Co(II), Cu(II), and Zn(II) thio-bis(benzimidazole) complexes induce apoptosis via mitochondrial pathway

The copper(II), cobalt(II), and zinc(II) complexes with 2-(1H-benzimidazol-2-ylmethylsulfanylmethyl)-1H-benzimidazole (tbb) and 2-[2-[2-(1H-benzimidazol-2-yl)ethylsulfanyl]ethyl]-1H-benzimidazole (tebb), [Cu(tbb)Cl2] (1), [Co(tbb)Cl2] (2), [Zn(tbb)Cl2] (3), [Cu(tebb)Cl(H2O)]Cl (4), [Co(tebb)Cl2]n·nCH3OH (5) and [Zn(tebb)Cl(H2O)]Cl (6), have been prepared and evaluated for antiproliferative activity. The structure of (4) was proved by X-ray diffraction crystallography. The coordination compounds were tested for their cytotoxic activities in cancer cell lines in vitro. The lower IC50 values were obtained for Co(II), Cu(II), and Zn(II) complexes with tebb in comparison with tbb complexes. Complex 2 showed strong antiproliferative selectivity for leukemia CEM cells and nontoxicity towards other tested cell lines and normal human cells (BJ and RPE-1). Proapoptotic activity of 2 and 5 were weaker than positive control cisplatin, but the big advantage of these complexes was their zero-cytotoxicity for normal healthy cells in contrast to the high cytotoxicity of cisplatin. The activation of apoptotic initiation phase was detected in neuroblastoma cancer cell line SH-SY5Y where 5 was cytotoxic without fragmentation of cells. Interestingly, complexes 5, 6, and tebb, together with cisplatin, dramatically impaired the mitochondrial membrane potential of SH-SY5Y after 72 h. Taken together, we demonstrated that our compounds trigger apoptosis via the mitochondrial pathway.