Studies on metabolism of bromazepam. VI. Reduction of 2-(2-amino-5-bromobenzoyl)pyridine, a metabolite of bromazepam, in the rabbit, rat, and guinea pig.

Three urinary metabolites that were formed by cleavage of the benzodiazepine ring of bromazepam, 2-(2-amino-5-bromobenzoyl)pyridine (ABBP), 2-(2-amino-5-bromo-3-hydroxybenzoyl)pyridine (3-OH-ABBP), and 2-amino-5-bromo-2'-azabenzhydrol (ABAB), were measured in urine of rabbits, rats, and guinea pigs. The major metabolite was 3-OH-ABBP in all animals given bromazepam orally. ABAB was also excreted in major amounts in the guinea pig, but was excreted in minor amounts in the rabbit and rat. Moreover, ABAB was excreted in the urine of all animals given ABBP orally. It may be concluded that ABAB was formed by reduction of the carbonyl group of ABBP. ABBP reduction was catalyzed by NADPH-dependent enzymes occurring in rabbit liver cytoplasm, and rat liver microsomes, and guinea pig liver cytoplasm and microsomes. The reductases were inhibited by sulfhydryl group reagents. The optimum pH of the cytoplasmic enzyme ranged from 7.2 to 7.8, and that of the microsomal enzyme was 6.5. The apparent KM value for the reduction of ABBP by guinea pig liver microsomes was the lowest among all of the liver preparations.