Steady-state pharmacokinetics and pharmacodynamics in methadone maintenance patients: Comparison of those who do and do not experience withdrawal and concentration-effect relationships

OBJECTIVE: To determine plasma racemic methadone concentration-effect relationships for subjective and objective responses and whether pharmacokinetic and/or pharmacodynamic factors influence withdrawal severity. METHODS: Eighteen patients enrolled in a public methadone maintenance program, nine of whom experienced significant withdrawal, received constant doses of methadone once daily for at least 2 months. During an interdosing interval, 13 blood samples were collected to measure plasma racemic methadone concentrations (patients); subjective (withdrawal severity, direct opioid effects, and pain threshold) and objective (pupil diameter and respiratory rate) opioid effects were quantified on 11 occasions (all participants). The sigmoid Emax model was used to relate plasma concentrations and effects and to calculate the slope factor (N). The rate of decline in plasma concentration during each hour from the peak to the trough concentration was calculated. RESULTS: There was an inverse relationship between plasma concentrations and withdrawal severity and pupil diameter, as well as a direct relationship with subjective opioid effects and pain threshold. The mean N values were 5.4+/-0.9 for withdrawal severity, 5.1+/-1.1 for subjective opioid effects, 1.2+/-0.1 for pupil diameter, and 2.8+/-0.7 for pain threshold. Withdrawal severity correlated with the maximum rate of decrease in plasma concentration (P < .01). There were no differences between those who reported significant withdrawal and those who did not with respect to mean area under the plasma concentration versus time curve and predose plasma concentration, but maximal rate of decline was greater in the former group (74.5 versus 42.1 ng/mL/h). CONCLUSIONS: In this group of long-term methadone-maintained recipients, opioid responses were strongly correlated with changes in plasma racemic methadone concentrations. For the subjective responses, notably withdrawal, small changes in plasma concentrations translate into relatively large changes in effect; therefore, clinically important withdrawal is a consequence of more rapid decline in methadone concentration.