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.