Generation of free radicals during the reductive metabolism of the nitroaromatic compound, nilutamide

Incubation of the antiandrogen nilutamide with rat liver microsomes and NADPH, under anaerobic conditions, led to the formation of the nitro anion free radical, as indicated by electron spin resonance spectroscopy. The steady-state concentration of the nitro anion free radical was not decreased by SKF 525-A, carbon monoxide or metyrapone (3 inhibitors of cytochrome P-450) but was decreased by NADP+ or p-chloromercuribenzoate (2 inhibitors of NADPH-cytochrome P-450 reductase). Under aerobic conditions, the nitro anion free radical was reoxidized by oxygen, and the electron spin resonance signal was not detected. This redox cycle was associated with NADPH oxidation, consumption of oxygen, and formation of superoxide anion, hydrogen peroxide and glutathione disulfide. Under anaerobic conditions, nilutamide was further reduced to chemically reactive metabolites. Anaerobic incubation of [3H]nilutamide (0.1 mM) with rat liver microsomes and a NADPH-generating system resulted in the in vitro covalent binding of [3H]nilutamide metabolites to microsomal proteins; covalent binding required NADPH; it was decreased in the presence of NADPH-cytochrome P-450 reductase inhibitors (methylene blue, 2'-adenosine monophosphate) or in the presence of the nucleophile glutathione, but was unaffected by cytochrome P-450 inhibitors (SKF 525-A, CO). Covalent binding was decreased markedly under aerobic conditions. We conclude that nilutamide is reduced by microsomal NADPH-cytochrome P-450 reductase into a nitro anion free radical. In the presence of oxygen, this nitro anion free radical undergoes redox cycling with oxygen, and forms reactive oxygen species. In anaerobiosis, it is reduced further to covalent binding species.