Ligand-Based Neutral Ruthenium(II) Arene Complex: Selective Anticancer Action

Ruthenium arene complexes with mono-carbonyl analogues of curcumin as pendant or bridging ligands: Synthesis, anti-cancer activity and interaction with quadruplex DNA

A series of half‐sandwich ruthenium(II) arene complexes [(η6‐p‐cymene)RuII(R‐BzTSC)Cl]Cl 1, 2, 3 (BzTSC = benzaldehyde thiosemicarbazone and R = H, CH3 and C6H5) have been synthesized and characterized by IR, 1H NMR, UV‐visible, electrospray ionization mass spectrometry and elemental analysis. The single‐crystal structures of 1 and 3 have been determined. The molecular orbitals and electronic absorption spectra of the compounds have been calculated using the DFT and TDDFT methods. The in vitro antiproliferative activities of these complexes have been evaluated against four human cancer cell lines (CNE, H292, SKBR3 and Hey1‐B), and 3 is proved to be the most efficient inhibitor, with IC50 values of 20, 31, 10 and 34 μm, respectively. Copyright © 2013 John Wiley & Sons, Ltd.

Water‐soluble ruthenium(II)–arene complexes [(η6‐arene)Ru(κ2‐L)]n+ (n = 0, 1) ([Ru]‐1–[Ru]‐10) containing pyridine‐based bischelating N,O/N,N donor ligands (L1–L5) are synthesized and employed for the catalytic C–H bond activation/arylation of a wide range of 2‐phenylpyridines and aryl halides in water, affording the corresponding mono‐ and biarylated products. Exploring the reactivity of the synthesized complexes, our investigations, including time‐dependent 1H NMR spectroscopic studies with ruthenium–arene catalysts, demonstrate a remarkable structure–activity relationship for the ligand‐tuned C–H activation/arylation of 2‐phenylpyridine, where the complexes with bischelating N,O donor‐based ligands (acteylpyridine and picolinate) outperform those with N,N donor ligands (iminopyridine). Moreover, among the N,O donor ligands, a distinct effect of the nature of the coordinating oxygen donor on the catalytic activity is also observed, where ruthenium–arene complexes having N,O donor ligands (acetylpyridine) with neutral oxygen‐donor atoms exhibit enhanced catalytic activity over those with anionic oxygen‐donor atoms (picolinate). The observed trend in the catalytic activity is attributed to the ligand‐promoted facile deprotonation and coordination–decoordination interconversion behavior. In addition, molecular structures for a few of the representative complexes ([Ru]‐2, [Ru]‐4, and [Ru]‐5) are authenticated by single‐crystal X‐ray diffraction studies.

Ru(II) eta6-arene complexes containing p-cymene (p-cym), tetrahydronaphthalene (thn), benzene (bz), or biphenyl (bip), as the arene, phenylazopyridine derivatives (C5H4NN:NC6H5R; R = H (azpy), OH (azpy-OH), NMe2 (azpy-NMe2)) or a phenylazopyrazole derivative (NHC3H2NN:NC6H5NMe2 (azpyz-NMe2)) as N,N-chelating ligands and chloride as a ligand have been synthesized (1-16). The complexes are all intensely colored due to metal-to-ligand charge-transfer Ru 4d6-pi* and intraligand pi -->pi* transitions (eta = 5000-63 700 M-1 cm-1) occurring in the visible region. In the crystal structures of [(eta6-p-cym)Ru(azpy)Cl]PF6 (1), [(eta6-p-cym)Ru(azpy-NMe2)Cl]PF6 (5), and [(eta6-bip)Ru(azpy)Cl]PF6 (4), the relatively long Ru-N(azo) and Ru-(arene-centroid) distances suggest that phenylazopyridine and arene ligands can act as competitive pi-acceptors toward Ru(II) 4d6 electrons. The pKa* values of the pyridine nitrogens of the ligands are low (azpy 2.47, azpy-OH 3.06 and azpy-NMe2 4.60), suggesting that they are weak sigma-donors. This, together with their pi-acceptor behavior, serves to increase the positive charge on ruthenium, and together with the pi-acidic eta6-arene, partially accounts for the slow decomposition of the complexes via hydrolysis and/or arene loss (t(1/2) = 9-21 h for azopyridine complexes, 310 K). The pKa* of the coordinated water in [(eta6-p-cym)Ru(azpyz-NMe2)OH2]2+ (13A) is 4.60, consistent with the increased acidity of the ruthenium center upon coordination to the azo ligand. None of the azpy complexes were cytotoxic toward A2780 human ovarian or A549 human lung cancer cells, but several of the azpy-NMe2, azpy-OH, and azpyz-NMe2 complexes were active (IC50 values 18-88 microM).

Five complexes of formula [Ru(η6-C6H5CH2COOH)(XY)Cl]Cl/Na (XY = ethylenediamine (1), o-phenylenediamine (2), phenanthroline (3), and oxalato (4)) and [Ru(η6:κ1-C6H5CH2COO)(tmen)]Cl (tmen = N, N, N', N'-tetramethylethylenediamine, 5C) have been synthesized and fully characterized. Five new X-ray crystal structures ([Ru(η6-C6H5CH2COOH)(μ-Cl)Cl]2, 1, 3, 4, and 5C·PF6) have been determined, which are the first examples of ruthenium(II) structures with phenylacetic acid as arene ligand. Furthermore, 5C·PF6 is the first example of a five-membered tether ring with a Ru(η6:κ1-arene:O) bond. The tether ring in these complexes opens in acidic pH (<5) and closes reversibly in aqueous solution. The chlorido open-form undergoes aquation, and the aqua adduct can be observed (prior to ring closure) by NMR. The speciation has an attractive complexity in the pH range 0-12, showing interconversion of the three species (chlorido, aqua, and closed tether), dependent on the proton concentration and the nature of the XY chelating ligand. The closed tether version of 3, complex 3C, with σ-donor/π-acceptor phenanthroline as chelating ligand, opens up more readily (pH 4), while the tether ring in complex 5C hardly opens even at pH as low as 1. We have determined the p Ka of the pendant carboxylic group and that of the aqua adduct (ca. 3 and ca. 7, respectively), which can be finely tuned by the appropriate choice of XY. Complexes 1 and 2, which predominate in their inactive (closed-tether) form in intracellular conditions, show some cytotoxic activity (IC50 130 and 117 μM, respectively) in A2780 ovarian cancer cells. Complex 1 catalyzes the reduction through transfer hydrogenation of pyruvate to lactate and NAD+ to NADH in the presence of formate as H-source. Co-incubation with sodium formate decreases the IC50 value of 1 in A2780 cancer cells significantly.

Synthesis, antiproliferative activity and apoptosis-promoting effects of arene ruthenium(II) complexes with N, O chelating ligands

Ruthenium(II)-arene complexes with monodentate aminopyridine ligands: Insights into redox stability and electronic structures and biological activity

Quadruplex nucleic acids - DNA/RNA secondary structures formed in guanine rich sequences - proved to have key roles in the biology of cancers and, as such, in recent years they emerged as promising targets for small molecules. Many reports demonstrated that metal complexes can effectively stabilize quadruplex structures, promoting telomerase inhibition, downregulation of the expression of cancer-related genes and ultimately cancer cell death. Although extensively explored as anticancer agents, studies on the ability of ruthenium arene complexes to interact with quadruplex nucleic acids are surprisingly almost unknown. Herein, we report on the synthesis and characterization of four novel Ru(ii) arene complexes with 1,3-dioxoindan-2-carboxamides ligands bearing pendant naphthyl-groups designed to bind quadruplexes by both stacking and coordinating interactions. We show how improvements on the hydrolytic stability of such complexes, by substituting the chlorido leaving ligand with pyridine, have a dramatic impact on their interaction with quadruplexes and on their cytotoxicity against ovarian cancer cells.

Organoruthenium complexes are potentalternatives for platinum-basedcomplexes because of their superior anticancer activity. In this investigation,a series of new Ru(II)-arene complexes with triarylamine–thiosemicarbazonehybrid ligands with higher anticancer activity than cisplatin arereported. The molecular structure of the ligands and complexes wasconfirmed spectroscopically and supported by single-crystal X-raycrystallography. These complexes adopted a three-leg piano stool geometry.All the Ru(II)-arene complexes were systematically investigated fortheir in vitro cytotoxicity against human cervical (HeLa S3), lung(A549) cancer, and human normal lung (IMR-90) cell lines using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide assay. Interestingly, a pyrrolidine-attached Ru(II)-benzenecomplex exhibited superior activity against cancer cells with lowIC50 values, and colony formation study showed completeinhibition at 5 and 10 μM concentration. Furthermore, morphologicalchanges assessed by acridine orange and propidium iodide stainingrevealed that the cell death occurred by apoptosis. In addition, theinteraction between synthesized Ru(II)-arene complexes and DNA/proteinwas explored by absorption and emission spectroscopy methods. Thesesynthesized new organoruthenium complexes can be used for developingnew metal-based anticancer drugs.

Human galectin-1 (hGal-1) is an abundant β-galactoside-binding animal lectin that plays an essential role in promoting the immunosuppressive tumor microenvironment. Although hGal-1 has been identified as a promising target for pharmacological inhibition, developing potent and selective hGal-1 inhibitors has been complicated by the high degree of sequence similarity of the glycan-binding site across the galectin family. Herein, we present potent nanomolar hGal-1 inhibitors with unprecedented selectivity of 2 to 3 orders of magnitude over human galectin-3 (hGal-3). Their primary structural feature is the modification of a thiodigalactoside scaffold at the 3- and 3'-positions with a half-sandwich ruthenium(II) arene complex containing a bidentate 4-(2-pyridyl)-1H-1,2,3-triazol-1-yl ligand. The most potent inhibitor in the series efficiently blocked the binding of hGal-1 to the surface of MDA-MB-231 tumor cells, reduced their viability, and completely suppressed hGal-1-induced phosphatidylserine exposure in Jurkat cells, a process previously described as preaparesis rather than classical apoptosis.

Three new ruthenium complexes with bidentate chloroquine analogue ligands, [Ru(η(6)-cym)(L(1))Cl]Cl (1, cym = p-cymene, L(1) = N-(2-((pyridin-2-yl)methylamino)ethyl)-7-chloroquinolin-4-amine), [Ru(η(6)-cym)(L(2))Cl]Cl (2, L(2) = N-(2-((1-methyl-1H-imidazol-2-yl)methylamino)ethyl)-7-chloroquinolin-4-amine) and [Ru(η(6)-cym)(L(3))Cl] (3, L(3) = N-(2-((2-hydroxyphenyl)methylimino)ethyl)-7-chloroquinolin-4-amine) have been synthesized and characterized. In addition, the X-ray crystal structure of 2 is reported. The antimalarial activity of complexes 1-3 and ligands L(1), L(2) and L(3), as well as the compound N-(2-(bis((pyridin-2-yl)methyl)amino)ethyl)-7-chloroquinolin-4-amine (L(4)), against chloroquine sensitive and chloroquine resistant Plasmodium falciparum malaria strains was evaluated. While 1 and 2 are less active than the corresponding ligands, 3 exhibits high antimalarial activity. The chloroquine analogue L(2) also shows good activity against both the chloroquine sensitive and the chloroquine resistant strains. Heme aggregation inhibition activity (HAIA) at an aqueous buffer/n-octanol interface (HAIR(50)) and lipophilicity (D, as measured by water/n-octanol distribution coefficients) have been measured for all ligands and metal complexes. A direct correlation between the D and HAIR(50) properties cannot be made because of the relative structural diversity of the complexes, but it may be noted that these properties are enhanced upon complexation of the inactive ligand L(3) to ruthenium, to give a metal complex (3) with promising antimalarial activity.

The dinuclear RuII arene complexes [{(eta6-arene)RuCl}2(mu-2,3-dpp)](PF6)2, arene=indan (1), benzene (2), p-cymene (3), or hexamethylbenzene (4) and 2,3-dpp=2,3-bis(2-pyridyl)pyrazine, have been synthesized and characterized. Upon irradiation with UVA light, complexes 1 and 2 readily underwent arene loss, while complexes 3 and 4 did not. The photochemistry of 1 was studied in detail. In the X-ray structure of [{(eta6-indan)RuCl}2(mu-2,3-dpp)](PF6)2 (1), 2,3-dpp bridges two RuII centers 6.8529(6) A apart. In water, aquation of 1 in the dark occurs with replacement of chloride with biexponential kinetics and decay constants of 100+/-1 min-1 and 580+/-11 min-1. This aquation was suppressed by 0.1 M NaCl. UV or visible irradiation of 1 in aqueous or methanolic solution led to arene loss. The fluorescence of the unbound arene is approximately 40 times greater than when it is complexed. Irradiation of 1 also had a significant effect on its interactions with DNA. The DNA binding of 1 is increased after irradiation. The non-irradiated form of 1 preferentially formed DNA adducts that only weakly blocked RNA polymerase, while irradiation of 1 transformed the adducts into stronger blocks for RNA polymerase. The efficiency of irradiated 1 to form DNA interstrand cross-links was slightly greater than that of cisplatin in both 10 mM NaClO4 and 0.1 M NaCl. In contrast, the interstrand cross-linking efficiency of non-irradiated 1 in 10 mM NaClO4 was relatively low. An intermediate amount of cross-linking was observed when the sample of DNA already modified by non-irradiated 1 was irradiated. DNA unwinding measurements supported the conclusion that both mono- and bifunctional adducts with DNA can form. These results show that photoactivation of dinuclear RuII arene complexes can simultaneously produce a highly reactive ruthenium species that can bind to DNA and a fluorescent marker (the free arene). Importantly, the mechanism of photoreactivity is also independent of oxygen. These complexes, therefore, have the potential to combine both photoinduced cell death and fluorescence imaging of the location and efficiency of the photoactivation process.

One pot conversion of acetyl chloride to dehydroacetic acid and its coordination in a ruthenium(II) arene complex

Organometallic ruthenium(II)-arene complexes are currently attracting increasing interest as anticancer compounds with the potential to overcome drawbacks of traditional drugs like cisplatin with respect to resistance, selectivity, and toxicity. Rational design of new potential pharmaceutical compounds requires a detailed understanding of structure–property relationships at an atomic level. We performed in vacuo density functional theory(DFT) calculations, classical MD, and mixed QM/MM Car-Parrinello MD explicit solvent simulations to rationalize the binding mode of two series of anticancer ruthenium(II) arene complexes to double-stranded DNA (dsDNA).Binding energies between the metal centers and the surrounding ligands as well as proton affinities were calculated using DFT. Our results support a pH-dependent mechanism for the activity of the RAPTA [Ru(?6-arene)X2(pta)] (pta = 1,3,5-triaza-7-phosphatricyclo[3.3.1.1]decane) compounds. Adducts of the bifunctional RAPTA and themonofunctional [Ru(?6-p-cymene)Xen]+ series of compounds with the DNA sequence d(CCTCTG*G*TCTCC)/d(GGAGACCAGAGG), where G* are guanosine bases that bind to the ruthenium compounds through their N(7) atom, have been investigated. The resulting binding sites were characterized in QM/MM molecular dynamics simulations showing that DNA can easily adapt to accommodate the ruthenium compounds.

New cationic, half-sandwich Ru(II) arene compounds of general formula [(η(6)-arene)RuCl(κ(2)-N,N-L)]X (where L are functionalized phenanthrolines such as 1,10-phenanthroline-5-amine (aphen); 5,6-epoxy-5,6-dihydro-[1,10]phenanthroline (ephen); or 4,7-dihydroxy-1,10-phenanthroline (dhphen)) have been prepared to study their anticancer potential. All the isolated complexes have been fully characterized by spectroscopic and analytical techniques. The structure of endo-[(η(6)-p-cymene)RuCl(κ(2)-N,N-ephen)]BF4, [2a](BF4), has been determined by X-ray crystallography. The in vitro cytotoxicity of the aphen and ephen phenanthrolines and their Ru derivatives [(η(6)-p-cymene)RuCl(κ(2)-N,N-L)]Cl ([1a]Cl and [2a]Cl, respectively) assessed in tumour cell lines has shown that the free ligands are more active than the organometallic products, with aphen being the most potent specimen. Furthermore, the binding interaction of both [1a]Cl and aphen with calf thymus DNA (CT-DNA) has been investigated using a variety of thermodynamic and kinetic techniques. The aphen free ligand intercalates into DNA at low ligand content, whereas [1a]Cl forms with DNA a bifunctional partially intercalated-covalent complex, in which the intercalation constant is nearly three orders of magnitude lower than that of aphen. This finding demonstrates that the covalent binding noticeably weakens the intercalation, a feature presumably related to the higher cytotoxic activity of aphen relative to that of [1a]Cl.