Biological activity of non-radical vs. stable radical of a pyridazine-sulfonamide aminophenol-type compound

Abstract

4-(3,5-Di-tert-butyl-2-hydroxyphenylamino)-N-(6-methoxypyridazin-3-yl)benzenesulfonamide (LSO_APH₂) is an 2-aminophenol-appended, redox-non-innocent molecule. It was characterized structurally by spectroscopy and its structure was confirmed by single-crystal X-ray diffraction. The o-iminosemiquinonate monoanion (LSO_ISQ) was obtained by one electron-oxidation and confirmed by electron paramagnetic resonance (EPR) spectroscopy and UV-vis spectroscopy. Cyclic voltammetry showed two consecutive redox couples: o-amidophenolate/o-iminosemiquinonate monoanion (LSO_APH₂/LSO_ISQ) and o-iminosemiquinonate monoanion/o-benoquinone (LSO_ISQ/LSO_IQ). The radical features of LSO_ISQ in air were supported by EPR spectroscopy (g = 2.005) in a solution at room temperature. Computation based on density functional theory revealed the spin density to be localized on LSO_ISQ due to formation of the o-iminosemiquinonatemonoanion radical. The in vitro toxicity of LSO_APH₂ and LSO_ISQ was estimated against human cancer cell lines (HepG2, MDA-MB 231, HeLa) and a human normal cell line (WI-38). The cellular metabolic activity of LSO_APH₂ and LSO_ISQ were determined using the MTT assay using the cell lines stated above. LSO_APH₂ and LSO_ISQ enhanced cellular levels of reactive oxygen species in human cancer cell lines, which culminated in the inactivation/death of cancer cells. LSO_ISQ was more effective than LSO_APH₂ as an anticancer agent.