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.