Interaction of methadone with substrates of human hepatic cytochrome P450 3A4

Combined in vivo metabolic effects of quetiapine and methadone in brain and blood of rats

Changes in pharmacokinetics and endogenous metabolites may underlie additive biological effects of concomitant use of antipsychotics and opioids. In this study, we employed untargeted metabolomics analysis and targeted analysis to examine the changes in drug metabolites and endogenous metabolites in the prefrontal cortex (PFC), midbrain, and blood of rats following acute co-administration of quetiapine and methadone. Rats were divided into four groups and received cumulative increasing doses of quetiapine (QTP), methadone (MTD), quetiapine + methadone (QTP + MTD), or vehicle (control). All samples were analyzed using liquid chromatography-mass spectrometry (LC-MS). Our findings revealed increased levels of the quetiapine metabolites: Norquetiapine, O-dealkylquetiapine, 7-hydroxyquetiapine, and quetiapine sulfoxide, in the blood and brain when methadone was present. Our study also demonstrated a decrease in methadone and its metabolite 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) in the rat brain when quetiapine was present. Despite these findings, there were only small differences in the levels of 225-296 measured endogenous metabolites due to co-administration compared to single administrations. For example, N-methylglutamic acid, glutaric acid, p-hydroxyphenyllactic acid, and corticosterone levels were significantly decreased in the brain of rats treated with both compounds. Accumulation of serotonin in the midbrain was additionally observed in the MTD group, but not in the QTP + MTD group. In conclusion, this study in rats suggests a few but important additive metabolic effects when quetiapine and methadone are co-administered.

Drug-Drug Interaction Between Oxycodone and Diazepam by a Combined in Silico Pharmacokinetic and Pharmacodynamic Modeling Approach

Opioids and benzodiazepines have complex drug-drug interactions (DDIs), which serve as an important source of adverse drug effects. In this work, we predicted the DDI between oxycodone (OXY) and diazepam (DZP) in the human body by applying in silico pharmacokinetic (PK) and pharmacodynamic (PD) modeling and simulation. First, we studied the PK interaction between OXY and DZP with a physiologically based pharmacokinetic (PBPK) model. Second, we applied molecular modeling techniques including molecular docking, molecular dynamics (MD) simulation, and the molecular mechanics/Poisson-Boltzmann surface area (MM-PBSA) free energy method to predict the PD-DDI between these two drugs. The PK interaction between OXY and DZP predicted by the PBPK model was not obvious. No significant interaction was observed between the two drugs at normal doses, though very high doses of DZP demonstrated a non-negligible inhibitory effect on OXY metabolism. On the contrary, the molecular modeling study shows that DZP has potential to compete with OXY at the same binding pocket of the active μ-opioid receptor (MOR) and κ-opioid receptor (KOR). MD simulation and MM-PBSA calculation results demonstrated that there is likely a synergetic effect between OXY and DZP binding to opioid receptors, as OXY is likely to target the active MOR while DZP selectively binds to the active KOR. Thus, pharmacokinetics contributes slightly to the DDI between OXY and DZP although an overdose of DZP has been brought to attention. Pharmacodynamics is likely to play a more important role than pharmacokinetics in revealing the mechanism of DDI between OXY and DZP.

Prediction of Drug-Drug Interactions Between Opioids and Overdosed Benzodiazepines Using Physiologically Based Pharmacokinetic (PBPK) Modeling and Simulation

Background

Researchers have long been interested in the potential drug–drug interactions (DDIs) between opioids and benzodiazepines. However, much remains unknown concerning the interactions between these two drug classes. The objective of this work is to study the mechanism underlying the DDIs between opioids and benzodiazepines from the perspective of their pharmacokinetic (PK) interactions. A PK interaction occurs when two drugs are metabolized by the same cytochrome P450 enzymes and is one of the most common reasons for DDIs.

Methods

We quantitatively predicted the DDIs between three opioids (fentanyl, oxycodone and buprenorphine) and four benzodiazepines (alprazolam, diazepam, midazolam and triazolam) using a physiologically based pharmacokinetic (PBPK) modeling approach. A set of PBPK models was first constructed for these common opioids and benzodiazepines using SimCYP software, and the DDIs between them were then explored at various dosages.

Results

Our simulation results suggested there were no PK interactions between normal doses of opioids and benzodiazepines; but weak interactions can be expected with the combination of opioids and overdosed benzodiazepines. Particular attention should be given to the combination of fentanyl and overdosed alprazolam since a PK interaction can be observed between them.

Conclusion

Our results appear to indicate that pharmacodynamics may play a more important role than PKs in causing DDIs between opioids and benzodiazepines. This study also demonstrated that molecular modeling can be a very useful tool to mitigate the problem of “missing metabolic reaction parameters” in PK modeling and simulation.

Electronic supplementary material

The online version of this article (10.1007/s40268-019-00282-3) contains supplementary material, which is available to authorized users.

Current Concepts in Methadone Metabolism and Transport

Methadone is a cornerstone therapy for opioid addiction and a public health strategy for HIV/AIDS and hepatitis C reduction. Methadone is also used for acute and chronic pain. As use for chronic pain has grown, so too have adverse events. Constitutive and acquired (drug interactions) inter- and intraindividual variability in methadone pharmacokinetics and pharmacodynamics confounds reliable clinical use. Identification of enzymes and transporters responsible for methadone disposition has been a long-sought ideal. Initial in vitro studies identified CYP3A4 as metabolizing methadone. Subsequently, by extrapolation, CYP3A4 was long assumed to be responsible for clinical methadone disposition. However, CYP2B6 is also a major catalyst of methadone metabolism in vitro. It has now been unequivocally established that CYP2B6, not CYP3A4, is the principal determinant of methadone metabolism, clearance, elimination, and plasma concentrations in humans. Methadone disposition is susceptible to inductive and inhibitory drug interactions. CYP2B6 genetics also influences methadone metabolism and clearance, which were diminished in CYP2B6*6 carriers and increased in CYP2B6*4 carriers. CYP2B6 genetics can explain, in part, interindividual variability in methadone metabolism and clearance. Thus, both constitutive variability due to CYP2B6 genetics, and CYP2B6-mediated drug interactions, can alter methadone disposition, clinical effect, and drug safety. Methadone is not a substrate for major influx or efflux transporters.

Opioid Drug-Drug Interactions: A Review

Opioid analgesics are among the most commonly prescribed medications. Frequently, they are combined with other therapeutic agents and pharmacodynamic or pharmacokinetic interactions may ensue. This review summarizes published case reports and studies of potential opioid drug interactions. A MED-LINE computer literature search (1966-1998) was undertaken to retrieve all pertinent case reports and studies of opioid drug interactions published in the English language. The results of the search indicate that numerous compounds from various therapeutic classes may participate in clinically significant pharmacodynamic and pharmacokinetic drug-drug interactions. Pharmacodynamic interactions usually involved additive central nervous system depression. Additionally, propoxyphene and tramadol can potentiate a hyperserotonergic state when coadministered with the SSRIs and MAOIs. Pharmacokinetic interactions typically involved inhibition or induction by specific hepatic cytochrome P-450 isoenzymes. Agents with enzyme inhibiting ability such as erythromycin, cimetidine, and selective serotonin reuptake inhibitors have been shown to potentiate the effects of certain opioid analgesics while codeine, which requires metabolic conversion via CYP 2D6 for pharmacological effectiveness, has reduced analgesic efficacy in the presence of inhibitors. The enzyme inducers rifampin and several anticonvulsants have been involved in the emergence of methadone withdrawal when added to existing methadone treatment. Additionally, enzyme inducers can increase the formation of the toxic metabolite of meperidine. Genetic polymorphism also potentially impacts the effectiveness of agents such as codeine since reduced active metabolite formation and analgesic efficacy has been demonstrated in individuals who lack CYP 2D6 activity.

Acute Pain Management

Acute pain management is improving steadily over the past few years, but training and professional education are still lacking in many professions. Untreated or undertreated acute pain could have detrimental effects on the patient in terms of comfort and recovery from trauma or surgery. Acute undertreated pain can decrease a patient's vascular perfusion, increase oxygen demand, suppress the immune system, and possibly risk increased incidence of venous thrombosis. Although acute postoperative pain needs to be managed aggressively, patients are most vulnerable during this period for developing adverse effects, and therefore, patient assessment and careful drug therapy evaluation are necessary processes in therapeutic planning. Acute pain management requires careful and thorough initial assessment and follow-up reassessment in addition to frequent dosage adjustments, and managing analgesic induced side effects. Analgesic selection and dosing must be based on the patient's past and recent analgesic exposure. There is no single acute pain management regimen that is suitable for all patients. Analgesics must be tailored to the individual patient.

[Use of methadone in the elderly with cancer pain: a systematic review]

OBJECTIVE

To identify the clinical use of methadone as an analgesic in the management of cancer pain in elderly patients.

MATERIAL AND METHODS

We performed a systemic review of the literature on the specific use of methadone in elderly with cancer pain in MEDLINE, COCHRANE DATABASE and SCOPUS. A second search was conducted in MEDLINE to look for clinical trials and systematic review of the use of methadone in cancer pain, selecting only those in which the mean age of patients was ≥ 65 years old.

RESULTS

Four articles were obtained in the first search, and from the second 7 clinical trials, none of them specific to methadone use in elderly patients with cancer.

CONCLUSIONS

There are insufficient data on the use of methadone as an analgesic in the elderly with cancer. Given its pharmacological characteristics it must be used by trained personnel. Several recommendations are proposed for its use as an analgesic in the treatment of cancer pain in the elderly.

Respiratory effects of diazepam/methadone combination in rats: a study based on concentration/effect relationships

BACKGROUND

Methadone may cause respiratory depression and fatalities. Concomitant use of benzodiazepines in methadone-treated patients for chronic pain or as maintenance therapy for opiate abuse is common. However, the exact contribution of benzodiazepines to methadone-induced respiratory toxicity remains debatable.

METHODS

We investigated the respiratory effects of the combination diazepam (20mg/kg)/methadone (5mg/kg) in the rat, focusing on methadone concentration/effect relationships. Respiratory effects were studied using arterial blood gases and whole-body plethysmography. Plasma concentrations of both R- and S-methadone enantiomers were measured using high-performance liquid chiral chromatography coupled to mass spectrometry. To clarify mechanisms of diazepam/methadone interaction, methadone metabolism was investigated in vitro using rat liver microsomes.

RESULTS

Diazepam/methadone co-administration significantly increased methadone-related effects on inspiratory time (p<0.001) but did not significantly alter the other respiratory parameters when compared with methadone alone, despite significant increase in the area under the curve of plasma R-methadone concentrations measured during 240 min (p<0.05). Diazepam/methadone co-incubation with microsomes in vitro resulted in a significant inhibition of methadone metabolism (p<0.01), with 50%-inhibitory diazepam concentrations of 25.02 ± 0.18 μmol/L and 25.18 ± 0.23 μmol/L for R- and S-methadone, respectively.

CONCLUSION

We concluded that co-administration of high-doses of diazepam and methadone in rats is not responsible for additional respiratory depression in comparison to methadone alone, despite significant metabolic interaction between the drugs. In humans, although our experimental data may suggest the relative safety of benzodiazepine/methadone co-prescription, physicians should remain cautious as other underlying conditions may enhance this drug-drug interaction.

Polydrug abuse: a review of opioid and benzodiazepine combination use

This paper reviews studies examining the pharmacological interactions and epidemiology of the combined use of opioids and benzodiazepines (BZDs). A search of English language publications from 1970 to 2012 was conducted using PubMed and PsycINFO(®). Our search found approximately 200 articles appropriate for inclusion in this paper. While numerous reports indicate that the co-abuse of opioids and BZDs is ubiquitous around the world, the reasons for the co-abuse of these medications are not entirely clear. Though the possibility remains that opioid abusers are using BZDs therapeutically to self-medicate anxiety, mania or insomnia, the data reviewed in this paper suggest that BZD use is primarily recreational. For example, co-users report seeking BZD prescriptions for the purpose of enhancing opioid intoxication or "high," and use doses that exceed the therapeutic range. Since there are few clinical studies investigating the pharmacological interaction and abuse liability of their combined use, this hypothesis has not been extensively evaluated in clinical settings. As such, our analysis encourages further systematic investigation of BZD abuse among opioid abusers. The co-abuse of BZDs and opioids is substantial and has negative consequences for general health, overdose lethality, and treatment outcome. Physicians should address this important and underappreciated problem with more cautious prescribing practices, and increased vigilance for abusive patterns of use.

Mechanism-based inactivation of cytochrome P450 2B6 by methadone through destruction of prosthetic heme

Methadone is a μ-opioid receptor agonist widely used in the treatment of narcotic addiction and chronic pain conditions. Methadone is metabolized predominantly in the liver by cytochromes P450 to its pharmacologically inactive primary metabolite 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine. Initial in vitro data suggested that CYP3A4 is the major isoform responsible for the in vivo clearance of methadone in humans. However, recent clinical data have indicated that CYP2B6 is actually the major isoform responsible for methadone metabolism and clearance in vivo. In this study, methadone was shown to act as a mechanism-based inactivator of CYP2B6. Methadone inactivates CYP2B6 in a time-, concentration-, and NADPH-dependent manner with a K(I) = 10.0 μM and k(inact) = 0.027 min⁻¹. The loss of CYP2B6 activity in the presence of methadone and NADPH occurred with concomitant loss of the reduced CO spectrum of the P450. Moreover, there was good correlation between the loss of CYP2B6 activity and the loss of the CO-binding spectrum. High-performance liquid chromatography analysis of the native heme of the inactivated CYP2B6 demonstrated that approximately 75% loss of heme was accompanied by comparable inactivation of CYP2B6. Liquid chromatography-mass spectrometry analysis did not reveal the formation of a protein adduct during the inactivation. The evidence strongly suggests that destruction of prosthetic heme is the underlying mechanism leading to the inactivation of CYP2B6 by methadone.

Methadone: a review of drug-drug and pathophysiological interactions

Numerous established and potential drug interactions with methadone are clinically important in people treated with methadone either for addiction or for chronic pain. Methadone users often have comorbidities and are prescribed drugs that may interact with methadone. Methadone is extensively metabolized by cytochrome P450 (CYP) 3A4 and to a lesser extent by CYP 1A2, 2D6, 2D8, 2C9/2C8, 2C19, and 2B6. Eighty-six percent of methadone is protein bound, predominately to α1-acid glycoprotein (AGP). Polymorphisms in or interactions with CYPs that metabolize methadone, changes in protein binding, and other pathophysiological conditions affect the pharmacokinetic properties of methadone. It is critical for health care providers who treat patients on methadone to have adequate information on the interactions of methadone with other drugs of abuse and other medications. We set out to describe drug-drug interactions as well as physiological and pathophysiological factors that may impact the pharmacokinetics of methadone. Using MEDLINE, we conducted a systematic search for papers and related abstracts published between 1966 and June 2010. Keywords that included methadone, drug-drug interactions, CYP P450 and AGP identified a total of 7709 papers. Other databases, including the Cochrane Database of Systematic Reviews and Scopus, were also searched; an additional 929 papers were found. Final selection of 286 publications was based on the relevance of each paper to the topic. Over 50 such interactions were found. Interactions of methadone with other drugs can lead to increased or decreased methadone drug levels in patients and result in potential overdose or withdrawal, respectively. The former can contribute to methadone's fatality. Prescribers of methadone and pharmacists should enquire about any new medications (including natural products and over-the-counter medications) periodically, and especially when an otherwise stable patient suddenly experiences drug craving, withdrawal or intoxication.

Pharmacokinetic drug interactions and adverse consequences between psychotropic medications and pharmacotherapy for the treatment of opioid dependence

BACKGROUND

Psychiatric comorbidities among opioid-dependent patients are common. Many medications used to treat both conditions are metabolized through complimentary cytochrome P450 isoenzymes. When medication-assisted treatment for opioid dependence is concurrently used with psychotropic medications, problematic pharmacokinetic drug interactions may occur.

METHODS

We reviewed relevant English language articles identified through the MedLine, Scopus, and Embase databases from 1950 to December 2009 using the specific generic names of medications and keywords such as pharmacokinetics and drug interactions with buprenorphine, methadone, and naltrexone. Selected references from these articles were reviewed. Additionally, a review was conducted of abstracts and conference proceedings from national and international meetings from 1990 to 2009. A total of 60 studies were identified and reviewed.

RESULTS

Clinical case series and carefully controlled pharmacokinetic interaction studies have been conducted between methadone, buprenorphine, or naltrexone and some psychoactive medications. Important pharmacokinetic drug interactions have been demonstrated within each class of medications affecting either methadone and buprenorphine or psychoactive drugs. Few studies, however, have been conducted with naltrexone.

CONCLUSIONS

Several interactions between methadone, buprenorphine, or naltrexone and psychoactive medications are described and may have important clinical consequences. To optimize care, clinicians must be alerted to these interactions.

Review: Pharmacogenetic aspects of the effect of cytochrome P450 polymorphisms on serotonergic drug metabolism, response, interactions, and adverse effects

The field of pharmacogenetics contains a wealth of potential for the enhancement of clinical practice by providing a more effective match between patient and drug, consequently reducing the probability of an adverse drug reaction. Although a relatively novel concept in the forensic context, pharmacogenetics has the capability to assist in the interpretation of drug related deaths, particularly in unintentional drug poisonings where the cause of death remains unclear. However, the complex pharmacology of the drugs when subjected to genetic variations in metabolism makes interpretation of the expected response and adverse events difficult. Many possess multiple metabolic pathways, narrow therapeutic indices and active metabolites or enantiomers which may be eliminated via different pathways to the parent drug. A number of these drugs, which are metabolised primarily by the CYP450 system, are also associated with serotonin syndrome, or serotonin toxicity, especially when used concomitantly with other serotonin active drugs which rely on the same metabolic pathways for drug elimination. A comprehensive understanding of polymorphic drug metabolism and its expected outcomes is therefore essential when interpreting the involvement of drugs in adverse reactions. This review examines the genetically variable CYP450-mediated metabolism of a number of serotonin-active drugs that are often implicated in cases of serotonin toxicity, to assess the impact of pharmacogenetics on drug metabolism, response, interactions and adverse effects.

Gender issues in the pharmacotherapy of opioid-addicted women: buprenorphine

Gender, a biological determinant of mental health and illness, plays a critical role in determining patients' susceptibility, exposure to mental health risks, and related outcomes. Regarding sex differences in the epidemiology of opioid dependence, one third of the patients are women of childbearing age. Women have an earlier age of initiation of substance use and a more rapid progression to drug involvement and dependence than men. Generally few studies exist which focus on the special needs of women in opioid maintenance therapy. The aim of this paper is to provide an overview of treatment options for opioid-dependent women, with a special focus on buprenorphine, and to look at recent findings related to other factors that should be taken into consideration in optimizing the treatment of opioid-dependent women. Issues addressed include the role of gender in the choice of medication assisted treatment, sex differences in pharmacodynamics and pharmacokinetics of buprenorphine drug interactions, cardiac interactions, induction of buprenorphine in pregnant patients, the neonatal abstinence syndrome and breastfeeding. This paper aims to heighten the awareness for the need to take gender into consideration when making treatment decisions in an effort to optimize services and enhance the quality of life of women suffering from substance abuse.

Methadone: a substrate and mechanism-based inhibitor of CYP19 (aromatase)

The peripheral conversion of testosterone to estradiol by aromatase is the primary source of endogenous estrogen in postmenopausal women. Studies indicating that placental aromatase is able to metabolize methadone to its primary metabolite, 2-ethylidene-1, 5-dimethyl-3, 3-diphenylpyrrolidin (EDDP), led us to test the hypothesis that methadone is able to act as an inhibitor of aromatase. Using recombinant human CYP19, we examined the ability of methadone to bring about either reversible or mechanism-based inhibition of the conversion of testosterone to estradiol. To test for reversible inhibition, racemic methadone or its metabolite EDDP or 2-ethyl-5-methyl-3, 3-diphenylpyrroline (EMDP) was incubated for 30 min with testosterone at the K(m) (4 microM). To test for mechanism-based inhibition, microsomal preincubations were performed for up to 30 min using racemic methadone (1-1000 microM), R- or S-methadone (0.5-500 microM), or EDDP or EMDP (10 and 100 microM) followed by incubation with testosterone at a V(max) concentration (50 microM). Racemic methadone, EDDP, and EMDP did not act as competitive inhibitors of CYP19. Preincubation of methadone, EDDP, or EMDP with CYP19 resulted in time- and concentration-dependent inhibition, indicating a mechanism-based reaction that destroys CYP19 activity. The K(I) and k(inact) values for racemic methadone were calculated to be 40.6 +/- 2.8 microM and 0.061 +/- 0.001 min(-1), respectively. No stereoselectivity was observed. Methadone is metabolized by CYP19 and may act as a potent inhibitor of CYP19 in vivo. These findings may contribute to variability in methadone clearance, to drug-drug interactions, and to side effects observed in individual patients.

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.

Validation of a LC–APCI-MS/MS method for quantification of methadone, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) and 2-ethyl-5-methyl-3,3-diphenylpyraline (EMDP) in infant plasma following protein precipitation

A validated, quantitative LC-APCI-MS/MS method for methadone, EDDP and EMDP in 200-microL plasma is presented. Specimen preparation was limited to protein precipitation and centrifugation. Chromatographic separation was achieved on a Synergi Hydro-RP 80A (50 mm x 2.0 m, 4 microm) column with gradient elution. The assay was linear from 1 to 500 ng/mL, with intra- and inter-assay accuracy >or=87.5% and intra- and inter-assay precision <13.4% R.S.D. and recovery >or=87.5% for all analytes at 40 ng/mL. This analytical method is suitable for the accurate and precise determination of methadone and metabolites in human plasma specimens.

The human cytochrome P450 sub-family: Transcriptional regulation, inter-individual variation and interaction networks

The Cytochrome P450 super-family is a fundamental requirement for the viability of most life, with Cytochrome P450 proteins having been identified in organisms ranging from bacteria to man. These enzymes may be subdivided into those that metabolise purely endogenous chemicals, and those that are involved in xenobiotic metabolism. Of the latter group it can be argued that CYP3A sub-family members rank as the most important; their high expression in the liver and wide substrate specificity mean that they are clinically important in the metabolism of many therapeutic drugs, and alteration in their activity is central to many clinically-relevant drug-drug interactions. In this review I will examine the human CYP3A enzymes, discussing their genome structure, common allelic variants and, in greatest detail, their transcriptional regulation. Through examination of these characteristics we will see both striking similarities and differences between the four human CYP3A enzymes, which may have important impacts on inter-individual response to chemical exposure. Finally, the role of nuclear receptors in regulating CYP3A gene expression, and indeed that of many other proteins involved in drug metabolism, will be examined: Such an examination will show the need to utilize a systems biology approach to understand fully how the human body responds to chemical exposure.

Effects of Ganoderma lucidum Polysaccharide on CYP2E1, CYP1A2 and CYP3A Activities in BCG-Immune Hepatic Injury in Rats

The purpose of the present study was to investigate the effect of Ganoderma lucidum polysaccharide (GLPS), a major active component in Chinese medicinal fungus, on cytochrome P450 metabolic activity in Bacillus Calmette Guérin (BCG)-induced immune hepatic injury in rats. The enzyme kinetics of the probes including chlorzoxazone (CYP2E1), phenacetin (CYP1A2) and nifedipine (CYP3A) were evaluated by HPLC. The results showed that BCG-pretreatment (125 mg/kg) significantly increased serum levels of alanine transaminase (ALT), nitrite and malondialdehyde (MDA), inhibited activities of superoxide dismutase (SOD) and decreased P450 total content in microsomes (p<0.05). Administration of GLPS (50 and 200 mg/kg) reversed above hepatic injury stimulated by BCG in vivo. Moreover, GLPS dose-dependently inhibited activities of CYP2E1, CYP1A2 and CYP3A in hepatic microsomes in vitro, suggesting that inhibition of GLPS on P450 oxidative metabolism might participate in the hepatoprotective mechanism, and also suggested that pharmacokinetics might be changed by drug-herb interaction.

Clinical Pharmacology and Pharmacotherapy of Opioid Switching in Cancer Patients

Pain is one of the most common and often most feared symptoms in patients with cancer. Ongoing or progressive pain is physically debilitating and has a marked impact on quality of life. Since a third of the population will die from cancer, and of these, 80% will experience severe pain in their final year of life, effective treatment of cancer-related pain remains both a high priority and an ongoing challenge in clinical practice. Individuals with moderate to severe cancer-related pain require treatment with strong analgesics, namely opioids. There is evidence to support the therapeutic maneuver of opioid switching in clinical practice, but further evidence is needed to elucidate the underlying mechanisms for interindividual differences in response to different opioids. Large, robust clinical trials will be needed if clinical differences among side-effect profiles of different opioids are to be clearly demonstrated. This review discusses candidate genes, which contribute to opioid response; many other genes have also been implicated in "pain" from animal or human studies. In order to continue to evaluate the genetic contributions to both pain susceptibility and analgesic response, further candidate genes need to be considered. Good pain control remains a high priority for clinicians and patients, and there is much work to be done to further individualize analgesic therapy for patients with cancer.

Lopinavir/ritonavir induces the hepatic activity of cytochrome P450 enzymes CYP2C9, CYP2C19, and CYP1A2 but inhibits the hepatic and intestinal activity of CYP3A as measured by a phenotyping drug cocktail in healthy volunteers

OBJECTIVE

The effect of lopinavir/ritonavir (LPV/r) administration on cytochrome P450 (CYP) enzyme activity was quantified using a phenotyping biomarker cocktail. Changes in CYP2C9, CYP2C19, CYP3A, CYP1A2, N-acetyltransferase-2 (NAT-2), and xanthine oxidase (XO) activities were evaluated using warfarin (WARF) + vitamin K, omeprazole (OMP), intravenous (IV) and oral (PO) midazolam (MDZ), and caffeine (CAF).

DESIGN

: Open-label, multiple-dose, pharmacokinetic study in healthy volunteers.

METHODS

Subjects (n = 14) simultaneously received PO WARF 10 mg, vitamin K 10 mg, OMP 40 mg, CAF 2 mg/kg, and IV MDZ 0.025 mg/kg on days (D) 1 and 14, and PO MDZ 5 mg on D2 and D15. LPV/r (400/100 mg twice daily) was administered on D4-17. CYP2C9 and CYP2C19 activities were quantified by S-WARF AUC0-inf and OMP/5-hydroxy OMP ratio, respectively. CYP1A2, NAT-2, and XO activities were quantified by urinary CAF metabolite ratios. Hepatic and intestinal + hepatic CYP3A activities were quantified by IV (CL) and PO (CL/F) MDZ clearance, respectively.

RESULTS

After LPV/r therapy, CYP2C9, CYP2C19, and CYP1A2 activity increased by 29%, 100%, and 43% (P = 0.001, 0.046, and 0.001), respectively. No changes were seen in NAT-2 or XO activity. Hepatic and intestinal + hepatic CYP3A activity decreased by 77% (P < 0.001) and 92% (P = 0.001), respectively.

CONCLUSION

LPV/r therapy results in modest induction of CYP1A2 and CYP2C9 and potent induction of CYP2C19 activity. Increasing doses of concomitant medications metabolized by these enzymes may be necessary. LPV/r inhibited intestinal CYP3A to a greater extent than hepatic CYP3A activity. Doses of concomitant CYP3A substrates should be reduced when combined with LPV/r, although intravenously administered compounds may require less of a relative dose reduction than orally administered compounds.

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.

Use of slow-release oral morphine for the treatment of opioid dependence

AIMS

In addition to methadone, other synthetic opioids are now available for the treatment of opioid dependence. The study investigated the treatment satisfaction of oral slow-release morphine for maintenance therapy in opioid-dependent patients in an open-label 3-week study.

DESIGN

We evaluated the treatment satisfaction of oral slow-release morphine hydrochloride for 3 weeks in 110 patients meeting the diagnosis of opioid dependence (DSM-IV 304.0) or polysubstance dependence (DSM-IV 304.9).

MEASUREMENTS

Primary outcome measures were the study retention rate, urinalysis for additional illicit consumption other than heroin, cravings and withdrawal symptoms 24 h after the last intake of the medication (duration of action of treatment).

FINDINGS

In total, 103 patients completed the study, representing a retention rate of 94%. Patients reported significant improvements in somatic complaints, as well as significant reductions in heroin and cocaine cravings (p < 0.0001) and in additional consumption of cocaine in supervised urinalysis (p = 0.0083). Additional illicit consumption of benzodiazepines remained unchanged.

CONCLUSIONS

The high study retention rate implies a good acceptance of slow-release acting oral morphine. However, randomised, double-blind, double-dummy studies with a longer investigational period are needed to meet criteria for evidence-based medicine.

Effect of Tenofovir Disoproxil Fumarate on the Pharmacokinetics and Pharmacodynamics of Total, R-, and S-Methadone

STUDY OBJECTIVE

To evaluate the potential effect of tenofovir disoproxil fumarate (DF) on the pharmacokinetics of methadone.

DESIGN

Phase I, open-label, fixed-sequence, pharmacokinetic drug-drug interaction study.

SETTING

Clinical research center.

SUBJECTS

Fourteen volunteers receiving stable methadone maintenance therapy who were not infected with the human immunodeficiency virus.

INTERVENTION

Tenofovir DF was added to the subjects' methadone regimens.

MEASUREMENTS AND MAIN RESULTS

The pharmacokinetics of total, R-, and S-methadone were evaluated at baseline and after 2 weeks of daily tenofovir DF coadministration with a light meal. Steady-state tenofovir DF pharmacokinetics were evaluated at day 15. Bioequivalence testing was conducted of total, R-, and S-methadone area under the serum or plasma concentration-time curve during the 24-hour dosing interval at steady state (AUCss) and maximum concentration in serum or plasma (Cmax). Subjects were evaluated for changes in methadone pharmacodynamics by the Short Opiate Withdrawal Scale (SOWS) and pupillary diameter measurements at frequent intervals. Coadministration with tenofovir DF did not affect the pharmacokinetics of methadone. Geometric mean R-methadone systemic exposures, AUCss and Cmax, differed by 5% or less when methadone was dosed with tenofovir DF. Similar results were observed for S-methadone and for total methadone. Both AUCss and Cmax met the strict criteria for bioequivalence between the two study periods for total, R-, and S-methadone, indicating a lack of drug interaction when tenofovir DF was coadministered with methadone. No significant changes in SOWS scores or pupillary diameter measurements occurred, and no notable clinical adverse events were reported.

CONCLUSION

Tenofovir DF pharmacokinetics were comparable to previously reported values of tenofovir DF in HIV-infected patients. Coadministration of methadone with tenofovir DF did not alter the pharmacokinetics or pharmacodynamics of total, R-, or S-methadone. Tenofovir DF may be given as part of a once-daily antiretroviral regimen in patients receiving methadone maintenance therapy.

Genetic Predictors of the Clinical Response to Opioid Analgesics

This review uses a candidate gene approach to identify possible pharmacogenetic modulators of opioid therapy, and discusses these modulators together with demonstrated genetic causes for the variability in clinical effects of opioids. Genetically caused inactivity of cytochrome P450 (CYP) 2D6 renders codeine ineffective (lack of morphine formation), slightly decreases the efficacy of tramadol (lack of formation of the active O-desmethyl-tramadol) and slightly decreases the clearance of methadone. MDR1 mutations often demonstrate pharmacogenetic consequences, and since opioids are among the P-glycoprotein substrates, opioid pharmacology may be affected by MDR1 mutations. The single nucleotide polymorphism A118G of the mu opioid receptor gene has been associated with decreased potency of morphine and morphine-6-glucuronide, and with decreased analgesic effects and higher alfentanil dose demands in carriers of the mutated G118 allele. Genetic causes may also trigger or modify drug interactions, which in turn can alter the clinical response to opioid therapy. For example, by inhibiting CYP2D6, paroxetine increases the steady-state plasma concentrations of (R)-methadone in extensive but not in poor metabolisers of debrisoquine/sparteine. So far, the clinical consequences of the pharmacogenetics of opioids are limited to codeine, which should not be administered to poor metabolisers of debrisoquine/sparteine. Genetically precipitated drug interactions might render a standard opioid dose toxic and should, therefore, be taken into consideration. Mutations affecting opioid receptors and pain perception/processing are of interest for the study of opioid actions, but with modern practice of on-demand administration of opioids their utility may be limited to explaining why some patients need higher opioid doses; however, the adverse effects profile may be modified by these mutations. Nonetheless, at a limited level, pharmacogenetics can be expected to facilitate individualised opioid therapy.

Potential inhibition of cytochrome P450 3A4 by propofol in human primary hepatocytes

AIM: Hepatic cytochrome P450 isoenzymes constitute a superfamily of hemoproteins that play a major role in the metabolism of endogenous compounds and in the detoxification of xenobiotic molecules. P450 3A4 is one of the most important forms in human being, and mediates the metabolism of around 70% of therapeutic drugs and endogenous compounds. Propofol, a widely used intravenous anesthetic drug, is known to inhibit cytochrome P450 activities in isolated rat hepatocytes. The goal of this study was to evaluate the potential efficacy of propofol on P450 3A4 in a dose-dependent manner to understand its drug-drug interaction.

METHODS: Hepatocytes were isolated from liver specimens from hepatic angioma patients undergone hepatic surgery. Primary incubated hepatocytes were treated with 0, 0.01, 0.05, 0.1, 0.5, and 1.0 mM propofol for 24 hours. P450 3A4 activity was measured with Nash’s colorimetry. The protein expression was assessed by Western blot analysis.

RESULTS: A dose-dependent inhibitory effect of propofol was observed in cytochrome P450 3A4 activity. A minimal dosage of propofol (0.01 mM) induced a significant inhibition of P450 3A4 activity, although its regular dosages (0.01-0.1 mM) showed no inhibitory effect on the cellular protein expression of P450 3A4.

CONCLUSION: Propofol may be a potential CYP3A4 inhibitor as this anesthetic can inhibit isoenzyme activity significantly and reduce the metabolic rate of CYP3A4 substrates. This inhibition occurs at post-expression level, and concentration of propofol used clinically does not affect CYP3A4 protein expression. propofol may thus induce drug interaction of cytochrome P450 3A4 activity at the dosage used clinically.

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.

Interactions between recreational drugs and antiretroviral agents

OBJECTIVE

To summarize existing data regarding potential interactions between recreational drugs and drugs commonly used in the management of HIV-positive patients.

DATA SOURCES

Information was obtained via a MEDLINE search (1966-August 2002) using the MeSH headings human immunodeficiency virus, drug interactions, cytochrome P450, medication names commonly prescribed for the management of HIV and related opportunistic infections, and names of commonly used recreational drugs. Abstracts of national and international conferences, review articles, textbooks, and references of all articles were also reviewed.

STUDY SELECTION AND DATA EXTRACTION

Literature on pharmacokinetic interactions was considered for inclusion. Pertinent information was selected and summarized for discussion. In the absence of specific data, prediction of potential clinically significant interactions was based on pharmacokinetic and pharmacodynamic properties.

RESULTS

All protease inhibitors (PIs) and nonnucleoside reverse transcriptase inhibitors are substrates and potent inhibitors or inducers of the cytochrome P450 system. Many classes of recreational drugs, including benzodiazepines, amphetamines, and opioids, are also metabolized by the liver and can potentially interact with antiretrovirals. Controlled interaction studies are often not available, but clinically significant interactions have been observed in a number of case reports. Overdoses secondary to interactions between the "rave" drugs methylenedioxymethamphetamine (MDMA) or gamma-hydroxybutyrate (GHB) and PIs have been reported. PIs, particularly ritonavir, may also inhibit metabolism of amphetamines, ketamine, lysergic acid diethylmide (LSD), and phencyclidine (PCP). Case series and pharmacokinetic studies suggest that nevirapine and efavirenz induce methadone metabolism, which may lead to symptoms of opiate withdrawal. A similar interaction may exist between methadone and the PIs ritonavir and nelfinavir, although the data are less consistent. Opiate metabolism can be inhibited or induced by concomitant PIs, and patients should be monitored for signs of toxicity and/or loss of analgesia. PIs should not be coadministered with midazolam and triazolam, since prolonged sedation may occur.

CONCLUSIONS

Interactions between agents commonly prescribed for patients with HIV and recreational drugs can occur, and may be associated with serious clinical consequences. Clinicians should encourage open dialog with their patients on this topic, to avoid compromising antiretroviral efficacy and increasing the risk of drug toxicity.

Different alterations of cytochrome P450 3A4 isoform and its gene expression in livers of patients with chronic liver diseases

AIM: To determine whether parenchymal cells or hepatic cytochrome P450 protein was changed in chronic liver diseases, and to compare the difference of CYP3A4 enzyme and its gene expression between patients with hepatic cirrhosis and obstructive jaundice, and to investigate the pharmacologic significance behind this difference.

METHODS: Liver samples were obtained from patients undergoing hepatic surgery with hepatic cirrhosis (n = 6) and obstructive jaundice (n = 6) and hepatic angeioma (controls, n = 6). CYP3A4 activity and protein were determined by Nash and western bloting using specific polychonal antibody, respectively. Total hepatic RNA was extracted and CYP3A4cDNA probe was prepared according the method of random primer marking, and difference of cyp3a4 expression was compared among those patients by Northern blotting.

RESULTS: Compared to control group, the CYP3A4 activity and protein in liver tissue among patients with cirrhosis were evidently reduced. (P < 0.01) Northern blot showed the same change in its mRNA levels. In contrast, the isoenzyme and its gene expression were not changed among patients with obstructive jaundice.

CONCLUSION: Hepatic levels of P450s and its CYP3A4 isoform activity were selectively changed in different chronic liver diseases. CYP3A4 isoenzyme and its activity declined among patients with hepatic cirrhosis as expression of cyp3a4 gene was significantly reduced. Liver's ability to eliminate many clinical therateutic drug substrates would decline consequently, These findings may have practical implications for the use of drugs in patients with cirrhosis and emphasize the need to understand the metabolic fate of therapeutic compounds. Elucidation of the reasons for these different changes in hepatic CYP3A4 may provide insight into more fundamental aspects and mechanisms of imparied liver function.

Optimizing long-term response to methadone maintenance treatment: a 152-week follow-up using higher-dose methadone

We report the clinical course over 152 weeks of 245 patients in methadone maintenance treatment: 144 high dose (HD) patients (> or = 100 mg/d, mean 211 mg/d), and 101 control (C) patients (< 100 mg/d, mean 65 mg/d). After 152 weeks the mean methadone doses were 284.9 mg/d (range 13-1100 mg/d) and 94.0 mg/d (range 10-500 mg/d), respectively. Overall retention in treatment was 59% over the 152 weeks, with the HD group having significantly better retention (61.1% vs. 46.3%) and lower rates of positive urine toxicologies (16.0% vs. 36.6%). Mortality was statistically the same for the HD group (2/144, 1.4%) and the C group (2/101, 1.9%) over the 152-week period. We conclude that doses of methadone exceeding 100 mg/d are safe and effective in long-term maintenance treatment. We attribute the favorable outcomes we report to a model of treatment that emphasizes medication management in the treatment of opioid addiction.

The interaction of medications used in palliative care

Palliative care uses several classes of drugs, which are handled by the CYP P450 system. Interaction of drugs in this setting requires ongoing vigilance by the physician. Phenocopying may be more common than previously realized.

Drug interactions with patient-controlled analgesia

Patient-controlled analgesia (PCA) has become standard procedure in the clinical treatment of pain. Its widespread use in patients with all kinds of diseases opens a variety of possible interactions between analgesics used for PCA and other drugs that might be administered concomitantly to the patient. Many of these drug interactions are of little clinical importance. However, some drug interactions have been reported to result in serious clinical problems. Drug interactions can either predominantly affect the pharmacokinetics or pharmacodynamics of the drug. Most important pharmacokinetic drug interactions occur at the level of drug metabolism or protein binding. Acceleration of methadone metabolism caused by cytochrome P450 (CYP) 3A4 induction by antiretroviral drugs or rifampicin (rifampin) has caused methadone withdrawal symptoms. Lack of morphine formation from codeine as a result of CYP2D6 inhibition by quinidine results in an almost complete loss of the analgesic effects of codeine. Alterations of methadone protein binding caused by an inhibition of alpha1-acid glycoprotein synthesis by alkylating substances are another possibility for predominantly pharmacokinetically based drug interactions during PCA. Furthermore, inhibition of P-glycoprotein by anticancer drugs could result in altered transmembrane transport of morphine, methadone or fentanyl, although this has not been shown to be of clinical relevance. Synergistic effects of systemically administered opioids with spinally or topically delivered opioids or anaesthetics have been reported frequently. The same is true for the opioid-sparing effects of coadministered non-opioid analgesics. Antidepressants, anticonvulsants or alpha2-adrenoreceptor agonists have also been shown to exert additive analgesic effects when administered together with an opioid. Inconsistent findings, however, are reported regarding the treatment of patients with opioid-induced nausea and sedation, since coadministration of antiemetics either increased or decreased the respective adverse effects or revealed additional unwanted drug effects.

Methadone and fluconazole: respiratory depression by drug interaction

A 60-year-old man with advanced gastric cancer achieved good pain control on a stable dose of methadone for 10 days. However, he developed respiratory depression 2 days after intravenous fluconazole was administrated for refractory oral candidiasis. Intravenous naloxone effectively reversed the respiratory depression. This case illustrates a significant interaction between methadone and fluconazole, and highlights the need for awareness of potential interactions between drugs used in palliative care.

Methadone use in cancer patients with pain: a review

In recent years a better understanding of the pharmacologic and pharmacokinetic properties of methadone, including equianalgesic ratios has led to its increased use as a second line opioid for the treatment of pain in patients with cancer. Methadone may be an important alternative for those who have side effects related to the use of other opioids because it has no known active metabolites, is well absorbed by oral and rectal routes, and also has the advantage of very low cost. However, it has a long, unpredictable half-life, which can result in accumulation and toxicity in some patients. In addition, rotation to methadone as a second line agent is more complex than with other opioids because of its increased potency in those patients who are opioid tolerant, particularly those who have been on higher doses of other opioids. Future research should address the use of methadone as a first-line agent in the management of cancer pain, its use in patients with neuropathic pain, and in those who develop rapid tolerance to other opioids. In some patients with cancer the long half-life of methadone offers the advantage of extended dosing intervals to 12 and even 24 hours, further research is also needed in this area.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

A single dose of methadone inhibits cytochrome P-4503A activity in healthy volunteers as assessed by the urinary cortisol ratio

AIMS

To examine the effect of a single oral dose of methadone on cytochrome P450 (CYP) 3A activity using the urinary 6beta-hydroxycortisol to cortisol ratio (UCR) as a marker of CYP3A activity.

METHODS

A single oral dose (0.2 mg kg-1) of rac-methadone was administered to eight healthy female volunteers. Frequent blood samples and all urine over seven time periods was collected for 96 h following dosing. The UCR and the concentration of the major CYP3A metabolite of methadone, EDDP, were measured in urine. Methadone enantiomer concentrations were determined in plasma and urine. All quantifications were performed by validated high performance liquid chromatography assays.

RESULTS

In all volunteers a significant decline of the UCR from immediately predose values was observed at the 4-8 and 8-12 h collection periods (P < 0.05, 95% CI for the differences: 0.4,16 and 0.6,16, respectively) with a return to immediately predose values after 2-3 days, suggesting methadone was an inhibitor of CYP3A. The UCR was found to be significantly correlated with the amount of EDDP excreted in the urine and with the area under the plasma concentration vs time profile for total (R + S) methadone, supporting in vitro data that CYP3A is primarily responsible for EDDP formation and has a significant influence on methadone disposition.

CONCLUSIONS

Methadone appears to be a CYP3A inhibitor in vivo following a single oral dose and measurements of the urinary cortisol ratio appear to be a useful index to follow this inhibition.

Methadone maintenance treatment: is it possible to adapt the daily doses to the metabolic activity of the patient?

Controversy still exists concerning the proper daily dose of methadone to be used in opiate dependency treatment. Because it is admitted that serum methadone concentration may be significantly correlated with the amount of drug available at the receptor level, it could be interesting to predict the methadone daily doses necessary to reach such a serum concentration. The authors have attempted to correlate the serum methadone level with the daily intake, considering the metabolic activity of the patients. A poor correlation was found between methadone doses and methadone serum concentrations (r2 = 0.0409, p = 0.048). The test used to determine the metabolic activity of patients is the 6-OH cortisol/17-OH corticosteroids ratio in urine. This urinary 6-OH cortisol/17-OH corticosteroids ratio was tested because cortisol is metabolized through the same P450 cytochromes as methadone, namely cytochrome P450 3A4. This determination could be of interest because it could be tested before methadone administration to predict optimal doses. But when the authors tried to correlate the methadone serum concentration with the steroid ratio, they failed to find a significant correlation (r2 = 0.0046, N.S.), even when they took into account the daily doses (r2 = 0.0015, N.S.), most probably because of some limitations of the cortisol ratio.