Organometallic chemistry, biology and medicine: ruthenium arene anticancer complexes

Ruthenium p-Cymene Complexes Incorporating Substituted Pyridine-Quinoline-Based Ligands: Synthesis, Characterization, and Cytotoxic Properties

Organometallic complexes of the formula [Ru(N^N)(p-cymene)Cl][X] (N^N = bidentate polypyridyl ligands, p-cymene = 1-methyl-4-(1-methylethyl)-benzene, X = counter anion), are currently studied as possible candidates for the potential treatment of cancer. Searching for new organometallic compounds with good to moderate cytotoxic activities, a series of mononuclear water-soluble ruthenium(II)-arene complexes incorporating substituted pyridine-quinoline ligands, with pending -CH2OH, -CO2H and -CO2Me groups in the 4-position of quinoline ring, were synthesized, for the first time, to study their possible effect to modulate the activity of the ruthenium p-cymene complexes. These include the [Ru(η6-p-cymene)(pqhyme)Cl][X] (X = Cl- (1-Cl), PF6- (1-PF6), pqhyme = 4-hydroxymethyl-2-(pyridin-2-yl)quinoline), [Ru(η6-p-cymene)(pqca)Cl][Cl] ((2-Cl), pqca = 4-carboxy-2-(pyridin-2-yl)quinoline), and [Ru(η6-p-cymene)(pqcame)Cl][X] (X = Cl- (3-Cl), PF6- (3-PF6), pqcame = 4-carboxymethyl-2-(pyridin-2-yl)quinoline) complexes, respectively. Identification of the complexes was based on multinuclear NMR and ATR-IR spectroscopic methods, elemental analysis, conductivity measurements, UV-Vis spectroscopic, and ESI-HRMS techniques. The solid-state structures of 1-PF6 and 3-PF6 have been elucidated by single-crystal X-ray diffraction revealing a three-legged piano stool geometry. This is the first time that the in vitro cytotoxic activities of these complexes are studied. These were conducted in HEK293T (human embryonic kidney cells) and HeLa cells (cervical cancer cells) via the MTT assay. The results show poor in vitro anticancer activities for the HeLa cancer cell lines and 3-Cl proved to be the most potent (IC50 > 80 μΜ). In both cell lines, the cytotoxicity of the ligand precursor pqhyme is significantly higher than that of cisplatin.

Promoting the Anticancer Activity with Multidentate Furan-2-Carboxamide Functionalized Aroyl Thiourea Chelation in Binuclear Half-Sandwich Ruthenium(II) Complexes

A new set of binuclear arene ruthenium complexes [Ru2(p-cymene)2(k4-N2OS)(L1-L3)Cl2] (Ru2L1-Ru2L3) encompassing furan-2-carboxamide-based aroylthiourea derivatives (H2L1-H2L3) was synthesized and characterized by various spectral and analytical techniques. Single-crystal XRD analysis unveils the N^O and N^S mixed monobasic bidentate coordination of the ligands constructing N, S, Cl/N, O, and Cl legged piano stool octahedral geometry. DFT analysis demonstrates the predilection for the formation of stable arene ruthenium complexes. In vitro antiproliferative activity of the complexes was examined against human cervical (HeLa), breast (MCF-7), and lung (A549) cancerous and noncancerous monkey kidney epithelial (Vero) cells. All the complexes are more efficacious against HeLa and MCF-7 cells with low inhibitory doses (3.86-11.02 μM). Specifically, Ru2L3 incorporating p-cymene and -OCH3 fragments exhibits high lipophilicity, significant cytotoxicity against cancer cells, and lower toxicity on noncancerous cells. Staining analysis indicates the apoptosis-associated cell morphological changes expressively in MCF-7 cells. Mitochondrial membrane potential (MMP) and reactive oxygen species (ROS) analyses reveal that Ru2L3 can raise ROS levels, reduce MMP, and trigger mitochondrial dysfunction-mediated apoptosis. The catalytic oxidation of glutathione (GSH) to its disulfide form (GSSG) by the complexes may simultaneously increase the ROS levels, alluding to their observed cytotoxicity and apoptosis induction. Flow cytometry determined the quantitative classification of late apoptosis and S-phase arrest in MCF-7 and HeLa cells. Western blotting analysis confirmed that the complexes promote apoptosis by upregulating Caspase-3 and Caspase-9 and downregulating BCL-2. Molecular docking studies unfolded the strong binding affinities of the complexes with VEGFR2, an angiogenic signaling receptor, and BCL2, Cyclin D1, and HER2 proteins typically overexpressed on tumor cells.

Exploiting Co(III)-Cyclopentadienyl Complexes To Develop Anticancer Agents

In recent years, organometallic complexes have attracted much attention as anticancer therapeutics aiming at overcoming the limitations of platinum drugs that are currently marketed. Still, the development of half-sandwich organometallic cobalt complexes remains scarcely explored. Four new cobalt(III)-cyclopentadienyl complexes containing N,N-heteroaromatic bidentate, and phosphane ligands were synthesized and fully characterized by elemental analysis, spectroscopic techniques, and DFT methods. The cytotoxicity of all complexes was determined in vitro by the MTS assay in colorectal (HCT116), ovarian (A2780), and breast (MDA-MB-231 and MCF-7) human cancer cell lines and in a healthy human cell line (fibroblasts). The complexes showed high cytotoxicity in cancer cell lines, mostly due to ROS production, apoptosis, autophagy induction, and disruption of the mitochondrial membrane. Also, these complexes were shown to be nontoxic in vivo in an ex ovo chick embryo yolk sac membrane (YSM) assay.

Strategies for the Nuclear Delivery of Metal Complexes to Cancer Cells

The nucleus is an essential organelle for the function of cells. It holds most of the genetic material and plays a crucial role in the regulation of cell growth and proliferation. Since many antitumoral therapies target nucleic acids to induce cell death, tumor-specific nuclear drug delivery could potentiate therapeutic effects and prevent potential off-target side effects on healthy tissue. Due to their great structural variety, good biocompatibility, and unique physico-chemical properties, organometallic complexes and other metal-based compounds have sparked great interest as promising anticancer agents. In this review, strategies for specific nuclear delivery of metal complexes are summarized and discussed to highlight crucial parameters to consider for the design of new metal complexes as anticancer drug candidates. Moreover, the existing opportunities and challenges of tumor-specific, nucleus-targeting metal complexes are emphasized to outline some new perspectives and help in the design of new cancer treatments.

Mononuclear η6-arene ruthenium(II) complexes with pyrazolyl-pyridazine ligands: synthesis, CT-DNA binding, reactivity towards glutathione, and cytotoxicity

Organometallic η6-arene ruthenium(II) complexes with 3-chloro-6-(1H-pyrazol-1-yl)pyridazine (Ru1, Ru2, and Ru5) and 3-chloro-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridazine (Ru3-4) N,N' heterocyclic and η6-arene (cymene (Ru1-4) or toluene (Ru 5)) have been synthesized. The ruthenium(II) complexes have common "three-legged piano-stool" pseudo-octahedral structures known for half-sandwich complexes. Evolution of their UV-Visible absorption spectra in PBS buffer or DMSO over 24 h confirmed their good solvolysis stability. Titrations of the complexes with the calf thymus DNA (CT-DNA) were monitored using UV-Visible absorption and fluorescence spectroscopies. The complexes interact moderately with CT-DNA and their binding constants are in the order of 104 M-1. Competitive binding of the complexes to a DNA-Hoechst 33,258 depicted competitive displacement of Hoechst from DNA's minor grooves. These complexes bind to glutathione forming GSH-adducts through S coordination by replacement of a halide, with the iodo-analogues having higher binding constants than the chloro-complexes. Cyclic voltammograms of the complexes exhibited one electron-transfer quasi-reversible process. Trends in the molecular docking data of Ru1-5/DNA were similar to those for DNA binding constants. Of the five, only Ru1, Ru3 and Ru5 showed some activity (moderate) against the MCF-7 breast cancer cells with IC50 values in the range of 59.2-39.9 for which Ru5 was the most active. However, the more difficult-to-treat cell line, MDA-MB 231 cell was recalcitrant to the treatment by these complexes.

Impact of the central atom and halido ligand on the structure, antiproliferative activity and selectivity of half-sandwich Ru(II) and Ir(III) complexes with a 1,3,4-thiadiazole-based ligand

Half-sandwich complexes [Ru(η6-pcym)(L1)X]PF6 (1, 3) and [Ir(η5-Cp*)(L1)X]PF6 (2, 4) featuring a thiadiazole-based ligand L1 (2-(furan-2-yl)-5-(pyridin-2-yl)-1,3,4-thiadiazole) were synthesized and characterized by varied analytical methods, including single-crystal X-ray diffraction (X = Cl or I, pcym = p-cymene, Cp* = pentamethylcyclopentadienyl). The structures of the molecules were analysed and interpreted using computational methods such as Density Functional Theory (DFT) and Quantum Theory of Atoms in Molecules (QT-AIM). A 1H NMR spectroscopy study showed that complexes 1-3 exhibited hydrolytic stability while 4 underwent partial iodido/chlorido ligand exchange in phosphate-buffered saline. Moreover, 1-4 demonstrated the ability to oxidize NADH (reduced nicotinamide adenine dinucleotide) to NAD+ with Ir(III) complexes 2 and 4 displaying higher catalytic activity compared to their Ru(II) analogues. None of the complexes interacted with reduced glutathione (GSH). Additionally, 1-4 exhibited greater lipophilicity than cisplatin. In vitro biological analyses were performed in healthy cell lines (CCD-18Co colon and CCD-1072Sk foreskin fibroblasts) as well as in cisplatin-sensitive (A2780) and -resistant (A2780cis) ovarian cancer cell lines. The results indicated that Ir(III) complexes 2 and 4 had no effect on human fibroblasts, demonstrating their selectivity. In contrast, complexes 1 and 4 exhibited moderate inhibitory effects on the metabolic and proliferation activities of the cancer cells tested (selectivity index SI > 3.4 for 4 and 2.6 for cisplatin; SI = IC50(A2780)/IC50(CCD-18Co)), including the cisplatin-resistant cancer cell line. Based on these findings, it is possible to emphasize that mainly complex 4 could represent a further step in the development of selective and highly effective anticancer agents, particularly against resistant tumour types.

Interactions of Mn complexes with DNA: the relevance of therapeutic applications towards cancer treatment

Manganese (Mn) is one of the most significant bio-metals that helps the body to form connective tissue, bones, blood clotting factors, and sex hormones. It is necessary for fat and carbohydrate metabolism, calcium absorption, blood sugar regulation, and normal brain and nerve functions. It accelerates the synthesis of proteins, vitamin C, and vitamin B. It is also involved in the catalysis of hematopoiesis, regulation of the endocrine level, and improvement of immune function. Again, Mn metalloenzymes like arginase, glutamine synthetase, phosphoenolpyruvate decarboxylase, and Mn superoxide dismutase (MnSOD) contribute to the metabolism processes and reduce oxidative stress against free radicals. Recent investigations have revealed that synthetic Mn-complexes act as antibacterial and antifungal agents. As a result, chemists and biologists have been actively involved in developing Mn-based drugs for the treatment of various diseases including cancer. Therefore, any therapeutic drugs based on manganese complexes would be invaluable for the treatment of cancer/infectious diseases and could be a better substitute for cisplatin and other related platinum based chemotherapeutic drugs. From this perspective, attempts have been made to discuss the interactions and nuclease activities of Mn(II/III/IV) complexes with DNA through which one can evaluate their therapeutic applications.

Ru(II)-p-Cymene Complexes of Furoylthiourea Ligands for Anticancer Applications against Breast Cancer Cells

Half-sandwich Ru(II) complexes containing nitro-substituted furoylthiourea ligands, bearing the general formula [(η⁶-p-cymene)RuCl₂(L)] (1-6) and [(η⁶-p-cymene)RuCl(L)(PPh₃)]⁺ (7--12), have been synthesized and characterized. In contrast to the spectroscopic data which revealed monodentate coordination of the ligands to the Ru(II) ion via a "S" atom, single crystal X-ray structures revealed an unusual bidentate N, S coordination with the metal center forming a four-membered ring. Interaction studies by absorption, emission, and viscosity measurements revealed intercalation of the Ru(II) complexes with calf thymus (CT) DNA. The complexes showed good interactions with bovine serum albumin (BSA) as well. Further, their cytotoxicity was explored exclusively against breast cancer cells, namely, MCF-7, T47-D, and MDA-MB-231, wherein all of the complexes were found to display more pronounced activity than their ligand counterparts. Complexes 7-12 bearing triphenylphosphine displayed significant cytotoxicity, among which complex 12 showed IC₅₀ values of 0.6 ± 0.9, 0.1 ± 0.8, and 0.1 ± 0.2 μM against MCF-7, T47-D, and MDA-MB-231 cell lines, respectively. The most active complexes were tested for their mode of cell death through staining assays, which confirmed apoptosis. The upregulation of apoptotic inducing and downregulation of apoptotic suppressing proteins as inferred from the western blot analysis also corroborated the apoptotic mode of cell death. The active complexes effectively generated reactive oxygen species (ROS) in MDA-MB-231 cells as analyzed from the 2',7'-dichlorofluorescein diacetate (DCFH-DA) staining. Finally, in vivo studies of the highly active complexes (6 and 12) were performed on the mice model. Histological analyses revealed that treatment with these complexes at high doses of up to 8 mg/kg did not induce any visible damage to the tested organs.

Ionic mononuclear [Fe] and heterodinuclear [Fe,Ru] bis(diphenylphosphino)alkane complexes: Synthesis, spectroscopy, DFT structures, cytotoxicity, and biomolecular interactions

Iron(II) and Ru(II) half-sandwich compounds encompass some promising pre-clinical anticancer agents whose efficacy may be tuned by structural modification of the coordinated ligands. Here, we combine two such bioactive metal centres in cationic bis(diphenylphosphino)alkane-bridged heterodinuclear [Fe²⁺, Ru²⁺] complexes to delineate how ligand structural variations modulate compound cytotoxicity. Specifically, Fe(II) complexes of the type [(η⁵-C₅H₅)Fe(CO)₂(κ¹-PPh₂(CH₂)_nPPh₂)]{PF₆} (n = 1-5), compounds 1-5, and heterodinuclear [Fe²⁺, Ru²⁺] complexes, [(η⁵-C₅H₅)Fe(CO)₂(μ-PPh₂(CH₂)_nPPh₂))(η⁶-p-cymene)RuCl₂]{PF₆} (n = 2-5) (compounds 7-10), were synthesized and characterised. The mononuclear complexes were moderately cytotoxic against two ovarian cancer cell lines (A2780 and cisplatin resistant A2780cis) with IC₅₀ values ranging from 2.3 ± 0.5 μM to 9.0 ± 1.4 μM. For 7-10, the cytotoxicity increased with increasing Fe⋅⋅⋅Ru distance, consistent with their DNA affinity. UV-visible spectroscopy suggested the chloride ligands in heterodinuclear 8-10 undergo stepwise substitution by water on the timescale of the DNA interaction experiments, probably affording the species [RuCl(OH₂)(η⁶-p-cymene)(PRPh₂)]²⁺ and [Ru(OH)(OH₂)(η⁶-p-cymene)(PRPh₂)]²⁺ (where PRPh₂ has R = [-(CH₂)₅PPh₂-Fe(C₅H₅)(CO)₂]⁺). One interpretation of the combined DNA-interaction and kinetic data is that the mono(aqua) complex may interact with dsDNA through nucleobase coordination. Heterodinuclear 10 reacts with glutathione (GSH) to form stable mono- and bis(thiolate) adducts, 10-SG and 10-SG₂, with no evidence of metal ion reduction (k₁ = 1.07 ± 0.17 × 10^-1 min^-1 and k₂ = 6.04 ± 0.59 × 10^-3 min^-1 at 37 °C). This work highlights the synergistic effect of the Fe²⁺/Ru²⁺ centres on both the cytotoxicity and biomolecular interactions of the present heterodinuclear complexes.

Combined theoretical and experimental insights on DNA and BSA binding interactions of Cu(ii) and Ni(ii) complexes along with the DPPH method of antioxidant assay and cytotoxicity studies

This present study delineates the syntheses, detailed characterization and anti-proliferative potential against SiHa (cervical cancer cell) of two mononuclear complexes of Cu(ii) and Ni(ii) using a Schiff base ligand (L) derived from 2-hydroxybenzaldehyde and N-methyl-propane 1,3-diamine. The crystallographic results show the centro-symmetric space group of orthorhombic nature (Pccn) for Cu(ii) complex (1) where the central Cu(ii) has an inversion center symmetry with six co-ordinations resulting in a distorted octahedral geometry. Whereas, in complex (2), the two independent Ni(ii) atoms present in the special position within version symmetry and form a distorted geometry of octahedral nature with six coordinations. Absorption spectral titrations with Calf Thymus (CT) DNA and the extent of the decrease in relative emission intensities of DNA-bound ethidium bromide (EB) upon adding the complexes reveal the parallel trend in DNA binding affinities for both the complexes but with a small extent of binding capabilities. Bovine serum albumin (BSA) interaction studies demonstrate that complex 1 exhibits more promiscuous binding with BSA as compared to complex 2 from the spectroscopic and theoretical approaches. α,α-Diphenyl-β-picrylhydrazyl (DPPH) free radical scavenging method shows a little antioxidant or free radical scavenging activity for both the studied complexes. Cytotoxicity studies against SiHa expressed that the percentage of cell viability was reduced with time whereas in the same concentration and conditions, the viability percentage was higher for 3T3-L1 (several normal cell lines of mouse). The fluorescence imaging obtained from acridine orange (AO) and ethidium bromide (EtBr) demonstrates that the colour of the cancer cells has changed gradually dictating the cell apoptosis from day 1 to day 3.

Recent Trends in the Development of Novel Metal-Based Antineoplastic Drugs

Since the accidental discovery of the anticancer properties of cisplatin more than half a century ago, significant efforts by the broad scientific community have been and are currently being invested into the search for metal complexes with antitumor activity. Coordination compounds of transition metals such as platinum (Pt), ruthenium (Ru) and gold (Au) have proven their effectiveness as diagnostic and/or antiproliferative agents. In recent years, experimental work on the potential applications of elements including lanthanum (La) and the post-transition metal gallium (Ga) in the field of oncology has been gaining traction. The authors of the present review article aim to help the reader "catch up" with some of the latest developments in the vast subject of coordination compounds in oncology. Herewith is offered a review of the published scientific literature on anticancer coordination compounds of Pt, Ru, Au, Ga and La that has been released over the past three years with the hope readers find the following article informative and helpful.

Utilization of Cancer Cell Line Screening to Elucidate the Anticancer Activity and Biological Pathways Related to the Ruthenium-Based Therapeutic BOLD-100

BOLD-100 (sodium trans-[tetrachlorobis(1H indazole)ruthenate(III)]) is a ruthenium-based anticancer compound currently in clinical development. The identification of cancer types that show increased sensitivity towards BOLD-100 can lead to improved developmental strategies. Sensitivity profiling can also identify mechanisms of action that are pertinent for the bioactivity of complex therapeutics. Sensitivity to BOLD-100 was measured in a 319-cancer-cell line panel spanning 24 tissues. BOLD-100's sensitivity profile showed variation across the tissue lineages, including increased response in esophageal, bladder, and hematologic cancers. Multiple cancers, including esophageal, bile duct and colon cancer, had higher relative response to BOLD-100 than to cisplatin. Response to BOLD-100 showed only moderate correlation to anticancer compounds in the Genomics of Drug Sensitivity in Cancer (GDSC) database, as well as no clear theme in bioactivity of correlated hits, suggesting that BOLD-100 may have a differentiated therapeutic profile. The genomic modalities of cancer cell lines were modeled against the BOLD-100 sensitivity profile, which revealed that genes related to ribosomal processes were associated with sensitivity to BOLD-100. Machine learning modeling of the sensitivity profile to BOLD-100 and gene expression data provided moderative predictive value. These findings provide further mechanistic understanding around BOLD-100 and support its development for additional cancer types.

Quantum Chemical Investigation on Hydrolysis of Orally Active Organometallic Ruthenium(II) and Osmium(II) Anticancer Drugs and Their Interaction with Histidine

Influence of the metal center on hydrolysis of organometallic anticancer complexes containing an N-phenyl-2-pyridinecarbothioamide (PCA) ligand, [M(η⁶-p-cymene)(N-phenyl-2-pyridinecarbothioamide)Cl]⁺ (M = Ru^II, 1A, and Os^II, 2A), as well as their N-fluorophenyl derivatives [M(η⁶-p-cymene)(N-fluorophenyl-2-pyridinecarbothioamide)Cl]⁺ (M = Ru^II, 1B, and Os^II, 2B) have been investigated using the DFT method in aqueous medium. The activation energy barriers for the hydrolysis of 1A (21.5 kcal/mol) and 1B (20.7 kcal/mol) are found to be significantly lower than those of their corresponding osmium analogs 2A (28.6 kcal/mol) and 2B (27.5 kcal/mol). DFT evaluated results reveal the inertness of Os(II)-PCA complex toward the hydrolysis that rationalizes the experimental observations. However, the incorporation of fluoride substituent slightly decreases the activation energy for the hydrolysis of Ru(II)- and Os(II)-PCA. In addition, the interaction of hydrolyzed Ru(II)-PCAs (1AH and 1BH) and Os(II)-PCAs (2AH and 2BH) complexes with the histidine (Hist) have also been investigated. The aquated 1BH and 2BH show an enhanced propensity toward the interaction with histidine, and their activation Gibbs free energies are calculated to be 15.9 and 18.9 kcal/mol, respectively. ONIOM (QM/MM) study of the resulting aquated complexes inside histone protein shows the maximum stability of the 2BH complex having a binding energy of -43.6 kcal/mol.

Ru(II)(ƞ6-p-cymene) Conjugates Loaded onto Graphene Oxide: An Effective pH-Responsive Anticancer Drug Delivery System

Graphene oxide-based nanodrug delivery systems are considered one of the most promising platforms to deliver therapeutic drugs at the target site. In this study, Ru(II)(ƞ6-p-cymene) complexes containing the benzothiazole ligand were covalently anchored on graphene oxide using the ultrasonication method. The nanoconjugates GO-NCD-1 and GO-NCD-2 were characterized by FT-IR, UV-visible, 1H NMR, TGA, SEM, and TEM techniques, which confirmed the successful loading of both the complexes (NCD 1 and NCD 2) on the carrier with average particle diameter sizes of 17 ± 6.9 nm and 25 ± 6.5 nm. In vitro DNA binding studies of the nanoconjugates were carried out by employing various biophysical methods to investigate the binding interaction with the therapeutic target biomolecule and to quantify the intrinsic binding constant values useful to understand their binding affinity. Our results suggest (i) high Kb and Ksv values of the graphene-loaded conjugates (ii) effective cleavage of plasmid DNA at a lower concentration of 7.5 µM and 10 µM via an oxidative pathway, and (iii) fast release of NCD 2 at an acidic pH that could have a good impact on the controlled delivery of drug. It was found that 90% of the drug was released in an acidic pH (5.8 pH) environment in 48 h, therefore suggesting pH-responsive behavior of the drug delivery system. Molecular docking, DFT studies, and cytotoxicity activity against three cancer cell lines by SRB assay were also performed.

The straightforward synthesis of N-coordinated ruthenium 4-aryl-1,2,3-triazolato complexes by [3 + 2] cycloaddition reactions of a ruthenium azido complex with terminal phenylacetylenes and non-covalent aromatic interactions in structures

The straightforward preparation of N-coordinated ruthenium triazolato complexes by [3 + 2] cycloaddition reactions of a ruthenium azido complex [Ru]-N3 (1, [Ru] = (η5-C5H5)(dppe)Ru, dppe = Ph2PCH2CH2PPh2) with a series of terminal phenylacetylenes is reported. The reaction products, N(2)-bound ruthenium 4-aryl-1,2,3-triazolato complexes such as [Ru]N3C2H(4-C6H4CN) (2), [Ru]N3C2H(4-C6H4CHO) (3), [Ru]N3C2H(4-C6H4F) (4), [Ru]N3C2H(Ph) (5) and [Ru]N3C2H(4-C6H4CH3) (6) were produced from 4-ethynylbenzonitrile, 4-ethynylbenzaldehyde, 1-ethynyl-4-fluorobenzene, phenylacetylene and 4-ethynyltoluene, respectively, at 80 °C or above under an atmosphere of air. To the best of our knowledge, this is the first example of the preparation of N-coordinated ruthenium aryl-substituted 1,2,3-triazolato complexes by the [3 + 2] cycloaddition of a metal-coordinated azido ligand and a terminal aryl acetylene, less electron-deficient terminal aryl alkynes. All of the compounds have been fully characterized and the structures of complexes 2, 3, 5 and 6 were confirmed by single-crystal X-ray diffraction analysis. Each compound participates in non-covalent aromatic interactions in the solid-state structure which can be favorable in the binding of DNA/biomolecular targets and has shown great potential in the development of biologically active anticancer drugs.

Systematic Study on the Cytotoxic Potency of Commonly Used Dimeric Metal Precursors in Human Cancer Cell Lines

The cytotoxicities of seven dimeric metal species of the general formula [M(arene)Cl2 ]2 , commonly used as precursors for complex synthesis and deemed biologically inactive, are investigated in seven commonly employed human cancer cell lines. Four of these complexes featured a ruthenium(II) core, where p-cymene, toluene, benzene and indane were used as arenes. Furthermore, the osmium(II) p-cymene dimer, as well as the Cp* dimers of rhodium(III) and its heavier analogue iridium(III) were included in this work (Cp*=1,2,3,4,5-pentamethylcyclopentadienide). While the cytotoxic potencies of the ruthenium(II) and osmium(II) dimers are very low (or not even detectable at applicable concentrations), surprising activity, especially in cells from ovarian malignancies (with one or two-digit micromolar IC50 values), have been found for the rhodium(III) and iridium(III) representatives. This publication is aimed at all researchers using synthetic procedures based on functionalization of these dimeric starting materials to rationalize changes in biological properties, especially cytotoxicity in cancer cells.

In-vitro anticancer profile of recent ruthenium complexes against liver cancer

Ruthenium complexes are considered as the most favorable alternatives to traditional platinum-based cancer drugs owing to their acceptable toxicity level, selectivity, variant oxidation states and ability to treat platinum-resistant cancer cells. They have similar ligand exchange kinetics as platinum drugs but can be tailored according to our desire by ligands influence. In the current study, we illustrate the in-vitro anticancer profile of some ruthenium complexes (2016–2021) against human hepatocellular carcinoma (HepG2). The anticancer activity of ruthenium complexes is determined by comparing their IC50 values with one another and positive controls. Fortunately, some ruthenium complexes including 3, 4, 6, 14, 15, 20, 42, and 48 exhibit surpassed in-vitro anticancer profile than that of positive controls promising as potential candidates against liver cancer. We also explored the structure-activity relationship (SAR) which is a key factor in the rational designing and synthesis of new ruthenium drugs. It covers the factors affecting anticancer activity including lipophilicity, planarity, area and bulkiness, the steric influence of different ligands, and electronic effects induced by ligands, stability, aqueous solubility and bioavailability to the target sites. The data reported here will provide strong support in the plausible design and synthesis of ruthenium anticancer drugs in the upcoming days.

Metal-Based Complexes as Potential Anti-cancer Agents

Metal based therapy is no new in biomedical research. In early days the biggest limitation was the inequality among therapeutical and toxicological dosages. Ever since, Barnett Rosenberg discovered cisplatin, a new era has begun to treat cancer with metal complexes. Platinum complexes such as oxaliplatin, cisplatin, and carboplatin, seem to be the foundation of metal/s-based components to challenge malignancies. With an advancement in the biomolemoecular mechanism, researchers have started developing non-classical platinum-based complexes, where a different mechanistic approach of the complexes is observed towards the biomolecular target. Till date, larger number of metal/s-based complexes was synthesized by overhauling the present structures chemically by substituting the ligand or preparing the whole novel component with improved cytotoxic and safety profiles. Howsoever, due to elevated accentuation upon the therapeutic importance of metal/s-based components, a couple of those agents are at present on clinical trials and several other are in anticipating regulatory endorsement to enter the trial. This literature highlights the detailed heterometallic multinuclear components, primarily focusing on platinum, ruthenium, gold and remarks on possible stability, synergism, mechanistic studies and structure activity relationships.

Recent Advances of Pyridinone in Medicinal Chemistry

Pyridinones have been adopted as an important block in medicinal chemistry that could serve as hydrogen bond donors and acceptors. With the help of feasible synthesis routes via established condensation reactions, the physicochemical properties of such a scaffold could be manipulated by adjustment of polarity, lipophilicity, and hydrogen bonding, and eventually lead to its wide application in fragment-based drug design, biomolecular mimetics, and kinase hinge-binding motifs. In addition, most pyridinone derivatives exhibit various biological activities ranging from antitumor, antimicrobial, anti-inflammatory, and anticoagulant to cardiotonic effects. This review focuses on recent contributions of pyridinone cores to medicinal chemistry, and addresses the structural features and structure–activity relationships (SARs) of each drug-like molecule. These advancements contribute to an in-depth understanding of the potential of this biologically enriched scaffold and expedite the development of its new applications in drug discovery.

Arene Ru(II) Complexes with Difluorinated Ligands Act as Potential Inducers of S-Phase Arrest via the Stabilization of c-myc G-Quadruplex DNA

Here, a series of half-sandwich arene Ru(II) complexes with difluorinated ligands [Ru(η⁶-arene)(L)Cl] (L₁ = 2-(2,3-difluorophenyl)imidazole[4,5f][1,10]-phenanthroline; L₂ = 2-(2,4-difluorophenyl)imidazole[4,5f][1,10]-phenanthroline; arene = benzene, toluene, and p-cymene) were synthesized and characterized. Molecular docking analysis showed that these complexes bind to c-myc G-quadruplex DNA through either groove binding or π–π stacking, and the relative difluorinated site in the main ligand plays a role in regulating the binding mode. The binding behavior of these complexes with c-myc G-quadruplex DNA was evaluated using ultraviolet–visible spectroscopy, fluorescence intercalator displacement assay, fluorescence resonance energy transfer melting assay, and polymerase chain reaction. The comprehensive analysis indicated that complex 1 exhibited a better affinity and stability in relation to c-myc G-quadruplex DNA with a DC₅₀ of 6.6 μM and ΔT_m values of 13.09 °C, than other molecules. Further activity evaluation results displayed that this class of complexes can also inhibit the growth of various tumor cells, especially complexes 3 and 6, which exhibited a better inhibitory effect against human U87 glioblastoma cells (51.61 and 23.75 μM) than other complexes, even superior to cisplatin (32.59 μM). Owing to a befitting lipophilicity associated with the high intake of drugs by tumor cells, complexes 3 and 6 had favorable lipid-water partition coefficients of −0.6615 and −0.8077, respectively. Moreover, it was found that complex 6 suppressed the proliferation of U87 cells mainly through an induced obvious S phase arrest and slight apoptosis, which may have resulted from the stabilization of c-myc G-quadruplex DNA to block the transcription and expression of c-myc. In brief, these types of arene Ru(II) complexes with difluorinated ligands can be developed as potential inducers of S-phase arrest and apoptosis through the binding and stabilization of c-myc G-quadruplex DNA, and could be used in clinical applications in the future.

Investigation of the binding site of ruthenium complexes to short single-stranded oligodeoxynucleotides using electrospray ionization tandem mass spectrometry

RATIONALE

In order to elucidate the nature of the interaction between metal complexes and DNA, use was made of short telomere single-stranded oligodeoxynucleotide (ODN) strand 5'-T₁ T₂ A₃ G₄ G₅ G₆ -3' (1) and strands 5'-T₁ C₂ A₃ G₄ G₅ G₆ -3' (2), 5'-T₁ T₂ A₃ C₄ G₅ G₆ -3' (3) and 5'-T₁ C₂ C₃ C₄ C₅ G₆ -3' (4) for the verification of the binding site with four different ruthenium complexes as possible anticancer drug candidates.

METHODS

The ability to form adducts between ruthenium complexes with short single-stranded 6-mers was investigated through the use of electrospray ionization mass spectrometry (ESI-MS). Full scan ESI mass spectra and collision-induced dissociation (CID) mass spectra were recorded on a high-resolution quadrupole time-of-flight mass spectrometer. The elemental compositions of the adducts and the most important product ions were calculated by exact mass measurements.

RESULTS

ESI-MS measurements showed that the mono-ruthenated ODNs were the main products produced under the conditions for the four ruthenium complexes and each of the ODNs. The CID results revealed that thymine and guanine are the preferred binding sites depending on the different compositions in the ODNs. However, for the ODN of the type: 5'-T₁ C₂ C₃ C₄ C₅ G₆ -3' the coordination site on cytosine was observed as well. The different ruthenium complexes interacted in the same way.

CONCLUSIONS

This study showed that the characterization of new ruthenium complexes with short single-stranded telomeric DNA (TTAGGG) and further different ODNs is possible with positive ESI-MS/MS measurement. The identification of thymine and cytosine besides guanine as possible binding sites suggests that the interaction site is highly affected by the ODN's structure.

Structural and morphological changes of breast cancer cells induced by iron(II) complexes

Metal-based complexes are well-established cancer chemotherapeutic drug candidates. Although our knowledge regarding their exact activity vs. toxicity profile is incomplete, changes in cell membrane biophysical properties and cytoskeletal structures have been implicated as part of the mechanism of action. Thus, in this work, we characterised the effects of iron(II)-based complexes on the structural and morphological properties of epithelial non-tumorigenic (MCF 10A) and tumorigenic (MDA-MB-231) breast cell lines using atomic force microscopy (AFM), flow cytometry and immunofluorescence microscopy. At 24 h of exposure, both the MCF 10A and MDA-MB-231 cells experienced a cell softening, and an increase in size followed by a re-stiffening at 96 h. In addition, the triple negative breast cancer cell line, MDA-MB-231, sustained a notable cytoskeletal and mitochondrial reorganization with increased actin stress fibers and cell-to-cell communication structures. An extensive all-atom molecular dynamic simulation suggests a possible direct and unassisted internalization of the metallodrug candidate, and confirmed that the cellular effects could not be ascribed to the simple physical interaction of the iron-based complexes with the biological membrane. These observations provide an insight into a link between the mechanisms of action of such iron-based complexes as anti-cancer treatment and cytoskeletal architecture.

Cytotoxic Ruthenium(II) Complexes Containing a Dangling Pyridine: Selectivity for Diseased Cells Mediated by pH-Dependent DNA Binding

Ruthenium(II) complexes of the type [Ru(bpy)₂(L¹/L²/L³)]PF₆ [where bpy = 2,2'-bipyridine, H(L¹) = N-(pyrid-2-yl)salicylaldimine (1), H(L²) = N-(6-methylpyrid-2-yl)salicylaldimine (2), and H(L³) = N-(4,6-dimethylpyrid-2-yl)salicylaldimine (3)] have been isolated. The X-ray structures of 1-3 reveal distorted octahedral coordination geometry with a planar ruthenium phenolate moiety. They exhibit interpair dimeric association in their solid state such as (a) π-π-stacking interactions (1-3) and (b) C-H···π interactions (2). The ¹H NMR spectral data shed light on the characteristics of metal-ligand bonding and chelate ring conformations. The complexes exhibit strong metal-to-ligand charge-transfer transitions in the visible region. The complexes also undergo two successive metal-based oxidative processes corresponding to the Ru^II/Ru^III and Ru^III/Ru^IV couples. Resonance Raman studies strongly suggest that the lowest unoccupied molecular orbital of 1-3 is localized at the bpy ligand. Absorption, emission, and circular dichroic spectral measurements for 1-3 with calf-thymus DNA reveal a groove binding mode of interaction. Interestingly, all of the complexes exhibit pH-dependent DNA damage, and the pH at which the damage is highest corresponds to the pH conditions of the cancer cells. The DNA damage is in the order of 3 > 2 > 1, in which a hydrolytic mechanism dominates. The protein binding properties of the complexes examined by the tryptophan quenching measurements suggest a static mechanism. The positive ΔH and ΔS values indicate that the force acting between the complexes and bovine serum albumin (BSA) is mainly a hydrophobic interaction, and thus BSA may act as a targeted drug-delivery vehicle for ruthenium(II) complexes (K ∼ 10⁵). It is noteworthy that 3 exhibits selectivity with high cytotoxicity against breast cancer cells (EVSA-T and MCF-7), and its potency is comparable to that of cisplatin.

The Significant Influence of a Second Metal on the Antiproliferative Properties of the Complex [Ru(η6 -C10 H14 )(Cl2 )(dmoPTA)]

Complexes [Ru(η6 -C10 H14 )(Cl2 )(HdmoPTA)](OSO2 CF3 ) (1), [Ru(η6 -C10 H14 )(Cl2 )(dmoPTA)] (2) and [Ru(η6 -C10 H14 )(Cl2 )-μ-dmoPTA-1κP:2κ2 N,N'-MCl2 ] (M=Zn (3), Co (4), Ni (5), dmoPTA=3,7-dimethyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane) have been synthesized and characterized by elemental analysis and spectroscopic techniques. The crystal structures of 1, 3 and 5 were obtained by single-crystal X-ray diffraction. The antiproliferative activity of the complexes was evaluated against colon cancer cell line Caco-2/TC7 by using the MTT protocol. The monometallic ruthenium complexes 1 and 2 were found to be inactive, but the bimetallic complexes 3, 4 and 5 display an increased activity (IC50 3: 9.07±0.27, 4: 5.40±0.19, 5: 7.15±0.30 μM) compared to cisplatin (IC50 =45.6±8.08 μM). Importantly, no reduction in normal cell viability was observed in the presence of the complexes. Experiments targeted to obtain information on the possible action mechanism of the complexes, such as cell cycle, ROS and gene expression studies, were performed. The results showed that the complexes display different properties and action mechanism depending on the nature of metal, M, bonded to the CH3 NdmoPTA atoms.

Syntheses, Structural and Serum Protein Protecting Activity of Ruthenium(II)-DMSO Complexes Containing Mercapto Ligand

Four new ruthenium(II) complexes [Ru(mpt)2(DMSO)2] (1), [Ru(mpt)2(bpy)] (2), [Ru(mpt)2(phen)] (3) and [Ru(mpt)2(tptz)] (4) have been synthesized and characterized by elemental analyses, IR, 1H and 13C NMR, and electronic absorption spectroscopy....

A Ru(ii)-arene-ferrocene complex with promising antibacterial activity

The evolution of high virulence bacterial strains has necessitated the development of novel therapeutic agents to treat resistant infections.

Ru(II)-Based Acetylacetonate Complexes Induce Apoptosis Selectively in Cancer Cells

The synthesis, chemical and biological characterization of seven Ru(II) polypyridyl complexes containing acetylacetonate (acac) ligands are reported. Electronic absorption spectra were determined and electrochemical potentials consistent with Ru(III/II) couples ranging from +0.60 to +0.73 V vs Ag/AgCl were measured. A series of complexes were screened against MDA-MB-231, DU-145, and MCF-10A cell lines to evaluate their cytotoxicities in cancer and normal cell lines. Although most complexes were either nontoxic or equipotent in cancer cells and normal cell lines, compound 1, [Ru(dpqy)(acac)(py)](PF6), where dqpy is 2,6-di(quinolin-2-yl)pyridine, showed up to 2.5:1.0 selectivity for cancer as compared to normal cells, along with nanomolar EC50 values in MDA-MB-231 cells. Lipophilicity, determined as the octanol/water partition coefficient, log Po/w, ranged from -0.33 (0.06) to 1.15 (0.10) for the complexes. Although cytotoxicity was not correlated with electrochemical potentials, a moderate linear correlation between lipophilicity and toxicities was observed. Cell death mechanism studies indicated that several of the Ru-acac compounds, including 1, induce apoptosis in MDA-MB-231 cells.

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.

Synthesize and multi-spectroscopic studies of zinc-naproxen nanodrug as DNA intercalator agent

The zinc-naproxen complex as a nano-drug (NanoD) was synthesized successfully via fast and effective ultrasound-assisted processes. The chemicophysical properties of the NanoD were determined using FT-IR, XRD, SEM, and EDX mapping analyses. The results confirmed the formation of the 55 nm NanoD laminates. The interaction of the obtained NanoD with calf thymus deoxyribonucleic acid (CT-DNA) was studied as well. Structural and topography changes of DNA in interaction with the NanoD were investigated by atomic force microscopy (AFM). The results of electronic absorption spectroscopy, the DNA-viscosity studies, and competition fluorescence spectroscopy showed that CT-DNA binds to the NanoD through the intercalative binding mode. The data of AFM analysis indicated swollen CT-DNA upon interaction with the NanoD. The in vitro investigation of cytotoxicity of the NanoD on HT-29 and Hep G2 cancer cells demonstrated high cytotoxicity activity of the NanoD than that of cisplatin in HT-29 cell line, especially at lower concentrations.

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.

Covalent and noncovalent interactions of coordination compounds with DNA: An overview

Deoxyribonucleic acid plays a central role in crucial cellular processes, and many drugs exert their effects through binding to DNA. Since the discovery of cisplatin and its derivatives considerable attention of researchers has been focused on the development of novel anticancer metal-based drugs. Transition metal complexes, due to their great diversity in size and structure, have a big potential to modify DNA through diverse types of interactions, making them the prominent class of compounds for DNA targeted therapy. In this review we describe various binding modes of metal complexes to duplex DNA based on covalent and noncovalent interactions or combination of both. Specific examples of each binding mode as well as possible cytotoxic effects of metal complexes in tumor cells are presented.

Fine Tuning of Cholinesterase and Glutathione-S-Transferase Activities by Organoruthenium(II) Complexes

Cholinesterases (ChEs) show increased activities in patients with Alzheimer’s disease, and remain one of the main therapeutic targets for treatment of this neurodegenerative disorder. A library of organoruthenium(II) complexes was prepared to investigate the influence of their structural elements on inhibition of ChEs, and on another pharmacologically important group of enzymes, glutathione S-transferases (GSTs). Two groups of organoruthenium(II) compounds were considered: (i) organoruthenium(II) complexes with p-cymene as an arene ligand, and (ii) organoruthenium(II) carbonyl complexes as CO-releasing molecules. Eight organoruthenium complexes were screened for inhibitory activities against ChEs and GSTs of human and animal origins. Some compounds inhibited all of these enzymes at low micromolar concentrations, while others selectively inhibited either ChEs or GSTs. This study demonstrates the importance of the different structural elements of organoruthenium complexes for their inhibitory activities against ChEs and GSTs, and also proposes some interesting compounds for further preclinical testing as ChE or GST inhibitory drugs.

Probing the Paradigm of Promiscuity for N-Heterocyclic Carbene Complexes and their Protein Adduct Formation

Metal complexes can be considered a "paradigm of promiscuity" when it comes to their interactions with proteins. They often form adducts with a variety of donor atoms in an unselective manner. We have characterized the adducts formed between a series of isostructural N-heterocyclic carbene (NHC) complexes with Ru, Os, Rh, and Ir centers and the model protein hen egg white lysozyme by X-ray crystallography and mass spectrometry. Distinctive behavior for the metal compounds was observed with the more labile Ru and Rh complexes targeting mainly a surface l-histidine moiety through cleavage of p-cymene or NHC co-ligands, respectively. In contrast, the more inert Os and Ir derivatives were detected abundantly in an electronegative binding pocket after undergoing ligand exchange of a chlorido ligand for an amino acid side chain. Computational studies supported the binding profiles and hinted at the role of the protein microenvironment for metal complexes eliciting selectivity for specific binding sites on the protein.

Antitumor Effect of Organometallic Half-Sandwich Ru(II)-Arene Complexes Bearing a Glutathione S-Transferase Inhibitor

The facile modification of the ligands in organometallic Ru(II)-arene complexes offers more opportunities to optimize their pharmacological profiles. Herein, three Ru(II)-arene complexes containing a glutathione S-transferase (GST) inhibitor (NBDHEX) in chelate ligand have been designed and synthesized in this study. In vitro results indicated that the ligation with NBDHEX significantly increased the activities and selectivities of the organometallic Ru(II)-arene complexes against tumor cells, especially complex 3, which was the most active compound among the tested compounds. DFT calculations and hydrolysis results demonstrated that complex 3 with more alkyl groups in the arene ligand has increased electron density at the Ru(II) center as compared with complexes 1 and 2, thus resulting in the improved hydrolysis rate, which may be responsible for its higher anticancer activity. Further studies showed that complexes 1-3 can cause the loss of the mitochondrial membrane potential and upregulate the expression of Bcl-2 and Bax in A549 cells, suggesting that complexes 1-3-induced cell death may be mediated via the mitochondrial apoptotic pathway. Thus, these findings suggested that simultaneous modification of the chelate ligands and arene rings in the organometallic Ru(II)-arene complexes is an effective way to improve their pharmacological properties.

Introducing N-, P-, and S-donor leaving groups: an investigation of the chemical and biological properties of ruthenium, rhodium and iridium thiopyridone piano stool complexes

A series of 15 piano-stool complexes featuring either a RuII, RhIII or IrIII metal center, a bidentate thiopyridone ligand, and different leaving groups was synthesized. The leaving groups were selected in order to cover a broad range of different donor atoms. Thus, 1-methylimidazole served as a N-donor, 1,3,5-triaza-7-phosphaadamantane (pta) as a P-donor, and thiourea as a S-donor. Additionally, three complexes featuring different halido leaving groups (Cl, Br, I) were added. Leaving group alterations were carried out with respect to a possible influence on pharmacokinetic and pharmacodynamic parameters, as well as the cytotoxicity of the respective compounds. The complexes were characterized via NMR spectroscopy, X-ray diffraction (where possible), mass spectrometry, and elemental analysis. Cytotoxicity was assessed in 2D cultures of human cancer cell lines by microculture and clonogenic assays as well as in multicellular tumor spheroids. Furthermore, cellular accumulation studies, flow-cytometric apoptosis and ROS assays, DNA plasmid assays, and laser ablation ICP-MS studies for analyzing the distribution in sections of multicellular tumor spheroids were conducted. This work demonstrates the importance of investigating each piano-stool complexes' properties, as the most promising candidates showed advantages over each other in certain tests/assays. Thus, it was not possible to single out one lead compound, but rather a group of complexes with enhanced cytotoxicity and activity.