Involvement of cytochrome P450 3A4 enzyme in the N-demethylation of methadone in human liver microsomes

Drug-Drug Interaction Studies of Esmethadone (REL-1017) Involving CYP3A4- and CYP2D6-Mediated Metabolism

BACKGROUND AND OBJECTIVE

Esmethadone (dextromethadone; d-methadone; S-methadone (+)-methadone; REL-1017) is the opioid inactive dextro-isomer of racemic methadone. Esmethadone is a low potency N-methyl-D-aspartate (NMDA) receptor channel blocker with higher affinity for GluN2D subtypes. Esmethadone showed robust, rapid, and sustained antidepressant effects in patients with major depressive disorder (MDD) with inadequate response to ongoing serotonergic antidepressant treatment.

METHODS

Here we described the results of in vitro and phase 1 clinical trials aimed at investigating the esmethadone metabolism and possible drug-drug interactions.

RESULTS

Esmethadone is primarily metabolized to EDDP (2-ethylene-1,5-dimethyl-3,3-diphenylpyrrolidine) by multiple enzymes, including CYP3A4/5 and CYP2B6. In vitro studies showed that esmethadone inhibits CYP2D6 with IC50 of 9.6 μM and is an inducer of CYP3A4/5. The clinical relevance of the inhibition of CYP2D6 and the induction of CYP3A4 were investigated by co-administering esmethadone and dextromethorphan (a substrate for CYP2D6) or midazolam (a substrate for CYP3A4) in healthy volunteers. The administration of esmethadone at the dosage of 75 mg (which is the loading dose administered to patients in MDD clinical trials) significantly increased the exposure (AUC) of both dextromethorphan and its metabolite dextrorphan by 2.71 and 3.11-fold, respectively. Esmethadone did not modify the pharmacokinetic profile of midazolam, while it increased Cmax and AUC of its metabolite 1'-hydroxymidazolam by 2.4- and 3.8-fold, respectively. A second study evaluated the effect of the CYP3A4 inhibitor cobicistat on the pharmacokinetics of esmethadone. Cobicistat slightly increase (+32%) the total exposure (AUC0-inf) of esmethadone.

CONCLUSIONS

In summary, esmethadone demonstrated a negligible effect on CYP3A4 induction and its metabolism was not meaningfully affected by strong CYP3A4 inhibitors while it increased exposure of CYP2D6-metabolized drugs.

Predicting disruptions to drug pharmacokinetics and the risk of adverse drug reactions in non-alcoholic steatohepatitis patients

The liver plays a central role in the pharmacokinetics of drugs through drug metabolizing enzymes and transporters. Non-alcoholic steatohepatitis (NASH) causes disease-specific alterations to the absorption, distribution, metabolism, and excretion (ADME) processes, including a decrease in protein expression of basolateral uptake transporters, an increase in efflux transporters, and modifications to enzyme activity. This can result in increased drug exposure and adverse drug reactions (ADRs). Our goal was to predict drugs that pose increased risks for ADRs in NASH patients. Bibliographic research identified 71 drugs with reported ADRs in patients with liver disease, mainly non-alcoholic fatty liver disease (NAFLD), 54 of which are known substrates of transporters and/or metabolizing enzymes. Since NASH is the progressive form of NAFLD but is most frequently undiagnosed, we identified other drugs at risk based on NASH-specific alterations to ADME processes. Here, we present another list of 71 drugs at risk of pharmacokinetic disruption in NASH, based on their transport and/or metabolism processes. It encompasses drugs from various pharmacological classes for which ADRs may occur when used in NASH patients, especially when eliminated through multiple pathways altered by the disease. Therefore, these results may inform clinicians regarding the selection of drugs for use in NASH patients.

Pharmacokinetics and pharmacodynamics of l-methadone in isoflurane-anaesthetized and mechanically ventilated ponies

OBJECTIVE

To evaluate the pharmacokinetics and selected pharmacodynamic effects of a commercially available l-methadone/fenpipramide combination administered to isoflurane anaesthetized ponies.

STUDY DESIGN

Prospective single-group interventional study.

ANIMALS

A group of six healthy adult research ponies (four mares, two geldings).

METHODS

Ponies were sedated with intravenous (IV) detomidine (0.02 mg kg^-1) and butorphanol (0.01 mg kg^-1) for an unrelated study. Additional IV detomidine (0.004 mg kg^-1) was administered 85 minutes later, followed by induction of anaesthesia using IV diazepam (0.05 mg kg^-1) and ketamine (2.2 mg kg^-1). Anaesthesia was maintained with isoflurane in oxygen. Baseline readings were taken after 15 minutes of stable isoflurane anaesthesia. l-Methadone (0.25 mg kg^-1) with fenpipramide (0.0125 mg kg^-1) was then administered IV. Selected cardiorespiratory variables were recorded every 10 minutes and compared to baseline using the Wilcoxon signed-rank test. Adverse events were recorded. Arterial plasma samples for analysis of plasma concentrations and pharmacokinetics of l-methadone were collected throughout anaesthesia at predetermined time points. Data are shown as mean ± standard deviation or median and interquartile range (p < 0.05).

RESULTS

Plasma concentrations of l-methadone showed a rapid initial distribution phase followed by a slower elimination phase which is best described with a two-compartment model. The terminal half-life was 44.3 ± 18.0 minutes, volume of distribution 0.43 ± 0.12 L kg^-1 and plasma clearance 7.77 ± 1.98 mL minute^-1 kg^-1. Mean arterial blood pressure increased from 85 (±16) at baseline to 100 (±26) 10 minutes after l-methadone/fenpipramide administration (p = 0.031). Heart rate remained constant. In two ponies fasciculations occurred at different time points after l-methadone administration.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of a l-methadone/fenpipramide combination to isoflurane anaesthetized ponies led to a transient increase in blood pressure without concurrent increases in heart rate. Pharmacokinetics of l-methadone were similar to those reported for conscious horses administered racemic methadone.

Development of template systems for ligand interactions of CYP3A5 and CYP3A7 and their distinctions from CYP3A4 template

Starting from established CYP3A4 Template (DMPK. 2019, and 2020), CYP3A5 and CYP3A7 Templates have been constructed to be reliable tools for verification of their distinct catalytic properties. A distinct occupancy was observed on CYP3A4-selective ligands, but not on the non-selective ligands, in simulation experiments. These ligands often invade into Bay-1 region during the migration from Entrance to Site of oxidation in simulation experiments. These results offered an idea of the distinct localization of Bay-1 residue on CYP3A5 Template, in which the Bay-1 residue stayed closely to Template border. The idea also accounted for the higher oxidation rates of CYP3A5, than of CYP3A4, of noscapine and schisantherin E through their enhanced sitting-stabilization. Typical CYP3A7 substrates such as zonisamide and retinoic acids took their placements without occupying a left side region of Template for their metabolisms. In turn, the occupancies of the left-side region were inevitably observed among poor ligands of CYP3A7. Altered extent of IJK-Interaction or localization of a specific residue at the left-side would thus explain distinct catalytic properties of CYP3A7 on Template. These data suggest the alteration of each one of Template region, from CYP3A4 Template, led to the distinct catalytic properties of CYP3A5 and CYP3A7 forms.

Pharmacogenomics of methadone: a narrative review of the literature

Background: Methadone, a synthetic opioid with longer duration of action and lower abuse potential compared with morphine, is used to prevent opioid withdrawal, as well as to manage chronic and acute surgical pain. The variability in response to methadone has been widely recognized. The purpose of this article is to review the literature on the pharmacogenetic factors underlying this variability. Materials & methods: This is a narrative overview of the literature on the genetic variants affecting pharmacodynamics and pharmacokinetics of methadone, retrieved from searches of databases such as PubMed and google scholar. Discussion: Clinical responses to methadone may be affected by genetic variants in the opioidergic, dopaminergic and neurotrophic pathways. Polymorphisms in genes related to disposition and elimination of methadone alter the pharmacokinetics, and possibly pharmacodynamics of methadone. Cytochrome P450 enzymes and P-glycoprotein variants contribute to the interindividual variability in methadone pharmacokinetics. Evidence for single gene variants affecting methadone response remains weak. Multiple genetic variants must be considered in conjunction to improve predictive ability. Conclusion: Evidence remains scarce at this time, to recommend pharmacogenetic testing before methadone administration. Well-powered clinical studies are needed with population pharmacokinetic-pharmacodynamic modeling and multigenetic signature-based predictions to enable tailored use of methadone in clinical practice.

Magnesium Salt, a Simple Strategy to Improve Methadone Analgesia in Chronic Pain: An Isobolographic Preclinical Study in Neuropathic Mice

Analgesic efficacy of methadone in cancer and chronic non-cancer pains is greater than that of other opioids, probably because of its unique pharmacokinetics properties and also because it targets glutamatergic receptors in addition to µ-opioid receptors. However, methadone has drawbacks which are clearly related to dosing and treatment duration. The authors hypothesized that the antinociceptive efficacy of methadone could be synergistically potentiated by magnesium and copper salts in a preclinical mouse model of chronic pain, using the intraplantar formalin test as algesimetric tool. The spared nerve injury mice model was used to generate mononeuropathy. A low dose (0.25%) formalin was injected in the neuropathic limb in order to give rise only to Phase I response, resulting from direct activation by formalin of nociceptive primary afferents. Licking/biting of the formalin-injected limb was evaluated as nociceptive behavior during a 35-min observation period. Dose-response curves for intraperitoneal magnesium sulfate (10, 30, 100, and 300 mg/kg i.p.), copper sulfate (0.1, 0.3, 1, and 3 mg/kg i.p.) and methadone (0.1, 0.3, 1, and 3 mg/kg i.p.) allowed to combine them in equieffective doses and to determine their interaction by isobolographic analysis. Magnesium sulfate, copper sulfate and methadone dose-dependently decreased the nociceptive response evoked by formalin injection, the respective ED₅₀ being 76.38, 1.18, and 0.50 mg/kg i.p. Isobolographic analysis showed a superadditive interaction for magnesium and methadone. Indeed, despite that both ED₅₀ are obviously equieffective, the ED₅₀ for the MgSO4/methadone combination contained less than one third of the methadone having the ED₅₀ for methadone alone. For the CuSO₄/methadone combination, the interaction was only additive. Extrapolated to clinical settings, the results suggest that magnesium salts might be used to improve synergistically the efficacy of methadone in neuropathy, which would allow to reduce the dose of methadone and its associated side effects.

The molecular mechanisms that underpin the biological benefits of full-spectrum cannabis extract in the treatment of neuropathic pain and inflammation

Cannabis has been shown to be beneficial in the treatment of pain and inflammatory diseases. The biological effect of cannabis is mainly attributed to two major cannabinoids, tetrahydrocannabinol and cannabidiol. In the majority of studies to-date, a purified tetrahydrocannabinol and cannabidiol alone or in combination have been extensively examined in many studies for the treatment of numerous disorders including pain and inflammation. However, few studies have investigated the biological benefits of full-spectrum cannabis plant extract. Given that cannabis is known to generate a large number of cannabinoids along with numerous other biologically relevant products including terpenes, studies involving purified tetrahydrocannabinol and/or cannabidiol do not consider the potential biological benefits of the full-spectrum cannabis extracts. This may be especially true in the case of cannabis as a potential treatment of pain and inflammation. Herein, we review the pre-clinical physiological and molecular mechanisms in biological systems that are affected by cannabis.

Opioid Toxidrome Following Grapefruit Juice Consumption in the Setting of Methadone Maintenance

BACKGROUND

Methadone is a synthetic mu-opioid receptor agonist used in the treatment of chronic pain and opioid dependence. Methadone is metabolized by several cytochrome P450 isoenzymes; primarily CYP3A4, CYP2B6, and CYP2D6 before renal and fecal elimination. Exposure to substances like grapefruit juice, that inhibit these isoenzymes may result in increased blood levels of methadone, and thus may manifest clinically as unexpected opioid toxicity.

CASE

A 51-year-old male was found unresponsive. He was hypoxic and bradypneic with pinpoint pupils. Multiple boluses followed by infusion of naloxone were required before improvement of respiratory status. Upon awakening, the patient reported participating in an opioid treatment program where he is administered 90 mg of oral methadone daily and denied any other substance use. On further questioning, he admitted to drinking grapefruit juice (estimated to be approximately 500 mL/day) every day for 3 consecutive days before presentation. The patient was discharged home after being counseled to stop drinking grapefruit juice.

DISCUSSION

Grapefruit juice is known to be an inhibitor of the CYP3A4 isoenzyme. Various studies demonstrate that through CYP3A4 inhibition, grapefruit juice increases serum levels of opioids, such as methadone, though no clinically significant effects have been reported.

CONCLUSIONS

Grapefruit juice inhibits the metabolism of methadone, raising its serum levels. To our knowledge, this is the first reported case in which the interaction between grapefruit juice and methadone was significant enough to cause an opioid toxidrome. It is, therefore, recommended that opioid treatment programs (OTPs) advise patients about this interaction before administering methadone.

Enantioselective capillary electrophoresis for pharmacokinetic analysis of methadone and 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine in equines anesthetized with ketamine and isoflurane

An enantioselective assay for the determination of methadone and its main metabolite 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine in equine plasma based on capillary electrophoresis with highly sulfated γ-cyclodextrin as chiral selector and electrokinetic analyte injection is described. The assay is based on liquid/liquid extraction of the analytes at alkaline pH from 0.1 mL plasma followed by electrokinetic sample injection of the analytes from the extract across a buffer plug without chiral selector. Separation occurs cationically at normal polarity in a pH 3 phosphate buffer containing 0.16% (w/v) of highly sulfated γ-cyclodextrin. The developed assay is precise (intra- and interday RSD < 4% and < 7%, respectively), is capable to determine enantiomer levels of methadone and 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine in plasma down to 2.5 ng/mL, and was successfully applied to monitor enantiomer drug and metabolite levels in plasma of a pony that was anesthetized with racemic ketamine and isoflurane and received a bolus of racemic methadone and a bolus followed by constant rate infusion of racemic methadone. The data suggest that the assay is well suited for pharmacokinetic purposes.

Interaction of Human Drug-Metabolizing CYP3A4 with Small Inhibitory Molecules

Binding of small inhibitory compounds to human cytochrome P450 3A4 (CYP3A4) could interfere with drug metabolism and lead to drug-drug interactions, the underlying mechanism of which is not fully understood due to insufficient structural information. This study investigated the interaction of recombinant CYP3A4 with a nonspecific inhibitor metyrapone, antifungal drug fluconazole, and protease inhibitor phenylmethanesulfonyl fluoride (PMSF). Metyrapone and fluconazole are classic type II ligands that inhibit CYP3A4 with medium strength by ligating to the heme iron, whereas PMSF, lacking the heme-ligating moiety, acts as a weak type I ligand and inhibitor of CYP3A4. High-resolution crystal structures revealed that the orientation of metyrapone is similar but not identical to that in the previously reported 1W0G model, whereas the flexible fluconazole adapts a conformer markedly different from that observed in the target CYP51 enzymes, which could explain its high potential for cross-reactivity. Besides hydrophobic and aromatic interactions with the heme and active site residues, both drugs establish water-mediated contacts that stabilize the inhibitory complexes. PMSF also binds near the catalytic center, with the phenyl group parallel to the heme. However, it does not displace the water ligand and is held in place via strong H-bonds formed by the sulfofluoride moiety with Ser119 and Arg212. Collectively, our data suggest that PMSF might have multiple binding sites and likely occupies the high-affinity site in the crystal structure. Moreover, its hydrolysis product, phenylmethanesulfonic acid, can also access and be retained in the CYP3A4 active site. Therefore, to avoid experimental artifacts, PMSF should be excluded from purification and assay solutions.

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.

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.

Inhibitory Effects of Juices Prepared from Individual Vegetables on CYP3A4 Activity in Recombinant CYP3A4 and LS180 Cells

Human intestinal absorption and drug metabolism vary to a large extent among individuals. For example, CYP3A4 activity has large individual variation that cannot be attributed to only genetic differences. Various flavonoids in vegetables, such as kaempferol and quercetin, possess inhibitory effects, and some vegetable and fruit juices have also been found to inhibit CYP3A4 activity. Therefore, differences in daily intake of flavonoid-containing vegetables may induce individual variation in intestinal bioavailability. To identify a vegetable that strongly inhibits CYP3A4, we investigated the effects of juices, prepared from individual vegetables, on CYP3A4 activity using recombinant CYP3A4 and LS180 cells in this study. Nine vegetable juices (cabbage, Japanese radish, onion, tomato, eggplant, carrot, Chinese cabbage, green pepper, and lettuce), were prepared and recombinant CYP3A4 and LS180 cells were used for evaluation of CYP3A4 activity. Metabolism to 6β-hydroxytestosterone by recombinant CYP3A4 was strongly inhibited by cabbage, onion, and green pepper juices, and cabbage and green pepper juices significantly inhibited CYP3A4 activity in a preincubation time-dependent manner. In addition, CYP3A4 activity in LS180 cells was significantly inhibited by cabbage and onion juices. In conclusion, this study showed that juices prepared from some individual vegetables could significantly inhibit CYP3A4 activity. Therefore, variation in the daily intake of vegetables such as cabbage and onion may be one of the factors responsible for individual differences in intestinal bioavailability.

Effects of cytochrome P450 single nucleotide polymorphisms on methadone metabolism and pharmacodynamics

Methadone is a synthetic, long-acting opioid with a single chiral center forming two enantiomers, (R)-methadone and (S)-methadone, each having specific pharmacological actions. Concentrations of (R)- and (S)-methadone above therapeutic levels have the ability to cause serious, life-threatening, and fatal side effects. This toxicity can be due in part to the pharmacogenetics of an individual, which influences the pharmacokinetic and pharmacodynamic properties of the drug. Methadone is primarily metabolized in the liver by cytochrome P450 (CYP) enzymes, predominately by CYP2B6, followed by CYP3A4, 2C19, 2D6, and to a lesser extent, CYP2C18, 3A7, 2C8, 2C9, 3A5, and 1A2. Single nucleotide polymorphisms (SNPs) located within CYPs have the potential to play an important role in altering methadone metabolism and pharmacodynamics. Several SNPs in the CYP2B6, 3A4, 2C19, 2D6, and 3A5 genes result in increases in methadone plasma concentrations, decreased N-demethylation, and decreased methadone clearance. In particular, carriers of CYP2B6*6/*6 may have a greater risk for detrimental adverse effects, as methadone metabolism and clearance are diminished in these individuals. CYP2B6*4, on the other hand, has been observed to decrease plasma concentrations of methadone due to increased methadone clearance. The involvement, contribution, and understanding the role of SNPs in CYP2B6, and other CYP genes, in methadone metabolism can improve the therapeutic uses of methadone in patient outcome and the development of personalized medicine.

Genetic Testing for Opioid Pain Management: A Primer

Patients see their primary care physicians (PCPs) for a variety of medical conditions, chronic pain being one of the most common. An increased use of prescription medications (especially opioids) has led to an increase in adverse drug reactions and has heightened our awareness of the variability in response to medications. Opioids and other pain adjuvants are widely used, and drug-drug interactions involving these analgesics can be problematic and potentially lethal. Pharmacogenetics has improved our understanding of drug efficacy and response, opened doors to individual tailoring of medical management, and created a series of ethical and economic considerations. Since it is a relatively new field, genetic testing has not been fully integrated into the primary care setting. The purpose of this paper is to review the metabolism of commonly prescribed opioids, discuss the economic and ethical issues, and provide PCPs with an understanding of how to incorporate genetic testing into routine use to improve clinical practice and patient management.

Use of Methadone as an Adjuvant Medication to Low-Dose Opioids for Neuropathic Pain in the Frail Elderly: A Case Series

Palliative care clinicians are increasingly involved in the care of elderly patients suffering from chronic malignant and nonmalignant illnesses, of which neuropathic pain is a prevalent problem. As a person becomes more frail, pain medications such as opioid analgesics and adjuvant pain medications can result in unwanted effects such as sedation, confusion, and increased risk of falls. Treating pain in patients with advanced dementia or neurodegenerative diseases that can affect swallowing is particularly challenging because most adjuvant pain medications used to ameliorate neuropathic pain must be taken orally. Furthermore, dosing of neuropathic medications is limited by renal function, which is often impaired in the elderly due to both normal aging and renal disease. Methadone is an opioid analgesic that is effective in the treatment of neuropathic pain, is excreted by the bowels, is highly lipophilic, and can be administered through the oral, buccal, or sublingual routes. We present three cases highlighting the use of low-dose adjuvant methadone to manage complex neuropathic pain in the frail elderly.

Role of Cytochrome P450 2C8 in Drug Metabolism and Interactions

During the last 10-15 years, cytochrome P450 (CYP) 2C8 has emerged as an important drug-metabolizing enzyme. CYP2C8 is highly expressed in human liver and is known to metabolize more than 100 drugs. CYP2C8 substrate drugs include amodiaquine, cerivastatin, dasabuvir, enzalutamide, imatinib, loperamide, montelukast, paclitaxel, pioglitazone, repaglinide, and rosiglitazone, and the number is increasing. Similarly, many drugs have been identified as CYP2C8 inhibitors or inducers. In vivo, already a small dose of gemfibrozil, i.e., 10% of its therapeutic dose, is a strong, irreversible inhibitor of CYP2C8. Interestingly, recent findings indicate that the acyl-β-glucuronides of gemfibrozil and clopidogrel cause metabolism-dependent inactivation of CYP2C8, leading to a strong potential for drug interactions. Also several other glucuronide metabolites interact with CYP2C8 as substrates or inhibitors, suggesting that an interplay between CYP2C8 and glucuronides is common. Lack of fully selective and safe probe substrates, inhibitors, and inducers challenges execution and interpretation of drug-drug interaction studies in humans. Apart from drug-drug interactions, some CYP2C8 genetic variants are associated with altered CYP2C8 activity and exhibit significant interethnic frequency differences. Herein, we review the current knowledge on substrates, inhibitors, inducers, and pharmacogenetics of CYP2C8, as well as its role in clinically relevant drug interactions. In addition, implications for selection of CYP2C8 marker and perpetrator drugs to investigate CYP2C8-mediated drug metabolism and interactions in preclinical and clinical studies are discussed.

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.

A Review of the Toxicity of HIV Medications II: Interactions with Drugs and Complementary and Alternative Medicine Products

For many patients today, HIV has become a chronic disease. For those patients who have access to and adhere to lifelong antiretroviral (ARV) therapy, the potential for drug-drug interactions has become a real and life-threatening concern. It is known that most ARV drug interactions occur through the cytochrome P450 (CYP) pathway. Medications for comorbid medical conditions, holistic supplements, and illicit drugs can be affected by CYP inhibitors and inducers and have the potential to cause harm and toxicity. Protease inhibitors (PIs) tend to inhibit CYP3A4, while most non-nucleoside reverse transcriptase inhibitors (NNRTIs) tend to induce the enzyme. As such, failure to adjust the dose of co-administered medications, such as statins and steroids, may lead to serious complications including rhabdomyolysis and hypercortisolism, respectively. Similarly, gastric acid blockers can decrease several ARV absorption, and warfarin doses may need to be adjusted to maintain therapeutic concentrations. Illicit drugs such as methylenedioxymethamphetamine (MDMA, "ecstasy") in combination with PIs lead to increased toxicity, while the concomitant administration of sedative drugs such as midazolam and alprazolam in patients taking PIs can result in prolonged sedation, delayed recovery, and increased length of stay. Even supplements like St. John's Wort can alter PI concentrations. In theory, any drug that is metabolized by CYP has potential for a pharmacokinetic drug-drug interaction with all PIs, cobicistat, and most NNRTIs. When adding a new medication to an ARV regimen, use of a drug-drug interaction software and/or consultation with a clinical pharmacist/pharmacologist or HIV specialist is recommended.

Fatal methadone toxicity: potential role of CYP3A4 genetic polymorphism

Methadone is difficult to administer as a therapeutic agent because of a wide range of interindividual pharmacokinetics, likely due to genetic variability of the CYP450 enzymes responsible for metabolism to its principal metabolite 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP). CYP3A4 is one of the primary CYP450 isoforms responsible for the metabolism of methadone to EDDP in humans. The purpose of this study was to evaluate the role of CYP3A4 genetic polymorphisms in accidental methadone fatalities. A study cohort consisting of 136 methadone-only and 92 combined methadone/benzodiazepine fatalities was selected from cases investigated at the West Virginia and Kentucky Offices of the Chief Medical Examiner. Seven single nucleotide polymorphisms (SNPs) were genotyped within the CYP3A4 gene. Observed allelic and genotypic frequencies were compared with expected frequencies obtained from The National Center for Biotechnology Information dbSNP database. SNPs rs2242480 and rs2740574 demonstrated an apparent enrichment within the methadone-only overdose fatalities compared with the control group and the general population. This enrichment was not apparent in the methadone/benzodiazepine cases for these two SNPs. Our findings indicate that there may be two or more SNPs on the CYP3A4 gene that cause or contribute to the methadone poor metabolizer phenotype.

Influence of HIV antiretrovirals on methadone N-demethylation and transport

Drug interactions involving methadone and/or HIV antiretrovirals can be problematic. Mechanisms whereby antiretrovirals induce clinical methadone clearance are poorly understood. Methadone is N-demethylated to 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) by CYP2B6 and CYP3A4 in vitro, but by CYP2B6 in vivo. This investigation evaluated human hepatocytes as a model for methadone induction, and tested the hypothesis that methadone and EDDP are substrates for human drug transporters. Human hepatocyte induction by several antiretrovirals of methadone N-demethylation, and CYP2B6 and CYP3A4 transcription, protein expression and catalytic activity, and pregnane X receptor (PXR) activation were evaluated. Methadone and EDDP uptake and efflux by overexpressed transporters were also determined. Methadone N-demethylation was generally not significantly increased by the antiretrovirals. CYP2B6 mRNA and activity (bupropion N-demethylation) were induced by several antiretrovirals, as were CYP3A4 mRNA and protein expression, but only indinavir increased CYP3A activity (alfentanil dealkylation). CYP upregulation appeared related to PXR activation. Methadone was not a substrate for uptake (OCT1, OCT2, OCT3, OATP1A2, OATP1B1, OATP1B3, OATP2B1) or efflux (P-gp, BCRP) transporters. EDDP was a good substrate for P-gp, BCRP, OCT1, OCT3, OATP1A2, and OATP1B1. OATP1A2- and OCT3-mediated EDDP uptake, and BCRP-mediated EDDP efflux transport, was inhibited by several antiretrovirals. Results show that hepatocyte methadone N-demethylation resembles expressed and liver microsomal metabolism more than clinical metabolism. Compared with clinical studies, hepatocytes underreport induction of methadone metabolism by HIV drugs. Hepatocytes are not a good predictive model for clinical antiretroviral induction of methadone metabolism and not a substitute for clinical studies. EDDP is a transporter substrate, and is susceptible to transporter-mediated interactions.

Cyclosporine-inhibitable cerebral drug transport does not influence clinical methadone pharmacodynamics

BACKGROUND

Interindividual variability and drug interaction studies suggest that blood-brain barrier drug transporters mediate human methadone brain biodistribution. In vitro and animal studies suggest that methadone is a substrate for the efflux transporter P-glycoprotein, and that P-glycoprotein-mediated transport influences brain access and pharmacologic effect. This investigation tested whether methadone is a transporter substrate in humans [corrected].

METHODS

Healthy volunteers received oral (N=16) or IV (N=12) methadone in different crossover protocols after nothing (control) or the validated P-glycoprotein inhibitor cyclosporine (4.5 mg/kg orally twice daily for 4 days, or 5 mg/kg IV over 2 h). Plasma and urine methadone and metabolite concentrations were measured by mass spectrometry. Methadone effects were measured by miosis and thermal analgesia (maximally tolerated temperature and verbal analog scale rating of discreet temperatures).

RESULTS

Cyclosporine marginally but significantly decreased methadone plasma concentrations and apparent oral clearance, but had no effect on methadone renal clearance or on hepatic N-demethylation. Cyclosporine had no effect on miosis or on R-methadone concentration-miosis relationships after either oral or IV methadone. Peak miosis was similar in controls and cyclosporine-treated subjects after oral methadone (1.4±0.4 and 1.3±0.5 mm/mg, respectively) and IV methadone (3.1±1.0 and 3.2±0.8 mm, respectively). Methadone increased maximally tolerated temperature, but analgesia testing was confounded by cyclosporine-related pain.

CONCLUSIONS

Cyclosporine did not affect methadone pharmacodynamics. This result does not support a role for cyclosporine-inhibitable transporters mediating methadone brain access and biodistribution.

Drug-drug interactions between anti-retroviral therapies and drugs of abuse in HIV systems

INTRODUCTION

Substance abuse is a common problem among HIV-infected individuals. Importantly, addictions as well as moderate use of alcohol, smoking, or other illicit drugs have been identified as major reasons for non-adherence to antiretroviral therapy (ART) among HIV patients. The literature also suggests a decrease in the response to ART among HIV patients who use these substances, leading to failure to achieve optimal virological response and increased disease progression.

AREAS COVERED

This review discusses the challenges with adherence to ART as well as observed drug interactions and known toxicities with major drugs of abuse, such as alcohol, smoking, methamphetamine, cocaine, marijuana, and opioids. The lack of adherence and drug interactions potentially lead to decreased efficacy of ART drugs and increased ART, and drugs of abuse-mediated toxicity. As CYP is the common pathway in metabolizing both ART and drugs of abuse, we discuss the possible involvement of CYP pathways in such drug interactions.

EXPERT OPINION

We acknowledge that further studies focusing on common metabolic pathways involving CYP and advance research in this area would help to potentially develop novel/alternate interventions and drug dose/regimen adjustments to improve medication outcomes in HIV patients who consume drugs of abuse.

Pharmacogenomics of methadone maintenance treatment

Methadone is the major opioid substitution therapy for opioid dependence. Dosage is highly variable and is often controlled by the patient and prescriber according to local and national policy and guidelines. Nevertheless many genetic factors have been investigated including those affecting its metabolism (CYP2B6-consistent results), efflux transport (P-gp-inconsistent results), target μ-opioid receptor (μ-opioid receptor-inconsistent results) and a host of other receptors (DRD2) and signaling elements (GIRK2 and ARRB2; not replicated). None by themselves have been able to substantially explain dosage variation (the major but not sole end point). When multiple genes have been combined such as ABCB1, CYP2B6, OPRM1 and DRD2 a greater contribution to dosage variation was found but not as yet replicated. As stabilization of dosage needs to be made rapidly, it is imperative that larger internationally based studies be instigated so that genetic contribution to dosage can be properly assessed, which may or may not tailor to different ethnic groups and each country’s policy towards an outcome that benefits all.

[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.

Pharmacokinetic drug interactions between drugs of abuse and antiretroviral medications: implications and management for clinical practice

Substance abuse and HIV/AIDS are two of the most serious, yet treatable diseases worldwide. Global access to HIV treatment continues to expand. In settings where both active illicit drug use and HIV treatment are concurrent, potentional problematic pharmacokinetic drug interactions may arise and complicate therapy. Clinical case series and carefully controlled pharmacokinetic interaction studies have been conducted between only a few drugs of abuse and approved antiretroviral therapies. Important pharmacokinetic drug interactions have been described for benzodiazepines, 3,4-methylenedioxymethamphetamine, methadone and buprenorphine; however, most have not been studied and few well-controlled studies have been conducted to adequately address the clinical implications of these interactions. The metabolism of drugs of abuse, description of the known interactions, and clinical implications and management of these interactions are reviewed. Certain interactions between drugs of abuse and antiretroviral therapies are known and others are likely based upon shared metabolic pathways. These may result in important clinical consequences. To optimize care, clinicians must be alert, knowledgeable about known and possible interactions and equipped to clinically manage the medical consequences. Moreover, there is considerable need for carefully controlled studies in this important and emerging area.

Evaluation of an enzyme immunoassay for the detection of methadone metabolite EDDP [2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine] in urine

BACKGROUND

We evaluated a new EDDP [2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine] enzyme immunoassay (EDDPI; Lin-Zhi International, Inc., Sunnyvale, CA) for the detection of this primary methadone urinary metabolite.

METHODS

All specimens were tested with two different cutoff calibrators at 150 and 300 ng/ml EDDP on an ADVIA 1200 Chemistry System auto-analyzer. Controls containing 0, -25% (negative control), and +25% (positive control) of the cutoff calibrators (Lin-Zhi) were analyzed with each batch. All urine specimens were then analyzed by high-pressure liquid chromatography/ mass spectrometry/mass spectrometry (HPLC-MS/MS) for EDDP.

RESULTS

Approximately, 42% (151) of the 362 specimens yielded positive results by the EDDP assay at 150 and/or 300 ng/ml cutoff values. Of these specimens, HPLC-MS/MS confirmed the presence of EDDP > 25 ng/ml in all 151 specimens. No specimen yielding negative EDDPI results contained EDDP by HPLC/MS/MS. At 150 ng/ml cutoff, the EDDPI demonstrated a sensitivity of 1.00, a specificity of 0.986, and an overall agreement of HPLC/MS/MS of >99%. At 300 ng/ml cutoff, the EDDPI demonstrated a sensitivity of 1.00, a specificity of 0.959, and an overall agreement of HPLC/MS/MS results of 97.5%.

CONCLUSION

The Lin-Zhi EDDPI provides a precise, reliable method for the routine detection of methadone metabolite in urine specimens, particularly in pain management compliance testing.

A review of pharmacological interactions between HIV or hepatitis C virus medications and opioid agonist therapy: implications and management for clinical practice

Global access to opioid agonist therapy and HIV/hepatitis C virus (HCV) treatment is expanding but when used concurrently, problematic pharmacokinetic and pharmacodynamic interactions may occur. Articles published from 1966 to 2012 in Medline were reviewed using the following keywords: HIV, AIDS, HIV therapy, HCV, HCV therapy, antiretroviral therapy, highly active antiretroviral therapy, drug interactions, methadone and buprenorphine. In addition, a review of abstracts from national and international meetings and conference proceedings was conducted; selected reports were reviewed as well. The metabolism of both opioid and antiretroviral therapies, description of their known interactions and clinical implications and management of these interactions were reviewed. Important pharmacokinetic and pharmacodynamic drug interactions affecting either methadone or HIV medications have been demonstrated within each class of antiretroviral agents. Drug interactions between methadone, buprenorphine and HIV medications are known and may have important clinical consequences. Clinicians must be alert to these interactions and have a basic knowledge regarding their management.

Investigation of the interactions between methadone and elvitegravir-cobicistat in subjects receiving chronic methadone maintenance

Interactions between HIV and opioid dependence therapies are known to occur. We sought to determine if such interactions occurred between methadone and elvitegravir boosted with cobicistat (EVG/COBI). We performed a within-subject open-label pharmacokinetic and pharmacodynamic study of 11 HIV-seronegative subjects stabilized on at least 2 weeks of methadone. Subjects underwent baseline and steady-state evaluation of the effect of elvitegravir 150 mg once a day (QD) boosted with 150 mg QD of cobicistat (EVG/COBI) on methadone pharmacokinetic parameters. Safety and pharmacodynamics were monitored throughout the study. Compared to baseline values, the R-methadone mean area under the concentration-time curve to the end of the dosing period (AUCtau) (5,550 versus 6,210 h · ng/ml) and mean maximum concentration of drug in serum (Cmax) (316 versus 337 ng/ml) did not significantly increase in the presence of EVG/COBI. Compared to baseline values, the S-methadone mean AUCtau (7,040 versus 7,540 h · ng/ml) and mean Cmax (446 versus 452 ng/ml) did not significantly increase in the presence of EVG/COBI. The AUCtau, Cmax, and Ctau of elvitegravir and cobicistat did not significantly differ from those of historical controls. Opioid withdrawal or overdose was not observed among subjects in this study. The addition of EVG/COBI to stabilized patients receiving methadone did not affect methadone pharmacokinetics and pharmacodynamics. These two agents can be safely coadministered.

Cytochrome P4503A does not mediate the interaction between methadone and ritonavir-lopinavir

Plasma concentrations of orally administered methadone are reduced by the human immunodeficiency virus protease inhibitor combination ritonavir and lopinavir, but the mechanism is unknown. Methadone metabolism, clearance, and drug interactions have been attributed to CYP3A4, but this remains controversial. This investigation assessed the effects of acute (2 days) and steady-state (2 weeks) ritonavir-lopinavir on intravenous and oral methadone metabolism and clearance, hepatic and intestinal CYP3A4/5 activity (using the probe substrate intravenous and oral alfentanil), and intestinal transporter activity (using oral fexofenadine) in healthy volunteers. Plasma and urine concentrations of methadone and metabolite enantiomers, and other analytes, were determined by mass spectrometry. Acute and chronic ritonavir-lopinavir reduced plasma methadone enantiomer concentrations in half, with an average 2.6- and 1.5-fold induction of systemic and apparent oral methadone clearances. Induction was attributable to stereoselectively increased hepatic methadone N-demethylation, hepatic extraction, and hepatic clearance, and there was a strong correlation between methadone N-demethylation and clearance. Methadone renal clearance was unchanged. Alfentanil's systemic clearance and hepatic extraction, apparent oral clearance, and intestinal extraction were reduced to 25%, 16%, and 35% of control, indicating strong inhibition of hepatic and intestinal CYP3A activities. Ritonavir-lopinavir (acute > chronic) increased fexofenadine exposure, suggesting intestinal P-glycoprotein inhibition. No correlation was found between methadone clearance and CYP3A activity. Acute and steady-state ritonavir-lopinavir stereoselectively induced methadone N-demethylation and clearance, despite significant inhibition of hepatic and intestinal CYP3A activity. Ritonavir-lopinavir inhibited intestinal transporters activity but had no effect on methadone bioavailability. These results do not support a significant role for CYP3A or ritonavir-lopinavir-inhibitable intestinal transporters in single-dose methadone disposition.

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.

Pharmacogenetics of opiates in clinical practice: the visible tip of the iceberg

Opioids are the cornerstone of analgesic therapy and are used as a substitution therapy for opiate addiction. Interindividual variability in response to opioids is a significant challenge in the management of pain and substitution. Therefore, treatment with opioids requires a careful individualization of dosage to achieve an appropriate balance of efficacy and adverse effects and, consequently, avoid toxicity, particularly respiratory depression, sedation and for some, cardiac ventricular fibrillations. Many studies have investigated the association between genetic factors and the variability of response to opioids. Variants in genes encoding proteins implied in opioid pharmacokinetics (absorption, distribution, metabolism, excretion and toxicity), together with those implied in opioids direct and indirect pharmacodynamics (genes of opioid receptors and monoaminergic systems), are the most studied. Many association studies have not been replicated. The purpose of this article is to summarize pharmacogenetic data associated with some opioids frequently encountered in managed care settings.

Metabolic and toxicological considerations of the opioid replacement therapy and analgesic drugs: methadone and buprenorphine

INTRODUCTION

Methadone and buprenorphine are maintenance replacement therapies for opioid dependence; they are also used for pain management. Methadone and buprenorphine (to a lesser extent) have seen sharp increases in mortality associated with their use. They have distinct routes of metabolism (mostly cytochrome P450 dependent), and distinct pharmacologic activity of metabolites. As such, metabolism may play a role in differences in their toxicity.

AREAS COVERED

This article reviews peer-reviewed literature obtained from PubMed searches and literature referenced within. The review considers first an overview of drug use and mortality over the past decade. It then provides extensive detail on the in vitro and in vivo human metabolism of methadone and buprenorphine. Using both human and experimental animal studies it then presents the pharmacodynamic activity of parent drug and metabolites at the mu-opioid receptor, as P-glycoprotein substrates and plasma/brain concentration ratios, and activity at the hERG K(+) channel. Lessons learned from drug interaction studies in humans are then examined in an attempt to bring together the combined information.

EXPERT OPINION

The use and misuse of these drugs contributes to the epidemic in opioid-associated mortalities. A better understanding of metabolism-, transport- and co-medication-induced changes will contribute to their safer use.

Role of cytochrome P4502B6 in methadone metabolism and clearance

Methadone N-demethylation in vitro is catalyzed by hepatic cytochrome P4502B6 (CYP2B6) and CYP3A4, but clinical disposition is often attributed to CYP3A4. This investigation tested the hypothesis that CYP2B6 is a prominent CYP isoform responsible for clinical methadone N-demethylation and clearance, using the in vivo mechanism-based CYP2B6 inhibitor ticlopidine, given orally for 4 days. A preliminary clinical investigation with the CYP3A4/5 substrate probe alfentanil established that ticlopidine did not inhibit intestinal or hepatic CYP3A4/5. Subjects received intravenous plus oral (deuterium-labeled) racemic methadone before and after ticlopidine. Ticlopidine significantly and stereoselectively (S > R) inhibited methadone N-demethylation, decreasing plasma metabolite/methadone area under the curve ratios and metabolite formation clearances. Ticlopidine also significantly increased the dose-adjusted plasma area under the curve for R- and S-methadone by 20% and 60%, respectively, after both intravenous and oral dosing. CYP2B6 inhibition reduces methadone N-demethylation and clearance, and alters methadone concentrations, demonstrating an important role for CYP2B6 in clinical methadone disposition.