = 3.7 μM, MTT test), while 7 is completely inactive against this cell line. The observed cytotoxicity of compounds 4-6 and 8 against this triple-negative cell line suggests off-target mechanisms rather than only ERα inhibition, for which these compounds were originally designed. Spectroscopic properties and electronic structures of the metal complexes were investigated for possible explanations of the biological activities.

Evaluating Ligand Modifications of the Titanocene and Auranofin Moieties for the Development of More Potent Anticancer Drugs

Over time platinum-based anticancer drugs have dominated the market, but their side effects significantly impact the quality of life of patients. Alternative treatments are being developed all over the world. The titanocene and auranofin families of compounds, discovered through an empirical search for other metal-based therapeutics, hold tremendous promise to improve the outcomes of cancer treatment. Herein we present a historical perspective of these compounds and review current efforts focused on the evolution of their ligands to improve their physiological solution stability, cancer selectivity, and antiproliferative performance, guided by a clear understanding of the coordination chemistry and aqueous speciation of the metal ions, of the cytotoxic mechanism of action of the compounds, and the external factors that limit their therapeutic potential. Newer members of these families of compounds and their combination in novel bimetallic complexes are the result of years of scientific research. We believe that this review can have a positive impact in the development and understanding of the metal-based drugs of gold, titanium, and beyond.

Estrogen signaling: An emanating therapeutic target for breast cancer treatment

Breast cancer, a most common malignancy in women, was known to be associated with steroid hormone estrogen. The discovery of estrogen receptor (ER) gave us not only a powerful predictive and prognostic marker, but also an efficient target for the treatment of hormone-dependent breast cancer with various estrogen ligands. ER consists of two subtypes i.e. ERα and ERβ, that are mostly G-protein-coupled receptors and activated by estrogen, specially 17β-estradiol. The activation is followed by translocation into the nucleus and binding with DNA to modulate activities of different genes. ERs can manage synthesis of RNA through genomic actions without directly binding to DNA. Receptors are tethered by protein-protein interactions to a transcription factor complex to communicate with DNA. Estrogens also exhibit nongenomic actions, a characteristic feature of steroid hormones, which are so rapid to be considered by the activation of RNA and translation. These are habitually related to stimulation of different protein kinase cascades. Majority of post-menopausal breast cancer is estrogen dependent, mostly potent biological estrogen (E2) for continuous growth and proliferation. Estrogen helps in regulating the differentiation and proliferation of normal breast epithelial cells. In this review we have investigated the important role of ER in development and progression of breast cancer, which is complicated by receptor's interaction with co-regulatory proteins, cross-talk with other signal transduction pathways and development of treatment strategies viz. selective estrogen receptor modulators (SERMs), selective estrogen receptor down regulators (SERDs), aromatase and sulphatase inhibitors.

In vitro and in vivo trackable titanocene-based complexes using optical imaging or SPECT

Two unprecedented titanocene-based theranostics have been synthesized, characterized, and tracked either in vitro (BODIPY probe) or in vivo (¹¹¹In-DOTA probe).

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

This work reports the in vitro activity against Plasmodium falciparumblood 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.

Inhibition of human topoisomerase I and II and anti-proliferative effects on MCF-7 cells by new titanocene complexes

The antitumor activity shown by many platinum complexes has produced a strong interest in research of new organometallic compounds having anticancer action. Among the many metal compounds synthesized and tested, those based on titanium have received considerable attention because of their cytotoxic activity against solid tumors. Particularly, new titanocene compounds containing aromatic groups linked to the Cp (cyclopentadienyl ring, C5H5) have been synthetized, such as the titanocene Y (bis-[(p-methoxybenzyl)cyclopentadienyl]titanium dichloride) that displayed promising medium-high cytotoxic activity on breast cancer cell lines. Other titanocene complexes recently synthesized, obtained by replacing the substituent methoxy-aryl of cyclopentadienes of titanocene Y with ethenyl-methoxide or ethenyl-phenoxide, showed increased cytotoxic activity on breast cancer cell lines being more stable compounds. In this paper, we report that new titanocene complexes holding lipophilic groups, for instance a methyl group on benzyl carbon, exhibit improved antiproliferative effect on breast cancer cell line MCF-7. Similar results have been obtained introducing a 5-methoxy naphthyl group to further stabilize the titanocene complexes. These inhibitory effects on breast cancer cells have been ascribed to human topoisomerase I and II inhibition as demonstrated by specific enzymatic assays.

Antiproliferative effects of copper(II)-polypyridyl complexes in breast cancer cells through inducing apoptosis

Although cisplatin has been used for decades to treat human cancer, some toxic side effects and resistance are observed. Previous investigations have suggested copper complexes as a novel class of tumor-cell apoptosis inducers. The present study aimed to evaluate the anti-breast cancer activities of two polypyridyl-based copper(II) complexes, [Cu(tpy)(dppz)](NO3)2 (1) and [Cu(tptz)2](NO3)2 (2) (tpy = 2,2':6',2″-terpyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine, tptz = 2,4,6-tris(2-pyridyl)-1,3,5-triazine), using human breast adenocarcinoma cell line (MCF-7). The ability of the complexes to cleave supercoiled DNA in the presence and absence of external agents was also examined. The apoptotic activities of the complexes were assessed using flow cytometry, fluorescence microscope and western blotting analysis. Our results indicated the high DNA affinity and nuclease activity of complexes 1 and 2. The cleavage mechanisms between the complexes and plasmid DNA are likely to involve a singlet oxygen or singlet oxygen-like entity as the reactive oxygen species. Complexes 1 and 2 also significantly inhibited the proliferation of MCF-7 cells in a dose-dependent manner (IC50 values = 4.57 and 1.98 μM at 24 h, respectively). Complex 2 remarkably induced MCF-7 cells to undergo apoptosis, which was demonstrated by cell morphology, annexin-V and propidium iodide staining. The caspase cascade was activated as shown by the proteolytic cleavage of caspase-3 after treatment of MCF-7 cells with complex 2. Additionally, complex 2 significantly increased the expression of the Bax-to-Bcl-2 ratio to induce apoptosis. In conclusion, these results revealed that complex 2 may be a potential and promising chemotherapeutic agent to treat breast cancer.

New tungstenocenes containing 3-hydroxy-4-pyrone ligands: antiproliferative activity on HT-29 and MCF-7 cell lines and binding to human serum albumin studied by fluorescence spectroscopy and molecular modeling methods

Three new water-soluble tungstenocene derivatives were synthesized and characterized using 3-hydroxy-4-pyrone ligands, which provide aqueous stability to the complexes. The antiproliferative activities of the complexes on HT-29 colon cancer and MCF-7 breast cancer cell lines were evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and showed the new tungstenocene derivatives have higher antiproliferative action than tungstenocene dichloride (Cp(2)WCl(2), where Cp is cyclopentadienyl). The binding interactions of the tungstenocenes with human serum albumin (HSA) were investigated using fluorescence spectroscopy and molecular modeling methods. Analysis of the fluorescence quenching spectra indicates that the tungstenocene complexes bind HSA by hydrophobic interactions and hydrogen bonding at fatty acid binding site 6 and drug binding site II. Docking studies provided a description of the hydrophobic interactions and hydrogen bonding by which the tungstenocenes become engaged with HSA. It was determined that the binding affinity of the tungstenoecenes for HSA is in the order Cp(2)WCl(2) < [Cp(2)W(ethyl maltolato)]Cl < [Cp(2)W(maltolato)]Cl < [Cp(2)W(kojato)]Cl, consistent with the hydrophobic interactions and the number of hydrogen bonds involved.

Bioorganometallic Chemistry of Molybdenocene Dichloride and Its Derivatives

The potential application of metallocene complexes into the cancer research was established by the pioneer work of Köpf-Maeir and Köpf in the late 1970s. The combination of organometallic chemistry and biochemistry created a new research area: bioorganometallic chemistry. Bioorganometallic chemistry has developed rapidly in the last thirty years leading to application of organometallic species into diagnostic, sensors, immunoassays and anticancer research among others. This review focuses on the bioorganometallic chemistry of molybdenocene dichloride and its derivatives as metal-based anticancer drugs. The anticancer properties of molybdenocene dichloride and its derivatives are described as well as the mechanism of action, aqueous and coordination chemistry, and molybdenocene-biomolecule interactions.

Half-sandwich ruthenium–arene complexes with thiosemicarbazones: synthesis and biological evaluation of [(η⁶-p-cymene)Ru(piperonal thiosemicarbazones)Cl]Cl complexes

The synthesis and characterization of a number of organometallic ruthenium(II) complexes containing a series of bidentate thiosemicarbazone ligands derived from piperonal is reported. The structure of compounds have been confirmed by spectroscopic analysis (IR and NMR) as well as X-ray crystallographic analysis of [(η⁶-p-cymene)Ru(pPhTSC)Cl]Cl (4) (pPhTSC is piperonal-N(4)-phenylthiosemicarbazone). The interaction of the complexes ([(η⁶-p-cymene)Ru(pEtTSC)Cl]Cl) (3) (pEtTSC is piperonal-N(4)-ethylthiosemicarbazone) and 4 with calf thymus DNA, human serum albumin (HSA) and pBR322 plasmid DNA were studied by spectroscopic, gel electrophoresis and hydrodynamic methods. The apparent binding constant for the interaction with DNA was determined to be 3.97×10³ M⁻¹ and 4.07×10³ M⁻¹ at 293 K for 3 and 4 respectively. The complexes bind strongly to HSA with binding constants of 2.94×10⁴ M⁻¹ and 12.2×10⁴ M⁻¹ at 296 K for 3 and 4 respectively. The in vitro anticancer activity of 3 and 4 has been evaluated against two human colon cancer cell line (HCT-116 and Caco-2) with IC50 values in the range of 26–150 μM. Both 3 and 4 show good activity as a catalytic inhibitor of human topoisomerase II at concentrations as low as 20 μM. The proficiency of 3 and 4 to act as antibacterial agents was also evaluated against six pathogenic bacterial strains with the best activity seen against Gram-positive strains.

TiCp2Cl-Catalyzed Living Radical and Ring Opening Polymerizations Initiated from Epoxides and Aldehydes in the Synthesis of Linear, Graft and Branched Polymers

Ti(III)Cp2Cl-catalyzed radical ring opening (RRO) of epoxides or single electron transfer (SET) reduction of aldehydes generates Ti alkoxides and carbon centered radicals which add to styrene, initiating a radical polymerization. This polymerization is mediate in a living fashion by the reversible termination of growing chains with the TiCp2Cl metalloradical. In addition, polymers or monomers containing pendant epoxide groups (glycidyl methacrylate) can be used as substrates for radical grafting or branching reactions by self condensing vinyl polymerization. In addition, Ti alkoxides generated in situ by both epoxide RRO and aldehyde SET initiate the living ring opening polymerization of ε-caprolactone. Thus, new initiators and catalysts are introduced for the synthesis of complex polymer architectures.

Synthesis and cytotoxicity studies of steroid-functionalized titanocenes as potential anticancer drugs: sex steroids as potential vectors for titanocenes

Six titanocenyls functionalized with steroidal esters have been synthesized and characterized by infrared, (1)H, and (13)C NMR spectroscopy and elemental analysis. Among those steroids, dehydroepiandrosterone, trans-androsterone, and androsterone are androgens and pregnenolone is a progesterone precursor. Clionasterol is a natural steroid compound. These steroid-functionalized titanocenyls were tested by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay for in vitro cytotoxicity for MCF-7 breast cancer and HT-29 colon cancer cells. All complexes exhibited more cytotoxicity than titanocene dichloride. The titanocenyls containing androgen and progesterone derivatives as pendant groups had higher antiproliferative activities than those with cholesterol steroid compounds. Of particular significance is titanocenyl-dehydroepiandrosterone complex, which is 2 orders of magnitude more cytotoxic than titanocene dichloride and also shows much more sensitivity and selectivity for the MCF-7 cell line.

Titanium(IV) complexes: cytotoxicity and cellular uptake of titanium(IV) complexes on caco-2 cell line

Replacement of the ancillary ligand in titanocene dichloride by amino acids provides titanocene species with high water solubility. As part of our research efforts in the area of titanium-based antitumor agents, we have investigated the cytotoxic activity of Cp(2)TiCl(2) and three water soluble titanocene-amino acid complexes - [Cp(2)Ti(aa)(2)]Cl(2) (aa=L-cysteine, L-methionine, and D-penicillamine) and one water soluble coordination compound, [Ti(4)(maltolato)(8)(micro-O)(4)] on the human colon adenocarcinoma cell line, Caco-2. At pH of 7.4 all titanocene species decompose extensively while [Ti(4)(maltolato)(8)(micro-O)(4)] is stable for over seven days. In terms of cytotoxicity, the [Cp(2)Ti(aa)(2)]Cl(2) and [Ti(4)(maltolato)(8)(micro-O)(4)] complexes exhibited slightly higher toxicity than titanocene dichloride at 24h, but at 72h titanocene dichloride and [Ti(4)(maltolato)(8)(micro-O)(4)] have higher cytotoxic activity. Cellular titanium uptake was quantified at various time intervals to investigate the possible relationship between Ti uptake and cellular toxicity. Results indicated that there was not a clear relationship between Ti uptake and cytotoxicity. A structure-activity relationship is discussed.

Water-soluble molybdenocene complexes with both proliferative and antiproliferative effects on cancer cell lines and their binding interactions with human serum albumin

Two water-soluble molybdenocene complexes containing oxygen chelating ligands, maltolato and malonate, have been synthesized to elucidate the role of the ancillary ligands in the molybdenocene cytotoxic activity. The structural characterizations of these species by 1H NMR and IR spectroscopies suggest that both molybdenocene complexes contain the ligands in a bidentate fashion and elemental analysis and mass spectrometry corroborate the proposed formula for the species to be Cp2Mo(malonate) and [Cp2Mo(maltolato)]Cl (Cp is cyclopentadienyl). Metal-albumin binding studies were pursued using UV-vis spectroscopy and cyclic voltammetric techniques. Whereas metal-albumin binding studies using UV-vis spectroscopy did not show any evidence of interaction, cyclic voltammetry experiments showed that molybdenocene complexes may be involved in weak binding interactions with albumin, most likely in hydrophobic interactions. The cytotoxic activities of Cp2Mo(malonate) and [Cp2Mo(maltolato)]Cl alone with Cp2MoCl2 were investigated in HT-29 colon cancer and MCF-7 breast cancer cell lines using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cell viability assay. Cp2Mo(malonate) and [Cp2Mo(maltolato)]Cl showed slight improvement in terms of cytotoxic activity as compared with Cp2MoCl2 in the HT-29 colon cancer cell line, whereas for MCF-7 all the molybdenocene species exhibited a proliferative profile. The molybdenocene-containing chelating ligands showed stronger proliferative effects than Cp2MoCl2. There is no correlation between the binding affinity of molybdenocenes for human serum albumin and cytotoxic activity toward HT-29 and MCF-7 cancer cells.

Antitumor activity of Titanocene Y against freshly explanted human breast tumor cells and in xenografted MCF-7 tumors in mice

Bis-[(p-methoxybenzyl)cyclopentadienyl] titanium dichloride, better known as Titanocene Y, is a newly synthesized transition metal-based anticancer drug. We studied the antitumor activity of Titanocene Y with concentrations of 2.1, 21 and 210 micromol/l against a freshly explanted human breast cancer, using an in-vitro soft agar cloning system. The sensitivity against Titanocene Y was highly remarkable in the breast cancer tumor in the full concentration range. Titanocene Y showed cell death induction at 2.1 micromol/l, well comparable to cisplatin, given at a concentration of 1.0 micromol/l. A further preclinical development of Titanocene Y was warranted and therefore an MCF-7 human breast cancer xenograft nonobese diabetic/severe combined immunodeficient mouse model was used. Titanocene Y was given for 21 days at 30 mg/kg/day (75% of the maximum tolerable dose of Titanocene Y), which resulted in the reduction of the tumor volume to around one-third, whereas no mouse was lost because of the surprisingly low toxicity of Titanocene Y.

Antitumour bis(cyclopentadienyl) metal complexes: titanocene and molybdocene dichloride and derivatives

This Perspective will focus on recent developments in the field of antitumour metallocenes structurally related to titanocene dichloride. Despite extensive testing of titanocene dichloride which culminated in phase I and II clinical trials, further trials have been abandoned. While DNA has been implicated as the major target related to anticancer activity, identification of the active species and mechanism of action has been poorly understood and hence the design of second generation titanocene derivatives has not been possible. Recent mechanistic studies have provided a plausible mechanism for delivery of Ti to cancer cells via transferrin mediated endocytosis. This mechanism requires the presence of labile Cp-Ti bonds that hydrolyse on a time scale to deliver Ti to transferrin. A large range of titanocene derivatives in which the cyclopentadienyl rings have been substituted by both electron withdrawing and donating groups, including aromatic, alkyl and cyclic amines, have been prepared and tested for activity in the last 5 years. These results have shown that subtle structural effects can have a significant effect on biological activity and that biological activity is highly cell line dependent. However, the biological chemistry and cellular studies required to determine the mechanism of action of these new titanocenes have not been reported. In contrast, the bioorganometallic chemistry and cellular studies of molybdocene dichloride have implicated interaction with cellular thiols as the key reaction related to biological activity. Tailoring of the pseudohalide ligands by tuning the strength of the Mo-S bonds provides the opportunity to enhance cell uptake. Further research is required to establish the origin of antitumour activity.

Metal complex SERMs (selective oestrogen receptor modulators). The influence of different metal units on breast cancer cell antiproliferative effects

The selective oestrogen receptor modulator tamoxifen is a leading agent in the adjuvant treatment of breast cancer. Several organometallic moieties have been vectorised with tamoxifen, in order to improve on the latter's antiproliferative properties by the addition of a potentially cytotoxic moiety, and have been evaluated versus both oestrogen receptor positive (MCF7) and oestrogen receptor negative (MDA-MB231) breast cancer cells. For tamoxifen analogues with ((R,R)-trans-1,2-diaminocyclohexane)platinum(II), cyclopentadienyl rhenium tricarbonyl, and ruthenocene tethers, there was no enhancement of the antiproliferative effect on oestrogen receptor positive cells, nor any cytotoxic effect on oestrogen receptor negative cells, while those containing cyclopentadienyl titanium dichloride showed an oestrogenic effect. However, compounds where ferrocene replaces tamoxifen's phenyl ring were strongly cytotoxic against both cell lines. The synthesis and biological results of these compounds is reviewed and placed in the historic context of inorganic compounds in therapy.

Enhancement of the cytotoxicity of titanocene dichloride by aging in organic co-solvent

Titanocene dichloride exhibits anti-proliferative activity in a wide spectrum of murine and human tumors. Although it is still unclear as to which species are active in biological media, they all readily deliver Ti(IV) to transferrin, the protein that transports iron in the blood. In this article, we report that aging of the complex in alcohols (namely methanol and ethanol) or dimethyl sulfoxide, the co-solvents used to prepare mother solution of the drug, leads to increased cytotoxic activity (i.e. lower IC(50) values) in HCT116 colon cancer cell lines, to a different extent. The TiCp(2)Cl(2) solvolysis was followed by (1)H NMR, ESI-MS, electrochemical and conductivity measurements, and the intracellular Ti(IV) uptake was evaluated.

Organometallic chemistry, biology and medicine: ruthenium arene anticancer complexes

Our work has shown that certain ruthenium(II) arene complexes exhibit promising anticancer activity in vitro and in vivo. The complexes are stable and water-soluble, and their frameworks provide considerable scope for optimising the design, both in terms of their biological activity and for minimising side-effects by variations in the arene and the other coordinated ligands. Initial studies on amino acids and nucleotides suggest that kinetic and thermodynamic control over a wide spectrum of reactions of Ru(II) arene complexes with biomolecules can be achieved. These Ru(II) arene complexes appear to have an altered profile of biological activity in comparison with metal-based anticancer complexes currently in clinical use or on clinical trial.

Estradiol derivatives bearing sulfur-containing substituents at the 11beta or 7alpha positions: versatile reagents for the preparation of estrogen conjugates

Estradiol derivatives bearing HS-, HSCH(2)-, HSCH(2)CH(2)-, MeS-, MeSCH(2)-, MeSCH(2)CH(2)-, or PhCH(2)SCH(2)CH(2)-groups at the 11beta position or an HS-group at the 7alpha position have been synthesized, and their binding affinity to the estrogen receptor (ER) determined. Nearly all of these substituted estrogens retain high binding affinity, and at the 11beta position, the sulfur atom has an effect on ER binding that is similar to that of a carbon atom. These thiol derivatives are promising intermediates for the preparation of a variety of estradiol conjugates. The methyl sulfides, in particular, might potentially be developed as (11)C-labeled agents for imaging ER-positive tumors by positron emission tomography.