ESR detection of free radical intermediates during autoxidation of 5-hydroxyprimaquine

Autoxidation of 5-hydroxyprimaquine, a putative metabolite of the antimalarial primaquine, was studied by oxygen consumption and ESR spectroscopy. 5-Hydroxyprimaquine underwent fast autoxidation under mild conditions (pH 7.4-8.5, 25 degrees C, and presence of 1 mM diethylenetriamine pentaacetic acid); each mol of the drug consumed 0.75 mol of oxygen and formed 0.5 mol of hydrogen peroxide. Direct-ESR experiments demonstrated that 5-hydroxyprimaquine autoxidation was accompanied by generation of a drug-derived free radical that is oxygen sensitive. Generation of hydroxyl radical was also established by spin-trapping experiments in the presence of 5,5-dimethyl-1-pyrroline N-oxide. The effect of antioxidant enzymes on hydroxyl radical adduct yield and analysis of autoxidation stoichiometry suggest that the main route for hydroxyl radical generation is the iron-catalyzed reaction between the drug-derived free radical and hydrogen peroxide.