Pharmacokinetics of methadone and its primary metabolite in 20 opiate addicts

Abstract

In a closed metabolic ward the pharmacokinetics of methadone and its primary metabolite (EDDP) were studied in 20 long-term opiate addicts. After administration of the daily oral dose of methadone HC1 (mean 60 rag, range 10-225 rag) blood samples were taken and analysed, using a newly developed high-performance liquid chromatography (HPLC) method. The steady-state plasma concentrations of the 20 subjects varied from 65-630 ng'ml -~ and from 5 to 55 ng'm1-1, whereas the peak concentrations were 124-1255 ng.m1-1 and 10 301 ng'ml -~ for methadone and EDDR respectively. The calculated ratios between the area under the curve (AUC(0_24 h)) for methadone and the AUC(0_24 h) for EDDP varied from 5.9 to 44.6, indicating interindividual differences in metabolic activity. In 19 out of 20 subjects the pharmacokinetics of methadone are best described using a two-compartment model. The mean body clearance was 1.64 ml.min-l.kg -~, whereas the mean elimination rate constant (/~) and plasma half-life (t,/fi) were 0.026'h -1 (range 0.013-0.053"h -~) and 31.2 h (range 13-53 h), respectively. Differences of gender were also found. A poor correlation was found between the methadone dose and the steady-state level. A much better correlation was found between the normalized steady-state level and the body clearance.Since methadone maintenance therapy (MMT) was introduced [1], many studies have been conducted to establish the pharmacokinetics of methadone. Unfortunately they have not led to a general consensus with regard to well-defined dosage schedules related to clinical and therapeutic efficacy. This might be explained in part by the very divergent results of most of the studies performed so far. For example, plasma half-lives of methadone in tolerant subjects under MMT have varied from 19 h [2] to even 75 h [3]. Both studies were performed in closed metabolic wards. The same variability has applied for bioavailability, which has ranged from 36% to 106% (mean 87%) among 12 tolerant opiate addicts [4], also in a closed metabolic ward. Another study revealed a mean bioavailability of 79% (range 41-99%) [5]. Furthermore, there has been no consensus as to whether the disposition of methadone follows a one-[6-8] or a two- [4,5,9] compartment model. However, most of these studies were performed under divergent, hardly comparable circumstances. The pharmacokinetics in non-tolerant ("naive") subjects might differ from those in tolerant subjects [10,11]. Also short-term treatment might give different results from long-term (steady-state) treatment [12,13]. However, it seems that determinations of plasma methadone in outdoor addicts show wider scatters in elimination half-lifes than those performed in closed metabolic wards: 17.8-63.8 h [8] and 18.9-43 h [2], respectively.The metabolism of methadone has been studied intensively. Many metabolites have been traced and identified in human urine [14,15]. Nevertheless essential knowledge about the formation and disposition of the major metabolite of methadone, EDDP (1,5-dimethyl-3,3-diphenyl-2-eth...

<i>L</i>-Methadone and <i>D,L</i>-Methadonein Methadone Maintenance Treatment: A Comparison of Therapeutic Effectiveness and Plasma Concentrations

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Comparison of the Effects of Methadone and Heroin on Human ether-à-go-go-Related Gene Channels

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Gender-Specific Implications for Cardiovascular Medication Use in the Elderly

“…One carefully conducted study of steady-state methadone pharmacokinetics in opiate addicts determined that sex also contributes to the variability with longer elimination times in women compared with men that could not be explained on the basis of body weight, comedications, or other variables. 106 Drug Transporters. A focus on investigations of drug transporters has been more recent than the study of metabolic enzymes and data on sex differences are limited.…”