We report a quantitative structure-activity relationship study of a new class of pyrazole-pyridine copper complexes that establishes a clear correlation between the ability to promote copper accumulation and cytotoxicity. Intracellular metal accumulation is maximized when ligand lipophilicity allows the complex to rapidly cross the membrane. Copper and ligand follow different uptake kinetics and reach different intracellular equilibrium concentrations. These results support a model in which the ligand acts as an ionophore for the metal ion, cycling between intra- and extracellular compartments as dissociated or complexed entities. When treating cancer cells with structurally unrelated disulfiram and pyrazole-pyridine copper complexes, as well as with inorganic copper, the same morphological and molecular changes were reproduced, indicating that copper overload is responsible for the cytotoxic effects. Copper-based treatments drive sensitive cancer cells toward paraptotic cell death, a process hallmarked by endoplasmic reticulum stress and massive vacuolization in the absence of apoptotic features. A lack of caspase activation, as observed in copper-treated dying cells, is a consequence of metal-mediated inhibition of caspase-3. Thus, copper acts simultaneously as an endoplasmic reticulum (ER) stress inducer and a caspase-3 inhibitor, forcing the cell into caspase-independent paraptotic death. The establishment of a mechanism of action common to different copper binding agents provides a rationale for the exploitation of copper toxicity as an anticancer tool.

This study reports the structure-activity relationship of a series of 8-hydroxoquinoline derivatives (8-HQs) and focuses on the cytotoxic activity of 5-Cl-7-I-8-HQ (clioquinol, CQ) copper complex (Cu(CQ)). 8-HQs alone cause a dose-dependent loss of viability of the human tumor HeLa and PC3 cells, but the coadministration of copper increases the ligands effects, with extensive cell death occurring in both cell lines. Cytotoxic doses of Cu(CQ) promote intracellular copper accumulation and massive endoplasmic reticulum vacuolization that precede a nonapoptotic (paraptotic) cell death. The cytotoxic effect of Cu(CQ) is reproduced in normal human endothelial cells (HUVEC) at concentrations double those effective in tumor cells, pointing to a potential therapeutic window for Cu(CQ). Finally, the results show that the paraptotic cell death induced by Cu(CQ) does not require nor involve caspases, giving an indication for the current clinical assessment of clioquinol as an antineoplastic agent.

The chemical properties of copper allow it to take part in many biological functions such as electron transfer, catalysis, and structural shaping. The ability to cycle between +1 and +2 oxidation state is one of the features that has been exploited by organisms throughout the evolutionary process. Since copper is potentially toxic to cells also a finely controlled mechanism for copper handling has evolved. On the other side, many copper complexes were synthesized and tested for their anticancer activity in vitro and in vivo. Their ability to kill cancer cells is mainly related to the induction of an oxidative stress, but recently it emerged their ability to inhibit the proteasome, a protein complex whose proteolitic activity is needed by several cellular process. It has generally been described that the toxic effects of copper complexes leads to cell death either by necrosis or through the activation of the apoptotic process. Evidences are rising about the ability of some copper compounds to induce alternative non-apoptotic form of programmed cell death. Since copper is indispensable for the formation of new blood vessels, angiogenesis, a different antitumor approach based on the administration of copper sequestering agents has been attempted and its effectiveness is currently under evaluation by clinical trials. The proven essentiality of copper for angiogenesis, together with the marked sensitivity shown by several cancer cell lines to the copper toxicity, open a new perspective in the anticancer strategy: exploiting the tumor need of copper to accumulate toxic amount of the metal inside its cells.

The copper(II) complex A0 induces a type of non-apoptotic cell death also known as paraptosis. Paraptosis involves extensive endoplasmic reticulum vacuolization in the absence of caspase activation. A wide panel of human cancer cell lines was used to demonstrate differences in cytotoxicity by the paraptosis-inducing drug A0 and the metal-based pro-apoptotic drug cisplatin. Gene expression profiling of the human fibrosarcoma HT1080 cells showed that, while cisplatin induced p53 targets, A0 up-regulated genes involved in the unfolded protein response (UPR) and response to heavy metals. The cytotoxic effects of A0 were associated with inhibition of the ubiquitinproteasome system and accumulation of ubiquitinylated proteins, in a manner dependent on protein synthesis. Cycloheximide inhibited the accumulation of ubiquitinylated proteins and hampered A0-induced cell death process. The occurrence of the UPR during A0-induced death process was shown by the increased abundance of spliced XBP1 mRNA, transient eIF2␣ phosphorylation, and a series of downstream events, including attenuation of global protein synthesis and increased expression of ATF4, CHOP, BIP, and GADD34. Mouse embryonic fibroblasts expressing a mutant eIF2␣, which could not be phosphorylated, were more resistant to A0 than wild type cells, pointing to a pro-death role of eIF2␣ phosphorylation. A0 may thus represent the prototypical member of a new class of compounds that cause paraptotic cell death via mechanisms involving eIF2␣ phosphorylation and the UPR.

Synthesis, characterization, NLO properties, and theoretical studies of the mixed-ligand dithiolene complexes of the nickel triad [M(II)(Bz(2)pipdt)(mnt)] (Bz(2)pipdt = 1,4-dibenzyl-piperazine-3,2-dithione, mnt = maleonitriledithiolato, M(II) = Ni, 1, Pd, 2, Pt, 3) are reported. Molecular structural characterization of 1-3 points out that four sulfur atoms are in a slightly distorted square-planar geometry. While the M-S bond distances are only slightly different, comparison of the C-C and C-S bonds in the C(2)S(2)MS(2)C(2) core allows us to point out a significant difference between the C-C and the C-S distances in Bz(2)pipdt and mnt. These findings suggest assigning a dithiolato character to mnt (pull ligand) and a dithione one (push ligand) to Bz(2)pipdt. Cyclic voltammetry of 1-3 exhibits two reversible reduction waves and a broad irreversible oxidation wave. These complexes are characterized in the visible region by a peak of moderately strong intensity, which undergoes negative solvatochromism. The molecular quadratic optical nonlinearities were determined by the EFISH technique, which provided the following values μβ(λ) (10(-48) esu) = -1436 (1), -1450 (2), and -1950 (3) converted in μβ(0) (10(-48) esu) = -463 (1), -684 (2), and -822 (3), showing that these complexes exhibit large negative second-order polarizabilities whose values depend on the metal, being highest for the Pt compound. DFT and TD-DFT calculations on 1-3 allow us to correlate geometries and electronic structures. Moreover, the first molecular hyperpolarizabilities have been calculated, and the results obtained support that the most appealing candidate as a second-order NLO chromophore is the platinum compound. This is due to (i) the most extensive mixture of the dithione/metal/dithiolato orbitals, (ii) the influence of the electric field of the solvent on the frontier orbitals that maximizes the difference in dipole moments between the excited and the ground state, and (iii) the largest oscillator strength in the platinum case vs nickel and palladium ones.

We report the first combined optical and structural investigation of the water free Er-quinolinolate complex, an organo-lanthanide system of interest for 1.5-microm telecom applications. Structural data demonstrate that the complex has a trinuclear structure (Er3Q9) which provides the Er metals with an octa-coordination by the organic ligand and prevents solvent and water molecules from entering the lanthanide coordination sphere. The results of the structural analysis allow us to infer that the strong Er luminescence quenching exhibited by the Er3Q9 complex is due uniquely to resonant energy transfer to the aromatic C-H vibrations of the ligand, providing the correct tools to design more efficient emitters.

Complexes [Au(III)X(2)(dtc-Sar-AA-O(t-Bu))] (AA = Gly, X = Br (1)/Cl (2); AA = Aib, X = Br (3)/Cl (4); AA = l-Phe, X = Br (5)/Cl (6)) were designed on purpose in order to obtain gold(III)-based anticancer peptidomimetics that might specifically target two peptide transporters (namely, PEPT1 and PEPT2) upregulated in several tumor cells. All the compounds were characterized by means of FT-IR and mono- and multidimensional NMR spectroscopy, and the crystal structure of [Au(III)Br(2)(dtc-Sar-Aib-O(t-Bu))] (3) was solved and refined. According to in vitro cytotoxicity studies, the Aib-containing complexes 3 and 4 turned out to be the most effective toward all the human tumor cell lines evaluated (PC3, DU145, 2008, C13, and L540), reporting IC(50) values much lower than that of cisplatin. Remarkably, they showed no cross-resistance with cisplatin itself and were proved to inhibit tumor cell proliferation by inducing either apoptosis or late apoptosis/necrosis depending on the cell lines. Biological results are here reported and discussed in terms of the structure-activity relationship.

The reactions of the triply bonded anion [Mo2Cp2(μ-PCy2)(μ-CO)2]- (Li+ salt) with [NH4]PF6, MeI, and PhCH2Cl give, with good yields, the corresponding hydride- or alkyl-bridged derivatives [Mo2Cp2(μ-X)(μ-PCy2)(CO)2] (X = H, Me, CH2Ph). The related phenyl complex [Mo2Cp2(μ-Ph)(μ-PCy2)(CO)2] can be obtained upon reaction of the above anion with Ph3PbCl. According to the corresponding X-ray diffraction studies, the latter complex displays its phenyl group bonded to the dimetal center exclusively through the ipso carbon atom, while the methyl and benzyl complexes adopt an asymmetric α-agostic structure whereby one of the C−H bonds of the bridgehead carbon is bound to one of the molybdenum atoms. The intermetallic distances remain quite short in all cases, 2.56−2.58 Å. In solution, the hydride complex exhibits dynamic behavior involving mutual exchange of the carbonyl ligands. The alkyl derivatives behave similarly to each other in solution and also exhibit dynamic behavior, possibly implying the presence of small amounts of a nonagostic structure in equilibrium with the dominant α-agostic structure. Density functional theory calculations (B3LYP, B3PW91) correctly reproduce the experimental structures, and predict an α-agostic structure for both the methyl and benzyl complexes. The bonding in the above hydride and hydrocarbyl complexes was analyzed using molecular orbital, atoms in molecules, and natural bond orbital methodologies. The intermetallic binding in the hydride complex can be thus described as composed of a tricentric (Mo2H) plus two bicentric (Mo2) interactions, the latter being of σ and π types. In the hydrocarbyl-bridged complexes, analogous tricentric (Mo2C), and bicentric (Mo2) interactions can be identified, but there are additional interactions reducing the strength of the intermetallic binding, these being the α-agostic bonding in the case of the alkyl complexes and a π-donor interaction from the π-bonding orbitals of the hydrocarbon ring into suitable metal acceptor orbitals, in the case of the phenyl complex. The strength of these additional interactions have been estimated by second-order perturbation analysis to be of 70.3 (Me), 89.2 (CH2Ph), and 52.2 (Ph) kJ mol-1, respectively.