Strong Influence of Ancillary Ligands Containing Benzothiazole or Benzimidazole Rings on Cytotoxicity and Photoactivation of Ru(II) Arene Complexes

Ligand Oxidation Activates a Ruthenium(II) Precatalyst for C-H Hydroxylation

A new class of Ru-sulfonamidate precatalysts for sp3 C-H hydroxylation is described along with a versatile process for assembling unique heteroleptic Ru(II) complexes. The latter has enabled structure-performance studies to identify an optimal precatalyst, 2h, bearing one 4,4'-di-tert-butylbipyridine (dtbpy) and one pyridylsulfonamidate ligand. Single-crystal X-ray analysis confirmed the structure and stereochemistry of this adduct. Catalytic hydroxylation reactions are conveniently performed in an aqueous, biphasic solvent mixture with 1 mol % 2h and ceric ammonium nitrate as the terminal oxidant and deliver oxidized products in yields ranging from 37 to 90%. A comparative mechanistic investigation of 2h against a related homoleptic precatalyst, [Ru(dtbpy)2(MeCN)2](OTf)2, convincingly establishes that the former generates one or more surprisingly long-lived active species under the reaction conditions, thus accounting for the high turnover numbers. Structure-performance, kinetics, mass spectrometric, and electrochemical analyses reveal that ligand oxidation is a prerequisite for catalyst activation. Our findings sharply contrast a large body of prior art showing that ligand oxidation is detrimental to catalyst function. We expect these results to stimulate future innovations in C-H oxidation research.

Trinuclear ruthenium(II) polypyridyl complexes: Evaluation as photosensitizers for enhanced cervical cancer treatment

Trinuclear ruthenium(II) polypyridyl complexes anchored to benzimidazole-triazine / trisamine scaffolds were investigated as photosensitizers for photodynamic therapy. The trinuclear complexes were noted to produce a significant amount of singlet oxygen in both DMF and aqueous media, are photostable and show appreciable emission quantum yields (ɸem). In our experimental setting, despite the moderate phototoxic activity in the HeLa cervical cancer cell line, the phototoxic indices (PI) of the trinuclear complexes are superior relative to the PIs of a clinically approved photosensitizer, Photofrin®, and the pro-drug 5-aminolevulinic acid (PI: >7 relative to PI: >1 and PI: 4.4 for 5-aminolevulinic acid and Photofrin®, respectively). Furthermore, the ruthenium complexes were noted to show appreciable long-term cytotoxicity upon light irradiation in HeLa cells in a concentration-dependent manner. Consequently, this long-term activity of the ruthenium(II) polypyridyl complexes embodies their ability to reduce the probability of the recurrence of cervical cancer. Taken together, this presents a strong motivation for the development of polymetallic complexes as anticancer agents.

Half-Sandwich Cyclometalated RhIII Complexes Bearing Thiolate Ligands: Biomolecular Interactions and In Vitro and In Vivo Evaluations

A class of cyclometalated RhIII complexes [Cp*Rh(ppy)(SR)] bearing thiolate ligands, Cp* = pentamethylcyclopentadienyl, ppy = 2-phenylpyridinate, and R = pyridyl (Spy, 2), pyrimidyl (SpyN, 3), benzimidazolyl (Sbi, 4), and benzothiazolyl (Sbt, 5), were produced and identified by means of spectroscopic methods. The in vitro cytotoxicity of the RhIII compounds in three different human mortal cancerous cell lines (ovarian, SKOV3; breast, MCF-7; lung, A549) and a normal lung (MRC-5) cell line were evaluated, indicating the selectivity of these cyclometalated RhIII complexes to cancer cells. Complex 5, selected for in vivo experiment, has shown an effective inhibition of tumor growth in SKOV3 xenograft mouse model relative to control (p-values < 0.05 and < 0.01). Importantly, the outcomes of H&E (hematoxylin and eosin) staining and hematological analysis revealed negligible toxicity of 5 compared to cisplatin on a functioning of the main organs of mouse. Molecular docking, UV-vis, and emission spectroscopies (fluorescence, 3D fluorescence, synchronous) techniques were carried out on 1-5 to peruse the mechanism of the anticancer activities of these complexes. The obtained data help to manifest the binding affinity between the rhodium compounds and calf thymus DNA (CT-DNA) through the interaction by DNA minor groove and moderate binding affinity with bovine serum albumin (BSA), particularly with the cavity in the subdomain IIA. It can be concluded that the Rh-thiolate complexes are highly promising leads for the development of novel effective DNA-targeted anticancer drugs.

Synthesis, structural insights, and biological screening of DNA targeted Ru(ii)(η6-p-cymene) complexes containing bioactive amino-benzothiazole ligand scaffolds

Two new drug candidates [Ru(p-cymene)(C7H4ClN2S)Cl2] and [Ru(p-cymene)(C7H5FN2S)Cl2] were synthesized and characterised. The in vitro cytotoxic activity of the complexes was assessed against five human cancer cell lines and anthelmintic activity was also investigated.

The Development of Ru(II)-Based Photoactivated Chemotherapy Agents

Photoactivated chemotherapy (PACT) is a novel cancer treatment method that has drawn increasing attention due to its high selectivity and low side effects by spatio-temporal control of irradiation. Compared with photodynamic therapy (PDT), oxygen-independent PACT is more suitable for treating hypoxic tumors. By finely tuning ligand structures and coordination configurations, many Ru(II) complexes can undergo photoinduced ligand dissociation, and the resulting Ru(II) aqua species and/or free ligands may have anticancer activity, showing their potential as PACT agents. In this mini-review, we summarized the progress in Ru(II)-based PACT agents, as well as challenges that researchers in this field still face.

Ru(ii), Ir(iii), Re(i) and Rh(iii) based complexes as next generation anticancer metallopharmaceuticals

Several anticancer drugs such as cisplatin, and its analogues, epirubicin, and doxorubicin are well known for their anticancer activity but the therapeutic value of these drugs comes with certain side effects and they cannot distinguish between normal and cancer cells. Thus, a major challenge for researchers around the world is to develop an anticancer drug with the least toxicity and more target specificity. With the successful reporting of NAMI-A and KP1019, a new path has emerged in the anticancer field. Recently, several Ru(ii) complexes have been reported for their anticancer activity due to their enhanced cellular uptake and selectivity towards cancer cells. Apart from the Ru(ii) complexes, a large amount of research has been carried out with Ir(iii), Re(i), and Rh(iii) based complexes, which exhibited promising anticancer activity. The present review reports various Ru(ii), Ir(iii), Re(i), and Rh(iii) based complexes for their anticancer activity based on their cytotoxicity profiles, biological targets and mechanism of action.

A 2-(benzothiazol-2-yl)-phenolato platinum(II) complex as potential photosensitizer for combating bacterial infections in lung cancer chemotherapy†

Cancer and antibiotic resistance are two global health threats that usually hamper clinical chemotherapeutic efficacy. Particularly for lung cancer, bacterial infections frequently arise thereby complicating the course of cancer treatment. In this sense, three new neutral luminescent cycloplatinated(II) photosensitizers of the type [Pt(dmba)(L)] (dmba = N,N-dimethylbenzylamine-κN,κC; L = 2-(benzo[d]oxazol-2-yl)-phenolato-κN,κO1, 2-(benzo[d]thiazol-2-yl)-phenolato-κN,κO2, and 2-(1-methyl-1H-benzo[d]imidazole-2-yl)phenolato-κN,κO3) have been characterized and developed to potentially eliminate both resistant bacteria and lung cancer cells. The phototherapeutic effects of complex 2 have been evaluated using low doses of blue light irradiation. Complex 2 exerted promising photoactivity against pathogenic Gram-positive bacteria strains of clinical interest, displaying a phototoxic index (PI) of 15 for methicillin-resistant Staphylococcus aureus, one of the major microorganisms predominating lung infections. Likewise, the anticancer activity of 2 was also increased upon light irradiation in human lung A549 cancer cells (PI = 36). Further in vitro experiments with this platinum(II) complex suggest that ROS-generating photodynamic reactions were involved upon light irradiation, thus providing a reasonable mechanism for its dual anticancer and antibacterial activities.

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.

Utilization of Guanidine-Based Ancillary Ligands in Arene-Ruthenium Complexes for Selective Cytotoxicity

A family of three water-soluble half-sandwich arene-ruthenium complexes, depicted as C ₁ -C ₃ , having the general formula [Ru(p-cymene)(L)Cl]Cl has been synthesized, where L represents (1H-benzo[d]imidazol-2-yl)guanidine (L ₁ ) or (benzo[d]oxazol-2-yl)guanidine (L ₂ ) or (benzo[d]thiazol-2-yl)guanidine (L ₃ ). The crystal structure of complex C ₃ has been determined. The complexes show several absorption bands in the visible and ultraviolet regions, and they also show prominent emission in the visible region while excited near 400 nm. Studies on the interaction of ligands L ₁ -L ₃ and complexes C ₁ -C ₃ with calf thymus DNA reveal that the complexes are better DNA binders than the ligands, which is attributable to the imposed planarity of the ruthenium-bound guanidine-based ligand, enabling it to serve as a better intercalator. Molecular docking studies show that the complexes effectively bind with DNA through electrostatic and H-bonding interactions and partial intercalation of the guanidine-based ligands. Cytotoxicity studies carried out on two carcinoma cell lines (PC3 and A549) and on two non-cancer cell lines (BPH1 and WI-38) show a marked improvement in antitumor activity owing to complex formation, which is attributed to improvement in cellular uptake on complex formation. The C ₁ complex is found to exhibit the most prominent activity against the PC3 cell line. Inclusion of the guanidine-based ligands in the half-sandwich ruthenium-arene complexes is found to be effective for displaying selective cytotoxicity to cancer cells and also for convenient tracing of the complexes in cells due to their prominent emissive nature.

Analysis of Ion Pairing in Solid State and Solution in p-Cymene Ruthenium Complexes

The importance of ion pairing in different fields of chemistry is widely recognized. In this work, we have synthesized a set of cationic p-cymene ruthenium complexes of general formula [(p-cym)Ru(L')(κ²-O^N-L)]X (p-cym = p-cymene; L' = N-methylimidazole (MeIm), N-ethylpiperidylimidazole (EpipIm), 1,3,5-triaza-7-phosphaadamantane (PTA); L = 2-(1H-benzimidazol-2-yl)phenolato (L1), 2-(1,3-benzothiazol-2-yl)phenolato (L2); X = Cl^-, BF₄^-, OTf^-, BPh₄^-). X-ray diffraction studies on selected complexes revealed relatively strong anion-cation interactions in the solid state mainly based on N-H···X (X = Cl, F, O) and C-H···π interactions, also observed in the DFT-modeled complexes in the gas phase. Moreover, NMR studies showed that they exist as intimate ion pairs in solution and, remarkably, as head-to-tail quadruples in the particular case of the cation [(p-cym)Ru(MeIm)(κ²- O^N-L1)]⁺ ([1]⁺) with Cl^- and BPh₄^- as counteranions. Furthermore, a value of ΔG = -2.9 kcal mol^-1 at 299 K has been estimated for the equilibrium {[1]BPh₄···[1]BPh₄} ⇆ 2{[1]⁺···BPh₄^-} in concentrated CDCl₃ solutions. In addition, preliminary studies concerning the cytotoxic properties against HeLa cell lines of the derivatives suggested a positive effect derived from the presence of the lipophilic BPh₄^- anion and also from the NH group of the benzimidazolyl fragment.

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.

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.