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The opioid antagonist nalmefene offers an alternative to traditional pharmacological treatments for alcoholism. The present study was designed to investigate the relationship between nalmefene plasma concentration and central mu-opioid receptor occupancy after a clinically effective dose (20 mg, orally). Pharmacokinetics and mu-opioid receptor occupancy of nalmefene after single and repeated dosing over 7 days was studied in 12 healthy subjects. Serial blood samples were obtained after both dosings, and pharmacokinetic parameters for nalmefene and main metabolites were determined. Central mu-opioid receptor occupancy of nalmefene was measured with positron emission tomography (PET) and [(11)C]carfentanil at four time points (3, 26, 50, 74 h) after both dosings. Nalmefene was rapidly absorbed in all subjects. The mean t(1/2) of nalmefene was 13.4 h after single and repeated dosing. The accumulation of nalmefene and its main metabolites in plasma during the repeated dosing period was as expected for a drug with linear pharmacokinetics, and steady-state was reached for all analytes. Both nalmefene dosings resulted in a very high occupancy at mu-opioid receptors (87-100%), and the decline in the occupancy was similar after both dosings but clearly slower than the decline in the plasma concentration of nalmefene or metabolites. High nalmefene occupancy (83-100%) persisted at 26 h after the dosings. The prolonged mu-opioid receptor occupancy by nalmefene indicates slow dissociation of the drug from mu-opioid receptors. These results support the rational of administering nalmefene when needed before alcohol drinking, and they additionally suggest that a high mu-opioid receptor occupancy can be maintained when nalmefene is taken once daily.

Prolonged Central μ-Opioid Receptor Occupancy after Single and Repeated Nalmefene Dosing