Studies on anticancer properties with varying co-ligands in a Ru(ii) arene benzimidazole system

Abstract

Recently in the field of chemotherapeutics, to combat the side effects of cisplatin, ruthenium complexes have been investigated extensively. In this work, a bidentate benzimidazole-based ligand, HL [HL = 2-(1H-benzo[d]imidazol-2-yl)-6-methoxyphenol], was utilized to obtain three Ru(II) arene complexes having a generalized formula [Ru(η⁶-p-cym)(L)(X)] or [Ru(η⁶-p-cym)(L)(X)]⁺ (where p-cym = p-cymene). The co-ligand X (X = (i) Cl, (ii) PPh₃ = triphenyl phosphine, and (iii) PTA = 1,3,5-triaza-7-phosphaadamantane) was varied in order to study the effect it has on the antitumor activity of the compounds. The synthesized compounds were thoroughly characterized using different analytical techniques, including ESI-MS, NMR, FTIR, UV-Vis, and fluorescence spectroscopy. A fluorescence quenching experiment with serum albumin proteins revealed good interactions between the complexes and HSA and BSA. An analysis of their lipophilic character via the shake flask method and a stability study using UV spectroscopy were conducted as well. The anticancer properties of the synthesized compounds were further explored by conducting a DNA binding study using absorption spectroscopy and fluorometric titration with DAPI to check the mode of binding with DNA. Interestingly, the complexes were also found to catalyze the oxidation of NADH to NAD⁺, giving rise to radical species in the cells. An immunoblot analysis strongly suggested that all three complexes can remarkably upregulate the expression of cleaved caspase-3 and downregulate the expression of the anti-apoptotic protein BCL_XL. It is important to note that such studies are yet to be reported for similar benzimidazole-based ruthenium complexes and therefore present a new direction for the investigation of antitumor ruthenium-based metallodrugs. Furthermore, Hoechst and AO/EtBr staining was used to analyze the morphological changes of the compound-treated cancer cells due to apoptosis, which was also confirmed by the IC₅₀ values obtained from the colorimetric assay (MTT) against different cancer cell lines.