Study of metabolic pathways of racemic ketamine and its (S)-enantiomer in rat blood plasma using CE-ESI/MS with partial filling of dual chiral selector system

Ketamine is a chiral drug used as anesthetic, analgesic and antidepressant. Its enantiomers and stereoisomers of its metabolites show different pharmacological and behavioral effects. To study the ketamine metabolic pathway and investigate these effects, highly sensitive and enantioselective methods are required. For that reason, in this study, a new CE method using a partial filling dual chiral selector system and ESI-MS detection has been developed and applied for separation and quantification of enantiomers of ketamine and its main metabolites, norketamine, hydroxynorketamine and dehydronorketamine, extracted by dichloromethane from the blood plasma of laboratory rats. The dual chiral selector system consisting of two zones of highly sulfated β-cyclodextrin (30 mg mL⁻¹) and highly sulfated γ-cyclodextrin (10 mg mL⁻¹) was introduced consecutively near the capillary outlet end. Both chiral selectors were dissolved in the background electrolyte composed of 10 mM ammonium hydroxide, 104 mM acetic acid, 10 % (v/v) ethanol, pH∗ 3.75. This system enabled enantioseparation of ketamine and its metabolites within a single CE run. High resolutions (3.99–17.61) of enantiomers of all above four analytes within a short time (11 min) were achieved in the fused silica capillary covalently coated with weakly negatively charged polyanionic copolymer (poly(acrylamide-co-sodium-2-acrylamido-2-methylpropanesulfonate), PAMAMPS). This coating minimized analyte sorption to the capillary and provided good repeatability of migration times. The limits of detection and quantification of the above analytes were in the range 108–238 nM and 361–792 nM, respectively. The method was linear within wide concentration range of 0.1–200 μM and the recovery was 91.3–105 %.