Copper(ii) complexes of the CuN4S core: selective cytotoxicity to cancerous cells, ROS generation and induction of apoptosis

Eight novel mixed-ligand copper(ii) complexes of the type [Cu(L)(phen)](ClO4) (1–8) synthesized from S-methyldithiocarbazate Schiff bases [H(L1)–H(L8)] and 1,10-phenanthroline (phen) ligands have been characterized using various physicochemical, spectroscopic and electrochemical methods. The structures of seven out of the eight Cu(ii) complexes have been determined by single crystal X-ray diffraction. The coordination geometry around the Cu(ii) ion is distorted square pyramidal (τ, 0.24–0.49). The heterocyclic and azomethine nitrogens along with thiolato sulphur of the deprotonated tridentate ligand (NNS) and one of the imine nitrogens of phen form the equatorial CuN3S square plane, while the other imine nitrogen atom of phen occupies the axial position. The geometry of the Cu(ii) center in the solid state is preserved in DMF solution, stabilizing to distorted square pyramidal, as suggested by the low-temperature EPR data g‖ > g⊥ > 2.0. Cyclic voltammetry showed that the Cu(ii) complexes display quasi-reversible electrochemistry. The efficiency of the complexes in killing the human lung epithelial adenocarcinoma cells (A549) has also been studied along with the cell viability assay against the noncancerous human lung epithelial cells (L132) under in vitro conditions. They were found to exhibit excellent cytotoxicity against A549 cancer cells without affecting the normal L132 cells. The activity of all the complexes was significantly superior to that of cisplatin. The 2′,7′-dichlorodihydrofluorescein diacetate (DCHF-DA) experiment indicated that cancer cells produce more reactive oxygen species (ROS) than normal cells. Further, they cause cell death mainly through apoptotic mode, as revealed by the observation of a higher percentage of apoptotic cells in acridine orange (AO) or ethidium bromide (EB) or 4′,6′-diamidino-2-phenylindole (DAPI) stained cancer cells. Thus, ROS generation in tumor cells is implicated in cytotoxicity, demonstrating the versatile nature of presently accessible mixed-ligand copper(ii) complexes for cancer therapy by means of ROS-induced apoptotic cell death.