Advances in Copper Complexes as Anticancer Agents

Targeting copper in cancer therapy: 'Copper That Cancer'

Copper is an essential micronutrient involved in fundamental life processes that are conserved throughout all forms of life. The ability of copper to catalyze oxidation-reduction (redox) reactions, which can inadvertently lead to the production of reactive oxygen species (ROS), necessitates the tight homeostatic regulation of copper within the body. Many cancer types exhibit increased intratumoral copper and/or altered systemic copper distribution. The realization that copper serves as a limiting factor for multiple aspects of tumor progression, including growth, angiogenesis and metastasis, has prompted the development of copper-specific chelators as therapies to inhibit these processes. Another therapeutic approach utilizes specific ionophores that deliver copper to cells to increase intracellular copper levels. The therapeutic window between normal and cancerous cells when intracellular copper is forcibly increased, is the premise for the development of copper-ionophores endowed with anticancer properties. Also under investigation is the use of copper to replace platinum in coordination complexes currently used as mainstream chemotherapies. In comparison to platinum-based drugs, these promising copper coordination complexes may be more potent anticancer agents, with reduced toxicity toward normal cells and they may potentially circumvent the chemoresistance associated with recurrent platinum treatment. In addition, cancerous cells can adapt their copper homeostatic mechanisms to acquire resistance to conventional platinum-based drugs and certain copper coordination complexes can re-sensitize cancer cells to these drugs. This review will outline the biological importance of copper and copper homeostasis in mammalian cells, followed by a discussion of our current understanding of copper dysregulation in cancer, and the recent therapeutic advances using copper coordination complexes as anticancer agents.

Aerial Oxidation of a V(IV)-Iminopyridine Hydroquinonate Complex: A Trap for the V(IV)-Semiquinonate Radical Intermediate

The reaction of 2,5-bis[N,N'-bis(2-pyridyl-aminomethyl)aminomethyl]-p-hydroquinone (H2bpymah) with VO(2+) salts in acetonitrile or water at a low pH (2.2-3.5) results in the isolation of [{V(IV)(O)(Cl)}2(μ-bpymah)], the p-semiquinonate complex [{V(IV)(O)(Cl)}2(μ-bpymas)](OH), the cyclic mixed-valent hexanuclear compound [{V(V)(O)(μ-O)V(IV)(O)}(μ-bpymah)]3, and [(V(V)O2)2(μ-bpymah)]. [{V(IV)(O)(Cl)}2(μ-bpymas)](OH) is an intermediate of the radical-mediated oxidation of [{V(IV)(O)(Cl)}2(μ-bpymah)] from O2. At lower pH values (2.2), a reversible intramolecular electron transfer from the metal to the ligand of [{V(IV)(O)(Cl)}2(μ-bpymas)](OH) is induced with the concurrent substitution of chlorine atoms by the oxygen-bridging atoms, resulting in the formation of [{V(V)(O)(μ-O)V(IV)(O)}(μ-bpymah)]3. The metal complexes were fully characterized by X-ray crystallography, infrared (IR) spectroscopy, and magnetic measurements in the solid state, as well as by conductivity measurements, UV-vis spectroscopy, and electrochemical measurements in solution. The oxidation states of the metal ions and ligands were determined by the crystallographic data. The [{V(IV)(O)(Cl)}2(μ-bpymah)]-[{V(IV)(O)(Cl)}2(μ-bpymas)](OH) redox process is electrochemically reversible. The V(IV) ion in the semiquinonate compound exhibits a surprisingly low oxophilicity, resulting in the stabilization of OH(-) counterions at acidic pH values. An investigation of the mechanism of this reaction reveals that these complexes induce the reduction of O2 to H2O2, mimicking the activity of enzymes incorporating two redox-active centers (metal-organic) in the active site.

Homoleptic phosphino copper(I) complexes with in vitro and in vivo dual cytotoxic and anti-angiogenic activity

Homoleptic, tetrahedral Cu(i) complexes of the type [Cu(P)4]BF4 (1-3), where P are the phosphine ligands, 1,3,5-triaza-7-phosphaadamantane (PTA), 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA) and 2-thia-1,3,5-triaza-phosphoaadamantane-2,2-dioxide (PTA-SO2), have been prepared. Novel complexes [Cu(DAPTA)4]BF42 and [Cu(PTA-SO2)4]BF43 have been fully characterized by means of spectroscopic methods, corroborated by XAS-EXAFS analysis of 2. In vitro cell culture experiments revealed a significant antiproliferative activity for Cu(i) compounds against several human cancer cell lines derived from solid tumors with preferential cell growth inhibition towards tumour compared to non-malignant cells. In vitro monitoring of migration and capillary-like tube formation of human umbilical vein endothelial cells (HUVECs) showed an anti-angiogenic effect of copper(i) complexes at sub-cytotoxic concentrations. In vivo studies on the antitumor efficacy and ability to inhibit angiogenesis confirmed the dual cytotoxic and anti-angiogenic properties of Cu(i) derivatives.

Cu2+-embedded carbon nanoparticles as anticancer agents

We report the synthesis of luminescent carbon nanoparticles (93 ± 50 nm) embedded with Cu²⁺. It was observed that at a relatively low concentration of Cu²⁺ (2.55 ppm), cervical cancer HeLa cells died due to apoptosis induced by the nanoparticles. Also, generation of reactive oxygen species in the cells, in the presence of the composite nanoparticles, has been attributed to their killing. The luminescence of the carbon nanoparticles was used for imaging of the cells.

Intercalation processes of copper complexes in DNA

The family of anticancer complexes that include the transition metal copper known as Casiopeínas® shows promising results. Two of these complexes are currently in clinical trials. The interaction of these compounds with DNA has been observed experimentally and several hypotheses regarding the mechanism of action have been developed, and these include the generation of reactive oxygen species, phosphate hydrolysis and/or base-pair intercalation. To advance in the understanding on how these ligands interact with DNA, we present a molecular dynamics study of 21 Casiopeínas with a DNA dodecamer using 10 μs of simulation time for each compound. All the complexes were manually inserted into the minor groove as the starting point of the simulations. The binding energy of each complex and the observed representative type of interaction between the ligand and the DNA is reported. With this extended sampling time, we found that four of the compounds spontaneously flipped open a base pair and moved inside the resulting cavity and four compounds formed stacking interactions with the terminal base pairs. The complexes that formed the intercalation pocket led to more stable interactions.

Synthesis, characterization, X-ray crystal structure, DFT calculation, DNA binding, and antimicrobial assays of two new mixed-ligand copper(II) complexes

Two new Cu(II) complexes, [Cu(L)(phen)] (1), [Cu(L)(bipy)] (2), where L(2-)=(3-methoxy-2oxidobenzylidene)benzohydrazidato, phen=1,10 phenanthroline, and bipy=2,2' bipyridine, were prepared and fully characterized using elemental analyses, FT-IR, molar conductivity, and electronic spectra. The structures of both complexes were also determined by X-ray diffraction. It was found that, both complexes possessed square pyramidal coordination environment in which, Cu(II) ions were coordinated by donor atoms of HL and two nitrogens of heterocyclic bases. Computational studies were performed using DFT calculations at B3LYP/6-311+G(d,p) level of theory. DNA binding activities of these complexes were also investigated using electronic absorption, competitive fluorescence titration and cyclic voltammetry studies. The obtained results indicated that binding of the complexes to DNA was of intercalative mode. Furthermore, antimicrobial activities of these compounds were screened against microorganisms.

A comparative study of cytotoxicity and interaction with DNA/protein of five transition metal complexes with Schiff base ligands

Five transition metal complexes NiL(1)2 (1), CuL(1)2 (2), ZnL(1)2 (3), [MnL(1)2(N3)]n·nCH2Cl2 (4), CuL(2)2 (5) {HL(1)=3-{[2-(2-hydroxy-ethoxy)-ethylimino]-methyl}-naphthalen-2-ol, HL(2)=2-{[2-(2-hydroxy-ethoxy)-ethylimino]-methyl}-phenol} have been synthesized and fully characterized. In all of the complexes, the ligands coordinated to the metal ion in a negative fashion via O and N donor atoms. The X-ray structures of nickel complex 1 and copper complexes 2 and 5 are four-coordinated monomers and show slightly distorted square-planar geometry in the vicinity of the central metal atom. Zinc complex 3 exhibits a four-coordinated tetrahedral structure. Differently, manganese complex 4 reveals a six-coordinated octahedral structure, one-dimensional chain is linked by azide in the end-to-end mode. In vitro cytotoxicity of these complexes to various tumor cell lines was assayed by the MTT method. The results showed that most of these metal-Schiff base complexes exhibited enhanced cytotoxicity than Schiff base ligands, which clearly implied a positive synergistic effect. Moreover, these complexes appeared to be selectively active against certain cell lines. The interactions of these metal complexes with CT-DNA were investigated by UV-vis, fluorescence and CD spectroscopy, the results indicated that these complexes are metallointercalators and can interact with CT-DNA. The study of interaction between complexes and BSA indicated that all of the complexes could quench the intrinsic fluorescence of BSA in a static quenching process.

Synthesis and spectral characterization of mono- and binuclear copper(II) complexes derived from 2-benzoylpyridine-N⁴-methyl-3-thiosemicarbazone: crystal structure of a novel sulfur bridged copper(II) box-dimer

Mononuclear and binuclear copper(II) complexes of 2-benzoylpyridine-N(4)-methyl thiosemicarbazone (HL) were prepared and characterized by a variety of spectroscopic techniques. Structural evidence for the novel sulfur bridged copper(II) iodo binuclear complex is obtained by single crystal X-ray diffraction analysis. The complex [Cu2L2I2], a non-centrosymmetric box dimer, crystallizes in monoclinic C2/c space group and it was found to have distorted square pyramidal geometry (Addison parameter, τ=0.238) with the square basal plane occupied by the thiosemicarbazone moiety and iodine atom whereas the sulfur atom from the other coordinated thiosemicarbazone moiety occupies the apical position. This is the first crystallographically studied system having non-centrosymmetrical entities bridged via thiolate S atoms with Cu(II)I bond. The tridentate thiosemicarbazone coordinates in mono deprotonated thionic tautomeric form in all complexes except in sulfato complex, [Cu(HL)(SO4)]·H2O (1) where it binds to the metal centre in neutral form. The magnetic moment values and the EPR spectral studies reflect the binuclearity of some of the complexes. The spin Hamiltonian and bonding parameters are calculated based on EPR studies. In all the complexes g||>g⊥>2.0023 and the g values in frozen DMF are consistent with the d(x2-y2) ground state. The thermal stabilities of some of the complexes were also determined.

Magnetic property, DFT calculation, and biological activity of bis[(μ(2)-chloro)chloro(1,10-phenanthroline)copper(II)] complex

The dinuclear complex bis[(μ(2)-chloro)chloro(1,10-phenanthroline)copper(II)] (1) was synthesized, and characterized by X-ray, FTIR and thermal analysis. The fitting of magnetic susceptibility and magnetization curve of (1) indicates the occurrence of weak antiferromagnetic exchange interaction between copper(II) ions. The electronic structure has been also determined by density functional theory (DFT) method. Complex (1) displayed potent anticancer activity against B16 (Melanoma), MDA-MB-32 (Breast Adenocarcinoma), A549 (Lung Adenocarcinoma), HT-29 (Colon Adenocarcinoma) and SF (Astrocytoma) cell lines with an average IC50 value of 0.726 μg/ml compared to 4.88 μg/ml for cisplatin. Complex (1) has a better therapeutic index and toxicological profile than cisplatin, and has demonstrated a potential chemotherapeutic property.

Electrospray ionization multi-stage mass spectrometric study of the interaction products of the cytotoxic complex [Cu(thp)₄][PF₆] with methionine-rich model peptides

RATIONALE

The cytotoxic activity of the copper(I) complex [Cu(thp)4][PF6] (CP) (thp = tris(hydroxymethyl) phosphine) is correlated with its high accumulation in cancer cells. Human copper transporter 1 (hCtr1) has been described as the main trans-membrane protein involved in cellular trafficking of physiological copper. Methionine-rich peptide sequences incorporated in the extracellular domain of hCtr1 play a key role in the cellular internalization of copper. We wish to investigate the interaction of CP with model peptides that mimic the extracellular domain of hCtr1.

METHODS

The interaction of CP with methionine-rich and methionine-free model peptides has been investigated by electrospray ionization mass spectrometry and the interaction products have been characterized by multiple collisional experiments, using an ion trap mass instrument.

RESULTS

The interaction of CP with selected methionine-rich model peptides, Ac-MMMMPMTFK-NH2 (P1) and Ac-MGMSYMDSK-NH2 (P2), shows that the native copper complex, after sequential loss of phosphines, induces the formation of [Cu(P1)(thp)](+) and [Cu(P1/P2)](+) adducts reasonably by inclusion of the Cu(I) ion in the peptide framework. Collisionally induced fragmentations (MS(n)) of [Cu(P1/P2)](+) give evidence that the metal is coordinated by the thioether-S of two adjacent methionine residues. Interaction of the same peptides with the isostructural complex [Ag(thp)4](+) or AgNO3 yields similar experimental evidence, leading to [Ag(P1/P2)](+).

CONCLUSIONS

Methionine sequences incorporated in model peptides are crucial for the recruitment of copper from CP. Such a metal-peptide interaction does not take place when methionine-free Ac-NleGNleSYNleDSK-NH2 (P3) is utilized. A mechanism for tumor cell internalization of CP involving: (i) chemically driven sequential loss of phosphines from the native tetrahedral complex, followed by (ii) transfer of Cu(I) to the methionine-rich sequences typical of the hCtr1 transporter, is proposed.

New tricopper(ii) cores self-assembled from aminoalcohol biobuffers and homophthalic acid: synthesis, structural and topological features, magnetic properties and mild catalytic oxidation of cyclic and linear C5–C8 alkanes

Two new compounds with distinct tricopper(ii) cores were synthesised and applied as bio-inspired pre-catalysts for mild oxidation of alkanes.

The effect of potential supramolecular-bond promoters on the DNA-interacting abilities of copper–terpyridine compounds

Supramolecular interactions prevail over DNA-cleaving abilities in the cytotoxicity behaviour of terpyridine-based copper(ii) complexes.

Supramolecular architecture and photophysical and biological properties of ruthenium(ii) polypyridyl complexes

A two-dimensional cyclic hybrid water–chloride anionic network has been structurally characterized in a metal–organic matrix. DNA interactions and the cytotoxicity of ruthenium(ii) complexes have been studied.

DNA/protein interaction, cytotoxic activity and magnetic properties of amino-alcohol Schiff base derived Cu(ii)/Ni(ii) metal complexes: influence of the nuclearity and metal ions

The reactions of amino-alcohol derived Schiff-base ligands with Cu(ii) and Ni(ii) salts have afforded four novel polynuclear complexes, which exhibited interesting magnetic and cytotoxic properties.

Targeted delivery of “copper carbonate” nanoparticles to cancer cells in vivo

Novel CuCO₃ nanoparticles induced apoptosis in a human cervical cancer cell line. The folic acid mediated targeting of the CuCO₃ receptor was studied in vitro & in vivo.

Synthesis, X-ray structure and in vitro cytotoxicity studies of Cu(i/ii) complexes of thiosemicarbazone: special emphasis on their interactions with DNA

The interactions of four Cu-TSC complexes with DNA & their cytotoxicity studies against the HeLa cell have been reported.

Synthesis and biological evaluation of copper(ii) pyrenethiosemicarbazone

A fluorescent Cu(ii) pyrenethiosemicarbazone complex exhibits enhanced DNA-cleavage and cytotoxicity on photoexcitation.

Synthesis and characterization of monometallic rhenium(i) complexes and their application as selective sensors for copper(ii) ions

Monometallic Re(i) complexes, selective and sensitive sensor of Cu(ii) ion, show substantial enhancement in the emission intensity, quantum yield, and lifetime due to the restriction of CN isomerization.

Convenient and efficient synthesis of disubstituted piperazine derivatives by catalyst-free, atom-economical and tricomponent domino reactions

One-pot, atom-economical, catalyst-free and tri-component domino reactions are applied to diversity-oriented synthesis (DOS) of disubstituted piperazine derivatives under mild conditions with moderate to high yields.

Copolymerization of an indazole ligand into the self-polymerization of dopamine for enhanced binding with metal ions

Synthesis and mussel-inspired polymerization of a new catechol monomer. The generated copolymer exhibits enhanced metal binding, due to the ligand nature of the new monomer, compared to polydopamine homopolymer.

Novel triazolium based 11th group NHCs: synthesis, characterization and cellular response mechanisms

The novel NHC ligand precursor 1,4-bis(4-nitrobenzyl)-1H-1,2,4-triazol-4-ium bromide, [HTz^(pNO2Bz)2]Br, has been synthesized and used in the synthesis of the corresponding metal complexes M[Tz^(pNO2Bz)2]Br (M = Cu(i), Ag(i) or Au(i)).

Fluorescent mixed ligand copper(ii) complexes of anthracene-appended Schiff bases: studies on DNA binding, nuclease activity and cytotoxicity

While the phen of [Cu(L1–L5)(phen)(ACN)]²⁺ partially inserts into DNA base pairs the anthracenyl moiety of L1–L5 interacts with DNA hydrophobically.

Influence of ionic strength on the interaction of THA and its Cu(ii) complex with DNA helps to explain studies on various breast cancer cells

Purpurin, a structural analogue of anthracycline anticancer drugs, was used to understand how environmental changes affect its biophysical interactions with DNA and along with its Cu(ii) complex, the activity it shows on breast cancer cells.