BackgroundSystemic therapy for cancer metastatic lesions is difficult and generally renders a poor clinical response. Structural analogs of cisplatin, the most widely used synthetic metal complexes, show toxic side-effects and tumor cell resistance. Recently, palladium complexes with increased stability are being investigated to circumvent these limitations, and a biphosphinic cyclopalladated complex {Pd2 [S(-)C2, N-dmpa]2 (μ-dppe)Cl2} named C7a efficiently controls the subcutaneous development of B16F10-Nex2 murine melanoma in syngeneic mice. Presently, we investigated the melanoma cell killing mechanism induced by C7a, and extended preclinical studies.MethodsB16F10-Nex2 cells were treated in vitro with C7a in the presence/absence of DTT, and several parameters related to apoptosis induction were evaluated. Preclinical studies were performed, and mice were endovenously inoculated with B16F10-Nex2 cells, intraperitoneally treated with C7a, and lung metastatic nodules were counted. The cytotoxic effects and the respiratory metabolism were also determined in human tumor cell lines treated in vitro with C7a.ResultsCyclopalladated complex interacts with thiol groups on the mitochondrial membrane proteins, causes dissipation of the mitochondrial membrane potential, and induces Bax translocation from the cytosol to mitochondria, colocalizing with a mitochondrial tracker. C7a also induced an increase in cytosolic calcium concentration, mainly from intracellular compartments, and a significant decrease in the ATP levels. Activation of effector caspases, chromatin condensation and DNA degradation, suggested that C7a activates the apoptotic intrinsic pathway in murine melanoma cells. In the preclinical studies, the C7a complex protected against murine metastatic melanoma and induced death in several human tumor cell lineages in vitro, including cisplatin-resistant ones. The mitochondria-dependent cell death was also induced by C7a in human tumor cells.ConclusionsThe cyclopalladated C7a complex is an effective chemotherapeutic anticancer compound against primary and metastatic murine and human tumors, including cisplatin-resistant cells, inducing apoptotic cell death via the intrinsic pathway.

Permeabilization of the mitochondrial membrane has been extensively associated with necrotic and apoptotic cell death. Similarly to what had been previously observed for B16F10-Nex2 murine melanoma cells, PdC (palladacycle compounds) obtained from the reaction of dmpa (N,N-dimethyl-1-phenethylamine) with the dppe [1,2-ethanebis(diphenylphosphine)] were able to induce apoptosis in HTC (hepatoma, tissue culture) cells, presenting anticancer activity in vitro. To elucidate cell site-specific actions of dmpa:dppe that could respond to the induction of apoptosis in cancer cells in the present study, we investigated the effects of PdC on isolated RLM (rat liver mitochondria). Our results showed that these palladacycles are able to induce a Ca2+-independent mitochondrial swelling that was not inhibited by ADP, Mg2+ and antioxidants. However, the PdC-induced mitochondrial permeabilization was partially prevented by pre-incubation with CsA (cyclosporin A), NEM (N-ethylmaleimide) and bongkreic acid and totally prevented by DTT (dithiothreitol). A decrease in the content of reduced thiol groups of the mitochondrial membrane proteins was also observed, as well as the presence of membrane protein aggregates in SDS/PAGE without lipid and GSH oxidation. FTIR (Fourier-transform IR) analysis of PdC-treated RLM demonstrated the formation of disulfide bonds between critical thiols in mitochondrial membrane proteins. Associated with the mitochondrial permeabilization, PdC also induced the release of cytochrome c, which is sensitive to inhibition by DTT. Besides the contribution to clarify the pro-apoptotic mechanism of PdC, this study shows that the catalysis of specific protein thiol cross-linkage is enough to induce mitochondrial permeabilization and cytochrome c release.

Phenothiazines (PTZ) are drugs widely used in the treatment of schizophrenia. Trifluoperazine, a piperazinic PTZ derivative, has been described as inhibitor of the mitochondrial permeability transition (MPT). We reported previously the antioxidant activity of thioridazine at relatively low concentrations associated to the inhibition of the MPT (Brit. J. Pharmacol., 2002;136:136-142). In this study, it was investigated the induction of MPT by PTZ derivatives at concentrations higher than 10 microM focusing on the molecular mechanism involved. PTZ promoted a dose-response mitochondrial swelling accompanied by mitochondrial transmembrane potential dissipation and calcium release, being thioridazine the most potent derivative. PTZ-induced MPT was partially inhibited by CsA or Mg(2+) and completely abolished by the abstraction of calcium. The oxidation of reduced thiol group of mitochondrial membrane proteins by PTZ was upstream the PTP opening and it was not sufficient to promote the opening of PTP that only occurred when calcium was present in the mitochondrial matrix. EPR experiments using DMPO as spin trapping excluded the participation of reactive oxygen species on the PTZ-induced MPT. Since PTZ give rise to cation radicals chemically by the action of peroxidases and cyanide inhibited the PTZ-induced swelling, we propose that PTZ bury in the inner mitochondrial membrane and the chemically generated PTZ cation radicals modify specific thiol groups that in the presence of Ca(2+) result in MPT associated to cytochrome c release. These findings contribute for the understanding of mechanisms of MPT induction and may have implications for the cell death induced by PTZ.

In this minireview, the more recent findings about the effects of peculiar reactive thiol drugs on mitochondria are presented. These include the following compounds: metallo meso-tetrakis porphyrins, palladacycles, telluranes and phenothiazines. Metallo meso-tetrakis porphyrins can exhibit both beneficial and deleterious effects on mitochodria that are modulated by the central metal, cell location, and availability of axial ligands. Therefore, these compounds have the versatility to be used for cell and mitochondria protection and death. The antioxidant activity of manganese porphyrins is related to a glutathione peroxidase-like activity. By attacking exclusively the membrane protein thiol groups without glutathione depletion, palladacycles are able to induce mitochondrial permeability transition (MPT) and cytochrome c release in the absence of oxidative stress. In hepatoma cells, the mitochondrial action of palladacycles was able to induce apoptotic death. As opposed to palladacycles, telluranes and phenothiazines are able to conjugate the capacity to promote the MPT in a dose-dependent manner in association with efficient antioxidant activity toward lipids. These studies demonstrated that the action of drugs on mitochondrial bioenergetics can be modulated by peculiar reactivity with thiol groups. Therefore, they contribute to studies of toxicity as well as the design of new drugs.