In situ reactivity of electrochemically generated semiquinone on Emodin and its CuII/MnII complexes with pyrimidine based nucleic acid bases and calf thymus DNA: Insight into free radical induced cytotoxicity of anthracyclines

Abstract

There is enough proof to believe that free-radical intermediates of anthracycline based anticancer agents are involved in different stages of drug action. Subtle therapeutic differences observed in the actions of different anthracyclines largely influence the mechanism of action of the drugs that distinguish one member from another. Redox properties control biological responses related either to the one-electron quinone/semiquinone couple or the two-electron quinone/quinone-dianion couple. Comproportionation also leads to generation of semiquinone. Hence, whatever the form of reduction of the quinone moiety, a substantial amount of semiquinone is eventually formed in the system. Immediately after formation, there is competition between natural radical-decay pathways and one-electron transfer reactions that generate the superoxide-radical anion. Prototropic properties control rate of radical decay while redox properties control rate of electron transfer to molecular oxygen. In aerated medium, semiquinone-radical anion and superoxide-radical anion co-exist while in de-aerated medium semiquinone-radical anion predominates. All the radicals are damaging to the biological system. Through this study, attempt was made to detect changes induced by the radicals on pyrimidine based nucleic acid bases and calf thymus DNA in aerated and de-aerated (Ar saturated) medium to know the mechanism by which Emodin, its CuII/MnII complexes might affect DNA. Semiquinone-radical anion was generated electrochemically maintaining a glassy carbon electrode at the first reduction potential of each compound. Since the chosen compound (Emodin), its complexes are analogues of anthracyclines, findings on them can be extrapolated to understand the differences in anticancer activity or of adverse drug reactions reported in an innumerable number of clinical studies related to anthracyclines where the difference in structure of different members is due to differences in the relative positioning of hydroxy groups on the hydroxy-9, 10-anthraquinone moiety of anthracyclines. The study helps to realize action of compounds of this class as anticancer agents.