Anticancer Activity and Catalytic Potential of Ruthenium(II)-Arene Complexes with N,O-Donor Ligands

Analysis of antiproliferative activity of new half-sandwich arene Ru(II) thiophene based aroylhydrazone complexes

Efforts in researching the efficient anti-tumor properties of three novel arene ruthenium(II) complexes incorporating thiophene-based aroylhydrazone ligands have been undertaken. The complexes' elemental composition was [(η6-p-cymene)Ru(L)Cl]. They were comprehensively characterized through elemental and spectroscopic analyses (FT-IR, UV-vis, NMR, and HR-MS). Single crystal X-ray diffraction studies revealed a pseudo-octahedral geometry with bidentate coordination of the ligands in a representative complex. The in vitro assessment of the complexes' cancer cell growth inhibition was conducted using the MTT assay against A549 (human lung carcinoma), HeLa (human cervical carcinoma), HuH-7 (hepatocellular carcinoma), and NIH-3T3 (mouse fibroblast non-cancerous cell line). Results indicated significant cytotoxicity across all cancer cell lines, with IC50 concentrations of complex 2 being 6.8 μM for A549, 11.6 μM for HeLa, and 9.4 μM for HuH-7, compared to cisplatin with IC50 values of 18.9 μM, 17.68 μM, and 24 μM respectively. Notably, complex 2 demonstrated particularly promising cytotoxicity against all tested cancerous cell lines. Fluorescent staining analysis such as acridine orange/ethidium bromide (AO-EB) and HOECHST 33342 revealed cell death mechanisms involving membrane disintegration and nuclear condensation following treatment with complex 2. Further studies were conducted to measure reactive oxygen species (ROS) levels using the dichlorodihydrofluorescein diacetate (DCFH-DA) assay, and mitochondrial membrane potential (MMP) was assessed using the JC-1 dye assay. These studies demonstrated that complex 2 increased ROS levels, decreased membrane potential, and promoted mitochondrial dysfunction-mediated cell death pathways. Additionally, flow cytometry analysis, utilizing dual staining of Annexin V-FITC and propidium iodide (PI), was employed to quantitatively study apoptosis induction.

Exploring the potential of ruthenium(II)-phosphine-mercapto complexes as new anticancer agents

The search for new metal-based anticancer drug candidates is a fundamental task in medicinal inorganic chemistry. In this work, we assessed the potential of two new Ru(II)-phosphine-mercapto complexes as potential anticancer agents. The complexes, with the formula [Ru(bipy)(dppen)(Lx)]PF6 [(1), HL1 = 2-mercapto-pyridine and (2), HL2 = 2-mercapto-pyrimidine, bipy = 2,2'-bipyridine, dppen = cis-1,2-bis(diphenylphosphino)-ethylene] were synthesized and characterized by nuclear magnetic resonance (NMR) [1H, 31P(1H), and 13C], high resolution mass spectrometry (HR-MS), cyclic voltammetry, infrared and UV-Vis spectroscopies. Complex 2 was obtained as a mixture of two isomers, 2a and 2b, respectively. The composition of these metal complexes was confirmed by elemental analysis and liquid chromatography-mass spectrometry (LC-MS). To obtain insights into their lipophilicity, their distribution coefficients between n-octanol/PBS were determined. Both complexes showed affinity mainly for the organic phase, presenting positive log P values. Also, their stability was confirmed over 48 h in different media (i.e., DMSO, PBS and cell culture medium) via HPLC, UV-Vis and 31P{1H} NMR spectroscopies. Since enzymes from the P-450 system play a crucial role in cellular detoxification and metabolism, the microsomal stability of 1, which was found to be the most interesting compound of this study, was investigated using human microsomes to verify its potential oxidation in the liver. The analyses by LC-MS and ESI-MS reveal three main metabolites, obtained by oxidation in the dppen and bipy moieties. Moreover, 1 was able to interact with human serum albumin (HSA). The cytotoxicity of the metal complexes was tested in different cancerous and non-cancerous cell lines. Complex 1 was found to be more selective than cisplatin against MDA-MB-231 breast cancer cells when compared to MCF-10A non-cancerous cells. In addition, complex 1 affects cell morphology and migration, and inhibits colony formation in MDA-MB-231 cells, making it a promising cytotoxic agent against breast cancer.

Naphthoyl benzhydrazine-decorated binuclear arene Ru(II) complexes as anticancer agents targeting human breast cancer cells

Breast cancer is the most dangerous type in women and its fatality rate has increased over the past decade. To develop more potent and target-specific breast cancer drugs, six arene ruthenium(II) complexes (1-6) containing naphthoyl benzhydrazine ligands (NL1-NL3) were synthesized and characterized by analytical and spectroscopic (infrared, UV-visible, NMR and HR-MS) methods. The SC-XRD analysis of 1 and 6 demonstrates the bis N^O bidentate binding nature of ligands to ruthenium ions and a pseudo-octahedral geometry around the Ru(II) ion. Solution stability studies using UV-Vis spectroscopy evidenced the instantaneous hydrolysis of the complexes to form monoaquated species in a solution of 1 : 9 (v/v) DMSO/phosphate buffer. All the complexes were screened for their in vitro antiproliferative activities against different human breast cancer cells, including MCF-7, SkBr3, MDA-MB-468, MDA-MB-231, and non-cancerous HEK-293 cells, by an MTT assay, and they displayed good cancer cell growth inhibitory capacity with low IC50 values. Notably, complexes 2 and 5 comprising methoxy and p-cymene groups exhibited excellent cytotoxicity towards SkBr3 cells compared to clinical drug cisplatin. AO-EB and HOECHST-33342 staining assays revealed apoptotic morphological changes in complex-treated cancer cells. Further, reactive oxygen species and mitochondrial membrane potential assays validated that the complexes induce apoptotic cell death via an intrinsic mitochondrial pathway with ROS production. In addition, the apoptotic induction and the quantification of late apoptosis were established with the aid of western blot and flow cytometry analysis, respectively.

Monofunctional dimetallic Ru(η6-arene) complexes inhibit NOTCH1 signaling pathway and synergistically enhance anticancer effect in combination with cisplatin or vitamin C

ONS donor ligands L1-L4 were utilized in the preparation of monofunctional dimetallic Ru(η6-arene) complexes (C1-C4). These ONS donor ligand based novel tricoordinated Ru(II) complexes bearing η6-arene co-ligand were prepared for the first time. The current methodology resulted in excellent isolated yields and these complexes were characterized in detail by different spectroscopic and spectrometric techniques. The structures of C1-C2 and C4 were characterized in solid state by single crystal X-ray analysis. The in vitro anticancer analyses showed these novel complexes suppressed the growth of breast (MCF-7), liver (HepG2) and lung (A549) cancer cells. C2 suppressed the growth of these cells in dose-dependent manner revealed form the MTT and crystal violet cell viability assays. Moreover, C2 was observed the most potent complex that was used further in detailed mechanistic analyses in cancer cells. C2 showed good cytotoxic activity at 10 μM dose level as compared to cisplatin or oxaliplatin in these cancer cells. We observed morphological changes in cancer cells upon treatment with C2. Moreover, C2 suppressed the invasion and migration ability of cancer cells. C2 induced cellular senescence to retard cell growth and suppressed the formation of cancer stem cells. Importantly, C2 showed synergistic anticancer effect in combination with cisplatin and Vitamin C to further inhibit cell growth which suggested the potential role of C2 in cancer therapy. Mechanistically, C2 inhibited NOTCH1 dependent signaling pathway to suppress cancer cell invasion, migration and cancer stem cells formation. Thus, these data suggested potential role of C2 in cancer therapy by targeting NOTCH1-dependent signaling to suppress tumorigenesis. The results obtained in this study for these novel monofunctional dimetallic Ru(η6-arene) complexes showed their high anticancer potency and this study will pave to further cytotoxicity exploration on this class of complexes.

Targeting cancer lactate metabolism with synergistic combinations of synthetic catalysts and monocarboxylate transporter inhibitors

Synthetic anticancer catalysts offer potential for low-dose therapy and the targeting of biochemical pathways in novel ways. Chiral organo-osmium complexes, for example, can catalyse the asymmetric transfer hydrogenation of pyruvate, a key substrate for energy generation, in cells. However, small-molecule synthetic catalysts are readily poisoned and there is a need to optimise their activity before this occurs, or to avoid this occurring. We show that the activity of the synthetic organometallic redox catalyst [Os(p-cymene)(TsDPEN)] (1), which can reduce pyruvate to un-natural D-lactate in MCF7 breast cancer cells using formate as a hydride source, is significantly increased in combination with the monocarboxylate transporter (MCT) inhibitor AZD3965. AZD3965, a drug currently in clinical trials, also significantly lowers the intracellular level of glutathione and increases mitochondrial metabolism. These synergistic mechanisms of reductive stress induced by 1, blockade of lactate efflux, and oxidative stress induced by AZD3965 provide a strategy for low-dose combination therapy with novel mechanisms of action.

Syntheses, Structural Characterization, and Cytotoxicity Assessment of Novel Mn(II) and Zn(II) Complexes of Aroyl-Hydrazone Schiff Base Ligand

This work describes the syntheses, structural characterization, and biological profile of Mn(II)- and Zn(II)-based complexes 1 and 2 derived from the aroyl-hydrazone Schiff base ligand (L1). The synthesized compounds were thoroughly characterized by elemental analysis, Fourier transform infrared spectroscopy (FTIR), UV-vis, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and single-crystal X-ray diffraction (s-XRD). Density functional theory (DFT) studies of complexes 1 and 2 were performed to ascertain the structural and electronic properties. Hirshfeld surface analysis was used to investigate different intermolecular interactions that define the stability of crystal lattice structures. To ascertain the therapeutic potential of complexes 1 and 2, in vitro interaction studies were carried out with ct-DNA and bovine serum albumin (BSA) using analytical and multispectroscopic techniques, and the results showed more avid binding of complex 2 than complex 1 and L1. The antioxidant potential of complexes 1 and 2 was examined against the 2,2-diphenyl picrylhydrazyl (DPPH) free radical, which revealed better antioxidant ability of the Mn(II) complex. Moreover, the antibacterial activity of synthesized complexes 1 and 2 was tested against Gram-positive and Gram-negative bacteria in which complex 2 demonstrated more effective bactericidal activity than L1 and complex 1 toward Gram-positive bacteria. Furthermore, the in vitro cytotoxicity assessment of L1 and complexes 1 and 2 was carried out against MDA-MB-231 (triple negative breast cancer) and A549 (lung) cancer cell lines. The cytotoxic results revealed that the polymeric Zn(II) complex exhibited better and selective cytotoxicity against the A549 cancer cell line as was evidenced by its low IC₅₀ value.

Cyanine-Functionalized 2,2'-Bipyridine Compounds for Photocatalytic Cancer Therapy

Recently, interest has been given to developing photocatalytic anticancer drugs. This area of research is dominated by metal complexes. Here, we report the potential of lysosome/mitochondria targeting cyanine appended bipyridine compounds as the organic photocatalytic anticancer agents. The organocatalyst (bpyPCN) not only exhibits light-induced NADH oxidation but also generates intracellular ROS to demonstrate anticancer activity. This is the first example of organic compound induced catalytic NADH photo-oxidation in an aqueous solution and in cancer cells.

Os(II) complexes for catalytic anticancer therapy: recent update

The recent dramatic enhancement in cancer-related mortality and the drawbacks (side effects and resistance) of Pt-based first-generation chemotherapeutics have escalated the need for new cancer medicines with unique anticancer activities for better human life. To overcome the demerits of Pt-based cancer drugs, the concept of catalytic anticancer agents has recently been presented in the field of anticancer metallodrug development research. Many intracellular transformations in cancer cells are catalyzed by metal complexes, including pyruvate reduction to lactate, NAD(P)⁺ reduction to NAD(P)H and vice versa, and the conversion of ³O₂ to reactive oxygen species (ROS). These artificial in-cell changes with non-toxic and catalytic dosages of metal complexes have been shown to disrupt several essential intracellular processes which ultimately cause cell death. This new approach could develop potent next-generation catalytic anticancer drugs. In this context, recently, several 16/18 electron Os(II)-based complexes have shown promising catalytic anticancer activities with unique anticancer mechanisms. Herein, we have delineated the catalytic anticancer activity of Os(II) complexes from a critical viewpoint. These catalysts are reported to induce the in-cell catalytic transfer hydrogenation of pyruvate and important quinones to create metabolic disorder and photocatalytic ROS generation for oxidative stress generation in cancer cells. Overall, these Os(II) catalysts have the potential to be novel catalytic cancer drugs with new anticancer mechanisms.

New Triazolyl N^N Bidentate Rh(III), Ir(III), Ru(II) and Os(II) Complexes: Synthesis and Characterization, Probing Possible Relations between Cytotoxicity with Transfer Hydrogenation Efficacy and Interaction with Model Biomolecules

Cisplatin and other metallodrugs have realised great success in clinical chemotherapeutic applications as anticancer drugs. However, severe toxicity to healthy cells and non-selectivity to cancer cells remains a challenge, warranting the further search for alternative agents. Herein, we report the anticancer potential of a series of complexes of the general formula [MCl(p-cym)(k2-N^N-L)]+ X− and [MCl(Cp*)(k2-N^N-L)]+ X−, where M is the metal centre (Ru(II), Os(II), Rh(III) or Ir(III)), L = 1-benzyl-4-pyridinyl-1-H-1,2,3-triazole for L1 and 1-picolyl-4-pyridinyl-1-H-1,2,3-triazole for L2 and X− = Cl−, BF4−, BPh4−. When evaluated for activity against some cancerous and non-cancerous cell lines (namely, HeLa, HEK293, A549 and MT4 cancer cells and the normal healthy kidney cells (BHK21)), most of the compounds displayed poor cytotoxicities against cancer cells except for complexes C2 ([RuCl(p-cym)(k2-N^N-L1)]+ BPh4−, EC50 = 9−16 µM and SI = 14), C7 ([RuCl(p-cym)(k2-N^N-L2)]+ BPh4−, EC50 = 17−53 µM and SI = 4) and C11 ([IrCl(Cp*)(k2-N^N-L2)]+ BF4−, EC50 < 5 µM and SI > 10). Selected complexes C1 ([RuCl(p-cym)(k2-N^N-L1)]+ BF4−), C5 ([IrCl(Cp*)(k2-N^N-L1)]+ BF4−) and C11 showed significant interactions with model biomolecules such as guanosine-5′-monophosphate (5′-GMP), bovine serum albumin (BSA) and amino acids under physiological conditions, possibly through carbenylation and N-coordination with 5′-GMP, N-coordination with L-Histidine and L-proline. While the compounds showed good activities in reducing pyruvate to lactate, there was no direct correlation between catalytic transfer hydrogenation of pyruvate and the observed cytotoxic activities. As observed in this work, the marked influence of single atom replacement in ligand may provide a pivotal approach to improving the cytotoxicity and fine-tuning the selectivity to cancer cells.

Impact of aliphatic acyl and aromatic thioamide substituents on the anticancer activity of Ru(II)-p-cymene complexes with acylthiourea ligands-in vitro and in vivo studies

Six different acylthiourea ligands (L1-L6) and their corresponding Ru(II)-p-cymene complexes (P1-P6) were designed to explore the structure-activity relationship of the complexes upon aliphatic chain and aromatic conjugation on the C- and N-terminals, respectively. The compounds were synthesized and adequately characterized using various analytical and spectroscopic techniques. The structures of P2-P6, solved using single crystal X-ray diffraction (XRD), confirmed the neutral monodentate coordination of the S atoms of the acylthiourea ligands to Ru(II) ions. In silico studies showed an increase of lipophilicity for the ligands with an increase in alkyl chain length or aromatic conjugation at the C- or N-terminal, respectively. Subsequently, mitogen-activated protein kinases (MAPK) were predicted as one of the primary targets for the complexes, which showed good binding affinity towards extracellular signal-regulated kinases (ERK1, ERK2 and ERK5), c-Jun N-terminal kinase (JNK) and p38 of the MAPK pathway. Henceforth, the complexes were tested for their anticancer activity in lung carcinoma (A549) and cisplatin-resistant lung carcinoma (cisA549R) cells and human umbilical vein epithelial normal cells (HUVEC). Interestingly, an increase in chain length or aromatic conjugation led to an increase in the activity of the complexes, with P5 (7.73 and 13.04 μM) and P6 (6.52 and 14.45 μM) showing the highest activity in A549 and cisA549R cells, which is better than the positive control, cisplatin (8.72 and 44.28 μM). Remarkably, we report the highest activity yet observed for complexes of the type [(η6-p-cymene)RuIICl2(S-acylthiourea)] in the tested cell lines. Aqueous solution studies showed that complexes P5 and P6 are rapidly hydrolyzed to produce solely aquated species that remained stable for 24 h. Staining assays and flow cytometric analyses of P5 and P6 in A549 cells revealed that the complexes induced apoptosis and arrested the cell cycle predominantly in the S phase. In vivo studies demonstrated the higher toxicity of cisplatin and a comparatively higher survival rate of mice injected with the most active complex P6. Histological analyses revealed that treatment with P6 at high doses of up to 8 mg kg-1 did not cause any palpable damage to the tested organs.

Synthesis, Structure and Cytotoxicity of N,N and N,O-Coordinated RuII Complexes of 3-Aminobenzoate Schiff Bases against Triple-negative Breast Cancer

Half-sandwich Ru^II complexes, [(YZ)Ru^II (η⁶ -arene)(X)]+, (YZ=chelating bidentate ligand, X=halide), with N,N and N,O coordination (1-9) show significant antiproliferative activity against the metastatic triple-negative breast carcinoma (MDA-MB-231). 3-aminobenzoic acid or its methyl ester is used in all the ligands while varying the aldehyde for N,N and N,O coordination. In the N,N coordinated complex the coordinated halide(X) is varied for enhancing stability in solution (X=Cl, I). Rapid aquation and halide exchange of the pyridine analogues, 2 and 3, in solution are a major bane towards their antiproliferative activity. Presence of free -COOH group (1 and 4) make complexes hydrophilic and reduces toxicity. The imidazolyl 3-aminobenzoate based N,N coordinated 5 and 6 display better solution stability and efficient antiproliferative activity (IC₅₀ ca. 2.3-2.5 μM) compared to the pyridine based 2 and 3 (IC₅₀ >100 μM) or the N,O coordinated complexes (7-9) (IC₅₀ ca. 7-10 μM). The iodido coordinated, 6, is resistant towards aquation and halide exchange. The N,O coordinated 7-9 underwent instantaneous aquation at pH 7.4 generating monoaquated complexes stable for at least 6 h. Complexes 5 and 6, bind to 9-ethylguanine (9-EtG) showing propensity to interact with DNA bases. The complexes may kill via apoptosis as displayed from the study of 8. The change in coordination mode and the aldehyde affected the solution stability, antiproliferative activity and mechanistic pathways. The N,N coordinated (5 and 6) exhibit arrest in the G2/M phase while the N,O coordinated 8 showed arrest in the G0/G1 phase.

Synthesis, characterisation and biological activity of the ruthenium(II) complexes of the N4-tetradentate (N4-TL), 1,6-di(2'-pyridyl)-2,5-dibenzyl-2,5-diazahexane (picenBz2)

A series of complexes of the type rac-cis-β-[Ru(N₄-T_L)(N₂-bidentates)]²⁺ (where N₄-T_L = 1,6-di(2'-pyridyl)-2,5-dibenzyl-2,5-diazahexane (picenBz₂, N₄-T_L-2) and N₂-bidentates = 1,10-phenanthroline (phen, Ru-2), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq, Ru-3), 7,8-dimethyl-dipyrido[3,2-a:2',3'-c] phenazine (dppzMe_2,Ru-4), 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline (phenpyrBz, Ru-5), 2-(p-tolyl)-1H-imidazo[4,5-f][1,10]phenanthroline (phenpyrBzMe, Ru-6), 2-(4-nitrophenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (phenpyrBzNO_2,Ru-7), were synthesised and characterised and X-ray crystallography of Ru-5 obtained. The in vitro cytotoxicity assays revealed that Ru-6 was 5, 2 and 19-fold more potent than oxaliplatin, cisplatin, and carboplatin, respectively displaying an average GI₅₀ value of ≈ 0.76 μM against a panel of 11 cancer cell lines.

Osmium(ii) tethered half-sandwich complexes: pH-dependent aqueous speciation and transfer hydrogenation in cells

Aquation is often acknowledged as a necessary step for metallodrug activity inside the cell. Hemilabile ligands can be used for reversible metallodrug activation. We report a new family of osmium(ii) arene complexes of formula [Os(η6-C6H5(CH2)3OH)(XY)Cl]+/0 (1-13) bearing the hemilabile η6-bound arene 3-phenylpropanol, where XY is a neutral N,N or an anionic N,O- bidentate chelating ligand. Os-Cl bond cleavage in water leads to the formation of the hydroxido/aqua adduct, Os-OH(H). In spite of being considered inert, the hydroxido adduct unexpectedly triggers rapid tether ring formation by attachment of the pendant alcohol-oxygen to the osmium centre, resulting in the alkoxy tethered complex [Os(η6-arene-O-κ1)(XY)] n+. Complexes 1C-13C of formula [Os(η6:κ1-C6H5(CH2)3OH/O)(XY)]+ are fully characterised, including the X-ray structure of cation 3C. Tether-ring formation is reversible and pH dependent. Osmium complexes bearing picolinate N,O-chelates (9-12) catalyse the hydrogenation of pyruvate to lactate. Intracellular lactate production upon co-incubation of complex 11 (XY = 4-Me-picolinate) with formate has been quantified inside MDA-MB-231 and MCF7 breast cancer cells. The tether Os-arene complexes presented here can be exploited for the intracellular conversion of metabolites that are essential in the intricate metabolism of the cancer cell.

Metal-Based Catalytic Drug Development for Next-Generation Cancer Therapy

Considering the high increase in mortality caused by cancer in recent years, cancer drugs with novel mechanisms of anticancer action are urgently needed to overcome the drawbacks of platinum-based chemotherapeutics. Recently, in the area of metal-based cancer drug development research, the concept of catalytic cancer drugs has been introduced with organometallic Ru^II , Os^II , Rh^III and Ir^III complexes. These complexes are reported as catalysts for many important biological transformations in cancer cells such as nicotinamide adenine dinucleotide (NAD(P)H) oxidation to NAD⁺ , reduction of NAD⁺ to NADH, and reduction of pyruvate to lactate. These unnatural intracellular transformations with catalytic and nontoxic doses of metal complexes are known to severely perturb several important biochemical pathways and could be the antecedent of next-generation catalytic cancer drug development. In this concept, we delineate the prospects of such recently reported organometallic Ru^II , Os^II , Rh^III and Ir^III complexes as future catalytic cancer drugs. This new approach has the potential to deliver new cancer drug candidates.

Investigation into antiproliferative activity and apoptosis mechanism of new arene Ru(ii) carbazole-based hydrazone complexes

Ruthenium complexes with bioactive ligands are becoming promising substitutes for platinum complexes due to their precise action against various cancers. In the present study, the synthesis of three new arene Ru(ii) complexes containing new carbazole-based hydrazone ligands of general formula [(η6-benzene)Ru(L)Cl] (1-3; L = carbazolone benzhydrazone ligands), and their anticancer properties are described. The structural characterization of the ligands and their ruthenium complexes has been realized with the aid of elemental analysis, IR, UV-vis, NMR and HR-MS techniques. The molecular structures of all three complexes have been elucidated by single crystal X-ray crystallography and reveal the existence of pseudo-octahedral geometry around the ruthenium. The in vitro cancer cell growth inhibition property of the complexes against A549 (lung carcinoma), A2780 (ovarian adenocarcinoma) and non-cancerous 16HBE (human lung bronchial epithelium) cells were examined by MTT assay. All the complexes display good cytotoxicity towards both of these types of cancer cell compared to the standard drug cisplatin, with low IC50 values. Remarkably, complex 3, which contains an electron-donating substituent, induces a significant reduction of viability in A2780 cells. The inhibition capacity of the complexes towards A2780 cells proliferation was further confirmed using 5-ethynyl-2-deoxyuridine (EdU) assay via minimal DNA synthesis. The result of the acridine orange-ethidium bromide (AO-EB) fluorescent staining assay establishes that the cytotoxicity of the complexes was mediated by apoptosis in cancer cells. Furthermore, flow cytometry using Annexin V-FITC/propidium iodide (PI) double staining determines the quantitative discrimination of early apoptosis by the externalization of phosphatidylserine. In addition, cell cycle distribution indicates that the complexes block the cell cycle progression in the S-phase. The outcome of our investigation shows the promising scope and potency of tailored arene ruthenium complexes for precise cancer chemotherapy beyond platinum drugs.

Transfer hydrogenation catalysis in cells

Hydrogenation reactions in biology are usually carried out by enzymes with nicotinamide adenine dinucleotide (NAD(P)H) or flavin mononucleotide (FAMH2)/flavinadenine dinucleotide (FADH2) as cofactors and hydride sources. Industrial scale chemical transfer hydrogenation uses small molecules such as formic acid or alcohols (e.g. propanol) as hydride sources and transition metal complexes as catalysts. We focus here on organometallic half-sandwich RuII and OsII η6-arene complexes and RhIII and IrIII η5-Cp x complexes which catalyse hydrogenation of biomolecules such as pyruvate and quinones in aqueous media, and generate biologically important species such as H2 and H2O2. Organometallic catalysts can achieve enantioselectivity, and moreover can be active in living cells, which is surprising on account of the variety of poisons present. Such catalysts can induce reductive stress using formate as hydride source or oxidative stress by accepting hydride from NAD(P)H. In some cases, photocatalytic redox reactions can be induced by light absorption at metal or flavin centres. These artificial transformations can interfere in biochemical pathways in unusual ways, and are the basis for the design of metallodrugs with novel mechanisms of action.

A novel biocompatible formate bridged 1D-Cu(ii) coordination polymer induces apoptosis selectively in human lung adenocarcinoma (A549) cells

Copper compounds are promising candidates for next-generation metal anticancer drugs. Therefore, we synthesized and characterized a formate bridged 1D coordination polymer [Cu(L)(HCOO)₂]_n, (L = 2-methoxy-6-methyl-3-((quinolin-8-ylimino)methyl)chroman-4-ol), PCU1, wherein the Cu(ii) center adopts a square pyramidal coordination environment with adjacent CuCu distances of 5.28 Å. Primarily, in vitro DNA interaction studies revealed a metallopolymer which possesses high DNA binding propensity and cleaves DNA via the oxidative pathway. We further analysed its potential on cancerous cells MCF-7, HeLa, A549, and two non-tumorigenic cells HEK293 and HBE. The selective cytotoxicity potential of PCU1 against A549 cells driven us to examine the mechanistic pathways comprehensively by carrying out various assays viz, cell cycle arrest, Annexin V-FTIC/PI assay, autophagy, intercellular localization, mitochondrial membrane potential 'MMP', antiproliferative assay, and gene expression of TGF-β and MMP-2.

DFT Investigation of the η6 ⇌ η6-Inter-ring Haptotropic Rearrangement of the Group 8 Metals Complexes [(graphene)MCp]+ (M = Fe, Ru, Os)

Metalcyclopentadienyl complexes (MCp)⁺ (M = Fe, Ru, Os) bound to the large polyaromatic hydrogenated hydrocarbon (PAH) C₉₆H₂₄ used as a model for pristine graphene have been studied using a density functional theory (DFT) generalized gradient approximation (PBE functional) to reveal their structural features and dynamic behavior. The inter-ring haptotropic rearrangements (IRHRs) for these complexes were shown to occur via two transition states and one intermediate. The energy barriers of the η⁶ ⇌ η⁶ IRHRs of the (MCp)⁺ unit were found to be 30, 27, and 29 kcal/mol for M = Fe, Ru, and Os, respectively. These values are significantly lower than the values found previously for smaller PAHs. Both polar and nonpolar solvents were found not to affect significantly the energy barrier heights. Investigated transition metal complexes could be used in general as catalysts in the design of novel derivatives or materials with promising properties. Metalcyclopentadienyl complexes (MCp)⁺ of PAHs show catalytic properties mainly due to their structural details as well as their important characteristic of inter-ring haptotropic rearrangement. IRHRs take place usually by intramolecular mechanisms. During IRHRs, the ML_n organometallic groups (OMGs) undergo shifting along the PAH plane and could coordinate additional reagents, which is important for catalysis. Large PAHs such as graphene, fullerenes, and nanotubes possess intrinsic anticancer activity, and numerous arene complexes of Ru and Os have been proven to have anticancer properties as well. We suppose that coordinating Ru or Os to very large PAHs could synergistically increase the anticancer activity of resulting complexes.

Novel NHC-coordinated ruthenium(II) arene complexes achieve synergistic efficacy as safe and effective anticancer therapeutics

There is an urgent need for more effective, less toxic cancer therapy agents. Motivated by this need, we synthesized a small panel of N-heterocyclic carbene (NHC)-coordinated ruthenium(II) arene complexes Ru1-Ru6 with the formula [Ru(p-cymene)(L)Cl]PF₆ (L = NHC ligand with varying substituents). Cell-based in vitro studies revealed that despite the structural similarity, Ru1-Ru6 exhibited distinct cytotoxic activities against cancer cells. In particular, Ru4 and Ru6, which bear n-octyl and pentamethylbenzyl motifs, respectively, were the most active at inducing apoptosis. In human ovarian A2780 cancer cells, Ru4 and Ru6 showed the highest cytotoxicities with IC₅₀ values of 2.74 ± 0.15 μM and 1.98 ± 0.10 μM, respectively, and they were approximately 2-fold more potent than cisplatin (IC₅₀ = 5.55 ± 0.37 μM). In addition to the cell killing capacity, inhibition of cell migration was validated by using these two optimized complexes. Mechanistic studies revealed that Ru4 and Ru6 complexes induced apoptosis in a caspase-dependent manner, primarily through intracellular reactive oxygen species (ROS) overproduction and cell cycle arrest at G1 phase. Furthermore, in a preclinical metastatic model of A2780 tumor xenograft, administration of Ru4 and Ru6 (20 μmol/kg) resulted in a marked inhibition of tumor progression and metastasis. Finally, a substantially alleviated systemic toxicity was observed for both complexes in comparison with cisplatin in animals. Overall, this study greatly increases our understanding of NHC-coordinated Ru(II) arene metallodrugs, aiding further investigation of their therapeutic potential in the treatment of metastatic cancers.

Effect of N,N Coordination and RuII Halide Bond in Enhancing Selective Toxicity of a Tyramine-Based RuII (p-Cymene) Complex

Ruthenium compounds are promising anticancer candidates owing to their lower side-effects and encouraging activities against resistant tumors. Half-sandwich piano-stool type Ru^II compounds of general formula [(L)Ru^II(η⁶-arene)(X)]⁺ (L = chelating bidentate ligand, X = halide) have exhibited significant therapeutic potential against cisplatin-resistant tumor cell lines. In Ru^II (p-cymene) based complexes, the change of the halide leaving group has led to several interesting features, viz., hydrolytic stability, resistance toward thiols, and alteration in pathways of action. Tyramine is a naturally occurring monoamine which acts as a catecholamine precursor in humans. We synthesized a family of N,N and N,O coordinated Ru^II (p-cymene) complexes, [(L)Ru^II(η⁶-arene)(X)]⁺ (1-4), with tyramine and varied the halide (X = Cl, I) to investigate the difference in reactivity. Our studies showed that complex 2 bearing N,N coordination with an iodido leaving group shows selective in vitro cytotoxicity against the pancreatic cancer cell line MIA PaCa-2 (IC₅₀ ca. 5 μM) but is less toxic to triple-negative breast cancer (MDA-MB-231), hepatocellular carcinoma (Hep G2), and the normal human foreskin fibroblasts (HFF-1). Complex 2 displays stability toward hydrolysis and does not bind with glutathione, as confirmed by ¹H NMR and ESI-HRMS experiments. The inert nature of 2 leads to enhancement of cytotoxicity (IC₅₀ = 5.3 ± 1 μM) upon increasing the cellular treatment time from 48 to 72 h.

Synthesis and biological assessment of a ruthenium(II) cyclopentadienyl complex in breast cancer cells and on the development of zebrafish embryos

Ruthenium-based complexes currently attract great attention as they hold promise to replace platinum-based drugs as a first line cancer treatment. Whereas ruthenium arene complexes are some of the most studied species for their potential anticancer properties, other types of ruthenium complexes have been overlooked for this purpose. Here, we report the synthesis and characterization of Ru(II) cyclopentadienyl (Cp), Ru(II) cyclooctadienyl (COD) and Ru(III) complexes bearing anastrozole or letrozole ligands, third-generation aromatase inhibitors currently used for the treatment of estrogen receptor positive (ER +) breast cancer. Among these complexes, Ru(II)Cp 2 was the only one that displayed a high stability in DMSO and in cell culture media and consequently, the only complex for which the in vitro and in vivo biological activities were investigated. Unlike anastrozole alone, complex 2 was considerably cytotoxic in vitro (IC50 values < 1 μM) in human ER + breast cancer (T47D and MCF7), triple negative breast cancer (TNBC) (MBA-MB-231), and in adrenocortical carcinoma (H295R) cells. Theoretical (docking simulation) and experimental (aromatase catalytic activity) studies suggested that an interaction between 2 and the aromatase enzyme was not likely to occur and that the bulkiness of the PPh3 ligands could be an important factor preventing the complex to reach the active site of the enzyme. Exposure of zebrafish embryos to complex 2 at concentrations around its in vitro cytotoxicity IC50 value (0.1-1 μM) did not lead to noticeable signs of toxicity over 96 h, making it a suitable candidate for further in vivo investigations. This study confirms the potential of Ru(II)Cp complexes for breast cancer therapy, more specifically against TNBCs that are usually not responsive to currently used chemotherapeutic agents.

Synthesis, characterization and biological evaluation of cationic organoruthenium(ii) fluorene complexes: influence of the nature of the counteranion

In this study, five ruthenium arene complexes with fluorene-bearing N,N-(1) and N,O-(2) donor Schiff base ligands were synthesized and fully characterized. Cationic ruthenium complexes 3[X], ([Ru(η⁶-C₆H₆)(Cl)(fluorene-N[double bond, length as m-dash]CH-pyridine)][X] (where X = BF₄, PF₆, BPh₄), were obtained by reacting ligand 1 with [Ru(η⁶-C₆H₆)Cl₂]₂ in the presence of NH₄X salts, whereas neutral complex 4, Ru(η⁶-C₆H₆)(Cl)(fluorene-N[double bond, length as m-dash]CH-naphtholate), was isolated by reacting ligand 2 with the same precursor. It was possible to obtain a cationic version of the latter, 5[BF₄], by reacting 4 with AgBF₄ in the presence of pyridine. All compounds were fully characterized by NMR and HR-ESI-MS whereas some of them were also analyzed by single crystal X-ray analysis. Their in vitro antiproliferative activity was also assessed in human breast cancer cell lines, notably MCF-7 and T47D. Complex 4 and its cationic counterpart 5[BF₄] were found to be the most cytotoxic compounds of the series (IC₅₀ = 6.2-16.2 μM) and displayed higher antiproliferative activities than cisplatin in both cell lines. It was found that 5[BF₄] undergoes a ligand exchange reaction and readily converts to 4 in the presence of 0.1 M NaCl, explaining the similarity in their observed cytotoxicities. Whereas 3[BF₄] and 3[PF₆] were found inactive at the tested concentrations, 3[BPh₄] displayed a considerable cytotoxicity (IC₅₀ = 16.7-27.8 μM). Notably, 3[BPh₄], 4 (and 5[BF₄]) were active against T47D, a cisplatin resistant cell line. Interestingly, 4 (16.4 μM) was found to be less cytotoxic than 3[BPh₄] and cisplatin (6.6 and 7.9 μM, respectively) in breast healthy cells (MCF-12A). However, in comparison to 4 and cisplatin (at 10 μM), a lower in vivo toxicity was observed for complex 3[BPh₄] on the development of zebrafish (Danio rerio) embryos.

Ruthenium(II) Complexes as Potential Apoptosis Inducers in Cancer Therapy

The compound cis-diamminedichloroplatinum(II) (cisplatin) is the most widely used anticancer drug, but due to its serious side effects (including gastrointestinal symptoms, renal tubular injury, neuromuscular complications, and ototoxicity), clinical applications of cisplatin are limited. Therefore, these limitations have provided an encouragement for further research into other transition metal complexes, with an aim to overcome the disadvantages related with cisplatin therapy. In the search for effective complexes that can be targeted against tumor cells, many research groups synthesized various ruthenium( II) complexes with different ligands. Also, newly synthesized ruthenium(II) complexes showed selective anticancer activity against different types of cancer cells. Activity of ruthenium(II) complexes in some cases was even higher than that of cisplatin against the same cells. Precise mechanism of action of ruthenium(II) complexes is not fully understood. The different examples mentioned in this review showed that ruthenium(II) complexes decreased viability of cancer cells by induction of apoptosis and/or by cell cycle arrest which implies their different mechanism of action against different types of cancer cells.

Binding mechanisms of half-sandwich Rh(III) and Ru(II) arene complexes on human serum albumin: a comparative study

Abstract

Various half-sandwich ruthenium(II) arene complexes and rhodium(III) arene complexes have been intensively investigated due to their prominent anticancer activity. The interaction of the organometallic complexes of Ru(η⁶-p-cymene) and Rh(η⁵-C₅Me₅) with human serum albumin (HSA) was studied in detail by a combination of various methods such as ultrafiltration, capillary electrophoresis, ¹H NMR spectroscopy, fluorometry and UV–visible spectrophotometry in the presence of 100 mM chloride ions. Binding characteristics of the organometallic ions and their complexes with deferiprone, 2-picolinic acid, maltol, 6-methyl-2-picolinic acid and 2-quinaldic acid were evaluated. Kinetic aspects and reversibility of the albumin binding are also discussed. The effect of low-molecular-mass blood components on the protein binding was studied in addition to the interaction of organorhodium complexes with cell culture medium components. The organometallic ions were found to bind to HSA to a high extent via a coordination bond. Release of the bound metal ions was kinetically hindered and could not be induced by the denaturation of the protein. Binding of the Ru(η⁶-p-cymene) triaqua cation was much slower (ca. 24 h) compared to the rhodium congener (few min), while their complexes interacted with the protein relatively fast (1–2 h). The studied complexes were bound to HSA coordinatively. The highly stable and kinetically inert 2-picolinate Ru(η⁶-p-cymene) complex bound in an associative manner preserving its original entity, while lower stability complexes decomposed partly or completely upon binding to HSA. Fast, non-specific and high-affinity binding of the complexes on HSA highlights their coordinative interaction with various types of proteins possibly decreasing effective drug concentration.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1007/s00775-019-01683-0) contains supplementary material, which is available to authorized users.

Crystal structure of trans-tetrakis(acetonitrile-κN)bis(triphenylphosphine-κP)ruthenium(II) dihexafluorophosphate - dichloroform (1/2), C46H44Cl6F12N4P4Ru

C46H44Cl6F12N4P4Ru, orthorhombic, Pbca (no. 61), a = 14.319(2) Å, b = 16.7358(5) Å, c = 22.6972(16) Å, Z = 4, V = 5439.1(9) Å3, R gt(F) = 0.0727, wR ref(F 2) = 0.1673, T = 235.38 K.

Anticancer activity and mechanism of bis-pyrimidine based dimetallic Ru(II)(η6-p-cymene) complex in human non-small cell lung cancer via p53-dependent pathway

Non-small cell lung cancer (NSCLC) is the most common cancer worldwide, which is related with poor prognosis and resistance to chemotherapy. Notably, ruthenium-based complexes have emerged as good alternative to the currently used platinum-based drugs for cancer therapy. In the present study, we synthesized a novel bis-pyrimidine based ligand 1,3-bis(2-methyl-6-(pyridin-2-yl)pyrimidin-4-yl)benzene (L) and used it in the synthesis of a dimetallic Ru(II) cymene complex [(Ru(η⁶-p-cymene)Cl)₂(1,3-bis(2-methyl-6-(pyridin-2-yl)pyrimidin-4-yl)benzene)] (L-Ru). We checked the stability of this complex in solution state in D₂O/DMSO‑d₆ mixture and found it to be highly stable under these conditions. We determined the anticancer activity and mechanism of action of L-Ru in human NSCLC A549 and A427 by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and related biological analyses. These results revealed that L-Ru exerted a strong inhibitory effect on the cells proliferation,G0/G1-arrest, accompanied with upregulation of p53, p21, p15, cleaved Poly (ADP-ribose) polymerase (PARP) protein and downregulation of cell cycle markers. L-Ru inhibited cell migration and invasion. The mitochondria-mediated apoptosis of NSCLC induced by L-Ru was also observed followed by the increase of apoptosis regulator B-cell lymphoma 2 associated X (BAX), and activation of caspase-3/-9. The effects of L-Ru on the cell viability, Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positive cells and Annexin V-positive cells apoptosis induction were remarkably attenuated. This complex induced DNA damage, cell cycle arrest and cell death via caspase-dependent apoptosis involving PARP activation and induction of p53-dependent pathway. These findings suggested that this ruthenium complex might be a potential effective chemotherapeutic agent in NSCLC therapy.

Mechanistic study on substitution reaction of a citrato(p-cymene)Ru(ii) complex with sulfur-containing amino acids

Thorough kinetic study revealed characteristics of the reaction mechanism for arene ruthenium(ii) complexes with bio-related ligands.

Luminescent fac-[Re(CO)3(phen)] carboxylato complexes with non-steroidal anti-inflammatory drugs: synthesis and mechanistic insights into the in vitro anticancer activity of fac-[Re(CO)3(phen)(aspirin)]

Luminescent fac-[Re(CO)₃(phen)(aspirin)]: insights into in vitro anticancer activity and confocal microscopy imaging in HeLa cells.

Dimetallic Ru(II) arene complexes appended on bis-salicylaldimine induce cancer cell death and suppress invasion via p53-dependent signaling

A series of bis-salicylaldimine ligands bearing two ON-donor functions were reacted with dichloro(p-cymene)ruthenium(II) dimer in the presence of base (NaOAc) and a series of four dimetallic Ru(II) arene complexes (Ru(p-cymene))₂(bis-salicylaldimine)Cl₂ (C1C4) were prepared. These complexes were obtained in excellent isolated yields and characterized in detail by using different spectroscopic techniques. The structure of C1 was also determined in solid state by single crystal X-ray analysis. These complexes were studied for their cytotoxic effect against three different types of human cancer cells including hepatocellular carcinoma (HepG2), non-small-cell lung cancer (A549) and breast cancer (MCF-7) cells by MTT assay. These complexes showed considerable cytotoxic effect in all the above-mentioned cell lines that was comparable to the effect of cisplatin. C1 and C2 showed moderate anticancer effect while C3 and C4 showed reasonable cytotoxicity. We found the cytotoxicity was increased in series from C1 to C4 representing the effect of ligand modification from small to bulky group at the amine functionality of the salicylaldimine. We selected C3 and C4 for mechanistic anticancer study in MCF-7 cells. The acridine orange/ethidium bromide and DAPI staining assays of MCF-7 cells treated with Ru(II) complexes showed apoptosis in cancer cells. Similarly, these complexes induced p53 protein expression in MCF-7 cells. Further, increased mRNA levels of p63, p73, PUMA, BAX and NOXA genes were observed in response to the treatment with C3 and C4, while cyclinD1, MMP3 and ID1 gene expression was significantly reduced. We found reduced invasion ability in breast cancer cells treated with C3 and C4. Taken together, we demonstrated that bis-salicylaldimine based dimetallic Ru-(p-cymene) complexes exerts anticancer effects by p53 pathway, suggesting the promising chemotherapeutic potentials of these Ru(II) complexes for the treatment of cancer. This study may further pave for their in depth in vitro or in vivo anticancer investigations.