A comparative study of the oral, intravenous, and subcutaneous administration of 3H-naltrexone to normal male volunteers.

3H-naltrexone was administered orally, intravenously, and subcutaneously to groups of normal, male, paid volunteers. The doses given were: 50 mg orally (specific activity 4 microCi/mg), 1 mg intravenously (specific activity 200 microCi/mg), and 5 mg subcutaneously (specific activity 30 microCi/mg). At these doses, the subjects did not experience any noticeable effects. Following intravenous injection, plasma levels of radioactivity were immediately high and declined rapidly during the first 30 minutes and declined gradually thereafter. Following oral or subcutaneous administration, maximal plasma levels were observed to occur one hour after dosing, and reached similar levels to those obtained when the drug was intravenously injected. This finding indicates the excellent bioavailability of naltrexone following oral or subcutaneous administration 3H-naltrexone and/or its metabolites were predominately excreted in the urine, and the renal excretion was similar for all three routes of administration. Fecal excretion is a minor pathway of elimination. The urinary and fecal excretion of 3H-naltrexone was studied in one subject for 133 hours after drug ingestion, and it was found that essentially all of the dose administered was excreted in this period.