Opioid plasma concentrations in methadone-and buprenorphine-maintained patients

This is the first trial to compare the relationship of opioid plasma concentrations in methadone-versus buprenorphine-maintained subjects. Sixty subjects (19 females and 41 males) seeking treatment who met Diagnostic and Statistical Manual version IV (DSM-IV) criteria for opioid dependence were recruited and treated at the Drug Addiction Outpatient Clinic at the University of Vienna. Of these, 44 (11 female and 33 male) were included in the analyses of plasma concentrations. Subjects received either daily sublingual buprenorphine (2 mg or 8 mg tablets; maximum daily dose: 8 mg) or oral methadone (racemic R-/S-methadone) and were maintained on a stable dose after an induction period of 2 weeks. Mean dose and mean plasma concentrations were correlated on an individual and collective basis. Correlation was 0.51 for buprenorphine, whereas the score for methadone was 0.69. Intra-individual variation was much higher for buprenorphine (p<0.0001), while the concentration-to-dose ratio was very small. Based on the differences of the pharmacokinetics of blood plasma of the two agents, we tried to explain the differences in the acceptance of treatment, which was significantly lower in the buprenorphine-maintained group. No such differences could be evaluated between completers and dropouts in buprenorphine-maintained subjects, neither concerning withdrawal scores nor dose, plasma concentration, concentration-to-dose ratios or intra-individual variation.