Photocytotoxic kinetically stable ruthenium(ii)-N,N-donor polypyridyl complexes of oxalate with anticancer activity against HepG2 liver cancer cells

Abstract

Liver cancer is one of the leading causes of death that motivating scientists worldwide to synthesize novel chemotherapeutics. Ru(II)-polypyridyl complexes are extensively studied for possible therapeutic and cellular applications due to their tunable coordination chemistry, structural diversity, ligand-exchange kinetics, accessible redox states, and rich photophysical or photochemical properties. Herein, we have synthesized a series of Ru(II) polypyridyl complexes [Ru^II(N^N)₂(ox)] (1–3), where ox is oxalate (C₂O₄²⁻) and N^N is 1,10-phenanthroline (phen) (1), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq) (2), and dipyrido[3,2,-a:2′,3′-c]phenazine (dppz) (3). Oxalate (ox²⁻) was opted as a bioactive dioxo ligand to prevent facile hydrolysis in aqueous media, thereby increasing the stability of the Ru(II)-polypyridyl complexes in physiological media. We thoroughly characterized all the complexes using ESI-MS, FT-IR, UV-vis, and ¹H NMR spectroscopy and other physicochemical methods. The complexes were stable under physiological conditions and under low-energy green LED light (λ_irr = 530 nm). However, the photoirradiation of complexes resulted in the efficient generation of singlet oxygen (¹O₂) as a major reactive oxygen species (ROS). The role of the extended aromatic conjugation of the N^N-donor ligands in the complexes was demonstrated by their binding propensities with CT-DNA and bovine serum albumin (BSA). Both DNA intercalation and groove binding were evidenced, while tryptophan (Trp) and tyrosine (Tyr) binding site preferences were revealed from the synchronous fluorescence spectra (SFS) of BSA. The cytotoxic profiling of the complexes performed on hepatocellular carcinoma cells (HepG2) in the dark and in the presence of green light indicated their dose-dependent cytotoxicity. The [Ru^II(N^N)₂(ox)] complexes exhibited enhanced photocytotoxicity mediated by efficient generation of cytotoxic ¹O₂ and effective interaction with DNA. All the complexes were internalized by the HepG2 liver cancer cells efficiently and localized to the cytoplasm and nucleus. The complexes exhibited potent anti-proliferative, anti-clonogenic, and anti-migratory effects on the cancer cells, suggesting their potential for therapeutic applications.