Arene ruthenium complexes with phosphinoferrocene amino acid conjugates: Synthesis, characterization and cytotoxicity

Preparation of new organo-ruthenium(II) complexes and their nucleic acid/albumin binding efficiency and in vitro cytotoxicity studies

Hetero-bimetallic ruthenium(II) complexes (PRAFIZ and PRBFIZ) containing acetyl ferrocene (AFIZ)/benzoyl ferrocene isonicotinic hydrazone ligands (BFIZ) were synthesized and characterized by various spectral and analytical techniques. The structure of acetyl ferrocene isonicotinic hydrazone (AFIZ) and the complex PRBFIZ was confirmed by X-ray crystallography. The hydrazide ligands coordinated in a bidentate monobasic fashion using their N1 hydrazinic nitrogen and enolic oxygen atoms. The binding interactions of the ligands and complexes were examined using Calf-Thymus DNA (CT-DNA) and bovine serum albumin (BSA). Scanning Electron Microscopic (SEM) experiments clarified the efficient binding interaction of the ligands and complexes with BSA. The results of in vitro cytotoxicity studies on MDA-MB-261 breast cancer cells and A549 human lung cancer cells and cell morphological analysis results through staining assays clearly indicated the cytotoxic nature of the complexes.

Triiron Complex with N-Ferrocenyl Aminocarbyne Ligand Bridging a Diiron Core: DFT, Electrochemical, and Biological Insights

The first N-ferrocenyl aminocarbyne complex, [Fe2Cp2(CO)2(μ-CO){μ-CN(Me)(Fc)}]CF3SO3 ([2]CF3SO3), was synthesized with an 88% yield from [Fe2Cp2(CO)4], isocyanoferrocene (CNFc), and methyl triflate. The synthesis proceeded through the intermediate formation of [Fe2Cp2(CO)3(CNFc)], 1. Multinuclear NMR experiments revealed the presence of cis and trans isomers for [2]CF3SO3 in organic solvents, in agreement with DFT outcomes. Electrochemical and spectroelectrochemical studies demonstrated one reduction process occurring prevalently at the diiron core and one oxidation involving the ferrocenyl substituent. The oxidation process is expected to favor the redox activation of [2]+ in a biological environment. Both [2]CF3SO3 and its phenyl analogue [Fe2Cp2(CO)2(μ-CO){μ-CN(Me)(Ph)}]CF3SO3 ([3]CF3SO3), prepared for comparison, exerted moderate antiproliferative activity against the human cancer cell lines A431, HCT-15, PSN-1, 2008, and U1285. However, [2]CF3SO3 exhibited a higher cytotoxicity than [3]CF3SO3, showed a substantial ability to induce intracellular ROS production, and outperformed cisplatin in a three-dimensional SCLC cell model.

Asymmetrically Coordinated Heterodimetallic Ir-Ru System: Synthesis, Computational, and Anticancer Aspects

Herein, we present an unprecedented formation of a heterodinuclear complex [{(ppy)₂Ir^III}(μ-phpy){Ru^II(tpy)}](ClO₄)₂ {[1](ClO₄)₂} using terpyridyl/phenylpyridine as ancillary ligands and asymmetric phpy as a bridging ligand. The asymmetric binding mode (N^∧N-∩-N^∧N^∧C^-) of the phpy ligand in {[1](ClO₄)₂} is confirmed by ¹H, ¹³C, ¹H-¹H correlated spectroscopy (COSY), high-resolution mass spectrum (HRMS), single-crystal X-ray crystallography techniques, and solution conductivity measurements. Theoretical investigation suggests that the highest occupied molecular orbital (HOMO) and the least unoccupied molecular orbital (LUMO) of [1]²⁺ are located on iridium/ppy and phpy, respectively. The complex displays a broad low energy charge transfer (CT) band within 450-575 nm. The time-dependent density functional theory (TDDFT) analysis suggests this as a mixture of metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT), where both ruthenium, iridium, and ligands are involved. Complex {[1](ClO₄)₂} exhibits Ru^IIIr^III/Ru^IIIIr^III- and Ru^IIIIr^III/Ru^IIIIr^IV-based oxidative couples at 0.83 and 1.39 V, respectively. The complex shows anticancer activity and selectivity toward human breast cancer cells (IC₅₀; MCF-7: 9.3 ± 1.2 μM, and MDA-MB-231: 8.6 ± 1.2 μM) over normal breast cells (MCF 10A: IC₅₀ ≈ 21 ± 1.3 μM). The Western blot analysis and fluorescence microscopy images suggest that combined apoptosis and autophagy are responsible for cancer cell death.

Potent Ruthenium-Ferrocene Bimetallic Antitumor Antiangiogenic Agent That Circumvents Platinum Resistance: From Synthesis and Mechanistic Studies to In Vivo Evaluation in Zebrafish

Emergence of resistance in cancer cells and dose-limiting side effects severely limit the widespread use of platinum (Pt) anticancer drugs. Multi-action hybrid anticancer agents that are constructed by merging two or more pharmacophores offer the prospect of circumventing issues of Pt drugs. Herein, we report the design, synthesis, and in-depth biological evaluation of a ruthenium-ferrocene (Ru-Fc) bimetallic agent [(η6-p-cymene)Ru(1,1,1-trifluoro-4-oxo-4-ferrocenyl-but-2-en-2-olate)Cl] and its five analogues. Along with aquation/anation chemistry, we evaluated the in vitro antitumor potency, Pt cross-resistance profile, and in vivo antiangiogenic properties. A structure activity analysis was performed to understand the impact of Fc, CF3, and p-cymene groups on the anticancer potency of the Ru-Fc hybrid. Finally, in addition to assessing cellular uptake and intracellular distribution, we demonstrated that the Ru-Fc hybrid binds to nucleophilic biomolecules and produces reactive oxygen species, which causes mitochondrial dysfunction and induces ER stress, leading to poly(ADP-ribose) polymerase-mediated necroptotic cell death.

Ferrocene-containing hybrids as potential anticancer agents: Current developments, mechanisms of action and structure-activity relationships

Cancer is one of the most fatal threatens to human health throughout the world. The major challenges in the control and eradication of cancers are the continuous emergency of drug-resistant cancer and the low specificity of anticancer agents, creating an urgent need to develop novel anticancer agents. Organometallic compounds especially ferrocene derivatives possess remarkable structural and mechanistic diversity, inherent stability towards air, heat and light, low toxicity, low cost, reversible redox, ligand exchange, and catalytic properties, making them promising drug candidates for cancer therapy. Ferrocifen, a ferrocene-phenol hybrid, has demonstrated promising anticancer properties on drug-resistant cancers. Currently, Ferrocifen is in pre-clinical trial against cancers. Obviously, ferrocene moiety is a useful template for the development of novel anticancer agents. This review will provide an overview of ferrocene-containing hybrids with potential application in the treatment of cancers covering articles published between 2010 and 2020. The mechanisms of action, the critical aspects of design and structure-activity relationships are also discussed.

Bimetallic DppfM(II) (M = Pt and Pd) dithiocarbamate complexes: Synthesis, characterization, and anticancer activity

A series of bimetallic dppfM(II) (dppf = 1,1’-bis (diphenyphosphino) ferrocene; M = Pt and Pd) dithiocarbamate complexes is synthesized and characterized by spectroscopic methods and single-crystal X-ray diffraction. Their antitumor activities in vitro are investigated by MTT assays against four cancer cell lines. The anticancer studies indicate most of the complexes display good to excellent antitumor activity. Remarkably, the platinum complex with a pyrrolidinyl substituent (3b) was identified as the most promising candidate due to its high potency and broad spectrum of activity.

Activation by Oxidation: Ferrocene-Functionalized Ru(II)-Arene Complexes with Anticancer, Antibacterial, and Antioxidant Properties

Organometallic Ru(II)-cymene complexes linked to ferrocene (Fc) via nitrogen heterocycles have been synthesized and studied as cytotoxic agents. These compounds are analogues of Ru(II)-arene piano-stool anticancer complexes such as RAPTA-C. The Ru center was coordinated by pyridine, imidazole, and piperidine with 0-, 1-, or 2-carbon bridges to Fc to give six bimetallic, dinuclear compounds, and the properties of these complexes were compared with their non-Fc-functionalized parent compounds. Crystal structures for five of the compounds, their Ru-cymene parent compounds, and an unusual trinuclear compound were determined. Cyclic voltammetry was used to determine the formal M^III/II potentials of each metal center of the Ru-cymene-Fc complexes, with distinct one-electron waves observed in each case. The Fc-functionalized complexes were found to exhibit good cytotoxicity against HT29 human colon adenocarcinoma cells, whereas the parent compounds were inactive. Similarly, antibacterial activity from the Ru-cymene-Fc compounds was observed against Bacillus subtilis, but not from the unfunctionalized complexes. In both cases, the IC₅₀ values correlated quantitatively with the Fc^+/0 reduction potentials. This is consistent with more facile oxidation to give ferrocenium, and subsequent generation of toxic reactive oxygen species, leading to greater cytotoxicity. The antioxidant properties of the complexes were quantified by a 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay. EC₅₀ values indicate that linking of the Ru and Fc centers promotes antioxidant activity.

Regulating the anticancer properties of organometallic dendrimers using pyridylferrocene entities: synthesis, cytotoxicity and DNA binding studies

Eight new heterometallic ferrocenyl-derived metal–arene metallodendrimers have been prepared and evaluated for anti-tumor activity.

Recent advances in iron complexes as potential anticancer agents

The iron complexes discussed in this review highlight their promising future as anticancer agents.

Induction of Cytotoxicity in Pyridine Analogues of the Anti-metastatic Ru(III) Complex NAMI-A by Ferrocene Functionalization

A series of novel ferrocene (Fc) functionalized Ru(III) complexes was synthesized and characterized. These compounds are derivatives of the anti-metastatic Ru(III) complex imidazolium [trans-RuCl4(1H-imidazole) (DMSO-S)] (NAMI-A) and are derived from its pyridine analogue (NAMI-Pyr), with direct coupling of Fc to pyridine at the 4 or 3 positions, or at the 4 position via a two-carbon linker, which is either unsaturated (vinyl) or saturated (ethyl). Electron paramagnetic resonance (EPR) and UV-vis spectroscopic studies of the ligand exchange processes of the compounds in phosphate buffered saline (PBS) report similar solution behavior to NAMI-Pyr. However, the complex with Fc substitution at the 3 position of the coordinated pyridine shows greater solution stability, through resistance to the formation of oligomeric species. Further EPR studies of the complexes with human serum albumin (hsA) indicate that the Fc groups enhance noncoordinate interactions with the protein and help to inhibit the formation of protein-coordinated species, suggesting the potential for enhanced bioavailability. Cyclic voltammetry measurements demonstrate that the Fc groups modestly reduce the reduction potential of the Ru(III) center as compared to NAMI-Pyr, while the reduction potentials of the Fc moieties of the four compounds vary by 217 mV, with the longer linkers giving significantly lower values of E1/2. EPR spectra of the compounds with 2-carbon linkers show the formation of a high-spin Fe(III) species (S = 5/2) in PBS with a distinctive signal at g = 4.3, demonstrating oxidation of the Fe(II) ferrocene center and likely reflecting degradation products. Density functional theory calculations and paramagnetic (1)H NMR describe delocalization of spin density onto the ligands and indicate that the vinyl linker could be a potential pathway for electron transfer between the Ru and Fe centers. In the case of the ethyl linker, electron transfer is suggested to occur via an indirect mechanism enabled by the greater flexibility of the ligand. In vitro assays with the SW480 cell line reveal cytotoxicity induced by the ruthenium ferrocenylpyridine complexes that is at least an order of magnitude higher than the unfunctionalized complex, NAMI-Pyr. Furthermore, migration studies with LNCaP cells reveal that Fc functionalization does not reduce the ability of the compounds to inhibit cell motility. Overall, these studies demonstrate that NAMI-A-type compounds can be functionalized with redox-active ligands to produce both cytotoxic and anti-metastatic activity.

Potential anticancer heterometallic Fe-Au and Fe-Pd agents: initial mechanistic insights

A series of gold(III) and palladium(II) heterometallic complexes with new iminophosphorane ligands derived from ferrocenylphosphanes [{Cp-P(Ph2)═N-Ph}2Fe] (1), [{Cp-P(Ph2)═N-CH2-2-NC5H4}2Fe] (2), and [{Cp-P(Ph2)═N-CH2-2-NC5H4}Fe(Cp)] (3) have been synthesized and structurally characterized. Ligands 2 and 3 afford stable coordination complexes [AuCl2(3)]ClO4, [{AuCl2}2(2)](ClO4)2, [PdCl2(3)], and [{PdCl2}2(2)]. The complexes have been evaluated for their antiproliferative properties in human ovarian cancer cells sensitive and resistant to cisplatin (A2780S/R), in human breast cancer cells (MCF7) and in a nontumorigenic human embryonic kidney cell line (HEK-293T). The highly cytotoxic trimetallic derivatives M2Fe (M = Au, Pd) are more cytotoxic to cancer cells than their corresponding monometallic fragments. Moreover, these complexes were significantly more cytotoxic than cisplatin in the resistant A2780R and the MCF7 cell lines. Studies of the interactions of the trimetallic compounds with DNA and the zinc-finger protein PARP-1 indicate that they exert anticancer effects in vitro based on different mechanisms of actions with respect to cisplatin.

A versatile diphosphine ligand: cis and trans chelation or bridging, with self association through hydrogen bonding

The diphosphine ligand, N,N'-bis(2-diphenylphosphinoethyl)isophthalamide, dpipa, contains two amide groups and can form cis or trans chelate complexes or cis,cis or trans,trans bridged complexes. The amide groups are likely to be involved in intramolecular or intermolecular hydrogen bonding. This combination of properties of the ligand dpipa leads to very unusual structural properties of its complexes, which often exist as mixtures of monomers and dimers in solution. In the complex [Au2(μ-dpipa)2]Cl2, the ligands adopt the trans,trans bridging mode, with linear gold(I) centers, and the amide groups hydrogen bond to the chloride anions. In [Pt2Cl4(μ-dpipa)2], the ligands adopt the cis,cis bridging mode, with square planar platinum(II) centers, and the amide groups form intermolecular hydrogen bonds to the chloride ligands to form a supramolecular one-dimensional polymer. Both the monomeric and dimeric complexes [PtMe2(dpipa)] and [Pt2Me4(μ-dpipa)2] have cis-PtMe2 units with cis chelating or cis,cis bridging dpipa ligands respectively; each forms a supramolecular dimer through hydrogen bonding between amide groups and each contains an unusual NH···Pt interaction. An attempted oxidative addition reaction with methyl iodide gave the complex [PtIMe(dpipa)], which contains trans chelating dpipa, while a reaction with bromine gave a disordered complex with approximate composition [Pt2Me3Br5(μ-dpipa)2], which contains trans,trans bridging dpipa ligands.

Identification of the structural determinants for anticancer activity of a ruthenium arene peptide conjugate

Organometallic Ru(arene)-peptide bioconjugates with potent in vitro anticancer activity are rare. We have prepared a conjugate of a Ru(arene) complex with the neuropeptide [Leu(5)]-enkephalin. [Chlorido(η(6)-p-cymene)(5-oxo-κO-2-{(4-[(N-tyrosinyl-glycinyl-glycinyl-phenylalanyl-leucinyl-NH2)propanamido]-1H-1,2,3-triazol-1-yl)methyl}-4H-pyronato-κO)ruthenium(II)] (8) shows antiproliferative activity in human ovarian carcinoma cells with an IC50 value as low as 13 μM, whereas the peptide or the Ru moiety alone are hardly cytotoxic. The conjugation strategy for linking the Ru(cym) (cym=η(6)-p-cymene) moiety to the peptide involved N-terminal modification of an alkyne-[Leu(5)]-enkephalin with a 2-(azidomethyl)-5-hydroxy-4H-pyran-4-one linker, using Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC), and subsequent metallation with the Ru(cym) moiety. The ruthenium-bioconjugate was characterized by high resolution top-down electrospray ionization mass spectrometry (ESI-MS) with regard to peptide sequence, linker modification and metallation site. Notably, complete sequence coverage was obtained and the Ru(cym) moiety was confirmed to be coordinated to the pyronato linker. The ruthenium-bioconjugate was analyzed with respect to cytotoxicity-determining constituents, and through the bioconjugate models [{2-(azidomethyl)-5-oxo-κO-4H-pyronato-κO}chloride (η(6)-p-cymene)ruthenium(II)] (5) and [chlorido(η(6)-p-cymene){5-oxo-κO-2-([(4-(phenoxymethyl)-1H-1,2,3-triazol-1-yl]methyl)-4H-pyronato-κO}ruthenium(II)] (6) the Ru(cym) fragment with a triazole-carrying pyronato ligand was identified as the minimal unit required to achieve in vitro anticancer activity.