Determination of naltrexone and its major metabolite, 6-β-naltrexol, in human plasma using liquid chromatography with electrochemical detection

A sensitive and specific high-performance liquid chromatographic method with electrochemical detection was developed for the simultaneous determination of naltrexone and its major metabolite, 6-β-naltrexol, in human plasma. After alkalinizing 2 ml plasma samples with pH 9 sodium carbonate buffer, naltrexone and 6-β-naltrexol were extracted into dichloromethane and then back-extracted into 0.017 M phosphoric acid. A portion of the acid extract was chromatographed on a YMC phenyl column using a mobile phase of methanol-phosphoric acid (50 mM) (20:80, v/v) (pH∗ 3.2) at a flow-rate of 1.2 ml min −1. Quantification was performed using an ESA Coulometric electrochemical detector. Acceptable intra-day and inter-day assay precision (RSD < 10%) and accuracy (< 16%) for both compounds were observed over concentration ranges of 0.25–50.0 ng ml −1 for naltrexone and 0.5–100 ng ml −1 for 6-β-naltrexol. No degradation of either naltrexone or 6-β-naltrexol was observed in frozen human plasma stored at − 20°C over an 8 month period. The method is sufficiently sensitive and selective to quantify plasma concentrations of naltrexone and 6-β-naltrexol after oral doses of 50 mg of naltrexone to healthy subjects or alcoholic patients.