Racemization kinetics of enantiomeric oxazepams and stereoselective hydrolysis of enantiomeric oxazepam 3-acetates in rat liver microsomes and brain homogenate.

Enantiomers of oxazepam and of 3-O-acyl, 1-N-acyl-3-O-acyl, and 3-O-methyl ether derivatives of oxazepam were resolved on HPLC columns packed with Pirkle's chiral stationary phases [CSP; (R)-N-(3,5-dinitrobenzoyl)phenylglycine or (S)-N-(3,5-dinitrobenzoyl)leucine] bonded either ionically or covalently to spherical particles of gamma-aminopropylsilanized silica, and on a column packed with poly-N-acryloyl-(S)-phenylalanine ethyl ester bonded covalently to silica gel (Chiraspher). Resolution was achieved, with several mobile phases of different solvent compositions and with varying chromatographic resolutions, on all of the chiral stationary phases tested. Resolved enantiomers of oxazepam undergo racemization, whereas enantiomers of 3-O-acyl and 3-O-methyl derivatives are stable. Racemization half-lives of oxazepam enantiomers were determined by monitoring changes in ellipticity as a function of time on a spectropolarimeter immediately (within 30 s) following resolution of enantiomers and were found to substantially vary, depending on the solvents used. Rates of hydrolysis of racemic and enantiomeric 3-O-acyl-oxazepams by esterases in liver microsomes and brain homogenate of rats were determined by a simple and sensitive CSP-HPLC method. The relative rate of hydrolysis was 3R greater than racemate much greater than 3S by rat liver microsomes and 3S greater than racemate much greater than 3R by rat brain homogenate.