Wolfgang Kandioller

Maltol‐Derived Ruthenium–Cymene Complexes with Tumor Inhibiting Properties: The Impact of Ligand–Metal Bond Stability on Anticancer Activity In Vitro

Organometallic ruthenium-arene compounds bearing a maltol ligand have been shown to be nearly inactive in in vitro anticancer assays, presumably due to the formation of dimeric Ru(II) species in aqueous solutions. In an attempt to stabilize such complexes, [Ru(eta(6)-p-cymene)(XY)Cl] (XY=pyrones or thiopyrones) complexes with different substitution pattern of the (thio)pyrone ligands have been synthesized, their structures characterized spectroscopically, and their aquation behavior investigated as well as their tumor-inhibiting potency. The aquation behavior of pyrone systems with electron-donating substituents and of thiopyrone complexes was found to be significantly different from that of the maltol-type complex reported previously. However, the formation of the dimer can be excluded as the primary reason for the inactivity of the complex because some of the stable compounds are not active in cancer cell lines either. In contrast, studies of their reactivity towards amino acids demonstrate different reactivities of the pyrone and thiopyrone complexes, and the higher stability of the latter probably renders them active against human tumor cells.

Physicochemical Studies and Anticancer Potency of Ruthenium η6-p-Cymene Complexes Containing Antibacterial Quinolones

With the aim of exploring the anticancer properties of organometallic compounds with bioactive ligands, Ru(arene) compounds of the antibacterial quinolones nalidixic acid (2) and cinoxacin (3) were synthesized, and their physicochemical properties were compared to those of chlorido(η6-p-cymene)(ofloxacinato-κ2O,O)ruthenium(II) (1). All compounds undergo a rapid ligand exchange reaction from chlorido to aqua species. 2 and 3 are significantly more stable than 1 and undergo minor conversion to an unreactive [(cym)Ru(μ-OH)3Ru(cym)]+ species (cym = η6-p-cymene). In the presence of human serum albumin 1−3 form adducts with this transport protein within 20 min of incubation. With guanosine 5′-monophosphate (5′-GMP; as a simple model for reactions with DNA) very rapid reactions yielding adducts via its N7 atom were observed, illustrating that DNA is a possible target for this compound class. A moderate capacity of inhibiting tumor cell proliferation in vitro was observed for 1 in CH1 ovarian cancer cells, whereas 2 and 3 turned out to be inactive.

Structure-activity relationships of targeted RuII(η6-p-cymene) anticancer complexes with flavonol-derived ligands.

RuII(arene) complexes have been shown to be promising anticancer agents, capable of overcoming major drawbacks of currently used chemotherapeutics. We have synthesized RuII(η6-arene) compounds carrying bioactive flavonol ligands with the aim to obtain multitargeted anticancer agents. To validate this concept, studies on the mode of action of the complexes were conducted which indicated that they form covalent bonds to DNA, have only minor impact on the cell cycle, but inhibit CDK2 and topoisomerase IIα in vitro. The cytotoxic activity was determined in human cancer cell lines, resulting in very low IC50 values as compared to other RuII(arene) complexes and showing a structure-activity relationship dependent on the substitution pattern of the flavonol ligand. Furthermore, the inhibition of cell growth correlates well with the topoisomerase inhibitory activity. Compared to the flavonol ligands, the RuII(η6-p-cymene) complexes are more potent antiproliferative agents, which can be explained by potential multitargeted properties.

Osmium(II)-versus ruthenium(II)-arene carbohydrate-based anticancer compounds: similarities and differences

The synthesis and in vitro anticancer activity of Os(II)-arene complexes with carbohydrate-derived phosphite co-ligands are reported. The compounds were characterized by standard methods and the molecular structure of dichlorido(eta(6)-p-cymene)(3,5,6-bicyclophosphite-1,2-O-isopropylidene-alpha-D-glucofuranoside)osmium(II) was determined by X-ray diffraction analysis. Complexes with chlorido leaving groups undergo hydrolysis by consecutive formation of aqua compounds, followed by cleavage of a P-O bond of sugar phosphite ligands, as demonstrated by NMR studies. These observations are similar to those of analogous Ru(II)-arene complexes; however the rate of hydrolysis is very slow for osmium compounds. The complexes with oxalato leaving groups resist hydrolysis; no hydrolytic species were detected by (31)P{(1)H} NMR spectroscopy over several days. Within this series of Os compounds, in vitro anticancer activity is highest for the most lipophilic chlorido complex dichlorido(eta(6)-p-cymene)(3,5,6-bicyclophosphite-1,2-O-cyclohexylidene-alpha-D-glucofuranoside)osmium(II).

Antitumor pentamethylcyclopentadienyl rhodium complexes of maltol and allomaltol: Synthesis, solution speciation and bioactivity

The reaction of the dimer [Rh(III)(pentamethylcyclopentadienyl)(μ-Cl)Cl]2 ([Rh(III)(Cp*)(μ-Cl)Cl]2) with the hydroxypyrone ligands maltol and allomaltol affords complexes of the general formula [Rh(III)(Cp*)(L)Cl] under standard and microwave conditions. The organometallic compounds were characterized by standard analytical methods and in the case of the allomaltol derivative in the solid state by single-crystal X-ray diffraction analysis. The complexes showed similar cytotoxicity profiles and were proved to be moderately active against various human cancer cell lines. The stoichiometry and stability of these complexes were determined in aqueous solution by pH-potentiometry, (1)H NMR spectroscopy and UV-visible spectrophotometry. Speciation was studied in the presence and in the absence of chloride ions. Hydrolysis of [Rh(III)(Cp*)(H2O)3](2+) gave dimeric mixed hydroxido species [(Rh(III)(Cp*))2(μ-OH)3](+) and [(Rh(III)(Cp*))2(μ-OH)2Z2] (Z=H2O/Cl(-)). Formation of the mononuclear complexes [Rh(III)(Cp*)(L)Z] of maltol and allomaltol with similar and moderate stability was found. These species predominate at physiological pH and decompose only partially at micromolar concentrations. In addition, hydrolysis of the aqua complex or a chlorido/hydroxido co-ligand exchange resulted in the formation of the mixed-hydroxido species [Rh(III)(Cp*)(L)(OH)] in the basic pH range. Replacement of the chlorido by an aqua ligand in the complex [Rh(III)(Cp*)(L)Cl] was monitored and with the help of the equilibrium constants the extent of aquation at various chloride concentrations of the extra- and intracellular milieu can be predicted. Complexation of these Rh(III) complexes was compared to analogous [Ru(II)(η(6)-p-cymene)] species and higher conditional stabilities were found in the case of the Rh(III) compounds at pH7.4.

Biomolecule binding vs. anticancer activity: Reactions of Ru(arene)[(thio)pyr-(id)one] compounds with amino acids and proteins

The interactions of the ruthenium(arene) complexes [chlorido(η(6)-p-cymene)(2-methyl-3-(oxo-κO)-4H-pyran-4-onato-κO)ruthenium(II)] 1, [chlorido(η(6)-p-cymene)(2-methyl-3-(oxo-κO)-4H-thiopyran-4-onato-κS)ruthenium(II)] 2 and [chlorido(η(6)-p-cymene){N-[(ethoxycarbonyl)methyl]-3-(oxo-κO)-1H-pyrid-2-onato-κO}ruthenium(II)] 3 with biomolecules such as l-methionine (Met) and ubiquitin (Ub) were investigated by electrospray ionization (ESI) ion trap mass spectrometry (MS). These Ru(II) compounds were shown to exhibit anticancer activity which varies depending on the (thio)pyr(id)onato ligands. Compounds 1 and 3 reacted readily with the model protein Ub to yield stable [Ub+Ru(p-cym)] adducts (p-cym=η(6)-p-cymene), whereas 2 was converted only to a minor degree. The protein adduct formation is reversible by incubation with N- and S-donor systems, the latter being more efficient. From these studies, an inverse correlation between metallodrug-protein interaction and cytotoxicity against human tumor cell lines was derived, where low protein binding ability is indicative of increased cytotoxic activity.

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

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

Influence of the Arene Ligand and the Leaving Group on the Anticancer Activity of (Thio)maltol Ruthenium(ii)–(η6-Arene) Complexes

Maltol and its metal complexes have shown promising applications in medicinal chemistry. We report here the synthesis and characterization of Ru(η6-arene)(halido) coordination compounds bearing maltol or thiomaltol ligands and studies on their behaviour in aqueous solution, their reactions with the DNA model guanosine 5′-monophosphate (5′-GMP) and their in vitro anticancer activity in human tumour cell lines. The compounds hydrolyze rapidly and quantitatively to the respective aqua species by exchange of the halido ligand. With pKa values of >8, such species would also be present in biological media and they proved reactive to 5′-GMP. The thiomaltol compounds show promising in vitro activity with IC50 values (50% inhibitory concentration) in the low micromolar range, whereas activity of the maltol complexes is marginal. Variation of the arene ligand (benzene, toluene, p-cymene or biphenyl) resulted only in minor changes in activity.

Novel thiosalicylate-based ionic liquids for heavy metal extractions

This study aims to develop novel ammonium and phosphonium ionic liquids (ILs) with thiosalicylate (TS) derivatives as anions and evaluate their extracting efficiencies towards heavy metals in aqueous solutions. Six ILs were synthesized, characterized, and investigated for their extracting efficacies for cadmium, copper, and zinc. Liquid-liquid extractions of Cu, Zn, or Cd with ILs after 1-24h using model solutions (pH 7; 0.1M CaCl2) were assessed using flame atomic absorption spectroscopy (F-AAS). Phosphonium-based ILs trihexyltetradecylphosphonium 2-(propylthio)benzoate [P66614][PTB] and 2-(benzylthio)benzoate [P66614][BTB] showed best extraction efficiency for copper and cadmium, respectively and zinc was extracted to a high degree by [P66614][BTB] exclusively.

Solution Equilibria of Anticancer Ruthenium(II)-(η6-p-Cymene)-Hydroxy(thio)pyr(id)one Complexes: Impact of Sulfur vs. Oxygen Donor Systems on the Speciation and Bioactivity

Stoichiometry and stability of antitumor ruthenium(II)-η(6)-p-cymene complexes of bidentate (O,O) hydroxypyrone and (O,S) hydroxythiopyr(id)one type ligands were determined by pH-potentiometry, (1)H NMR spectroscopy and UV-Vis spectrophotometry in aqueous solution and in dependence of chloride ion concentration. Formation of mono-ligand complexes with moderate stability was found in the case of the hydroxypyrone ligands (ethyl maltol and allomaltol) predominating at the physiological pH range. These complexes decompose to the dinuclear tri-hydroxido bridged species [{Ru(II)(η(6)-p-cymene)}2(OH)3](+) and to the metal-free ligand at basic pH values. In addition, formation of a hydroxido [Ru(II)(η(6)-p-cymene)(L)(OH)] species was found. The hydroxythiopyr(id)one ligands (thiomaltol, thioallomaltol, 3-hydroxy-1,2-dimethyl-thiopyridone) form complexes of significantly higher stability compared with the hydroxypyrones; their complexes are biologically more active, the simultaneous bi- and monodentate coordination of the ligands in the bis complexes (ML2 and ML2H) was also demonstrated. In the case of thiomaltol, formation of tris complexes is also likely at high pH. The replacement of the chlorido by the aqua ligand in the [Ru(II)(η(6)-p-cymene)(L)(Cl)] species was monitored, which is an important activation step in the course of the mode of action of the complexes, facilitating binding to biological targets.