Giordana Marcon

Crystal structure and solution chemistry of the cytotoxic complex 1,2-dichloro(o-phenanthroline)gold(III) chloride

Abstract The crystal structure of the cytotoxic complex 1,2-dichloro(o-phenanthroline)gold(III) chloride ([AuphenCl2]Cl) has been solved through single crystal X-ray diffraction methods. The complex is square planar and exhibits a quite regular geometry. Crystals of the compound belong to the space group P21/n with a=12.632(5), b=16.916(3), c=12.902(6) A, β=91.31(3)° and Z=8. The coordination of the two gold(III) ions in the asymmetric unit is completed by two chloride ions at 2.972(3) and 3.043(3) A, respectively, forming a distorted square pyramid. The behavior in solution of [AuphenCl2]Cl was further analyzed through 1H NMR spectroscopy. Results point out that the [Au(III)phen]3+ molecular fragment is stable in solution for several hours, even under physiological conditions, whereas the two chloride ligands are released within approximately 30 min after dissolution in the buffer, at 25°C. The gold(III) chromophore is easily and quickly reduced by addition of stoichiometric amounts of sodium ascorbate; metallic gold is formed and free phenanthroline liberates. The implications of these findings for the biological properties of the [Au(III)phen]3+ species are discussed.

Modeling of copper(II) sites in proteins based on histidyl and glycyl residues

The complexes between copper(II) and four synthetic tetrapeptides bearing a single histidine residue within the sequence (AcHGGG, AcGHGG, AcGGHG and AcGGGH, respectively), have been investigated by potentiometric and spectroscopic methods (UV-Vis, circular dichroism and electron paramagnetic resonance). Potentiometric studies in the pH range 4-12 allowed identification and quantitative determination of the species present in solution for each copper-peptide complex. In all cases, upon raising pH, copper(II) coordination starts from the imidazole nitrogen of the His; afterwards three deprotonated amide nitrogens are progressively involved in copper coordination, except in the case of AcGHGG. Based on the potentiometric and spectroscopic results, detailed molecular structures are proposed for the dominant copper(II) tetrapeptide species existing in solution, either at neutral or alkaline pH. The structural consequences of the presence and of the location of a unique histidine residue within the tetrameric sequence are specifically analyzed. Results are discussed in relation to the modeling of copper(II) binding sites in proteins, particular emphasis being devoted to the copper complexes of the prion protein.

Gold(III) Compounds as New Family of Anticancer Drugs

Gold(III) complexes are emerging as a new class of metal complexes with outstanding cytotoxic properties and are presently being evaluated as potential antitumor agents. This renewed interest is the result of recent studies in which various gold(III) complexes have been shown to be stable under physiological conditions and to manifest relevant antiproliferative properties against selected human tumor cell lines. The pharmacological investigation of some representative gold(III) complexes has been extended to consider their effects on the cell cycle and to reveal induction of apoptosis. Remarkably, preliminary studies suggest that the interactions in vitro of gold(Ill) complexes with calf thymus DNA are weak whereas significant binding to model proteins takes place. Our findings imply that the mechanism of action of cytotoxic gold(Ill) complexes might be substantially different from that of clinically established platinum compounds.

Cytotoxicity and DNA binding properties of a chloro glycylhistidinate gold(III) complex (GHAu)

The chloro glycylhistidinate gold(III) complex (GHAu) is shown to be fairly cytotoxic towards the established A2780 ovarian carcinoma human cell line either sensitive or resistant to cisplatin. Remarkably, GHAu is far more cytotoxic than the corresponding zinc(II), palladium(II), platinum(II) and cobalt(II) complexes implying that cytotoxicity is essentially to be ascribed to the presence of a gold(III) center. Circular dichroism (CD) spectra, atomic absorption measurements and DNA melting profiles suggest that GHAu in vitro is able to bind DNA, the presumed target for several antitumor metal complexes, and to modify its conformation, even if the observed changes are generally small. Implications of these findings for the mechanism of action of cytotoxic gold(III) complexes are discussed.

Molecular structure, solution chemistry and biological properties of the novel [ImH][trans-IrCl4(Im)(DMSO)], (I) and of the orange form of [(DMSO)2H][trans-IrCl4(DMSO)2], (II), complexes

The new iridium(III) complex, imidazolium[trans(DMSO,imidazole)tetrachloroiridate(III)], (I) (DMSO=dimethyl sulfoxide), and the orange form of [(DMSO)(2)H][trans(DMSO)(2)tetrachloroiridate(III)], (II) have been prepared and characterized, both in the solid state and in solution, by X-ray diffraction and by various physicochemical techniques. Single crystal X-ray diffraction studies point out that complex (II) is isomorphous to the ruthenium(III) analogue, [(DMSO)(2)H][trans-RuCl(4)(DMSO)(2)], (III). Crystallographic data are the following: a=16.028(2) A, b=24.699(3) A, c=8.262(1) A, in space group Pbca (Z=8) for (imidazolium)[trans(DMSO,imidazole)tetrachloroiridate(III)], (I); and a=9.189(2) A, b=16.511(4) A, c=14.028(3) A, beta=100.82(2) degrees in space group P2/n (Z=4) for [(DMSO)(2)H][trans(DMSO)(2)tetrachloroiridate(III)], (II). Visible absorption spectra show that both complexes are stable for several days, at pH 7.4, at room temperature. No significant chloride hydrolysis is observed, even at high temperature (70 degrees C), over 24 h. The extreme stability of these iridium(III) complexes within a physiological buffer was further assessed by (1)H NMR; in addition, cyclic voltammetry measurements evidenced a high stability of the oxidation state +3. Preliminary biological studies show that both complexes do not bind appreciably bovine serum albumin nor inhibit significantly the proliferation of representative human tumor cell lines, suggesting that hydrolysis of coordinated chlorides is a crucial feature for the biological properties and the antitumor activity of the parent ruthenium(III) complexes.

Synthesis, molecular structure and solution chemistry of the iridium(III) complex imidazolium [trans(bisimidazole)tetrachloro iridate(III)] (IRIM)

Abstract The iridium(III) complex imidazolium [trans(bisimidazole)tetrachloro iridate(III)], (IRIM), isostructural to the well known ruthenium(III) analogue, (ICR), has been prepared and characterised, both in the solid state and in solution, by X-ray diffraction and by a variety of physico-chemical techniques. Single crystal X-ray diffraction studies point out that this complex is isomorphous with ICR and with the rhodium(III) analogue. IRIM is moderately soluble in water and within a physiological phosphate buffer. Electronic spectra in the visible, show that the complex is stable for days at pH 7.4; notably no significant chloride hydrolysis is observed over a period of 24 h at 25°C. Stability of IRIM within a physiological buffer was further tested and confirmed by 1 H NMR spectra. The complex is stable toward treatment with either hydrogen peroxide or ascorbic acid or silver nitrate. The chemical behaviour in solution and the reactivity of IRIM are compared to those of ICR; implications of the present results for possible pharmacological applications of IRIM and for a better understanding of the mechanism of action of ICR are discussed.

Cytotoxicity, DNA damage, and cell cycle perturbations induced by two representative gold(III) complexes in human leukemic cells with different cisplatin sensitivity.

The gold(III) complexes [Au(phen)Cl2]Cl and [Au(dien)Cl]Cl2 were recently shown to exert important cytotoxic effects in vitro on human tumor cell lines. To elucidate the biochemical mechanisms leading to cell death, the effects produced by these gold(III) complexes on the leukemic CCRF-CEM cell line--either sensitive (CCRF-CEM) or resistant to cisplatin (CCRF-CEM/CDDP)--were analyzed in detail by various techniques. For comparison purposes the effects produced by equitoxic concentrations of cisplatin were also analyzed. First, the dependence of the IC50 values of either complex on the incubation time was investigated. Cytotoxicity experiments confirmed that both gold(III) compounds retain their efficacy against the cisplatin-resistant line: only minimal cross-resistance with cisplatin was detected. Notably, [Au(phen)Cl2]Cl is more cytotoxic than [Au(dien)Cl]Cl2, with IC50 values of 7.4 and 6.0 M at 24 and 72 h, respectively, on the resistant line. Results of the COMET assay point out that both gold(III) complexes directly damage nuclear DNA. Remarkably, DNA damage inferred by either gold(III) complex in the two cell lines is larger than that produced by equitoxic cisplatin concentrations. Finally, the effects that either gold(III) complex produces on the cell cycle were investigated by flow cytometry. It was found that both complexes cause only moderate and transient cell cycle perturbations. Larger cell cycle perturbations are induced by equitoxic concentrations of cisplatin. The implications of the present results for the mechanism of action of cytotoxic gold(III) complexes are discussed.

Molecular recognition of metal complexes by DNA: a comparative study of the interactions of the parent complexes [PtCl(TERPY)]Cl and [AuCl(TERPY)]Cl2 with double stranded DNA.

The interactions of the parent complexes [AuCl(Terpy)]Cl2 and [PtCl(Terpy)]Cl with DNA were analysed by various physicochemical methods. Surprisingly, these metal complexes produce different interaction patterns with DNA in spite of their profound structural similarity. Indeed, important modifications are detected in the characteristic UV-Vis bands of [PtCl(Terpy)]Cl upon addition of ct-DNA, while the spectrum of [AuCl(Terpy)]Cl2 is almost unaffected. Gel electrophoresis studies confirm these findings: [PtCl(Terpy)]Cl — but not [AuCl(Terpy)]Cl2 — retards significantly the mobility of the supercoiled form of the pHV14 plasmid after a short incubation time. Ultrafiltration studies indicate that the affinity of [PtCl(Terpy)]Cl for ct-DNA is significantly greater than that of [AuCl(Terpy)]Cl2. On the other hand, both [AuCl(Terpy)]Cl2 and [PtCl(Terpy)]Cl induce important changes in the CD spectrum of ct-DNA, at high concentration, and increase its Tm value. Remarkably, the analysed metal-complex/DNA interaction patterns depend critically on the incubation times. We propose that [PtCl(Terpy)]Cl quickly intercalates DNA; then, formation of coordinative bonds progressively takes place with time. At variance, [AuCl(Terpy)]Cl2 first interacts electrostatically with the DNA surface, with subsequent slow formation of some coordinative bonds.

Gold(III) complexes as a new family of cytotoxic and antitumor agents.

In recent years, owing to the contributions of a few research groups, some new gold(III) compounds – either simple coordination complexes or organogold compounds – have been prepared that are sufficiently stable under physiological conditions and are promising candidates for pharmacological testing as cytotoxic and antitumor agents. In vitro pharmacological studies point out that some of these novel gold(III) complexes are highly cytotoxic toward cultured human tumor cell lines and are able to overcome resistance to platinum. Significant differences in the spectrum of action were observed compared with cisplatin. Studies are in progress to elucidate the mechanism of action of these compounds. The cellular effects of two representative gold(III) complexes are described. Preliminary results on binding to DNA in vitro are presented, pointing out that the interactions are generally weak. The implications of these results for the development of gold(III) complexes as a new family of cytotoxic and antitumor agents are discussed.

Biological Properties of IRIM, the Iridium(III) Analogue of (Imidazolium (Bisimidazole) Tetrachlororuthenate) (ICR)

Some biological aspects of the new complex imidazolium bisimidazole tetrachloro iridate(III)-IRIM- the iridium(III) analogue of ICR, were considered. More in detail the conformational effects produced by IRIM on DNA and the cytotoxic properties of IRIM on some selected human cell lines were measured. Dialysis experiments and DNA thermal denaturation studies are suggestive of poor binding of IRIM to DNA; formation of interstrand crosslinks is not observed. In any case CD measurements suggest that addition of increasing amounts of IRIM to calf thymus DNA results into significant spectral changes, that are diagnostic of a direct interaction with DNA. A number of experiments carried out on the A2780 human ovarian carcinoma, B16 murine melanoma, MCF7 and TS mammary adenocarcinoma tumor cell lines strongly point out that IRIM does not exhibit significant growth inhibition effects within the concentration range 10-4-10-6 M. It is suggested that the lower biological effects of IRIM compared to ICR are a consequence of the larger kinetic inertness of the iridium(III) center with respect to ruthenium(III).