Microassay for ketamine and metabolites in plasma and serum based on enantioselective capillary electrophoresis with highly sulfated γ-cyclodextrin and electrokinetic analyte injection

For the assessment of stereoselective aspects of the metabolism of ketamine, an enantioselective CE-based microassay for determination of the stereoisomers of ketamine and three of its major metabolites in plasma and serum was developed. The assay is based on liquid/liquid extraction of the analytes of interest at alkaline pH from 0.05 mL plasma or serum followed by electrokinetic sample injection of the analytes from the extract across a buffer plug without chiral selector. Separation occurs cationically at normal polarity in a pH 3.0 phosphate buffer containing 0.66% of highly sulfated gamma-cyclodextrin (HS-gamma-CD). Key parameters for optimization are identified as being the amount of HS-gamma-CD in the BGE, the length of the buffer plug and its concentration, the duration of electrokinetic injection, and the extraction medium. Diluted buffer in the plug is employed to ascertain sufficient analyte stacking due to a combination of field amplification and complexation. The newly developed microassay is robust (intraday and interday RSD < 5% and < 9%, respectively) and well suited to determine enantiomer levels of ketamine and its metabolites down to 10 ng/mL. It is more sensitive, uses less plasma or serum, organic solvent, and analysis time compared to previous CE-based assays and was successfully applied to monitor ketamine, norketamine, 5,6-dehydronorketamine (DHNK), and 6-hydroxynorketamine (6HNK) stereoisomer levels in plasma of a Beagle dog that received a bolus of racemic ketamine or S-ketamine after sevoflurane anesthesia. The data suggest that the formation of DHNK and 6HNK occur stereoselectively.