Distribution of nitroreductive activity toward nilutamide in rat

Nilutamide is a pneumotoxic and hepatotoxic nitroaromatic (R-NO2) antiandrogen used in the treatment of prostate carcinoma in man. Previously, we established that in the rat lung, the drug is metabolized into the corresponding hydroxylamine (R-NHOH) and amine (R-NH2) derivatives. These results evidenced a cytosolic oxygen-sensitive (type II) nitroreductase activity in lung. In the present studies, we extended the characterization of nilutamide metabolism in liver, brain, kidney, heart, blood, intestine (small, cecum, and large, and their respective luminal contents) of male Sprague-Dawley rats. Subcellular fractions for all tissues (except blood) examined (postmitochondrial, cytosolic, and microsomal) were prepared by differential ultracentrifugation. Blood and intestinal contents were sonicated before investigation. Incubations were run in the presence or absence of O2 to assess type I and II nitroreductase activities. Organic extracts were analyzed by HPLC methods and results were expressed as pmoles of R-NH2 formed per milligram protein per minute. Four distinct nitroreductive activities were evidenced. Cytosolic and microsomal type II nitroreductase activities were detected in all tissue samples studied. Type I NR activity was not observed in any of the cytosols, but was detected in the small intestine, lung, kidney, and liver microsomes. Nilutamide was also reduced in the intestinal lumen, possibly by a bacterial type I nitroreductase. Highest activities were observed in cytosols and were oxygen sensitive. These results evidence and characterize previously unknown nitroreductive activities toward nilutamide in rat tissues that might provide some explanation to the side effects of nilutamide and other nitroaromatic compounds observed in human therapeutics