Methadone: a review of its pharmacokinetic/pharmacodynamic properties

Screening for drugs of abuse: which matrix, oral fluid or urine?

Urine is recognized as the prime matrix for drug test screening with well-established methods and testing protocols. Its major limitation is with regard to the inconvenience of sample collection and lack of integrity due to adulteration, dilution, drug spiking or sample exchange. The question is whether oral fluid, with its apparent better sample integrity, can replace urine for drug screening. This review examines the sample integrity problems and the advantages and limitations of oral fluid and urine in drug screening programmes. The variety of sample collection devices for oral fluid is shown to be a problem with recovery and detection for some drugs. This is examined in relation to the pharmacokinetics of drug metabolism and excretion in this matrix. Buccal contamination with drugs in oral fluid may also cause problems with interpretation. The clinical advantages of oral fluid analysis compared with urine testing are highlighted. Parent drugs are often found in oral fluid where only their metabolites may be found in urine, for example the benzodiazepines. 6-Monoacetylmorphine, an indicative marker of heroin, has a high prevalence in oral fluid from users of this drug but its detection in urine is limited due to its short half-life. Advances in analytical techniques, particularly chromatography linked to tandem mass spectrometry, are helping to promote oral fluid analysis. However, the lack of concordance studies examining both urine and oral fluid drug levels and kinetics in the clinical setting is of some concern.

Prenatal exposure to methadone and buprenorphine: a review of the potential effects on cognitive development

The amount of opioid users receiving opioid maintenance therapy has increased significantly over the last few years. As a result, an increasing number of children are prenatally exposed to long-lasting opioids such as methadone and buprenorphine. This article reviews the literature on the cognitive development of children born to mothers in opioid maintenance therapy. Topics discussed are the effects of prenatal exposure on prematurity, somatic growth, brain volume, myelination, and the endocrine and neurotransmitter system. Social-environmental factors, including parental functioning, as well as genetic factors are also described. Areas requiring further research are identified.

There is no age limit for methadone: a retrospective cohort study

Background

Data from the US indicates that methadone-maintained populations are aging, with an increase of patients aged 50 or older. Data from European methadone populations is sparse. This retrospective cohort study sought to evaluate the age trends and related developments in the methadone population of Basel-City, Switzerland.

Methods

The study included methadone patients between April 1, 1995 and March 31, 2003. Anonymized data was taken from the methadone register of Basel-City. For analysis of age distributions, patient samples were split into four age categories from '20-29 years' to '50 years and over'. Cross-sectional comparisons were performed using patient samples of 1996 and 2003.

Results

Analysis showed a significant increase in older patients between 1996 and 2003 (p < 0.001). During that period, the percentage of patients aged 50 and over rose almost tenfold, while the proportion of patients aged under 30 dropped significantly from 52.8% to 12.3%. The average methadone dose (p < 0.001) and the 1-year retention rate (p < 0.001) also increased significantly.

Conclusions

Findings point to clear trends in aging of methadone patients in Basel-City which are comparable, although less pronounced, to developments among US methadone populations. Many unanswered questions on medical, psychosocial and health economic consequences remain as the needs of older patients have not yet been evaluated extensively. However, older methadone patients, just as any other patients, should be accorded treatment appropriate to their medical condition and needs. Particular attention should be paid to adequate solutions for persons in need of care.

A high-throughput method for assessing chemical toxicity using a Caenorhabditis elegans reproduction assay

The National Research Council has outlined the need for non-mammalian toxicological models to test the potential health effects of a large number of chemicals while also reducing the use of traditional animal models. The nematode Caenorhabditis elegans is an attractive alternative model because of its well-characterized and evolutionarily conserved biology, low cost, and ability to be used in high-throughput screening. A high-throughput method is described for quantifying the reproductive capacity of C. elegans exposed to chemicals for 48 h from the last larval stage (L4) to adulthood using a COPAS Biosort. Initially, the effects of exposure conditions that could influence reproduction were defined. Concentrations of DMSO vehicle <or=1% did not affect reproduction. Previous studies indicated that C. elegans may be influenced by exposure to low pH conditions. At pHs greater than 4.5, C. elegans reproduction was not affected; however below this pH there was a significant decrease in the number of offspring. Cadmium chloride was chosen as a model toxicant to verify that automated measurements were comparable to those of traditional observational studies. EC(50) values for cadmium for automated measurements (176-192 microM) were comparable to those previously reported for a 72-h exposure using manual counting (151 microM). The toxicity of seven test toxicants on C. elegans reproduction was highly correlative with rodent lethality suggesting that this assay may be useful in predicting the potential toxicity of chemicals in other organisms.

Differential involvement of P-glycoprotein (ABCB1) in permeability, tissue distribution, and antinociceptive activity of methadone, buprenorphine, and diprenorphine: in vitro and in vivo evaluation

Conclusions based on either in vitro or in vivo approach to evaluate the P-gp affinity status of opioids may be misleading. For example, in vitro studies indicated that fentanyl is a P-gp inhibitor while in vivo studies indicated that it is a P-gp substrate. Quite the opposite was evident for meperidine. The objective of this study was to evaluate the P-gp affinity status of methadone, buprenorphine and diprenorphine to predict P-gp-mediated drug-drug interactions and to determine a better candidate for management of opioid dependence. Two in vitro (P-gp ATPase and monolayer efflux) assays and two in vivo (tissue distribution and antinociceptive evaluation in mdr1a/b (-/-) mice) assays were used. Methadone stimulated the P-gp ATPase activity only at higher concentrations, while verapamil and GF120918 inhibited its efflux (p < 0.05). The brain distribution and antinociceptive activity of methadone were enhanced (p < 0.05) in P-gp knockout mice. Conversely, buprenorphine and diprenorphine were negative in all assays. P-gp can affect the PK/PD of methadone, but not buprenorphine or diprenorphine. Our report is in favor of buprenorphine over methadone for management of opioid dependence. Buprenorphine most likely is not a P-gp substrate and concerns regarding P-gp-mediated drug-drug interaction are not expected.

Methadone: a new old drug with promises and pitfalls

Although methadone is not a new medication, its use in pain management has increased rapidly over the past decade. This article reviews the unique pharmacologic properties of methadone, including its long-acting nature, highly variable clearance rate, and its antagonism of the N-methyl-D-aspartate receptor. We discuss potential benefits and risks of methadone over other opioid medications. Preclinical studies suggest methadone may reduce abuse potential, tolerance development, and sensitization of nociceptive pathways. Pharmacologic properties of methadone suggest potential greater risk of dangerous or fatal side effects from overdose, QT interval prolongation, and drug interactions. However, clinical studies have yet to confirm that methadone produces either better clinical outcomes or higher rates of adverse events than other opioid analgesics. Clinicians who understand the special properties of methadone and follow recommended precautionary prescribing and monitoring practices can safely and effectively use methadone for pain treatment.

A systematic review and empirical analysis of the relation between dose and duration of drug action

There is a log-linear relation between the dose and duration of action of drugs with single-compartment pharmacokinetics and direct, reversible mechanisms of action. However, it has been suggested that this relation does not extend to drugs whose metabolites are active or slowly eliminated, drugs with saturable kinetics, and drugs with hit-and-run effects. The purpose of this study is to test this hypothesis and to quantify the relationship by way of a systematic review coupled to an empirical analysis. All issues of 4 clinical pharmacology journals from 1980 to 2005 are hand-searched for articles that present pharmacodynamic response versus time curves for 4 or more different doses. Data on duration of action, dose, and area under the plasma concentration versus time curve from zero to infinity (AUC) are abstracted and analyzed by panel data regression modeling, with within-study fixed effects. Duration of drug action is defined as the time during which a pharmacodynamic effect (or response) exceeds a nominal threshold. The generalized models of all observations from 33 publications, with duration of action as the dependent variable and the logarithm of the dose (or AUC) as the explanatory variable, yield significant log-linear relationships. The regressions for individual studies are correctly specified in 27 cases; there are insufficient data for analysis in 10 studies, and a log-linear specification is deemed inappropriate in 6. Analysis of published dose-ranging studies shows that the duration of action of a drug is directly proportional to the logarithm of dose across a wide range of different drugs, extending a result that was previously documented for very few compounds.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Interindividual variability of methadone response: impact of genetic polymorphism

Methadone, an opioid analgesic, is used clinically in pain therapy as well as for substitution therapy in opioid addiction. It has a large interindividual variability in response and a narrow therapeutic index. Genetic polymorphisms in genes coding for methadone-metabolizing enzymes, transporter proteins (p-glycoprotein; P-gp), and mu-opioid receptors may explain part of the observed interindividual variation in the pharmacokinetics and pharmacodynamics of methadone. Cytochrome P450 (CYP) 3A4 and 2B6 have been identified as the main CYP isoforms involved in methadone metabolism. Methadone is a P-gp substrate, and, although there are inconsistent reports, ABCB1 genetic polymorphisms also contribute slightly to the interindividual variability of methadone kinetics and influence dose requirements. Genetic polymorphism is the cause of high interindividual variability of methadone blood concentrations for a given dose; for example, in order to obtain methadone plasma concentrations of 250 ng/mL, doses of racemic methadone as low as 55 mg/day or as high as 921 mg/day can be required in a 70-kg patient without any co-medication. The clinician must be aware of the pharmacokinetic properties and pharmacological interactions of methadone in order to personalize methadone administration. In the future, pharmacogenetics, at a limited level, can also be expected to facilitate individualized methadone therapy.

Actual and potential drug interactions associated with methadone

OBJECTIVE

To identify and characterize methadone-related drug interactions, as well as factors accounting for the variability in manifesting these interactions clinically.

DESIGN

Systematic review of the primary literature.

METHODS

Over 200 articles, reports of clinical trials, and case reports were reviewed. Studies and case reports were included if they revealed either quantitative or qualitative methods to identify, evaluate severity of, or compare methadone-related drug interactions.

RESULTS OF DATA SYNTHESIS

The evidence base associated with methadone drug interactions is underdeveloped in general, as the majority of references found were case reports or case series. Most of the studies and reports focused on inpatients receiving methadone maintenance treatment (MMT) that were between 20 and 60 years of age, taking 200 mg/day of methadone or less. Evidence supporting the involvement of lesser known cytochrome P450 enzymes such as 2B6 is emerging, which may partially explain the inconsistencies previously found in studies looking specifically at 3A4 in vitro and in vivo. Genetic variability may play a role in the pharmacokinetics and pharmacodynamics of many medications, including methadone.

CONCLUSIONS

Drug interactions associated with methadone and their clinical significance are still poorly understood in general. Many tertiary drug information references and review articles report interactions associated with methadone in a general sense, much of which is theoretical and not verified by case reports, much less well-designed clinical trials. The majority of drug interaction reports that do exist were performed in the MMT population, which may differ significantly from chronic pain or cancer pain populations.

The effects of maternally administered methadone, buprenorphine and naltrexone on offspring: review of human and animal data

Most women using heroin are of reproductive age with major risks for their infants. We review clinical and experimental data on fetal, neonatal and postnatal complications associated with methadone, the current "gold standard", and compare these with more recent, but limited, data on developmental effects of buprenorphine, and naltrexone. Methadone is a micro-opioid receptor agonist and is commonly recommended for treatment of opioid dependence during pregnancy. However, it has undesired outcomes including neonatal abstinence syndrome (NAS). Animal studies also indicate detrimental effects on growth, behaviour, neuroanatomy and biochemistry, and increased perinatal mortality. Buprenorphine is a partial micro-opioid receptor agonist and a kappa-opioid receptor antagonist. Clinical observations suggest that buprenorphine during pregnancy is similar to methadone on developmental measures but is potentially superior in reducing the incidence and prognosis of NAS. However, small animal studies demonstrate that low doses of buprenorphine during pregnancy and lactation lead to changes in offspring behaviour, neuroanatomy and biochemistry. Naltrexone is a non-selective opioid receptor antagonist. Although data are limited, humans treated with oral or sustained-release implantable naltrexone suggest outcomes potentially superior to those with methadone or buprenorphine. However, animal studies using oral or injectable naltrexone have shown developmental changes following exposure during pregnancy and lactation, raising concerns about its use in humans. Animal studies using chronic exposure, equivalent to clinical depot formulations, are required to evaluate safety. While each treatment is likely to have maternal advantages and disadvantages, studies are urgently required to determine which is optimal for offspring in the short and long term.

Socio-demographic and clinical characteristics of 3129 heroin users in the first methadone maintenance treatment clinic in China

BACKGROUND

This study aimed to determine the characteristics of heroin users in the first methadone maintenance treatment (MMT) clinic in China.

METHODS

In a retrospective chart review, the notes of 3127 heroin users who received both detoxification and MMT at the clinic were analyzed. Their socio-demographic and clinical data were collected, and the frequency of human immunodeficiency virus (HIV), Hepatitis C virus (HCV), and syphilis infections was investigated.

RESULTS

The main findings are as follows: (1) 66.5% of the patients were younger than 35 years; (2) 55.1% were married at admission; (3) 32% were non-local residents; (4) the majority had high school level education, a history of smoking and alcohol consumption prior to the initial heroin use, and did not have stable jobs; (5) 28.4% were self-employed; (6) 5.4% of the sample had total hearing loss coupled with loss of speech; (7) 83.4% sniffed heroin or injected it intravenously at the time of admission, but 87.2% had smoked the drug when they first began abusing it; (8) a significant proportion of the patients were infected with HIV, HCV, and syphilis.

CONCLUSIONS

Some of the above findings are not consistent with the results of previous studies conducted in Western countries and China. The unique socio-cultural and clinical characteristics of heroin abusers in different regions of China should be considered when MMT services are planned.

Going beyond efficacy: strategies for cancer pain management

Despite great advances in the fields of pain management and palliative care, pain directly or indirectly associated with a cancer diagnosis remains significantly undertreated. The present paper reviews the current standard for cancer pain management and highlights new treatments and targeted interventional techniques.

Case report: quantification of methadone-induced respiratory depression using toxicokinetic/toxicodynamic relationships

Introduction

Methadone, the most widely delivered maintenance therapy for heroin addicts, may be responsible for life-threatening poisonings with respiratory depression. The toxicokinetics and the toxicokinetic/toxicodynamic (TK/TD) relationships of methadone enantiomers have been poorly investigated in acute poisonings. The aim of this study was to describe the relationships between methadone-related respiratory effects and their corresponding concentrations.

Methods

We report a 44-year-old methadone-maintained patient who ingested a 240-mg dose of methadone. He was found comatose with pinpoint pupils and respiratory depression. He was successfully treated with intravenous naloxone infusion over the course of 31 hours at a rate adapted to maintain normal consciousness and respiratory rate. We performed a TK/TD analysis of the naloxone infusion rate needed to maintain his respiratory rate at more than 12 breaths per minute (as toxicodynamics parameter) versus plasma R,S- and R-methadone concentrations (as toxicokinetics parameter), determined using an enantioselective high-performance liquid chromatography assay.

Results

Initial plasma R,S-methadone concentration was 1,204 ng/ml. Decrease in plasma R- and S-methadone concentrations was linear and demonstrated a first-order pharmacokinetics (maximal observed concentrations 566 and 637 ng/ml, half-lives 16.1 and 13.2 hours, respectively). TK/TD correlation between naloxone infusion rate and R,S- and R-methadone concentrations fitted well a sigmoidal E_max model (concentration associated with a half-maximum effect [EC₅₀] 334 and 173 ng/ml, Hill coefficient 10.0 and 7.8, respectively). In our chronically treated patient, EC₅₀ values were in the range of previously reported values regarding methadone analgesic effects, suggesting that plasma methadone concentrations to prevent withdrawal are lower than those associated with methadone analgesic effects.

Conclusion

After the ingestion of a toxic dose of a racemic mixture, plasma R- and S-enantiomer concentrations decreased in parallel. Despite large inter-individual variability in methadone toxicokinetics and toxicodynamics, TK/TD relationships would be helpful for providing quantitative data regarding the respiratory response to methadone in poisonings. However, further confirmatory TK/TD data are needed.

The effect of multiple doses of peginterferon alfa-2b on the steady-state pharmacokinetics of methadone in patients with chronic hepatitis C undergoing methadone maintenance therapy

This multicenter, open-label study evaluated the effects of multiple doses of peginterferon alfa-2b on the steadystate pharmacokinetics of methadone in 20 adults with hepatitis C virus infection who were enrolled in a methadone maintenance program. All subjects received peginterferon alfa-2b 1.5 mug/kg/wk for 4 weeks and maintained their normal methadone regimen. Serial blood samples were collected immediately before the first and after the fourth peginterferon alfa-2b dose (day 23). At day 23, exposure to the active methadone R-enantiomer increased by approximately 15% following administration of peginterferon alfa-2b, with 90% confidence intervals just outside the bioequivalence criteria (range, 80%-125%). Similar increases in exposure (C(max), AUC(0-24), and AUC(last)) were observed with S-methadone and total methadone. Peginterferon alfa-2b was well tolerated. Peginterferon alfa-2b is associated with minor increases in exposure to methadone in individuals with hepatitis C virus infection; however, these increases are unlikely to be clinically meaningful and are not associated with any safety concerns.

Modeling methadone pharmacokinetics in rats in presence of P-glycoprotein inhibitor valspodar

PURPOSE

To quantify the in vivo role of P-glycoprotein (P-gp) in the pharmacokinetics of methadone after intravenous and oral administration, using valspodar as a P-gp inhibitor.

MATERIALS AND METHODS

Methadone plasma concentrations after intravenous (0.35 mg/kg) and oral (6 mg/kg) administration were analyzed, in absence and presence of valspodar, using nonlinear mixed effects modeling (NONMEM V). Non-parametric bootstrap analysis and posterior predictive check were employed as model evaluation techniques.

RESULTS

The pharmacokinetics of methadone in the rat was successfully modeled using a two-compartmental model with a linear elimination from the central compartment and a first-order absorption process with lag time. Valspodar increased methadone F by 122% (95%CI: 34-269%) and decreased the V ( c ) and V ( p ) by 35% (95%CI: 16-49%) and 81% (95%CI: 63-93%), respectively. No effect of valspodar on other pharmacokinetic parameters was discernible. The non-parametric bootstrap analysis confirmed the absence of bias on the parameter estimates, and visual predictive check evidence the adequacy of the model to reproduce the observed time course of methadone plasma concentrations.

CONCLUSION

Valspodar increased methadone's bioavailability as consequence of P-gp inhibition, which resulted in an increased analgesic effect of methadone.

Blockade of morphine-induced behavioral sensitization by a combination of amisulpride and RB101, comparison with classical opioid maintenance treatments

BACKGROUND AND PURPOSE

Maintenance treatments with methadone or buprenorphine are more or less efficient procedures for helping heroin addicts to stop or reduce drug abuse. Another approach to treat opiate dependence could be to target the endogenous opioid system by enhancing the effects of enkephalins by protecting them from enzymic degradation by the dual peptidase inhibitor RB101.

EXPERIMENTAL APPROACH

As chronic treatment with the dopamine D2 antagonist amisulpride facilitates RB101-induced behavioral effects, we chose in this study to treat mice previously sensitized to the hyperlocomotor effects induced by morphine with a combination of amisulpride and RB101.

KEY RESULTS

Expression of morphine-induced locomotor sensitization was abolished after combined treatment with amisulpride (20 mg x kg(-1), i.p.) and RB101 (80 mg x kg(-1), i.p.), whereas these drugs were not effective when used alone. We then compared these results with the effects of amisulpride combined with buprenorphine (0.1 mg x kg(-1), i.p.) or methadone (2.5 mg x kg(-1), i.p.) upon morphine-induced behavioral sensitization. Whereas the combination of amisulpride and buprenorphine partially blocked the expression of morphine sensitization, amisulpride+methadone was not effective in this paradigm.

CONCLUSIONS AND IMPLICATIONS

The combination of amisulpride+RB101 appears to be very efficient in blocking the expression of morphine-induced behavioral sensitization. This could reflect a reinstatement of a balance between the function of the dopamine and opioid systems and could represent a new approach in maintenance treatments for opiate addiction.

Interpretation of oral fluid tests for drugs of abuse

Oral fluid testing for drugs of abuse offers significant advantages over urine as a test matrix. Collection can be performed under direct observation with reduced risk of adulteration and substitution. Drugs generally appear in oral fluid by passive diffusion from blood, but also may be deposited in the oral cavity during oral, smoked, and intranasal administration. Drug metabolites also can be detected in oral fluid. Unlike urine testing, there may be a close correspondence between drug and metabolite concentrations in oral fluid and in blood. Interpretation of oral fluid results for drugs of abuse should be an iterative process whereby one considers the test results in the context of program requirements and a broad scientific knowledge of the many factors involved in determining test outcome. This review delineates many of the chemical and metabolic processes involved in the disposition of drugs and metabolites in oral fluid that are important to the appropriate interpretation of oral fluid tests. Chemical, metabolic, kinetic, and analytic parameters are summarized for selected drugs of abuse, and general guidelines are offered for understanding the significance of oral fluid tests.

Long term methadone for chronic pain: a pilot study of pharmacokinetic aspects

Methadone is used as an alternative opioid when first line opioids fail to provide adequate pain control. Highly variable morphine:methadone dose ratios make switching challenging and little is known about the pharmacokinetics of long lasting methadone treatment for pain. Twelve patients treated with morphine for chronic non-malignant pain were switched to methadone. Seven of these patients continued with methadone throughout the nine months study period and only minor dose adjustments were performed. Serum concentrations of morphine, methadone and their metabolites were measured at baseline, day one and two, after dose titration and one week, five weeks, three months and nine months after the end of dose titration. Serum concentrations of methadone and its metabolite EDDP did not change significantly from the end of dose titration and during the nine months (repeated measures ANOVA: p=0.88 and p=0.06). Very low correlation between dose ratios and serum concentration ratios between morphine and methadone was observed. Large interindividual differences in serum concentrations and metabolism were observed. Our findings contradict that autoinduction of methadone metabolism takes place during long term treatment and supports that a 3-day opioid switch from morphine to methadone followed by a one week titration seems pharmacologically sound.

Méthadone : de la pharmacocinétique à la pharmacologie clinique

In January 2002, the official French methadone legislation prescription was modified. Thus, the number of clinicians authorized to introduce methadone substitution was increased. Knowledge of the pharmacokinetic and pharmacological properties of this compound remains particularly important for its appropriate prescription. Bearing this in mind, we linked methadone pharmacokinetics to its pharmacological use in this article. METHADONE PHARMACOLOGY: Methadone is a synthetic opiate. Its mean bioavailability is around 75%. Cytochrome P450 3A4 and 2D6 are involved in its hepatic metabolism. Its volume of distribution is of around 4 L/kg. The value of half-life elimination is of around 22 hours. These pharmacokinetic properties (long half-life, steady state concentration) are in favour of substitution use of this opiate. In practice, clinicians progressively introduce this substitution therapy to reach 80 mg +/- 20 mg per day, once daily. Therapeutic clinical goals are mainly to reduce craving, withdrawal symptoms, and to manage psychosocial problems and psychiatric co-morbidity. Practitioners should bear the latter in mind once substitution therapy has been appropriately initiated and stabilized. However, wide, interpatient, interindividual variability impacts on pharmacokinetic parameters. Subjects may be either high or poor metabolizers. Thus, bioavibility ranges from 36 to 100%. Induction or inhibition of CYP450 significantly modifies methadone pharmacodynamic properties. Genetic variability and medication can induce non response to substitution, craving, or withdrawal symptoms. PHARMACOLOGICAL INTERACTIONS: We describe here a large number of medications involved in pharmacokinetic or pharmacological interactions. Classical enzymatic inductors, such as antiepileptic molecules (phenobarbital, carbamazepin), antituberculosis compounds (rifampicin), or antiretroviral therapy (efavirenz, nevirapin, ritonavir), could possibly lead to respiratory depression for example. Metabolism inhibitors such as selective serotonin reuptake inhibitors (fluvoxamine, fluoxetine, paroxetine, sertraline) or antifungals of the azol groups could enhance plasma concentration and may sometimes lead to respiratory depression or death. Nevertheless, clinicians should know methadone pharmacokinetic properties and pharmacological interactions for the optimal opiate-dependant patients' management.

CLINICAL USE

Clinicians can use plasma concentrations as a useful indicator to reach substitution goals. The methadone plasmatic target value of 400 microg/ml can be recommended for therapeutic drug monitoring. This dosage not only facilitates interaction detection, but also hand encourages communication with the patient.

Methadone for treatment of cancer pain

Methadone is a unique mu opioid agonist, which also has delta receptor affinity and properties of N-methyl-D-aspartate receptor antagonism and monoamine reuptake inhibition. It is mainly used in the setting of uncontrolled pain or dose-limiting toxicity. Caution is advised when switching to methadone, especially from high doses of previous opioid, due to its variable conversion ratio and the potential for delayed toxicity due to its long half-life. Increasing evidence of risk also exists for a prolonged QT interval and torsades de pointes with very large doses of methadone. Methadone is likely safer when used at lower doses as a first-line opioid, but its potential as such has not received enough formal evaluation. Randomized controlled trials are needed to assess the effectiveness and safety of methadone compared with other opioids and to further evaluate its role in the treatment of neuropathic pain.

Population-based analysis of methadone distribution and metabolism using an age-dependent physiologically based pharmacokinetic model

Limited pharmacokinetic (PK) and pharmacodynamic (PD) data are available to use in methadone dosing recommendations in pediatric patients for either opioid abstinence or analgesia. Considering the extreme inter-individual variability of absorption and metabolism of methadone, population-based PK would be useful to provide insight into the relationship between dose, blood concentrations, and clinical effects of methadone. To address this need, an age-dependent physiologically based pharmacokinetic (PBPK) model has been constructed to systematically study methadone metabolism and PK. The model will facilitate the design of cost-effective studies that will evaluate methadone PK and PD relationships, and may be useful to guide methadone dosing in children. The PBPK model, which includes whole-body multi-organ distribution, plasma protein binding, metabolism, and clearance, is parameterized based on a database of pediatric PK parameters and data collected from clinical experiments. The model is further tailored and verified based on PK data from individual adults, then scaled appropriately to apply to children aged 0-24 months. Based on measured variability in CYP3A enzyme expression levels and plasma orosomucoid (ORM2) concentrations, a Monte-Carlo-based simulation of methadone kinetics in a pediatric population was performed. The simulation predicts extreme variability in plasma concentrations and clearance kinetics for methadone in the pediatric population, based on standard dosing protocols. In addition, it is shown that when doses are designed for individuals based on prior protein expression information, inter-individual variability in methadone kinetics may be greatly reduced.

The Acute Disposition of (R)- and (S)-Methadone in Brain and Lung of Sheep

The cerebral and lung kinetics of the enantiomers of methadone were quantified using a conscious chronically instrumented sheep preparation, as these organs are the major organs governing the peak brain concentrations (and therefore effects) of methadone after ivbolus administration. Seven sheep were administered intravenous infusions of rac-methadone (30 mg over 4 min). Whole blood (R)- and (S)-methadone concentrations were measured using stereoselective HPLC. Methadone transiently increased cardiac output (CO) and mean arterial pressure, but did not alter cerebral blood flow (CBF) or cause significant respiratory depression. Using physiologically based kinetic models, cerebral kinetics were inferred from arterio-sagittal sinus concentration gradients and CBF, lung kinetics from pulmonary artery-aortic gradient and CO. Lung and cerebral kinetics were best described by a partially membrane-limited model for both enantiomers. Lung kinetics displayed clear stereoselectivity, due to the smaller apparent volume of the deep lung compartment for (R)-methadone (45 l) compared to (S)-methadone (79 l). This resulted in systemic differences in the concentrations of the enantiomers. Minimal stereoselectivity was observed in cerebral kinetics. The brain:blood equilibration of methadone was slow (half-life of 18 min) due to intermediate permeability and large apparent cerebral distribution volumes. However, the permeability term was sufficiently high that cerebral kinetics were affected by CBF. Simulations demonstrated that if CBF was doubled, the equilibration half-life of methadone with brain tissue decreased by 30%, and there was a 25% increase in the peak brain concentrations. Future studies are needed to confirm the role of cerebral blood flow alterations in the exposure of the brain to methadone, especially in the case of respiratory depression. In conclusion, pharmacokinetic modelling of methadone confirmed a large equilibration delay between brain and blood.

Review article: Perioperative pain management of patients on methadone therapy

PURPOSE

Methadone, an opioid traditionally associated with the management of opioid addictive disorders, has been prescribed increasingly as an analgesic for the management of various chronic pain conditions. Despite the increasing popularity of methadone, most anesthesiologists are not familiar with its complex pharmacology. The purpose of this article is to review the pharmacology of methadone and to suggest a management algorithm for the perioperative care of methadone patients.

SOURCE

A Medline search was performed to obtain the published literature on the pharmacology of methadone and its use perioperatively.

PRINCIPAL FINDINGS

The complexity of methadone's pharmacology is characterized by a high inter-individual variability, a potential for interaction with other medications, and a long elimination half-life. The postoperative management of methadone patients may be difficult as they are often 'opioid-tolerant' but may be 'pain-intolerant'. For those patients who are taking part in methadone-maintenance programs, there is a potential for the problematic use of opioids or other substances. The management plan for patients taking methadone may differ depending on the type of surgery and the associated perioperative differences in fasting status and gastrointestinal function. In consideration of all the factors listed above, a management algorithm is outlined for the perioperative care of methadone patients.

CONCLUSION

Methadone is an opioid with complex properties, and a patient that is taking methadone can represent a unique challenge to the anesthesiologist. A good understanding of the pharmacology of methadone and of the type of patients on this medication will help to improve their perioperative care.

Pharmacologic management of relapse prevention in addictive disorders

Substance use disorders are an important public health problem associated with significant mortality and morbidity. Effective maintenance pharmacotherapies are available for tobacco, alcohol, and opioid use disorders. For optimum treatment response, these medications should be used in conjunction with behavioral interventions. For other drugs of abuse, especially for cocaine, medication development is an active area of research. Further research is needed to develop new pharmacotherapies for substance use disorders and establish clinical guidelines on how to use these medications most effectively.

Opioid receptors

Opioid receptors belong to the large superfamily of seven transmembrane-spanning (7TM) G protein-coupled receptors (GPCRs). As a class, GPCRs are of fundamental physiological importance mediating the actions of the majority of known neurotransmitters and hormones. Opioid receptors are particularly intriguing members of this receptor family. They are activated both by endogenously produced opioid peptides and by exogenously administered opiate compounds, some of which are not only among the most effective analgesics known but also highly addictive drugs of abuse. A fundamental question in addiction biology is why exogenous opioid drugs, such as morphine and heroin, have a high liability for inducing tolerance, dependence, and addiction. This review focuses on many aspects of opioid receptors with the aim of gaining a greater insight into mechanisms of opioid tolerance and dependence.

Compliance with methadone-based substitutive treatment: a proposed model based on immunoassay urinary sample screening

Methadone (MTD) maintenance treatment is a recognized method to reduce illicit opiate abuse. Because of the difficulties of collecting 24-hour urines routinely, the monitoring of MTD compliance is currently done with random urinary screening. However, monitoring of MTD compliance by random urinary screening lacks accuracy because of its highly variable pharmacokinetics, leading to false positive or negative results. This study's objective was to identify factors influencing the reliability of urinary screening of methadone for MTD compliance monitoring in a field setting involving usual care for opiate-dependent patients. In a cross-sectional population-based study, 1981 urine samples obtained from 68 patients in parallel with drug dose, gender, and weight were analyzed by MTD enzyme immunoassay (EMIT). Urinary pH was measured, and positive threshold was determined experimentally by box-plot analysis. Multivariate determinants of MTD excretion were established with stepwise multiple regression analysis. On this basis, adjusted values for MTD excretion were proposed and verified with an (S)-2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) assay from independent urine samples that were negative or doubtful by the MTD assay. MTD excretion was higher in men, decreased with increased urinary pH, and increased with daily dosage of MTD; these factors explain 32% of the total variance of urinary MTD. Adjustment on these 3 variables (urinary pH, sex, daily dosage) improved the prediction of compliance to MTD treatment. Threshold was stable across pH values and in agreement with EDDP results. The influence of simple variables such as gender, urinary pH, and daily dosage on urinary MTD excretion could be put in evidence and accounted for. Adjusted values of urinary MTD are more reliable than the raw values for monitoring compliance to MTD treatment.

The effect of quinidine, used as a probe for the involvement of P-glycoprotein, on the intestinal absorption and pharmacodynamics of methadone

AIMS

There is considerable unexplained interindividual variability in the methadone dose-effect relationship. The efflux pump P-glycoprotein (P-gp) regulates brain access and intestinal absorption of many drugs. Evidence suggests that methadone is a P-gp substrate in vitro, and P-gp affects methadone analgesia in animals. However the role of P-gp in human methadone disposition and pharmacodynamics is unknown. This investigation tested the hypothesis that the intestinal absorption and pharmacodynamics of oral and intravenous methadone are greater after inhibition of intestinal and brain P-gp, using the P-gp inhibitor quinidine as an in vivo probe.

METHODS

Two randomized, double-blind, placebo-controlled, balanced crossover studies were conducted in healthy subjects. Pupil diameters and/or plasma concentrations of methadone and the primary metabolite EDDP were measured after 10 mg intravenous or oral methadone HCl, dosed 1 h after oral quinidine (600 mg) or placebo.

RESULTS

Quinidine did not alter the effects of intravenous methadone. Miosis t(max) (0.3 +/- 0.3 vs 0.3 +/- 0.2 h (-0.17, 0.22)), peak (5.3 +/- 0.8 vs 5.1 +/- 1.0 mm (0.39, 0.84)) and AUC vs time (25.0 +/- 5.7 vs 26.8 +/- 7.1 mm h (-6.1, 2.5)) were unchanged (placebo vs quinidine (95% confidence interval on the difference)). Quinidine increased (P < 0.05) plasma methadone concentrations during the absorptive phase, decreased t(max) (2.4 +/- 0.7 vs 1.6 +/- 0.9 h (0.33, 1.2)), and increased peak miosis (3.2 +/- 1.5 vs 4.3 +/- 1.6 mm (-1.96, -0.19)) after oral methadone. The C(max) (55.6 +/- 10.3 vs 59.4 +/- 14.1 ng ml(-1) (-8.5, 0.65)) and AUC of methadone (298 +/- 46 vs 316 +/- 74 ng ml(-1) h (-54, 18)) were unchanged, as were the EDDP : methadone AUC ratios. Quinidine had no effect on the rate constant for transfer of methadone between plasma and effect compartment (k(e0)) (2.6 +/- 2.6 vs 2.5 +/- 1.4 h(-1) (-3.5, 4.2)).

CONCLUSIONS

Quinidine increased the plasma concentrations of oral methadone in the absorptive phase and the miosis caused by methadone, suggesting that intestinal P-gp affects oral methadone absorption and hence its clinical effects. Quinidine had no effect on methadone pharmacodynamics after intravenous administration, suggesting that if quinidine is an effective inhibitor of brain P-gp, then P-gp does not appear to be a determinant of the access of methadone to the brain.

Brain penetration of methadone (R)- and (S)-enantiomers is greatly increased by P-glycoprotein deficiency in the blood-brain barrier of Abcb1a gene knockout mice

RATIONALE

Methadone maintenance treatment is complicated by the wide variability of efficacy among patients. The large interindividual variability of the plasma concentrations of methadone was previously thought to be responsible for the variable therapeutic efficacy. However, recent studies suggested that methadone may be a substrate of P-glycoprotein (P-gp). Therefore, the function of P-gp in blood-brain barrier (BBB) may affect the concentration of methadone at its site(s) of action in the central nervous system, thereby contributing to its therapeutic efficacy and/or adverse events.

OBJECTIVE

To investigate the effect of P-gp on brain penetration of methadone (R)- and (S)-enantiomers and their major oxidative metabolite 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP).

METHODS

We compared the tissue distribution of methadone (R)- and (S)-enantiomers and EDDP in the Abcb1a-/- gene knockout mice and the Abcb1a+/+ wild-type mice 1 h following intraperitoneal administration of 15 microg Rac-methadone/g mouse.

RESULTS

Plasma concentrations of (R)- and (S)-methadone were similar between the two animal groups. However, the brain concentrations of (R)- and (S)-methadone in the Abcb1a-/- mice were markedly higher (15- and 23-fold, respectively, P<0.0001) than those of the Abcb1a+/+ wild-type mice. No statistically significant difference was found for other organs between the mutants and controls. No organ difference was found for EDDP between the mutants and controls.

CONCLUSIONS

(R)- and (S)-methadone are substrates of P-gp. The P-gp in BBB greatly limits the brain entry of (R)- and (S)-methadone to their central nervous system acting sites. The interindividual variation in expression of P-gp in BBB may represent a source of variation for the access and effects of methadone in the brain.

Methadone implants for methadone maintenance treatment. In vitro and in vivo animal studies

Methadone implant formulations elaborated with polylactide-co-glycolide (PLGA) and polylactic acid (PLA) for 1 week and 1 month release duration, respectively, were evaluated in vitro and in vivo. One-week implants prepared with methadone clorhydrate, methadone clorhydrate/methadone base blend or methadone base were tested in vitro. Results showed that the methadone release rate decreased as the methadone base increased. The best release profile was achieve when the methadone base implants, made by compression of a 50:50 PLGA (12 kDa) and methadone base mix, were coated with PLA (30 kDa). For 1-month implants, the methadone base load was increased to 65% and PLA of 30 kDa was used as a matrix component. In this case the implants were coated with the same polymer. Deconvolution methods could not be used for in vivo release estimation because an increase in methadone clearance was observed with methadone clorhydrate solution multiple-dose treatment. Therefore the amount of drug remaining within the implants was evaluated and the deconvolution was only used to establish the release profile range. The upper limit was estimated applying the absorption-disposition function obtained after multiple-dose administrations while the lower curve was estimated using the single-dose function. Methadone serum levels were maintained around 200 ng/ml during 1 week and approximately 5 weeks with the optimised implants. In vivo-in vitro correlations were always very good with slopes near 1.

Addiction, physical dependence, and tolerance: precise definitions to help clinicians evaluate and treat chronic pain patients

Pain is among the most common complaints for which people seek medical care; yet pain is also among the most undertreated patient complaints. Reasons for this include reluctance by clinicians to prescribe and support the use of opioids, often due to a fear of addiction. To address this issue, three major health professional organizations that deal with the treatment of pain and addiction, the American Academy of Pain Medicine, the American Pain Society, and the American Society of Addiction Medicine, formed the Liaison Committee on Pain and Addiction (LCPA). The first mission of the LCPA was to formulate precise definitions of the terms addiction, physical dependence, and tolerance. This report explains these definitions and discusses how they apply to clinical practice.

Methadone and the hospice patient: prescribing trends in the home-care setting

OBJECTIVE

To identify frequency and utilization patterns of methadone by hospice patients in the home-care setting.

PATIENTS AND SETTING

All hospice patients admitted to a North American palliative care specialty pharmacy and dispensed methadone from November 1, 2001 to October 31, 2002 were analyzed. We also analyzed all hospice patients dispensed long-acting opioids during that same time period.

DESIGN

A retrospective analysis of the pharmacy database was performed for patients dispensed methadone. Data was compared to the long acting opioid cohort to be able to identify any difference in terminal diagnoses present, and the presence of neuropathic pain in both groups. Methadone daily dosage was also analyzed during this study.

RESULTS

Four hundred sixteen hospice patients were dispensed methadone over a twelve-month period of time. For comparison, 21,219 patients were prescribed a long-acting opioid preparation (sustained-release morphine, sustained-release oxycodone, or transdermal fentanyl). The most common terminal diagnosis for both groups was lung carcinoma. The distribution of terminal diagnoses was similar in both groups. The group prescribed methadone was found to have a higher incidence of neuropathic pain (30.5% of patients) when compared to the long-acting opioid group (16.9%). Most patients (61.3%) were prescribed daily methadone doses of 100 mg or less.

CONCLUSIONS

Despite its potential clinical and economic benefits, methadone is not commonly prescribed for the hospice patient in the home-care setting. Clinicians may be more aware of the usefulness of methadone in the treatment of neuropathic pain.

Chronic pelvic pain: an integrated approach to diagnosis and treatment

Chronic pelvic pain affects upward of 15% of women and is a frustrating condition for both patients and physicians. Chronic pelvic pain is not a disease, but a syndrome that results from a complex interaction between neurologic, musculoskeletal, and endocrine systems that is further influenced by behavioral and psychologic factors. Traditional approaches to this disorder have been surgical, although long-term success rates have been disappointing. Placebo response to surgery is common, and many conditions that contribute to the pain cannot be identified or treated with a surgical approach. Many patients will require a combination of both pharmacologic and nonpharmacologic treatments in addition to various types of invasive procedures. It is now recognized that many disorders contribute to the chronic pelvic pain symptom complex; thus, an integrated multidisciplinary approach to diagnosis and treatment is essential to achieve the greatest success.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians.

LEARNING OBJECTIVES

After completion of this article, the reader will be able to describe the pathophysiology of chronic pelvic pain, to outline the evaluation of a patient with chronic pelvic pain, and to explain the treatment options for patients with chronic pelvic pain.

Incorporating the assessment of abuse liability into the drug discovery and development process

Evaluation of abuse liability is one of many obligations incurred by industrial sponsors in the development of medications acting on substrates in the central nervous system. In addition to providing the information necessary for a scheduling recommendation in the marketing application, the abuse liability assessment allows sponsors to estimate safety and commercial risks associated with scheduling, as well as to tailor their pre- and post-approval programs to collect information relevant to product misuse, illicit diversion and physical dependence. There are several important factors to consider before embarking on an abuse liability assessment, including the compound's primary and secondary biochemical activities, its absorption and metabolism, its final formulation, and its intended clinical population. Each of these factors will temper the timing and extent of the abuse liability program in animals and humans. Although every drug development program is unique in some way, a decision-making process may be applied to abuse liability assessment that will serve to better utilize limited resources and inform decisions regarding subsequent steps in the process. The emerging properties of the product will define the unique procedures best applied to assess it.

Involvement of CYP3A4, CYP2C8, and CYP2D6 in the metabolism of (R)- and (S)-methadone in vitro

To clarify the oxidative metabolism of methadone (R)- and (S)-enantiomers, the depletion of parent (R)- and (S)-methadone and the formation of racemic 2-ethylidene-1,5-dimethyl-3,3-diphe-nylpyrolidine were studied using human liver microsomes and recombinant cytochrome P450 enzymes. Based on studies with isoform-selective chemical inhibitors and expressed enzymes, CYP3A4 was the predominant enzyme involved in the metabolism of (R)-methadone. However, it has different stereoselectivity toward (R)- and (S)-methadone. In recombinant CYP3A4, the metabolic clearance of (R)-methadone was about 4-fold higher than that of (S)-methadone. CYP2C8 is also involved in the metabolism of methadone, but its contribution to the metabolism of (R)-methadone was smaller than that of CYP3A4. But for the metabolism of (S)-methadone, the roles of CYP2C8 and CYP3A4 appeared equal. Although CYP2D6 is involved in the metabolism of (R)- and (S)-methadone, its role was smaller compared with CYP3A4 and CYP2C8. Using clinically relevant concentrations of ketoconazole (1 microM, selective CYP3A4 inhibitor), trimethoprim (100 microM, selective CYP2C8 inhibitor), and paroxetine (5 microM, potent CYP2D6 inhibitor), these inhibitors decreased the hepatic metabolism of (R)-[(S)-]methadone by 69% (47%), 22% (51%), and 41% (77%), respectively. However, inhibition of the metabolism of (R)- and (S)-methadone by paroxetine was due to inhibition not only of CYP2D6, but also CYP3A4 and, to a minor extent, CYP2C8. The present in vitro findings indicated that CYP3A4, CYP2C8, and CYP2D6 are all involved in the stereoselective metabolism of methadone (R)- and (S)-enantiomers. These data suggest that coadministration of inhibitors of CYP3A4 and CYP2C8 may produce clinically significant drug-drug interactions with methadone.

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.

An ad libitum schedule for conversion of morphine to methadone in advanced cancer patients: an open uncontrolled prospective study in a Chinese population

Methadone has been used as an alternative strong opioid to morphine in the management of cancer pain. The conversion of morphine to methadone is not straightforward because of the high individual variability and unpredictability in the pharmacokinetics of methadone. An ad libitum schedule for conversion of morphine to methadone was used in 37 cancer patients who had intolerable morphine-related side effects or had pain not satisfactorily controlled by morphine. Oral morphine was discontinued on the day of conversion. Methadone was given at a dose calculated as one-twelfth of the total daily dose of morphine, up to a maximum of 30 mg/dose. Methadone was administered at patient-controlled intervals not more frequent than three hours, the need of which was indicated by the presence of pain of moderate intensity or above as rated by a verbal rating scale. When the demand for methadone was stabilized, the total daily dose was given regularly in divided doses. Pain control on day 7 was taken as the primary endpoint. Twenty-seven patients completed the study. Twenty-four patients (88.9%) were in good pain control on day 7, and all reached good pain control by day 11. The median time required to achieve good pain control was three days (range 1-11 days). A majority (88.6%) of morphine-related adverse effects improved or resolved after conversion to methadone. This ad libitum schedule is effective in conversion of morphine to methadone in these patients.

Methadone Reincarnated: Novel Clinical Applications with Related Concerns

Methadone has numerous advantages as an analgesic, which have supported its recent increase in use. However, methadone also has a pharmacological profile as an opioid that differentiates it from other, better known or more widely used opioids. It also has unusual pharmacodynamics, pharmacokinetics, and metabolism that must be considered for safe use of methadone as an analgesic. This review looks at the history of methadone use as an analgesic and its properties that distinguish it as an unusual, and potentially, unstable opioid.

Prescribing to drug misusers in practice--often effective, but rarely straightforward

Many reviews describe the effectiveness of methadone treatment in reducing illicit drug use and associated behaviours among opiate misusers. The strongest evidence includes social outcomes such as reduced debt and crime, and relates overwhelmingly to maintenance rather than detoxification treatment. Drug clinics are often dominated by individuals unable to withdraw fully from methadone, while the "harm reduction" model accepts some ongoing drug use, with attendant risks. Security measures are necessary to avoid abuse of treatments, but these may be undermined by the agenda of "partnerships with patients" in decision-making. Buprenorphine appears both safer and less addictive than methadone, and lofexidine is effective as a non-substitute detoxification method. Naltrexone can clearly reduce relapse rates, provided consumption is assured, while for individuals unable to detoxify or avoid euphoriant opiates, morphine and diamorphine are sometimes used. In non-opiate misuse, clinical studies of a wide range of medications have produced relatively few positive findings.

Toxicologic aspects of heroin substitution treatment

Heroin abuse is an international problem with which all countries must continually cope. Many countries have implemented heroin substitution therapy as an effective means of decreasing illicit heroin use, crime, HIV risk, and death, and in improving employment and social adjustment. Although methadone is the most commonly used medication for heroin substitution, other agonists in current use include levomethadyl acetate (LAAM), buprenorphine, and pharmaceutical-grade heroin. This report reviews toxicologic issues that arise in these programs. A broad array of testing methodologies are available that allow selection of on-site testing or laboratory-based methodology. Urine specimens may be monitored for nonprescribed drugs on a qualitative or semiquantitative basis. Methods for differentiating opiate sources by urinalysis have been proposed to distinguish poppy seed consumption from heroin abuse and for distinguishing pharmaceutical-grade heroin from illicit heroin. Therapeutic drug monitoring for methadone in plasma continues to be evaluated for use in establishing adequate dosing and detecting diversion, and new methods have been devised for measurement of the optical isomers of methadone in plasma. Biologic specimens, in addition to plasma and urine, have been evaluated for use in drug monitoring, including sweat, hair, and oral fluid, with promising results. Overall, the many recent developments in testing methodology provide more effective means to assess patients in heroin substitution programs and should contribute to improvements in public health.