DETERMINATION OF NALTREXONE AND ITS MAJOR METABOLITE, 6-BETA-NALTREXOL, IN HUMAN PLASMA USING LIQUID-CHROMATOGRAPHY WITH ELECTROCHEMICAL DETECTION

A sensitive and specific high-performance liquid chromatographic metho d with electrochemical detection was developed for the simultaneous de termination of naltrexone and its major metabolite, 6-beta-naltrexol, in human plasma. After alkalinizing 2 ml plasma samples with pH 9 sodi um carbonate buffer, naltrexone and 6-beta-naltrexol were extracted in to dichloromethane and then back-extracted into 0.017 M phosphoric aci d. A portion of the acid extract was chromatographed on a YMC phenyl c olumn 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 p erformed using an ESA Coulometric electrochemical detector. Acceptable intra-day and inter-day assay precision (RSD < 10%) and accuracy (< 1 6%) 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-beta-naltre xol. No degradation of either naltrexone or 6-beta-naltrexol was obser ved in frozen human plasma stored at -20 degrees C over an 8 month per iod. The method is sufficiently sensitive and selective to quantify pl asma concentrations of naltrexone and 6-beta-naltrexol after oral dose s of 50 mg of naltrexone to healthy subjects or alcoholic patients.