HPLC determination of ketamine, norketamine, and dehydronorketamine in plasma with a high-purity reversed-phase sorbent.

We developed an isocratic, selective, and very sensitive HPLC method for the determination of ketamine and its two main metabolites in plasma. The compounds were extracted from plasma by a liquid-liquid extraction with a dichloromethane:ethyl acetate mixture followed by an acidic back-extraction. Separation was achieved on a new stationary phase, Purospher RP-18 endcapped, with a mobile phase containing acetonitrile:0.03 mol/L phosphate buffer (23:77 by vol) adjusted to pH 7.2. Because of the high column efficiency and the significant improvement of peak symmetry, the quantification limit could be down to 5 micrograms/L for ketamine and norketamine (NK). The intraday and interday CVs ranged from 1.7% to 5.8% and 3.1% to 10.2% for all compounds respectively. The method is sensitive enough for monitoring ketamine, NK, and dehydroketamine in plasma during pharmacokinetic studies after an intravenous bolus of a low dose of ketamine.