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      The Effects of Chiral Isolates of Methadone on the Cardiac Potassium Channel IKr

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          Abstract

          Objectives: Methadone is a synthetic opioid, an analgesic and an antiaddictive. QT prolongation as well as torsade de pointes ventricular tachycardia and death have been reported with methadone. Methadone’s proarrhythmic toxicity is related to the inhibition of cardiac IKr channel and prolongation of the action potential. We hypothesized that the 2 isomers of methadone may have different effects on the IKr channel. Methods: The effects of the isomers on IKr were evaluated by using an oocytes system with heterogeneously expressed human ether-a-go-go-related gene (HERG) using the 2 electrode voltage clamp technique. r- and s-methadone were obtained by employing chiral high-performance liquid chromatography, separating methadone into 2 isolates, with optical rotations of –141 and +143 degrees. Results: At concentrations of 0.01, 0.03, 0.1, 1 and 3 m M, r/s-methadone produced a dose-dependent inhibition of HERG by 17 ± 5, 23 ± 4, 40 ± 4, 57 ± 3, 69 ± 3 and 80 ± 1%, respectively. The IC<sub>50</sub> of r/s-methadone was 0.21 ± 0.02 m M. At 0.1, 0.3 and 1 m M, s-methadone reduced HERG current by 50 ± 4, 76 ± 5 and 87 ± 5%, respectively, while r-methadone reduced HERG current by 26 ± 4, 53 ± 3 and 77 ± 3%, respectively. Conclusions: There was a significant difference (p < 0.01) in the percentage of current inhibition between r- and s-methadone, at 0.1 and 0.3 m M (52% reduction). Thus, r-methadone may be a safer agent due to less QT effect.

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          Interindividual variability of the clinical pharmacokinetics of methadone: implications for the treatment of opioid dependence.

          Methadone is widely used for the treatment of opioid dependence. Although in most countries the drug is administered as a racemic mixture of (R)- and (S)- methadone, (R)-methadone accounts for most, if not all, of the opioid effects. Methadone can be detected in the blood 15-45 minutes after oral administration, with peak plasma concentration at 2.5-4 hours. Methadone has a mean bioavailability of around 75% (range 36-100%). Methadone is highly bound to plasma proteins, in particular to alpha(1)-acid glycoprotein. Its mean free fraction is around 13%, with a 4-fold interindividual variation. Its volume of distribution is about 4 L/kg (range 2-13 L/kg). The elimination of methadone is mediated by biotransformation, followed by renal and faecal excretion. Total body clearance is about 0.095 L/min, with wide interindividual variation (range 0.02-2 L/min). Plasma concentrations of methadone decrease in a biexponential manner, with a mean value of around 22 hours (range 5-130 hours) for elimination half-life. For the active (R)-enantiomer, mean values of around 40 hours have been determined. Cytochrome P450 (CYP) 3A4 and to a lesser extent 2D6 are probably the main isoforms involved in methadone metabolism. Rifampicin (rifampin), phenobarbital, phenytoin, carbamazepine, nevirapine, and efavirenz decrease methadone blood concentrations, probably by induction of CYP3A4 activity, which can result in severe withdrawal symptoms. Inhibitors of CYP3A4, such as fluconazole, and of CYP2D6, such as paroxetine, increase methadone blood concentrations. There is an up to 17-fold interindividual variation of methadone blood concentration for a given dosage, and interindividual variability of CYP enzymes accounts for a large part of this variation. Since methadone probably also displays large interindividual variability in its pharmacodynamics, methadone treatment must be individually adapted to each patient. Because of the high morbidity and mortality associated with opioid dependence, it is of major importance that methadone is used at an effective dosage in maintenance treatment: at least 60 mg/day, but typically 80-100 mg/day. Recent studies also show that a subset of patients might benefit from methadone dosages larger than 100 mg/day, many of them because of high clearance. In clinical management, medical evaluation of objective signs and subjective symptoms is sufficient for dosage titration in most patients. However, therapeutic drug monitoring can be useful in particular situations. In the case of non-response trough plasma concentrations of 400 microg/L for (R,S)-methadone or 250 microg/L for (R)-methadone might be used as target values.
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            HERG, a human inward rectifier in the voltage-gated potassium channel family

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              The d- and l-isomers of methadone bind to the non-competitive site on the N-methyl-D-aspartate (NMDA) receptor in rat forebrain and spinal cord.

              Racemic (dl)-methadone has antagonist activity at the N-methyl-D-aspartate (NMDA) receptor. We evaluated dl-methadone, the opioid active (l-) and the opioid inactive (d-) isomers in competition binding assays. dl-Methadone and its d- and l- isomers exhibited low micromolar affinities for the [3H]MK-801-labeled non-competitive site of the NMDA receptor in both rat forebrain and spinal cord synaptic membranes, with Ki values and displacement curves similar to those of dextromethorphan, an established NMDA receptor antagonist. They lacked affinity at the [3H]CGS-19755-labeled competitive site of the NMDA receptor. Therefore, both methadone and its the d- and l- isomers differ from morphine, hydromorphone, and naltrexone in that they have non-competitive antagonist activity at the NMDA receptor. A non-opioid NMDA receptor antagonist, such as d-methadone, may improve the efficacy of morphine by attenuating the development of tolerance.
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                Author and article information

                Journal
                CRD
                Cardiology
                10.1159/issn.0008-6312
                Cardiology
                S. Karger AG
                0008-6312
                1421-9751
                2009
                March 2009
                04 November 2008
                : 113
                : 1
                : 59-65
                Affiliations
                aRush University, Chicago, Ill., and bAmerican Institute of Therapeutics, Lake Bluff, Ill., USA
                Article
                167043 Cardiology 2009;113:59–65
                10.1159/000167043
                18984955
                bd253905-a094-4548-85b8-30c12aba71fd
                © 2008 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                History
                : 01 July 2008
                : 07 July 2008
                Page count
                Figures: 6, References: 45, Pages: 7
                Categories
                Original Research

                General medicine,Neurology,Cardiovascular Medicine,Internal medicine,Nephrology
                Ventricular arrhythmias,Proarrhythmia,Prolonged repolarization,Methadone,QT interval,Pharmacology, cardiovascular

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