Elimination of Radioactivity Following Administration of [15,16-3H]naltrexone to Rats and Guinea Pigs

The elimination of radioactivity after [15,16-3H]naltrexone administration was studied in rats and guinea pigs. An average of 42% of the dose was eliminated in urine and 55% in feces following administration of 1 mg/kg iv to each of three rats. Analysis of radioactivity in the excreta of one rat that received the same dose im yielded similar results. On the other hand, four guinea pigs that received 1 mg/kg iv excreted only 14% of the dose in feces and 84% in urine. Similar results were obtained following im administration to guinea pigs at 1 and 20 mg/kg doses. In guinea pig excreta, an average of 64% of the dose corresponded to naltrexone and conjugates, 19% to beta-naltrexol and conjugates, and 2% to alpha-naltrexol and conjugates. In urine, the radioactivity corresponding to alpha-naltrexol and naltrexone was present mainly in conjugated form, whereas apparent beta-naltrexol was mainly unconjugated. The radioactivity in feces corresponded principally to unconjugated naltrexone and beta-naltrexol.