The urinary excretion profiles of naltrexone in man, monkey, rabbit, and rat

A gas-chromatographic method has been developed for the simultaneous determination of naltrexone, alpha-naltrexol, and beta-naltrexol as trimethylsiyl derivatives. Analysis of urine from rabbit, monkey, and rat demonstrated that, like man, these species reduce naltrexone primarily to beta-naltrexol. In naltrexone maintenance patients receiving 125 mg po three times per week, an average of 37% of the dose was recovered in 48-hr urine as free naltrexone (0.8%), conjugated naltrexone (7.6%), free beta-naltrexol (16.8%), and conjugated beta-naltrexol (11.8%). Thirty-four percent of the dose appeared in 0-24 hr and 3% during 24-48 hr. The ratio of beta-naltrexol to naltrexone rose from 2 at 0-4 hr to 34-48 hr. Monkeys receiving a daily dose of 12 mg/kg po, chronically, excreted very little free beta-naltrexol and exhibited an apparent sex-related difference in excretion patterns, with females excreting more than twice as much total base as males. Rabbits given a dose of 30 mg/kg ip for 4 days excreted conjugated naltrexone as the predominant urinary metabolite, accounting for 80% of total base recovered in 24 hr. In rats receiving 100 mg/kg po, less than 1% of the administered dose could be accounted for in the 24-hr urine, indicating that although the beta-naltrexol is produced as a urinary metabolite, other means of disposition of the drug must exist. Thus, in man and the monkey, beta-naltrexol is the predominant and persistent urinary metabolite. Urinary excretion profiles of naltrexone differ greatly between species commonly examined for chronic toxicity studies.