Free radical production and DNA cleavage by copper chelating peptide-anthraquinones

Two pseudopeptides incorporating a peptide metal-chelating moiety (Gly-His-Lys) and a polyhydroxy anthraquinone ring related to the nuclei of anti-tumor drugs such as mitoxantrone and ametantrone, have been synthesized. The goal was to conjugate the redox effects of a quinone ring with the iron-chelating properties of the peptide in order to generate free radical species capable of damaging DNA. Indeed quinone-containing drugs undergo, in vivo, one-electron reduction to the corresponding semiquinone radicals which, in the presence of molecular oxygen, produce a superoxide anion radical, hydrogen peroxide and ultimately, in the presence of metal, hydroxyl radical (Fenton reaction). Hydroxyl radicals (OH.) are short-lived and extremely reactive with their bioenvironment. The interaction of both drugs with DNA has been studied by fluorescence quenching and DNA melting experiments. Spectroscopic and e.s.r. studies demonstrated that several types of Cu-complex are formed depending on the copper-drug ratio. The production of free radicals, as evidenced by spin-trapping, is optimum with a Cu/drug ratio of 0.1; in this case the metal ion is chelated by the peptide moiety. This latter complex is able to induce DNA breakage at a high level. Thus, it appears that the proposed concept works but that care must be taken in the choice of the relative concentration of copper.