Duration of occupancy of opiate receptors by naltrexone

To determine the duration of blockade of mu-opiate receptors by naltrexone, we measured the binding of [¹¹C]carfentanil in the brain of five normal volunteers with a positron radiation detection system before and 1, 48, 72, 120, and 168 hr after naltrexone administration. The half-time of blockade by naltrexone in the brain ranged from 72 to 108 hr which is greater than the fast plasma clearance components (4-12 hr) of naltrexone or its metabolite, beta-naltrexol, but corresponds well to the half-time of the terminal phase of plasma naltrexone clearance (96 hr). These results are consistent with the duration of the pharmacologic effects of naltrexone in response to heroin administration and indicate that 50 mg/day of oral naltrexone results in plasma levels in excess of that needed to saturate opiate receptors. This is the first example of the use of a simple dual-detector system with positron-emitting radioactive drugs to provide information regarding the duration of action of the drug on its specific receptor site. The plasma clearance half-time of a drug may not give an accurate reflection of action of the drug on a specific neuroceptor site. Direct measurement of drug effects on recognition sites greatly extend current studies of pharmacokinetics.