Characterization of the stereoselective metabolism of thiopental and its metabolite pentobarbital via analysis of their enantiomers in human plasma by capillary electrophoresis

Using capillary zone electrophoresis (CZE) with a 75 mM phosphate buffer at pH 8.5 containing 5 mM hydroxypropyl-gamma-cyclodextrin (OHP-gamma-CD) as chiral selector, the separation of the enantiomers of thiopental and its ox ybarbiturate metabolite, pentobarbital, is reported. Enantiomer assignment was performed via preparation of enantiomerically enriched fractions using chiral recycling isotachophoresis (rITP) processing of racemic barbiturates and analysis of rITP fractions by chiral CZE and circular dichroism spectr oscopy. Thiopental and pentobarbital enantiomers in plasma were extracted a t low pH using dichloromethane and extracts were reconstituted in acetonitr ile or 10-fold diluted, achiral running buffer. The stereoselectivity of th e thiopental and pentobarbital metabolism was assessed via analysis of 12 p lasma samples that stemmed from patients undergoing prolonged or having com pleted long-term racemic thiopental infusion. The data obtained revealed a modest stereoselectivity with R-(+)-thiopental /S-(-)-thiopental and R-(+)- pentobarbital /S-(-)-pentobarbital plasma ratios being <1 (P <0.05 compared to data obtained with racemic controls) and >1 (P < 0.001), respectively. The total S-(-)-thiopental plasma concentration was found to be on average about 248 higher compared to the concentration of R-(+)-thiopental, whereas the total R-(+)-pentobarbital plasma level was observed to be on average 2 9% higher compared to the S-(-)-pentobarbital concentration. (C) 1999 Elsev ier Science B.V. All rights reserved.