Within- and between- subject variability in methadone pharmacokinetics and pharmacodynamics in methadone maintenance subjects

Aims To investigate within- and between-subject variability of the pharmacodynamics and pharmacokinetics of (R)- and (S)-methadone in methadone maintenance subjects at steady-state. Methods Six non-holder subjects were studied on three occasions at 7–16 day intervals; doses (20–170 mg/day) remained unchanged. Blood samples and pharmacodynamic data were collected 10–12 times over a 24-h inter-dosing interval. All pharmacodynamic data were expressed as the area under the end-point versus time curve. Using analyses of variance with mixed effects, best estimates were made of the ratio of between- to within-subject variation, with corresponding 95% confidence intervals (CI) for within-subject variation at the average value. Results Subjects were relatively consistent between occasions, whereas there was much greater between-subject variability (P < 0.02) for all measures. Estimates of the ratio of between- to within-subject variation ranged from 2.2–12.8 for pharmacodynamic measures, and 1.3–7.9 for pharmacokinetic parameters. For pain, total mood disturbance, withdrawal, pupil size and respiration rate, 95% CI for within-subject measures ranged ≤ 2-fold, while this was greater for subjective direct opioid effects (4.2-fold). For CL/F of the active (R)-methadone, the variance ratio was 4.9 (P < 0.0003), with 95% CI for within-subject measures ranging ≤ 2-fold. (S)-methadone CL/F demonstrated greater within-subject variability (3.4-fold), possibly contributing to a smaller (2.7; P < 0.0003) ratio of between- to within-subject variance. Conclusions Non-holder methadone maintenance treatment participants appear to respond consistently with respect to pharmacokinetics and pharmacodynamics over a 1–2 month period. Such knowledge may help prescribers to determine whether alternative dosing regimens or treatments might be more appropriate in this population.