Analysis of naltrexone urinary metabolites

A reversed-phase HPLC method using ion-pair formation has been developed for the simultaneous determination of naltrexone and three urinary metabolites. The extraction of the free and conjugated metabolites was studied by liquid—solid procedures using styrene—divinylbenzene copolymers (Amberlite XAD-2) and bonded octadecyl silica supports (ODS-silica). Optimum recovery was obtained with ODS-silica extraction using 25% acetonitrile in a 5 mM diammonium phosphate buffer pH 2.1 as elution solvent. The chromatographic behaviour of naltrexone metabolites and naloxone (internal standard) was examined by varying the mobile phase composition. Increments of both the diammonium phosphate buffer concentration and the percentage of organic solvent in the eluent decreases the retention of compounds in a non-linear manner. Increments of the dodecyl sulphate (counter-ion) concentration, increases the retention time. The method was applied to determine the urinary levels of major naltrexone metabolites in a volunteer receiving a 50 mg oral dose. This is the first method reported which permits the simultaneous quantitative determination of naltrexone and its metabolites, 6β-naltrexol, naltrexone glucuronide and 6β-naltrexol glucuronide, in urine.