Light-Stimulated Generation of Free Radicals by Quinones-Chelators

The role of metal ions in the mechanism of light-stimulated redox activity of potential anticancer agent 2-phenyl-4-(butylamino) naphtha[2,3-h] quinoline-7,12-dione (Qc) has been studied by CIDNP (chemically induced dynamic nuclear polarization) and EPR methods. The photo-induced oxidation of NADH and its synthetic analog - substituted dihydropyridine (DHP) - by quinone Qc was used as a model. The Qc capability of producing chelating complexes with divalent metal ions of Fe, Zn and Ca was studied quantitatively by optical absorption spectroscopy. A significant decrease of electrochemical reduction potential of Qc (Delta E = 0.4 - 0.6 eV for ACN and ACN/PBS solutions) in chelating complexes and in protonated form of Qc was observed. A pronounced increase in efficiency of DHP oxidation in chelating complexes with Zn2+ and Ca2+ ions compared with free Qc was demonstrated. The yields of free radicals, including reactive oxygen species (ROS) and reaction products, were a few times higher than those in the absence of metal ions. Application of such chelating compounds to enhance ROS generation looks very promising for anti-cancer therapy, including the photodynamic therapy.