The identification and measurement of two new metabolites of naltrexone in human urine.

Two metabolites of naltrexone were identified in human and rat urine following drug administration. The metabolites, 2-hydroxy-3-0-methylnatrexone and 2-hydroxy-3-0-methyl-6 beta-naltrexol, were identified by comparison of their respective gas chromatographic retention times and methane chemical ionization spectral patterns with authentic standards. The standards were obtained by chemical synthesis from naltrexone. Urinary excretion of the two metabolites was measured by single ion recording of the pseudomolecular ions of the metabolites and an internal standard. Excretion data was collected from 5 human subjects over a period of 5 days following a single dose of naltrexone. The metabolites, 2-hydroxy-3-0-methylnatrexone and 2-hydroxy-3-0-methyl-6 beta-natrexol were found to account for an average (+/- standard error) of 0.45 +/- 0.08% and 4.6 +/- 2.1% of the administered dose, respectively. An unusually long half-life of elimination of the latter metabolite (mean of 20.2 +/- 1.8 hr) was observed. Acid-hydrolysis of the samples failed to produce a significant increase of either metabolite indicating that the major protion of these metabolites is excreted in the free form.