Pharmacokinetic interaction between voriconazole and methadone at steady state in patients on methadone therapy

Natural Products Inhibition of Cytochrome P450 2B6 Activity and Methadone Metabolism

Methadone is cleared predominately by hepatic cytochrome P450 (CYP) 2B6-catalyzed metabolism to inactive metabolites. CYP2B6 also catalyzes the metabolism of several other drugs. Methadone and CYP2B6 are susceptible to pharmacokinetic drug-drug interactions. Use of natural products such as herbals and other botanicals is substantial and growing, and concomitant use of prescription medicines and non-prescription herbals is common and may result in interactions, often precipitated by CYP inhibition. Little is known about herbal product effects on CYP2B6 activity, and CYP2B6-catalyzed methadone metabolism. We screened a family of natural product compounds used in traditional medicines, herbal teas, and synthetic analogs of compounds found in plants, including kavalactones, flavokavains, chalcones and gambogic acid, for inhibition of expressed CYP2B6 activity and specifically inhibition of CYP2B6-mediated methadone metabolism. An initial screen evaluated inhibition of CYP2B6-catalyzed 7-ethoxy-4-(trifluoromethyl) coumarin O-deethylation. Hits were further evaluated for inhibition of racemic methadone metabolism, including mechanism of inhibition and kinetic constants. In order of decreasing potency, the most effective inhibitors of methadone metabolism were dihydromethysticin (competitive, K i 0.074 µM), gambogic acid (noncompetitive, K i 6 µM), and 2,2'-dihydroxychalcone (noncompetitive, K i 16 µM). Molecular modeling of CYP2B6-methadone and inhibitor binding showed substrate and inhibitor binding position and orientation and their interactions with CYP2B6 residues. These results show that CYP2B6 and CYP2B6-catalyzed methadone metabolism are inhibited by certain natural products, at concentrations which may be clinically relevant. SIGNIFICANCE STATEMENT: This investigation identified several natural product constituents which inhibit in vitro human recombinant CYP2B6 and CYP2B6-catalyzed N-demethylation of the opioid methadone. The most potent inhibitors (K i) were dihydromethysticin (0.074 µM), gambogic acid (6 µM) and 2,2'-dihydroxychalcone (16 µM). Molecular modeling of ligand interactions with CYP2B6 found that dihydromethysticin and 2,2'-dihydroxychalcone bound at the active site, while gambogic acid interacted with an allosteric site on the CYP2B6 surface. Natural product constituents may inhibit CYP2B6 and methadone metabolism at clinically relevant concentrations.

In vitro-in silico-based prediction of inter-individual and inter-ethnic variations in the dose-dependent cardiotoxicity of R- and S-methadone in humans

New approach methodologies predicting human cardiotoxicity are of interest to support or even replace in vivo-based drug safety testing. The present study presents an in vitro–in silico approach to predict the effect of inter-individual and inter-ethnic kinetic variations in the cardiotoxicity of R- and S-methadone in the Caucasian and the Chinese population. In vitro cardiotoxicity data, and metabolic data obtained from two approaches, using either individual human liver microsomes or recombinant cytochrome P450 enzymes (rCYPs), were integrated with physiologically based kinetic (PBK) models and Monte Carlo simulations to predict inter-individual and inter-ethnic variations in methadone-induced cardiotoxicity. Chemical specific adjustment factors were defined and used to derive dose–response curves for the sensitive individuals. Our simulations indicated that Chinese are more sensitive towards methadone-induced cardiotoxicity with Margin of Safety values being generally two-fold lower than those for Caucasians for both methadone enantiomers. Individual PBK models using microsomes and PBK models using rCYPs combined with Monte Carlo simulations predicted similar inter-individual and inter-ethnic variations in methadone-induced cardiotoxicity. The present study illustrates how inter-individual and inter-ethnic variations in cardiotoxicity can be predicted by combining in vitro toxicity and metabolic data, PBK modelling and Monte Carlo simulations. The novel methodology can be used to enhance cardiac safety evaluations and risk assessment of chemicals.

Determination of Voriconazole in Human Plasma Using RP-HPLC/UV-VIS Detection: Method Development and Validation; Subsequently Evaluation of Voriconazole Pharmacokinetic Profile in Pakistani Healthy Male Volunteers

In this research work, an isocratic, reversed-phase high-performance liquid chromatography-ultraviolet/visible detector method was developed for analysis of voriconazole standard (stock-solution) and in plasma samples. Optimization and validation of the method was carried out as per international guidelines. The method offered a simple liquid–liquid extraction technique, which exhibited best recovery of voriconazole along with fluconazole, i.e., internal standard. Different experimental conditions were tried and ultimately, the best outcomes were accomplished utilizing C-18 Perkin-Elmer® column with particulars of 150 mm length, 4.6 mm inner diameter and 5 μm particle size, protected by an RP-18 Perkin-Elmer® Pre-column guard cartridge with specifications of 10 μm particle size, 30 mm length and 4.6 mm inner diameter, utilizing mobile-phase of acetonitrile-water (ACN: H2O) in proportion of 60: 40 v/v, having a flow rate of 1.5 mL/min, and wavelength of 254 nm. All the analytes were observed to be separated in ≤7 min. A linear calibration curve was obtained at concentration range of 01–10 μg/mL of voriconazole. The correlation coefficient of voriconazole was observed to be 0.999, and average recovery (in percent) was 97.4%, whereas the relative standard deviation value was ≤2%. The lower limit of detection was 0.01 μg/mL, whereas, lower limit of quantification was 0.03 μg/mL, respectively. This developed method provided outstanding results of all validation parameters, i.e., recovery, accuracy, selectivity, precision and reproducibility. The method proposed for voriconazole analysis was applied effectively for further research investigation of voriconazole in human-plasma samples (to assess the pharmacokinetic parameters), pharmaceutical formulations and pharmacokinetic drug–drug interaction’s.

Interaction between methadone and clarithromycin as the suspected cause of an opioid toxidrome

A 40-year-old patient was admitted through the acute medical take with pleuritic chest pain and rigours. He had a medical history of opiate dependence and was receiving 60 mg of methadone once daily. He was diagnosed with a community-acquired pneumonia and treated with amoxicillin and clarithromycin. After administration of only two concomitant doses of methadone and oral clarithromycin, he developed an opioid toxidrome with type-2 respiratory failure, a decreased level of consciousness and pinpoint pupils. The patient was treated with naloxone and his symptoms improved. Retrospectively, it was suspected that an interaction between clarithromycin and methadone might have contributed to the toxidrome. Respiratory failure has not been previously prescribed for this combination of medication and is of high importance for physicians and pharmacists around the world.

Integrating in vitro data and physiologically based kinetic modeling-facilitated reverse dosimetry to predict human cardiotoxicity of methadone

Development of novel testing strategies to detect adverse human health effects is of interest to replace in vivo-based drug and chemical safety testing. The aim of the present study was to investigate whether physiologically based kinetic (PBK) modeling-facilitated conversion of in vitro toxicity data is an adequate approach to predict in vivo cardiotoxicity in humans. To enable evaluation of predictions made, methadone was selected as the model compound, being a compound for which data on both kinetics and cardiotoxicity in humans are available. A PBK model for methadone in humans was developed and evaluated against available kinetic data presenting an adequate match. Use of the developed PBK model to convert concentration–response curves for the effect of methadone on human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) in the so-called multi electrode array (MEA) assay resulted in predictions for in vivo dose–response curves for methadone-induced cardiotoxicity that matched the available in vivo data. The results also revealed differences in protein plasma binding of methadone to be a potential factor underlying variation between individuals with respect to sensitivity towards the cardiotoxic effects of methadone. The present study provides a proof-of-principle of using PBK modeling-based reverse dosimetry of in vitro data for the prediction of cardiotoxicity in humans, providing a novel testing strategy in cardiac safety studies.

Methadone Metabolism and Drug-Drug Interactions: In Vitro and In Vivo Literature Review

Methadone is utilized for the treatment of individuals with opiate dependence. Methadone undergoes N-demethylation by multiple cytochrome P450 (CYP) enzymes including CYP3A4, CYP2B6, CYP2C19, CYP2D6, CYP2C9, and CYP2C8. In vivo, polymorphism effects on methadone systemic exposure have been noted for CYP2B6, CYP3A4, and CYP2D6. Clinical drug interaction studies with antiviral drugs in methadone maintenance treatment patients yield varying results on methadone pharmacokinetics and pharmacodynamics. In general, CYP inhibitors altered methadone exposure with no adverse effects. CYP inducers generally decreased methadone exposure with some reports of withdrawal symptoms in the subjects. Interaction studies with antiviral drug combinations yielding differing results depend on the enzyme(s) affected. For certain antiviral medicines which are dual inhibitor(s) and inducer(s) for CYP enzymes, their effect on methadone pharmacokinetics can change with time since the effect of induction is usually delayed compared to the effect of inhibition.

Opioid analgesics-related pharmacokinetic drug interactions: from the perspectives of evidence based on randomized controlled trials and clinical risk management

BACKGROUND

Multimorbidity results in complex polypharmacy which may bear a risk of drug interactions. A better understanding of opioid analgesics combination therapy used for pain management could help warrant medication safety, efficacy, and economic relevance. Until now there has been no review summarizing the opioid analgesics-related pharmacokinetic drug interactions from the perspective of evidence based on randomized controlled trials (RCTs).

METHOD

A literature search was performed using PubMed, MEDLINE, and the Cochrane Library, using a PRISMA flowchart.

RESULTS

Fifty-two RCTs were included for data interpretation. Forty-two RCTs (80.8%) were conducted in healthy volunteers, whereas 10 RCTs (19.2%) enrolled true patients. None of the opioid-drug/herb pairs was listed as contraindications of opioids involved in this review. Circumstances in which opioid is comedicated as a precipitant drug include morphine-P2Y12 inhibitors, morphine-gabapentin, and methadone-zidovudine. Circumstances in which opioid is comedicated as an object drug include rifampin-opioids (morphine, tramadol, oxycodone, methadone), quinidine-opioids (morphine, fentanyl, oxycodone, codeine, dihydrocodeine, methadone), antimycotics-opioids (buprenorphine, fentanyl, morphine, oxycodone, methadone, tilidine, tramadol), protease inhibitors-opioids (ritonavir, ritonavir/lopinavir-oxycodone, ritonavir-fentanyl, ritonavir-tilidine), grapefruit juice-opioids (oxycodone, fentanyl, methadone), antidepressants-opioids (paroxetine-tramadol, paroxetine-hydrocodone, paroxetine-oxycodone, escitalopram-tramadol), metoclopramide-morphine, amantadine-morphine, sumatriptan-butorphanol nasal sprays, ticlopidine-tramadol, St John's wort-oxycodone, macrolides/ketolides-oxycodone, and levomepromazine-codeine. RCTs investigating the same combination, almost unanimously, drew consistent conclusions, except two RCTs on amantadine-intravenous morphine combination where a different amantadine dose was used and two RCTs on morphine-ticagrelor combination where healthy volunteers and true patients were enrolled, respectively. RCTs investigating in true patients may reflect a realistic clinical scenario and overcome the limitation of RCTs performed in healthy volunteers under standardized conditions. Further research opportunities are also presented in this review.

CONCLUSION

Effective and safe combination therapy of opioids can be achieved by promoting the awareness of potential changes in therapeutic efficacy and toxicities, prescribing alternatives or changing administration strategy, tailoring dose, reviewing the appropriateness of orders, and paying attention to medication monitoring.

Inhibition of Cytochrome P450 2B6 Activity by Voriconazole Profiled Using Efavirenz Disposition in Healthy Volunteers

Cytochrome P450 2B6 (CYP2B6) metabolizes clinically important drugs and other compounds. Its expression and activity vary widely among individuals, but quantitative estimation is hampered by the lack of safe and selective in vivo probes of CYP2B6 activity. Efavirenz, a nonnucleoside HIV-1 reverse transcriptase inhibitor, is mainly cleared by CYP2B6, an enzyme strongly inhibited in vitro by voriconazole. To test efavirenz metabolism as an in vivo probe of CYP2B6 activity, we quantified the inhibition of CYP2B6 activity by voriconazole in 61 healthy volunteers administered a single 100-mg oral dose of efavirenz with and without voriconazole administration. The kinetics of efavirenz metabolites demonstrated formation rate-limited elimination. Compared to control, voriconazole prolonged the elimination half-life (t_1/2) and increased both the maximum concentration of drug in serum (C_max) and the area under the concentration-time curve from 0 h to t (AUC_0-t) of efavirenz (mean change of 51%, 36%, and 89%, respectively) (P < 0.0001) with marked intersubject variability (e.g., the percent change in efavirenz AUC_0-t ranged from 0.4% to ∼224%). Voriconazole decreased efavirenz 8-hydroxylation by greater than 60% (P < 0.0001), whereas its effect on 7-hydroxylation was marginal. The plasma concentration ratio of efavirenz to 8-hydroxyefavirenz, determined 1 to 6 h after dosing, was significantly increased by voriconazole and correlated with the efavirenz AUC_0-t (Pearson r = >0.8; P < 0.0001). This study demonstrates the mechanisms of voriconazole-efavirenz interaction, establishes the use of a low dose of efavirenz as a safe and selective in vivo probe for phenotyping CYP2B6 activity, and identifies several easy-to-use indices that should enhance understanding of the mechanisms of CYP2B6 interindividual variability. (This study is registered at ClinicalTrials.gov under identifier NCT01104376.).

Voriconazole more likely than posaconazole increases plasma exposure to sublingual buprenorphine causing a risk of a clinically important interaction

PURPOSE

This study aimed to determine possible effects of voriconazole and posaconazole on the pharmacokinetics and pharmacological effects of sublingual buprenorphine.

METHODS

We used a randomized, placebo-controlled crossover study design with 12 healthy male volunteers. Subjects were given a dose of 0.4 mg (0.6 mg during placebo phase) sublingual buprenorphine after a 5-day oral pretreatment with either (i) placebo, (ii) voriconazole 400 mg twice daily on the first day and 200 mg twice daily thereafter or (iii) posaconazole 400 mg twice daily. Plasma and urine concentrations of buprenorphine and its primary active metabolite norbuprenorphine were monitored over 18 h and pharmacological effects were measured.

RESULTS

Compared to placebo, voriconazole increased the mean area under the plasma concentration-time curve (AUC_0-∞) of buprenorphine 1.80-fold (90 % confidence interval 1.45-2.24; P < 0.001), its peak concentration (C_max) 1.37-fold (P < 0.013) and half-life (t _½ ) 1.37-fold (P < 0.001). Posaconazole increased the AUC0_0-∞ of buprenorphine 1.25-fold (P < 0.001). Most of the plasma norbuprenorphine concentrations were below the limit of quantification (0.05 ng/ml). Voriconazole, unlike posaconazole, increased the urinary excretion of norbuprenorphine 1.58-fold (90 % confidence interval 1.18-2.12; P < 0.001) but there was no quantifiable parent buprenorphine in urine. Plasma buprenorphine concentrations correlated with the pharmacological effects, but the effects did not differ significantly between the phases.

CONCLUSIONS

Voriconazole, and to a minor extent posaconazole, increase plasma exposure to sublingual buprenorphine, probably via inhibition of cytochrome P450 3 A and/or P-glycoprotein. Care should be exercised in the combined use of buprenorphine with triazole antimycotics, particularly with voriconazole, because their interaction can be of clinical importance.

Pharmacokinetic Effects of Isavuconazole Coadministration With the Cytochrome P450 Enzyme Substrates Bupropion, Repaglinide, Caffeine, Dextromethorphan, and Methadone in Healthy Subjects

This report describes phase 1 clinical trials performed to assess interactions of oral isavuconazole at the clinically targeted dose (200 mg, administered as isavuconazonium sulfate 372 mg, 3 times a day for 2 days; 200 mg once daily [QD] thereafter) with single oral doses of the cytochrome P450 (CYP) substrates: bupropion hydrochloride (CYP2B6; 100 mg; n = 24), repaglinide (CYP2C8/CYP3A4; 0.5 mg; n = 24), caffeine (CYP1A2; 200 mg; n = 24), dextromethorphan hydrobromide (CYP2D6/CYP3A4; 30 mg; n = 24), and methadone (CYP2B6/CYP2C19/CYP3A4; 10 mg; n = 23). Compared with each drug alone, coadministration with isavuconazole changed the area under the concentration‐time curves (AUC_∞) and maximum concentrations (C_max) as follows: bupropion, AUC_∞ reduced 42%, C_max reduced 31%; repaglinide, AUC_∞ reduced 8%, C_max reduced 14%; caffeine, AUC_∞ increased 4%, C_max reduced 1%; dextromethorphan, AUC_∞ increased 18%, C_max increased 17%; R‐methadone, AUC_∞ reduced 10%, C_max increased 3%; S‐methadone, AUC_∞ reduced 35%, C_max increased 1%. In all studies, there were no deaths, 1 serious adverse event (dextromethorphan study; perioral numbness, numbness of right arm and leg), and adverse events leading to study discontinuation were rare. Thus, isavuconazole is a mild inducer of CYP2B6 but does not appear to affect CYP1A2‐, CYP2C8‐, or CYP2D6‐mediated metabolism.

Drug-drug interactions and QT prolongation as a commonly assessed cardiac effect - comprehensive overview of clinical trials

Background

Proarrhythmia assessment is one of the major concerns for regulatory bodies and pharmaceutical industry. ICH guidelines recommending preclinical tests have been established in attempt to eliminate the risk of drug-induced arrhythmias. However, in the clinic, arrhythmia occurrence is determined not only by the inherent property of a drug to block ion currents and disturb electrophysiological activity of cardiac myocytes, but also by many other factors modifying individual risk of QT prolongation and subsequent proarrhythmia propensity. One of those is drug-drug interactions. Since polypharmacy is a common practice in clinical settings, it can be anticipated that there is a relatively high risk that the patient will receive at least two drugs mutually modifying their proarrhythmic potential and resulting either in triggering the occurrence or mitigating the clinical symptoms. The mechanism can be observed either directly at the pharmacodynamic level by competing for the molecular targets, or indirectly by modifying the physiological parameters, or at the pharmacokinetic level by alteration of the active concentration of the victim drug.

Methods

This publication provides an overview of published clinical studies on pharmacokinetic and/or pharmacodynamic drug-drug interactions in humans and their electrophysiological consequences (QT interval modification). Databases of PubMed and Scopus were searched and combinations of the following keywords were used for Title, Abstract and Keywords fields: interaction, coadministration, combination, DDI and electrocardiographic, QTc interval, ECG. Only human studies were included. Over 4500 publications were retrieved and underwent preliminary assessment to identify papers accordant with the topic of this review. 76 papers reporting results for 96 drug combinations were found and analyzed.

Results

The results show the tremendous variability of drug-drug interaction effects, which makes one aware of complexity of the problem, and suggests the need for assessment of an additional risk factors and careful ECG monitoring before administration of drugs with anticipated QT prolongation.

Conclusions

DDIs can play significant roles in drugs’ cardiac safety, as evidenced by the provided examples. Assessment of the pharmacodynamic effects of the drug interactions is more challenging as compared to the pharmacokinetic due to the significant diversity in the endpoints which should be analyzed specifically for various clinical effects. Nevertheless, PD components of DDIs should be accounted for as PK changes alone do not allow to fully explain the electrophysiological effects in clinic situations.

Electronic supplementary material

The online version of this article (doi:10.1186/s40360-016-0053-1) contains supplementary material, which is available to authorized users.

Methadone Pharmacogenetics: CYP2B6 Polymorphisms Determine Plasma Concentrations, Clearance, and Metabolism

BACKGROUND

Interindividual variability in methadone disposition remains unexplained, and methadone accidental overdose in pain therapy is a significant public health problem. Cytochrome P4502B6 (CYP2B6) is the principle determinant of clinical methadone elimination. The CYP2B6 gene is highly polymorphic, with several variant alleles. CYP2B6.6, the protein encoded by the CYP2B6*6 polymorphism, deficiently catalyzes methadone metabolism in vitro. This investigation determined the influence of CYP2B6*6, and other allelic variants encountered, on methadone concentrations, clearance, and metabolism.

METHODS

Healthy volunteers in genotype cohorts CYP2B6*1/*1 (n = 21), CYP2B6*1/*6 (n = 20), and CYP2B6*6/*6 (n = 17), and also CYP2B6*1/*4 (n = 1), CYP2B6*4/*6 (n = 3), and CYP2B6*5/*5 (n = 2) subjects, received single doses of IV and oral methadone. Plasma and urine methadone and metabolite concentrations were determined by tandem mass spectrometry.

RESULTS

Average S-methadone apparent oral clearance was 35 and 45% lower in CYP2B6*1/*6 and CYP2B6*6/*6 genotypes, respectively, compared with CYP2B6*1/*1. R-methadone apparent oral clearance was 25 and 35% lower in CYP2B6*1/*6 and CYP2B6*6/*6 genotypes, respectively, compared with CYP2B6*1/*1. R- and S-methadone apparent oral clearance was threefold and fourfold greater in CYP2B6*4 carriers. IV and oral R- and S-methadone metabolism was significantly lower in CYP2B6*6 carriers compared with that of CYP2B6*1 homozygotes and greater in CYP2B6*4 carriers. Methadone metabolism and clearance were lower in African Americans in part because of the CYP2B6*6 genetic polymorphism.

CONCLUSIONS

CYP2B6 polymorphisms influence methadone plasma concentrations, because of altered methadone metabolism and thus clearance. Genetic influence is greater for oral than IV methadone and S- than R-methadone. CYP2B6 pharmacogenetics explains, in part, interindividual variability in methadone elimination. CYP2B6 genetic effects on methadone metabolism and clearance may identify subjects at risk for methadone toxicity and drug interactions.

Expansion of a PBPK model to predict disposition in pregnant women of drugs cleared via multiple CYP enzymes, including CYP2B6, CYP2C9 and CYP2C19

AIM

Conducting PK studies in pregnant women is challenging. Therefore, we asked if a physiologically-based pharmacokinetic (PBPK) model could be used to predict the disposition in pregnant women of drugs cleared by multiple CYP enzymes.

METHODS

We expanded and verified our previously published pregnancy PBPK model by incorporating hepatic CYP2B6 induction (based on in vitro data), CYP2C9 induction (based on phenytoin PK) and CYP2C19 suppression (based on proguanil PK), into the model. This model accounted for gestational age-dependent changes in maternal physiology and hepatic CYP3A, CYP1A2 and CYP2D6 activity. For verification, the pregnancy-related changes in the disposition of methadone (cleared by CYP2B6, 3A and 2C19) and glyburide (cleared by CYP3A, 2C9 and 2C19) were predicted.

RESULTS

Predicted mean post-partum to second trimester (PP : T2 ) ratios of methadone AUC, Cmax and Cmin were 1.9, 1.7 and 2.0, vs. observed values 2.0, 2.0 and 2.6, respectively. Predicted mean post-partum to third trimester (PP : T3 ) ratios of methadone AUC, Cmax and Cmin were 2.1, 2.0 and 2.4, vs. observed values 1.7, 1.7 and 1.8, respectively. Predicted PP : T3 ratios of glyburide AUC, Cmax and Cmin were 2.6, 2.2 and 7.0 vs. observed values 2.1, 2.2 and 3.2, respectively.

CONCLUSIONS

Our PBPK model integrating prior physiological knowledge, in vitro and in vivo data, allowed successful prediction of methadone and glyburide disposition during pregnancy. We propose this expanded PBPK model can be used to evaluate different dosing scenarios, during pregnancy, of drugs cleared by single or multiple CYP enzymes.

Voriconazole pharmacokinetics and exposure-response relationships: assessing the links between exposure, efficacy and toxicity

The triazole antifungal voriconazole (VCZ) exhibits broad-spectrum antifungal activity and is the first-line treatment for invasive aspergillosis. Highly variable, non-linear pharmacokinetics, metabolism via the polymorphic drug-metabolising enzyme CYP2C19, and a range of serious adverse events (AEs) including hepatotoxicity and neurotoxicity complicate the clinical utility of VCZ. As interest in optimising VCZ treatment has increased, a growing number of studies have examined the relationships between VCZ exposure and efficacy in the treatment and prevention of invasive fungal infections, as well as associations with VCZ-related AEs. This review provides a critical analysis of VCZ pharmacokinetics and exposure-response (E-R) relationships, assessing the links between VCZ exposure, efficacy and toxicity. Low VCZ exposure has frequently been associated with a higher incidence of treatment failure; fewer studies have addressed E-R relationships with prophylactic VCZ. VCZ-related neurotoxicity appears common at high VCZ concentrations and can be minimised by maintaining concentrations below the recommended upper concentration thresholds; hepatotoxicity appears to be associated with increased VCZ exposure but is also prevalent at low concentrations. Further research should aim to inform and optimise the narrow therapeutic range of VCZ as well as develop interventions to individualise VCZ dosing to achieve maximal efficacy with minimal toxicity.

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.

Ventricular bigeminy associated with voriconazole, methadone and esomeprazole

CASE

We report a case of ventricular bigeminy with concomitant administration of methadone, voriconazole and esomeprazole in a Caucasian woman aged 26 with acute lymphoblastic leukaemia. Plasma concentrations of voriconazole and methadone were high, 12.4 mg/l (therapeutic range: 1-4 mg/l) and 1.6 mg/l (therapeutic range: 0.2-0.4 mg/l), respectively. In the absence of esomeprazole, no more episode of cardiac arrhythmia occurred and 7 days after, methadone plasma concentration fell at 0.57 ml/l while voriconazole concentration was at 5.5 mg/l. We speculate that a pharmacokinetic interaction between methadone and voriconazole was amplified by the addition of esomeprazole. This led to the large increase of the plasma concentration of methadone and was potentially responsible for its cardiac toxicity.

CONCLUSION

Physicians should be aware of the potential interaction between voriconazole, esomeprazole and methadone leading to arrhythmia. The inhibitory potential of voriconazole is possibly increased by esomeprazole.

Is oral methadone better than placebo or other oral/transdermal opioids in the management of pain?

AIM

To address the question: is oral methadone better than placebo, or other oral/transdermal opioids in the management of cancer pain?

METHOD

A literature search was performed to identify relevant studies. Search strategies included: (1) methadone (title) AND placebo (title or abstract) AND pain (title or abstract); (2) methadone (title) AND randomized (title or abstract) AND pain (title or abstract) AND cancer (title or abstract). Papers were reviewed for relevance to first-line opioid therapy.

RESULTS

No studies were identified comparing methadone to placebo for cancer pain. A single study compared methadone to placebo for neuropathic pain and demonstrated evidence of analgesic effect at a dose of 20 mg/day but not at a dose of 10 mg/day. Four studies compared oral methadone to either oral morphine, or oral morphine and transdermal fentanyl in a first-line setting: Gourlay 1986 (N = 18), Ventafridda 1986 (N =54), Bruera 2004 (N = 106) and Mercadante 2008 (N = 108). All studies demonstrated comparable, but not superior, analgesia with, overall, a comparable adverse effect profile. The duration of the study period for the three largest studies was 28 days. Two of these studies, Ventafridda 1986 and Mercadante 2008, indicated that, over time, the opioid escalation index was lower for methadone than for morphine. One study that used a 2:1 dose ratio between morphine and methadone was associated with a high attrition rate in the first week because of excessive sedation. This effect was not seen in the study that used a 4:1 morphine to methadone dose ratio with dose titration.

CONCLUSION

This limited data suggests that (1) methadone may be an equally effective candidate for first-line opioid therapy, (2) that it is possibly less expensive, (3) that there may be a propensity to sedation and dose accumulation unless there is close monitoring and conservative dose selection and (4) that it should be initiated with a calculated dose based on a morphine to methadone dose ratio of not less than 4:1.

[Level of evidence for therapeutic drug monitoring of voriconazole]

Voriconazole is a major antifungal drug with activity against endemic fungi, Candida and Aspergillus species in immunocompromised patients. Voriconazole has a good bioavailability, an high protein binding percentage in plasma and is metabolized in liver via CYP2C19. It presents important neuro- and hepatotoxicities. Some studies determined trough concentrations of voriconazole in plasma using liquid chromatography coupled with UV or tandem mass detection. These studies showed a relationship between trough concentrations of voriconazole and efficacy or toxicity. Indeed, some studies reported a relationship between a lack of clinical response and concentrations below 1 or 2 μg/mL according to the localization of infection, while toxicities are frequently observed at concentrations above 5 μg/mL. Some particular populations will have to be taken into account such as children, patients with hemodialysis-dependent renal deficiency or hepatic insufficiency, cystic fibrosis patients or those treated concomitantly with interfering drugs. According to our survey, therapeutic drug monitoring of voriconazole appears recommended. However, controlled studies are still necessary to validate it prospectively and to evaluate pharmacokinetically-based methods proposed for individual dose adjustment.

Pharmacogenomics of the triazole antifungal agent voriconazole

Genetic polymorphisms in drug-metabolizing enzymes are frequently responsible for high variability in the pharmacokinetics of certain drugs leading to large variations in drug efficacy and adverse drug effects, or large ranges of the doses required for optimal drug efficacy. Voriconazole is a triazole antifungal agent which has been available for several years and has potent in vitro and in vivo activity against a broad spectrum of medically important pathogens, including Aspergillus, Cryptococcus and Candida. Voriconazole is extensively metabolized by the cytochrome P450 system with CYP2C19 being the major route for elimination. Thus, polymorphisms in the CYP2C19 gene have substantial impact on the pharmacokinetics of voriconazole and its interactions with other drugs. This article summarizes the current knowledge regarding CYP2C19 and discusses the influences of other drug-metabolizing enzymes and drug transporters on voriconazole disposition.

Update on the optimal use of voriconazole for invasive fungal infections

Voriconazole is an extended-spectrum triazole with excellent bioavailability that has now become the treatment of choice for aspergillosis. It has a unique side effect profile compared with other azoles, as well as a number of clinically important drug-drug interactions. These factors, along with a correlation between increased serum levels and improved outcomes, have prompted an interest in therapeutic drug monitoring of this agent. The pharmacology and clinical outcomes data of voriconazole are presented in this review.

Methadone for Cancer Pain

Pain is one of the most common and incapacitating symptoms experienced by patients with advanced cancer. Methadone is a potent opioid with strong affinity for the µ opioid receptor. In addition to being a potent µ opioid receptor ligand, methadone blocks the N-methyl-D-aspartic acid receptor and modulates neurotransmitters involved in descending pain modulation. These 3 properties enhance analgesic activity. Methadone's lack of active metabolites makes it an attractive option when opioid responsiveness declines and renal insufficiency complicates opioid therapy. A lipophilic opioid, methadone can be given by multiple routes. Clinical trial data show equivalence with morphine as an analgesic in moderate to severe cancer pain. Further investigations are needed to define the role of methadone in the management of breakthrough pain and neuropathic pain and to determine whether it is truly superior to morphine, the gold standard of cancer analgesia.

Torsades de pointes associated with methadone and voriconazole

This report concerns a case of torsades de pointes (TdP) associated with the concomitant administration of methadone and voriconazole in a patient with comorbid medical conditions. A 57-year-old man, with a medical history of human immunodeficiency virus, infective endocarditis, hepatitis C and orbital Aspergillus infection, was admitted to the intensive care unit following several episodes of TdP. The patient was being treated with methadone for opioid addiction and had started taking voriconazole 2 weeks prior for orbital Aspergillosis. He experienced multiple episodes of TdP with a prolonged QTc interval (>600 ms). The pronounced inhibitory impact of voriconazole on methadone metabolism via the cytochrome P450 (CYP)2B6 isoenzyme was identified as a probable cause of the arrhythmia. Voriconazole was subsequently temporarily withheld and the methadone dose was significantly reduced. The patient received an implantable cardioverter-defibrillator, did not experience additional episodes of TdP during hospitalisation, and was discharged from the hospital on day 13.

Comprehensive in vitro analysis of voriconazole inhibition of eight cytochrome P450 (CYP) enzymes: major effect on CYPs 2B6, 2C9, 2C19, and 3A

Voriconazole is an effective antifungal drug, but adverse drug-drug interactions associated with its use are of major clinical concern. To identify the mechanisms of these interactions, we tested the inhibitory potency of voriconazole with eight human cytochrome P450 (CYP) enzymes. Isoform-specific probes were incubated with human liver microsomes (HLMs) (or expressed CYPs) and cofactors in the absence and the presence of voriconazole. Preincubation experiments were performed to test mechanism-based inactivation. In pilot experiments, voriconazole showed inhibition of CYP2B6, CYP2C9, CYP2C19, and CYP3A (half-maximal [50%] inhibitory concentrations, <6 microM); its effect on CYP1A2, CYP2A6, CYP2C8, and CYP2D6 was marginal (<25% inhibition at 100 microM voriconazole). Further detailed experiments with HLMs showed that voriconazole is a potent competitive inhibitor of CYP2B6 (K(i) < 0.5), CYP2C9 (K(i) = 2.79 microM), and CYP2C19 (K(i) = 5.1 microM). The inhibition of CYP3A by voriconazole was explained by noncompetitive (K(i) = 2.97 microM) and competitive (K(i) = 0.66 microM) modes of inhibition. Prediction of the in vivo interaction of voriconazole from these in vitro data suggests that voriconazole would substantially increase the exposure of drugs metabolized by CYP2B6, CYP2C9, CYP2C19, and CYP3A. Clinicians should be aware of these interactions and monitor patients for adverse effects or failure of therapy.