Cytotoxic behavior of binuclear silver N-heterocyclic carbenes in HCT 116 cells and influence of substitution on cytotoxicity

Ag (I)-N-heterocyclic carbene (Ag–NHC) complexes ( S1 – S6 ) were synthesized from di-cationic ionic liquids ( DCILs-1–6 ) and characterized by NMR, FT-IR, and EI-MS studies, and evaluated for cytotoxicity against human colorectal carcinoma cell line (HCT-116) using methylthiazolyldiphenyl-tetrazolium bromide assay. The DCILs displayed good patterns of cytotoxicity which was improved greatly upon metallation and were found to be in a direct correlation with the increase in the alkyl chain that correlates with lipophilicity. Amongst the Ag–NHCs studied, S1 , S5 , and S6 have shown 85.95, 99.01 and 99.70% growth inhibition at the concentrations of 120, 120 and 60 µM, respectively. In order to depict the putative mechanism adopted by S6 , HCT-116 cells were treated with S6 and stained using acridine orange/ethidium bromide (AO/EtBr), and the results displayed a significant induction of apoptosis in HCT-116 cells as evident from the green-orange fluorescence. Furthermore, DAPI staining confirmed that the S6 -treated HCT-116 cells underwent chromatin condensation and DNA fragmentation. Since mitochondria play a prominent role in apoptosis, we further examined the mitochondrial membrane potential (MMP) and reactive oxygen species (ROS) formation. Interestingly, the S6 -treated HCT-116 cells demonstrated a higher ROS and depolarization of MMP, which suggests that S6 can induce mitochondria-mediated intrinsic apoptosis in cancer cells. Besides these consequences, HCT-116 cells also exhibited a lower level of oxidant defensive protein catalase. Taken altogether, the current study suggests that Ag–NHCs can reduce anti-oxidative protein catalase following an increase in ROS level, thereby triggering apoptosis in cancer cells.