Metabolism of prazepam by rat liver microsomes: stereoselective formation and N-dealkylation of 3-hydroxyprazepam.

The metabolism of prazepam (7-chloro-1-(cyclopropylmethyl)-1,3-dihydro-2H-1, 4-benzodiazepin-2-one) (PZ) was studied using liver microsomes prepared from untreated, phenobarbital (PB)-treated, and 3-methylcholanthrene (3MC)-treated male Sprague-Dawley rats. Relative rates of PZ metabolism by liver microsomes prepared from rats were PB-treated greater than untreated greater than 3MC-treated. Metabolites of PZ were separated by normal phase high performance liquid chromatography and the relative amounts of major metabolites were found to be N-desalkylprazepam (also known as N-desmethyldiazepam and nordiazepam) greater than 3-hydroxy-PZ (3-OH-PZ) greater than oxazepam. Enantiomers of 3-OH-PZ were resolved by high performance liquid chromatography on an analytical column packed with Pirkle's chiral stationary phase, (R)-N-(3,5-dinitrobenzoyl)phenylglycine covalently bonded to spherical particles of gamma-aminopropylsilanized silica. 3-OH-PZ formed in the metabolism of PZ by liver microsomes prepared from rats was found to have 3R/3S enantiomer ratios of 84:16 (untreated), 85:15 (PB-treated), and 84:16 (3MC-treated), respectively. Relative rates of N-dealkylation of PZ by three rat liver microsomal preparations were PB-treated greater than untreated greater than 3MC-treated. N-Dealkylation of 3-OH-PZ by rat liver microsomes was substrate enantioselective; the 3S-enantiomer was N-dealkylated faster than the 3R-enantiomer. The results indicated that both C3-hydroxylation of PZ and N-dealkylation of 3-OH-PZ catalyzed by rat liver microsomes were stereoselective, resulting in the formation of a 3-OH-PZ highly enriched in the 3R-enantiomer.