Plasma concentrations during naltrexone implant treatment of opiate-dependent patients.

Aims: To evaluate individual variations in plasma concentrations over time in patients with naltrexone implants.

Methods: Ten opioid-dependent patients received up to four implants. Plasma samples were collected regularly for the analyses of naltrexone and the metabolite beta-naltrexol.

Results: The median naltrexone C(max) was 12.3 (range 5.8-22.1) ng ml(-1), the median T(max) was 1 day (range 3 h to 35 days), and the median length of time that plasma concentrations were above 1 ng ml(-1) was 55 (range 30-80) days. Two patients reported heroin use without experiencing any effect. Tissue reactions were recorded in two patients after repeated implantation.

Conclusion: Marked individual and intraindividual variations in naltrexone concentrations were observed. Further studies should be performed to evaluate the need for therapeutic drug monitoring during naltrexone implant treatment.