Species Difference in Nisoldipine Oxidation Activity in the Small Intestine

Species difference in nisoldipine oxidation activities was investigated using small intestinal microsomes of rats, guinea pigs, dogs, monkeys and humans. The oxidation activities were estimated by measuring metabolites formation (BAY o 3199 and BAY r 9425) of nisoldipine. For the preparation of small intestinal microsomes of various animal species, the effect of protease inhibitors was preliminarily investigated. The formation of BAY o 3199 significantly increased in the rat small intestinal microsomes prepared with trypsin inhibitor. Using the trypsin inhibitor-treated small intestinal microsomes of various animals, metabolic intrinsic clearances (CLint, in vitro) for BAY o 3199 and BAY r 9425 formations were estimated based on an Eadie-Hofstee plot. The total CLint, in vitro estimated by the sum of CLint, in vitro for both formations in the small intestines of all species was much lower than that in the liver. There was a marked species difference in the nisoldipine oxidation activities in the small intestines, with the rank order being humans = monkeys > dogs > rats > guinea pigs, versus the following order in the liver: rats > monkeys = guinea pigs > humans > dogs. The formations of both BAY o 3199 and BAY r 9425 in the human intestinal microsomes were inhibited by pretreatment with troleandomycin (TAO) and antiserum against CYP3A4. Similar inhibition profile by TAO was obtained from the monkey intestinal microsomes. These results implied that monkeys would be a good predictor of human small intestinal metabolism for CYP3A4 substrates.