Differences in Methadone Metabolism by CYP2B6 Variants

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.

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.

A pharmacogenetic precision medicine approach to analgesia and sedation optimization in critically ill adults

Precision medicine is a growing field in critical care. Research increasingly demonstrated pharmacogenomic variability to be an important determinant of analgesic and sedative drug response in the intensive care unit (ICU). Genome-wide association and candidate gene finding studies suggest analgesic and sedatives tailored to an individual's genetic makeup, environmental adaptations, in addition to several other patient- and drug-related factors, will maximize effectiveness and help mitigate harm. However, the number of pharmacogenetic studies in ICU patients remains small and no prospective studies have been published using pharmacogenomic data to optimize analgesic or sedative therapy in critically ill patients. Current recommendations for treating ICU pain and agitation are based on controlled studies having low external validity, including the failure to consider pharmacogenomic factors affecting response. Use of a precision medicine approach to individualize pharmacotherapy focused on optimizing ICU patient comfort and safety may improve the outcomes of critically ill adults. Additionally, benefits and risks of analgesic and/or sedative therapy in an individual may be informed with large, standardized datasets. The purpose of this review was to describe a precision medicine approach focused on optimizing analgesic and sedative therapy in individual ICU patients to optimize clinical outcomes and reduce safety concerns.

The Evaluation of CYP2D6, CYP2C9, CYP2C19, and CYP2B6 Phenoconversion in Post-Mortem Casework: The Challenge of Forensic Toxicogenetics

In toxicogenetics, an integrative approach including the prediction of phenotype based on post-mortem genotyping of drug-metabolising enzymes might help explain the cause of death (CoD) and manner of death (MoD). The use of concomitant drugs, however, might lead to phenoconversion, a mismatch between the phenotype based on the genotype and the metabolic profile actually observed after phenoconversion. The aim of our study was to evaluate the phenoconversion of CYP2D6, CYP2C9, CYP2C19, and CYP2B6 drug-metabolising enzymes in a series of autopsy cases tested positive for drugs that are substrates, inducers, or inhibitors of these enzymes. Our results showed a high rate of phenoconversion for all enzymes and a statistically significant higher frequency of poor and intermediate metabolisers for CYP2D6, CYP2C9, and CYP2C19 after phenoconversion. No association was found between phenotypes and CoD or MoD, suggesting that, although phenoconversion might be useful for a forensic toxicogenetics approach, more research is needed to overcome the challenges arising from the post-mortem setting.

Multidisciplinary Insights into the Structure-Function Relationship of the CYP2B6 Active Site

Cytochrome P450 2B6 (CYP2B6) is a highly polymorphic human enzyme involved in the metabolism of many clinically relevant drugs, environmental toxins, and endogenous molecules with disparate structures. Over the last 20-plus years, in silico and in vitro studies of CYP2B6 using various ligands have provided foundational information regarding the substrate specificity and structure-function relationship of this enzyme. Approaches such as homology modeling, X-ray crystallography, molecular docking, and kinetic activity assays coupled with CYP2B6 mutagenesis have done much to characterize this originally neglected monooxygenase. However, a complete understanding of the structural details that make new chemical entities substrates of CYP2B6 is still lacking. Surprisingly little in vitro data has been obtained about the structure-function relationship of amino acids identified to be in the CYP2B6 active site. Since much attention has already been devoted to elucidating the function of CYP2B6 allelic variants, here we review the salient findings of in silico and in vitro studies of the CYP2B6 structure-function relationship with a deliberate focus on the active site. In addition to summarizing these complementary approaches to studying structure-function relationships, we note gaps/challenges in existing data such as the need for more CYP2B6 crystal structures, molecular docking results with various ligands, and data coupling CYP2B6 active site mutagenesis with kinetic parameter measurement under standard expression conditions. Harnessing in silico and in vitro techniques in tandem to understand the CYP2B6 structure-function relationship will likely offer further insights into CYP2B6-mediated metabolism. SIGNIFICANCE STATEMENT: The apparent importance of cytochrome P450 2B6 (CYP2B6) in the metabolism of various xenobiotics and endogenous molecules has grown since its discovery with many in silico and in vitro studies offering a partial description of its structure-function relationship. Determining the structure-function relationship of CYP2B6 is difficult but may be aided by thorough biochemical investigations of the CYP2B6 active site that provide a more complete pharmacological understanding of this important enzyme.

Effect of deuteration on the single dose pharmacokinetic properties and postoperative analgesic activity of methadone

Although methadone is effective in the management of acute pain, the complexity of its absorption-distribution-metabolism-excretion profile limits its use as an opioid of choice for perioperative analgesia. Because deuteration is known to improve the pharmacokinetic, pharmacodynamic and toxicological properties of some drugs, here we characterized the single dose pharmacokinetic properties and post-operative analgesic efficacy of d9-methadone. The pharmacokinetic profiles of d9-methadone and methadone administered intravenously to CD-1 male mice revealed that deuteration leads to a 5.7- and 4.4-fold increase in the area under the time-concentration curve and maximum concentration in plasma, respectively, as well as reduction in clearance (0.9 ± 0.3 L/h/kg vs 4.7 ± 0.8 L/h/kg). The lower brain-to-plasma ratio of d9-methadone compared to that of methadone (0.35 ± 0.12 vs 2.05 ± 0.62) suggested that deuteration decreases the transfer of the drug across the blood-brain barrier. The estimated LD50 value for a single intravenous dose of d9-methadone was 2.1-fold higher than that for methadone. Moreover, d9-methadone outperformed methadone in the efficacy against postoperative pain by primarily activating peripheral opioid receptors. Collectively, these data suggest that the replacement of three hydrogen atoms in three methyl groups of methadone altered its pharmacokinetic properties, improved safety, and enhanced its analgesic efficacy.

Methadone pharmacogenetics in vitro and in vivo: Metabolism by CYP2B6 polymorphic variants and genetic variability in paediatric disposition

AIMS

Methadone metabolism and clearance are determined principally by polymorphic cytochrome P4502B6 (CYP2B6). Some CYP2B6 allelic variants affect methadone metabolism in vitro and disposition in vivo. We assessed methadone metabolism by CYP2B6 minor variants in vitro. We also assessed the influence of CYP2B6 variants, and P450 oxidoreductase (POR) and CYP2C19 variants, on methadone clearance in surgical patients in vivo.

METHODS

CYP2B6 and P450 oxidoreductase variants were coexpressed with cytochrome b5 . The metabolism of methadone racemate and enantiomers was measured at therapeutic concentrations and intrinsic clearances were determined. Adolescents receiving methadone for surgery were genotyped for CYP2B6, CYP2C19 and POR, and methadone clearance and metabolite formation clearance were determined.

RESULTS

In vitro, CYP2B6.4 was more active than wild-type CYP2B6.1. CYPs 2B6.5, 2B6.6, 2B6.7, 2B6.9, 2B6.17, 2B6.19 and 2B6.26 were less active. CYPs 2B6.16 and 2B6.18 were inactive. CYP2B6.1 expressed with POR variants POR.28, POR.5 and P228L had lower rates of methadone metabolism than wild-type reductase. In vivo, methadone clinical clearance decreased linearly with the number of CYP2B6 slow metabolizer alleles, but was not different in CYP2C19 slow or rapid metabolizer phenotypes, or in carriers of the POR*28 allele.

CONCLUSIONS

Several CYP2B6 and POR variants were slow metabolizers of methadone in vitro. Polymorphisms in CYP2B6, but not CYP2C19 or P450 reductase, affected methadone clearance in vivo. CYP2B6 polymorphisms 516G>T and 983T>C code for canonical loss of function variants and should be assessed when considering genetic influences on clinical methadone disposition. These complementary translational in vitro and in vivo results inform on pharmacogenetic variability affecting methadone disposition in patients.

The Role of Pharmacogenomics in Opioid Prescribing

Pharmacogenomics is increasingly important to guide objective, safe, and effective individualised prescribing. Personalised prescribing has revolutionised treatments in the past decade, allowing clinicians to maximise drug efficacy and minimise adverse effects based on a person’s genetic profile. Opioids, the gold standard for cancer pain relief, are among the commonest medications prescribed in palliative care practice. This narrative review examines the literature surrounding opioid pharmacogenomics and its applicability to the palliative care cancer population. There is currently limited intersection between the fields of palliative care and pharmacogenomics, but growing evidence presents a need to build linkages between the two disciplines. Pharmacogenomic evidence guiding opioid prescribing is currently available for codeine and tramadol, which relates to CYP2D6 gene variants. However, these medications are prescribed less commonly for pain in palliative care. Research is accelerating with other opioids, where oxycodone (CYP2D6) and methadone (CYP2B6, ABCB1) already have moderate evidence of an association in terms of drug metabolism and downstream analgesic response and side effects. OPRM1 and COMT are receiving increasing attention and have implications for all opioids, with changes in opioid dosage requirements observed but they have not yet been studied widely enough to be considered clinically actionable. Current evidence indicates that incorporation of pharmacogenomic testing into opioid prescribing practice should focus on the CYP2D6 gene and its actionable variants. Although opioid pharmacogenomic tests are not widely used in clinical practice, the progressively reducing costs and rapid turnover means greater accessibility and affordability to patients, and thus, clinicians will be increasingly asked to provide guidance in this area. The upsurge in pharmacogenomic research will likely discover more actionable gene variants to expand international guidelines to impact opioid prescribing. This rapidly expanding area requires consideration and monitoring by clinicians in order for key findings with clinical implications to be accessible, meaningfully interpretable and communicated.

Effect of HIV, antiretrovirals, and genetics on methadone pharmacokinetics: Results from the methadone antiretroviral pharmacokinetics study

BACKGROUND

Methadone treatment of opioid use disorder in HIV-infected individuals is complicated by drug-drug interactions. Genetic and other cofactors further contribute to interindividual variability in methadone pharmacokinetics. We used population pharmacokinetics to estimate the effect of drug-drug interactions, genetics, and other cofactors on methadone pharmacokinetics in a methadone maintained population in Vietnam.

METHODS

Plasma R- and S-methadone levels were measured in 309 methadone maintained individuals just before and 2-5 h following methadone dosing. A linear one-compartment population pharmacokinetic model with first-order conditional estimation with interaction was used to evaluate methadone clearance (CL/F) and volume of distribution (V/F). The influence of covariates on parameter estimates was evaluated using stepwise covariate modeling. Covariates included HIV status, antiretroviral use (efavirenz or nevirapine), weight, BMI, age, methadone dose, and 8 single nucleotide polymorphisms in across the CYP2B6, ABCB1, and NR1I3 genes.

RESULTS

Taking either efavirenz or nevirapine increased R-methadone CL/F 220%. Nevirapine and efavirenz increased S-methadone CL/F by 404% and 273%, respectively. Variants in NR1I3 increased R- and S-methadone CL/F by approximately 20% only in patients taking efavirenz. Different alleles in ABCB1 rs2032582 either increased or decreased R-methadone CL/F by 10%. The CYP 2B6*4 variant decreased S-methadone CL/F by 18%. HIV-infection increased R- and S-methadone CL/F and V/F by 24%-39%.

CONCLUSIONS

The HIV antiretrovirals nevirapine and efavirenz significantly increase methadone clearance. Variants inNR1I3 increased the effect of efavirenz on methadone clearance. Other variants affecting methadone CL/F were also confirmed. To our knowledge, this is the first report of HIV itself affecting methadone pharmacokinetics.

Inter-ethnic genetic variations and novel variant identification in the partial sequences of CYP2B6 gene in Pakistani population

Background

Some single nucleotide polymorphisms (SNPs) in the cytochrome P450 (CYP)2B6 gene lead to decreased enzyme activity and have an impact on drug metabolism. The present study was designed to investigate the patterns of genetic distinction across a hypervariable region of the CYP2B6 gene, known to contain important SNPs, i.e. rs4803419 and rs3745274, among five major ethnic groups of the Pakistani population.

Methods

Arlequin v3.5.DnaSPv6.12. and network 5 resources were used to analyze population genetic variance in the partial CYP2B6 gene sequences obtained from 104 human samples belonging to Punjabi, Pathan, Sindhi, Seraiki and Baloch ethnicities of Pakistan. The partial CYP2B6 gene region analyzed in the current study is previously known to possess important SNPs.

Results

The data analyses revealed that genetic variance among samples mainly came from differentiation within the ethnic groups. However, significant genetic variation was also found among the various ethnic groups. The high pairwise Fst genetic distinction was observed between Seraiki and Sindhi ethnic groups (Fst = 0.13392, P-value = 0.026) as well as between Seraiki and Balochi groups (Fst = 0.04303, P-value = −0.0030). However, the degree of genetic distinction was low between Pathan and Punjabi ethnic groups. Some SNPs, including rs3745274 and rs4803419, which are previously shown in strong association with increased plasma Efavirenz level, were found in high frequency. Besides, a novel SNP, which was not found in dbSNP and Ensemble databases, was identified in the Balochi ethnicity. This novel SNP is predicted to affect the CYP2B6 splicing pattern.

Conclusion

These results may have significant implications in Pakistani ethnicities in the context of drugs metabolized by CYP2B6, especially in Seraiki and Balochi ethnicity. The novel heterogeneous SNP, found in the present study, might lead to altered drug-metabolizing potential of CYP2B6 and, therefore, may be implicated in non-responder phenomenon.

CYP2B6 Functional Variability in Drug Metabolism and Exposure Across Populations-Implication for Drug Safety, Dosing, and Individualized Therapy

Adverse drug reactions (ADRs) are one of the major causes of morbidity and mortality worldwide. It is well-known that individual genetic make-up is one of the causative factors of ADRs. Approximately 14 million single nucleotide polymorphisms (SNPs) are distributed throughout the entire human genome and every patient has a distinct genetic make-up which influences their response to drug therapy. Cytochrome P450 2B6 (CYP2B6) is involved in the metabolism of antiretroviral, antimalarial, anticancer, and antidepressant drugs. These drug classes are commonly in use worldwide and face specific population variability in side effects and dosing. Parts of this variability may be caused by single nucleotide polymorphisms (SNPs) in the CYP2B6 gene that are associated with altered protein expression and catalytic function. Population variability in the CYP2B6 gene leads to changes in drug metabolism which may result in adverse drug reactions or therapeutic failure. So far more than 30 non-synonymous variants in CYP2B6 gene have been reported. The occurrence of these variants show intra and interpopulation variability, thus affecting drug efficacy at individual and population level. Differences in disease conditions and affordability of drug therapy further explain why some individuals or populations are more exposed to CYP2B6 pharmacogenomics associated ADRs than others. Variabilities in drug efficacy associated with the pharmacogenomics of CYP2B6 have been reported in various populations. The aim of this review is to highlight reports from various ethnicities that emphasize on the relationship between CYP2B6 pharmacogenomics variability and the occurrence of adverse drug reactions. In vitro and in vivo studies evaluating the catalytic activity of CYP2B6 variants using various substrates will also be discussed. While implementation of pharmacogenomic testing for personalized drug therapy has made big progress, less data on pharmacogenetics of drug safety has been gained in terms of CYP2B6 substrates. Therefore, reviewing the existing evidence on population variability in CYP2B6 and ADR risk profiles suggests that, in addition to other factors, the knowledge on pharmacogenomics of CYP2B6 in patient treatment may be useful for the development of personalized medicine with regards to genotype-based prescription.

Precision dosing of methadone during pregnancy: A pharmacokinetics virtual clinical trials study

BACKGROUND

Methadone use for the management of opioid dependency during pregnancy is commonplace. Methadone levels are altered during pregnancy due to changes in maternal physiology. Despite this, a paucity of data exist regarding the most appropriate optimal dosing regimens during pregnancy.

METHODS

This study applied a pharmacokinetic modeling approach to examine gestational changes in R- and S-methadone concentrations in maternal plasma and fetal (cord) blood. This study did so to derive a theoretical optimal dosing regimen during pregnancy, and to identify the impact of Cytochromes P450 (CYP) 2B6 and 2C19 polymorphisms on methadone maternal and fetal pharmacokinetics.

RESULTS

The study noted significant decreases in maternal R- and S-methadone plasma concentrations during gestation, with concomitant increases in fetal levels. At a dose of 90 mg once daily, 75% (R-) and 94% (S-) of maternal methadone trough levels were below the lower therapeutic window at term (week 40). The developed optimal dosing regimen escalated doses to 110 mg by week 5, followed by 10 mg increments every 5 weeks up to a maximum of 180 mg once daily near term. This increase resulted in 27% (R-) and 11% (S-) of subjects with trough levels below the lower therapeutic window at term. CYP2B6 poor metabolizers (PM) and either CYP2C19 extensive metabolizers (EM), PM, or ultra-rapid (UM) metabolizer phenotypes demonstrated statistically significant increases in concentrations when compared to their matched CYP2B6 EM counterparts.

CONCLUSIONS

Specific and gestation-dependent dose titrations are required during pregnancy to reduce the risks associated with illicit drug use and to maintain fetal safety.

Allelic and genotype frequencies of major CYP2B6 polymorphisms in the Pakistani population

BACKGROUND

Cytochrome P450 (CYP2B6) is an important enzyme that metabolizes about 3.0% of therapeutic drugs. Drugs metabolized mainly by CYP2B6 include artemisinin, bupropion, cyclophosphamide, efavirenz, ketamine, and methadone. The genetic polymorphisms in the CYP2B6 gene have earlier been studied in many populations, but the data are lacking for the Pakistani population. This research study aimed to determine the frequencies of the three of the most important variant alleles and genotypes of the CYP2B6 gene in the Pakistani population.

METHODS

Blood was withdrawn from healthy volunteers after taking informed consent. DNA was extracted using commercial kits, and allelic and genotype frequencies were determined after PCR amplification followed by restriction fragment length polymorphism (RFLP) and gel electrophoresis.

RESULTS

Our results show a minor allele frequency of 33.8% for CYP2B6*6, 25.8% for CYP2B6*4, 6.5% for CYP2B6*3, whereas wild-type genotype frequency was 48.57% for CYP2B6*6, 59.79% for CYP2B6*4, and 90.20% for CYP2B6*3. A significant interethnic variation was also observed.

CONCLUSIONS

Our results suggest that the frequency of poor metabolizers of CYP2B6, especially *6 variant, is significant enough in the Pakistani population to be given an important consideration when drugs metabolized by this enzyme are prescribed.

Investigating the CYP2B6 rs3745274 and rs3211371 polymorphisms in Methadone-Responder and Non-Responder Addicts in Iran

Background

Methadone therapy is a major protocol in opioid addiction cases in many health care systems. Population-based studies have shown that in addicted people, the genetic profile affects their response to methadone therapy. Therefore, this study designed to examine the frequency of two SNPs of the CYP2B6 gene (rs3745274 and rs3211371) in addicted cases in two methadone-responders and methadone non-responders groups.

Methods

A total of 199 opioid-addicted individuals and 117 unaffected control subjects were genotyped for rs3745274 and rs3211371 polymorphisms of the CYP2B6 gene using the tetra-primer amplification refractory mutation system-polymerase chain reaction.

Results

Results of this study revealed the significant association of rs3745274 GG (p < 0.001; OR = 0.027; 95% CI = 0.14-0.49) and GT (p < 0.001; OR = 4.04; 95% CI = 2.26-7.21) genotypes with the risk of addiction in methadone-responders. Also, a significant association between rs3745274 GG (p < 0.001; OR = 0.28; 95% CI = 0.15-0.51) and GT (p < 0.001; OR = 5.1; 95% CI = 2.8-5.28) genotypes and addiction relapse was found in methadone non-responders.

Conclusion

Based on our findings, we can conclude that rs3745274 variant of CYP2B6 gene could serve as a potential biomarker, to evaluate the prognosis of addicted people fate under treatment with methadone.

Sources of Interindividual Variability

The efficacy, safety, and tolerability of drugs are dependent on numerous factors that influence their disposition. A dose that is efficacious and safe for one individual may result in sub-therapeutic or toxic blood concentrations in others. A significant source of this variability in drug response is drug metabolism, where differences in presystemic and systemic biotransformation efficiency result in variable degrees of systemic exposure (e.g., AUC, C_max, and/or C_min) following administration of a fixed dose.Interindividual differences in drug biotransformation have been studied extensively. It is recognized that both intrinsic factors (e.g., genetics, age, sex, and disease states) and extrinsic factors (e.g., diet , chemical exposures from the environment, and the microbiome) play a significant role. For drug-metabolizing enzymes, genetic variation can result in the complete absence or enhanced expression of a functional enzyme. In addition, upregulation and downregulation of gene expression, in response to an altered cellular environment, can achieve the same range of metabolic function (phenotype), but often in a less predictable and time-dependent manner. Understanding the mechanistic basis for variability in drug disposition and response is essential if we are to move beyond the era of empirical, trial-and-error dose selection and into an age of personalized medicine that will improve outcomes in maintaining health and treating disease.

Potential Pharmacokinetic Drug-Drug Interactions between Cannabinoids and Drugs Used for Chronic Pain

Choosing an appropriate treatment for chronic pain remains problematic, and despite the available medication for its treatment, still, many patients complain about pain and appeal to the use of cannabis derivatives for pain control. However, few data have been provided to clinicians about the pharmacokinetic drug-drug interactions of cannabinoids with other concomitant administered medications. Therefore, the aim of this brief review is to assess the interactions between cannabinoids and pain medication through drug transporters (ATP-binding cassette superfamily members) and/or metabolizing enzymes (cytochromes P450 and glucuronyl transferases).

Pharmacogenomics, concepts for the future of perioperative medicine and pain management: A review

Pharmacogenomics is the study of how genetic differences between individuals affect pharmacokinetics and pharmacodynamics. These differences are apparent to clinicians when taking into account the wide range of responses to medications given in clinical practice. A review of literature involving pharmacogenomics and pain management was performed. The implementation of preoperative pharmacogenomics will allow us to better care for our patients by delivering personalized, safer medicine. This review describes the current state of pharmacogenomics as it relates to many aspects of clinical practice and how clinicians can use these tools to improve patient outcomes.

Toward precision prescribing for methadone: Determinants of methadone deposition

BACKGROUND

Despite the World Health Organization listing methadone as an essential medication, effective dose selection is challenging, especially in racial and ethnic minority populations. Subtherapeutic doses can result in withdrawal symptoms while supratherapeutic doses can result in overdose and death. Although CYP3A4 was conventionally considered the principal methadone metabolizing enzyme, more recent data have identified CYP2B6 as the principal enzyme. CYP2B6 has ethnically-associated polymorphisms that affect the metabolic rate. Our objective was to investigate the effects of genetic and nongenetic factors on methadone metabolism.

METHODS

We measured trough plasma methadone levels in 100 participants with opioid use disorder. We assessed methadone metabolism by calculating the metabolite ratio (major metabolite: 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine [EDDP] divided by methadone concentration). We assessed hepatic fibrosis and steatosis by transient elastography and CYP2B6 alleles, principally responsible for methadone metabolism. Mixed effects models modeled the data in 97 participants.

RESULTS

Participants were largely male (58%), minority (61% African American) and non-Hispanic (68%). Forty percent were HCV mono-infected, 40% were uninfected, and 20% were HCV/HIV co-infected. Female sex had significant effects on (R)- and (S)-methadone metabolism (p = 0.016 and p = 0.044, respectively). CYP2B6 loss of function (LOF) alleles significantly affected (S)-methadone metabolism (p = 0.012). Body mass index (BMI) significantly affected (R)-methadone metabolism (p = 0.034). Methadone metabolism appeared to be lower in males, in individuals with LOF alleles, and elevated BMI.

CONCLUSIONS

Genetic analysis, especially in minority populations, is essential to delivering individualized treatments. Although the principal methadone metabolizing enzyme remains controversial, our results suggest that sex, CYP2B6 genotype, and BMI should be incorporated into multivariate models to create methadone dosing algorithms. Methadone dosing algorithms should facilitate medication delivery, improve patient satisfaction, and diminish overdose potential.

Drugs in pregnancy: Pharmacologic and physiologic changes that affect clinical care

Pharmacologic interventions play a major role in obstetrical care throughout pregnancy, labor and delivery and the postpartum. Traditionally, obstetrical providers have utilized standard dosing regimens developed for non-obstetrical indications based on pharmacokinetic knowledge from studies in men or non-pregnant women. With the recognition of pregnancy as a special pharmacokinetic population in the late 1990s, investigators have begun to study drug disposition in this unique patient dyad. Many of the basic physiologic changes that occur during pregnancy have significant impact on drug absorption, distribution and clearance. Activity of Phase I and Phase II drug metabolizing enzymes are differentially altered by pregnancy, resulting in drug concentrations sufficiently different for some medications that efficacy or toxicity is affected. Placental transporters play a major dynamic role in determining fetal drug exposure. In the past two decades, we have begun to expand our understanding of obstetrical pharmacology; however, to truly optimize pharmacologic care of our pregnant patients and their developing fetus, additional research is critically needed.

Stereoselective Bupropion Hydroxylation by Cytochrome P450 CYP2B6 and Cytochrome P450 Oxidoreductase Genetic Variants

Bioactivation of the antidepressant and smoking cessation drug bupropion is catalyzed predominantly by CYP2B6. The metabolite hydroxybupropion derived from t-butylhydroxylation is considered to contribute to the antidepressant and smoking-cessation effects of the parent drug. Bupropion hydroxylation is the canonical in vitro and in vivo probe for CYP2B6 activity. P450 also requires obligate partnership with P450 oxidoreductase (POR). Human CYP2B6 and POR genes are highly polymorphic. Some CYP2B6 variants affect bupropion disposition. This investigation evaluated the influence of several human CYP2B6 and POR genetic variants on stereoselective bupropion metabolism, using an insect cell coexpression system containing CYP2B6, POR, and cytochrome b ₅ Based on intrinsic clearances (Cl_ints), relative activities for S,S-hydroxybupropion formation were in the order CYP2B6.4 > CYP2B6.1 > CYP2B6.17 > CYP2B6.5 > CYP2B6.6 ≈ CYP2B6.26 ≈ CYP2B6.19 > CYP2B6.7 > CYP2B6.9 > > CYP2B6.16 and CYP2B6.18; relative activities for R,R-hydroxybupropion formation were in the order CYP2B6.17 > CYP2B6.4 > CYP2B6.1 > CYP2B6.5 ≈ CYP2B6.19 ≈ CYP2B6.26 > CYP2B6.6 > CYP2B6.7 ≈ CYP2B6.9 > > CYP2B6.16 and CYP2B6.18. Bupropion hydroxylation was not influenced by POR variants. CYP2B6-catalyzed bupropion hydroxylation is stereoselective. Though V_max and K_m varied widely among CYP2B6 variants, stereoselectivity was preserved, reflected by similar Cl_int(S,S-hydroxybupropion)/Cl_int(R,R-hydroxybupropion) ratios (1.8-2.9), except CYP2B6.17, which was less enantioselective. Established concordance between human bupropion hydroxylation in vitro and in vivo, together with these new results, suggests additional CYP2B6 variants may influence human bupropion disposition. SIGNIFICANCE STATEMENT: Bupropion pharmacokinetics, metabolism, and clinical effects are affected by the CYP2B6*6 polymorphism. Other expressed CYP2B6 polymorphisms had diminished (*5, *6, *7, *9, *19, *26) or defective (*16, *18) in vitro bupropion hydroxylation. P450 oxidoreductase genetic variants had no effect on metabolism, suggesting no clinical consequence of this polymorphism. These CYP2B6 polymorphisms may portend diminished in vivo bupropion hydroxylation and predict additional clinically important variant alleles.

Retention of polybrominated diphenyl ethers and hydroxylated metabolites in paired human serum and milk in relation to CYP2B6 genotype

Polybrominated diphenyl ethers (PBDEs) and their hydroxylated metabolites (OH-BDEs) are endocrine disrupting compounds prevalent in human serum and breast milk. Retention of PBDEs and OH-BDEs in humans may be affected by differences in PBDE metabolism due to variants in cytochrome P450 2B6 (CYP2B6). The objectives of this study are to assess the partitioning profiles of PBDEs and OH-BDEs in forty-eight paired human serum and milk samples, and to evaluate the relationship between variants in CYP2B6 genotype and PBDE and OH-BDE accumulation in humans. Results show that the geometric mean (GM) concentrations of PBDEs are similar in serum (GM = 43.4 ng/g lipid) and milk samples (GM = 52.9 ng/g lipid), while OH-BDEs are retained primarily in serum (GM = 2.31 ng/g lipid), compared to milk (GM = 0.045 ng/g lipid). Participants with CYP2B6*6 genotype had a greater relative retention of PBDEs in serum and milk, and significant relationships (p <  0.05) were also observed for PBDE-47, 5-OH-BDE-47 and 6-OH-BDE-47 concentrations relative to CYP2B6*5 and CYP2B6*6 genotypes. These results are the first to show that CYP2B6 genotype is significantly related to the relative retention of PBDEs in humans, which may have direct implications for variability in the susceptibility of individuals to the potential adverse effects of these contaminants.

Emerging Challenges to the Safe and Effective Use of Methadone for Cancer-Related Pain in Paediatric and Adult Patient Populations

Methadone continues to be an important medication for the treatment of paediatric and adult cancer-related pain. Appropriate patient selection to ensure safe and effective treatment by a team of clinicians who appreciate and are familiar with methadone and its unique pharmacology is crucial. Unlike morphine and other more common opioids, methadone is purported to have involvement with delta-opioid receptor and higher affinity as an N-methyl-D-aspartate-receptor antagonist. Clinically this gives it the advantage of being effective for both nociceptive and neuropathic pain, but also may be useful in the setting of tolerance to other opioids. Methadone also comes in multiple available formulations that can be administrated through a variety of routes beyond the oral route. Challenges with methadone in treating cancer-related pain include drug interactions specifically as it relates to new targeted cancer therapies. Recent guidelines recommend electrocardiogram monitoring with methadone and there is potential for additive cardiac toxicity in the oncology setting. Appropriate dosing of methadone for pain management given age, organ dysfunction, and patients who are on methadone maintenance therapy are also key factors. This article aims to provide clinicians with evidence and clinical practice guidelines for safe and appropriate use of methadone including indication, initiation, and monitoring given its complexity for management of pain in the dynamic oncology setting.

Utilizing Pediatric Physiologically Based Pharmacokinetic Models to Examine Factors That Contribute to Methadone Pharmacokinetic Variability in Neonatal Abstinence Syndrome Patients

Chronic intrauterine exposure to psychoactive drugs often results in neonatal abstinence syndrome (NAS). NAS is the symptomatic drug withdrawal in newborns that generally occurs after in utero chronic opioid exposure. Methadone is an opioid analgesic commonly prescribed for pharmacologic management of NAS. It exhibits high pharmacokinetic (PK) variability. The current study used physiologically based PK modeling to predict the PK profile of methadone in 20 newborns treated for NAS. The physiologically based PK simulations adequately predicted the PK profile of the clinical data for 45% of the patients. Sensitivity analyses were conducted to explore contributing factors to methadone PK variability. The data suggest that P450 enzymatic activity impacts the clearance of methadone in virtual adults and neonates, while the contribution of cardiac output may be negligible. Understanding maturational and/or pharmacogenetic changes in cytochrome P450 enzymatic activity may further explain the large PK variability of methadone in newborns with NAS and will help individualized treatment.

Opioid Safety and Concomitant Benzodiazepine Use in End-Stage Renal Disease Patients

Background

Opioid use is common in end-stage renal disease (ESRD) patients. However, safety of individual opioids and concomitant benzodiazepine use has not been studied.

Objective

To study the epidemiology of opioid and concomitant benzodiazepine use in ESRD population. To study the clinical safety profile of individual opioids in patients on hemodialysis.

Design

Retrospective analysis of the U.S. Renal Data System. A comprehensive review of the current literature was performed to update currently used opioid safety classification.

Participants

ESRD patients ≥18 years on hemodialysis who were enrolled in Medicare A and B and Part D between 2006 and 2012, excluding those with malignancy.

Main Measures

Hospital admission with diagnosis of prescription opioid overdose within 30, 60, and 90 days of prescription; death due to opioid overdose.

Results

Annually, the percentage of patients prescribed any opioid was 52.2%. Overall trend has been increasing except for a small dip in 2011, despite which the admissions due to opioid overdose have been rising. 30% of those who got a prescription for opioids also got a benzodiazepine prescription. 56.5% of these patients received both prescriptions within a week of each other. Benzodiazepine use increased the odds of being on opioids by 3.27 (CI 3.21–3.32) and increased the odds of hospitalization by 50%. Opioids considered safe such as fentanyl and methadone were associated with 3 and 6 folds higher odds of hospitalization within 30 days of prescription. Hydrocodone had the lowest odds ratio (1.9, CI 1.8–2.0).

Conclusions

Concurrent benzodiazepine use is common and associated with higher risk of hospitalization due to opioid overdose. Possible opioid-associated hospital admission rate is 4-5 times bigger in ESRD population than general population. Current safety classification of opioids in these patients is misleading, and even drugs considered safe based on pharmacokinetic data are associated with moderate to very high risk of hospitalization. We propose a risk-stratified classification of opioids and suggest starting to use them in all ESRD patients.

Stereoselective Ketamine Metabolism by Genetic Variants of Cytochrome P450 CYP2B6 and Cytochrome P450 Oxidoreductase

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Human ketamine N-demethylation to norketamine in vitro at therapeutic concentrations is catalyzed predominantly by the cytochrome P4502B6 isoform (CYP2B6). The CYP2B6 gene is highly polymorphic. CYP2B6.6, the protein encoded by the common variant allele CYP2B6*6, exhibits diminished ketamine metabolism in vitro compared with wild-type CYP2B6.1. The gene for cytochrome P450 oxidoreductase (POR), an obligatory P450 coenzyme, is also polymorphic. This investigation evaluated ketamine metabolism by genetic variants of human CYP2B6 and POR.

METHODS

CYP2B6 (and variants), POR (and variants), and cytochrome b5 (wild-type) were coexpressed in a cell system. All CYP2B6 variants were expressed with wild-type POR and b5. All POR variants were expressed with wild-type CYP2B6.1 and b5. Metabolism of R- and S-ketamine enantiomers, and racemic RS-ketamine to norketamine enantiomers, was determined using stereoselective high-pressure liquid chromatography-mass spectrometry. Michaelis-Menten kinetic parameters were determined.

RESULTS

For ketamine enantiomers and racemate, metabolism (intrinsic clearance) was generally wild-type CYP2B6.1 > CYP2B6.4 > CYP2B6.26, CYP2B6.19, CYP2B6.17, CYP2B6.6 > CYP2B6.5, CYP2B6.7 > CYP2B6.9. CYP2B6.16 and CYP2B6.18 were essentially inactive. Activity of several CYP2B6 variants was less than half that of CYP2B6.1. CYP2B6.9 was 15 to 35% that of CYP2B6.1. The order of metabolism was wild-type POR.1 > POR.28, P228L > POR.5. CYP2B6 variants had more influence than POR variants on ketamine metabolism. Neither CYP2B6 nor POR variants affected the stereoselectivity of ketamine metabolism (S > R).

CONCLUSIONS

Genetic variants of CYP2B6 and P450 oxidoreductase have diminished ketamine N-demethylation activity, without affecting the stereoselectivity of metabolism. These results suggest candidate genetic polymorphisms of CYP2B6 and P450 oxidoreductase for clinical evaluation to assess consequences for ketamine pharmacokinetics, elimination, bioactivation, and therapeutic effects.

Characterization of the Safety and Pharmacokinetic Profile of D-Methadone, a Novel N-Methyl-D-Aspartate Receptor Antagonist in Healthy, Opioid-Naive Subjects: Results of Two Phase 1 Studies

PURPOSE/BACKGROUND

N-methyl-D-aspartate (NMDA) receptor (NMDAR) antagonists are potential agents for the treatment of several central nervous system disorders including major depressive disorder. Racemic methadone, L-methadone, and D-methadone all bind the NMDAR with an affinity similar to that of established NMDAR antagonists, whereas only L-methadone and racemic methadone bind to opioid receptors with high affinity. Therefore, D-methadone is expected to have no clinically significant opioid effects at therapeutic doses mediated by its NMDAR antagonism.

METHODS

We conducted 2 phase 1, double-blind, randomized, placebo-controlled, single- and multiple-ascending-dose studies to investigate the safety and tolerability of oral D-methadone and to characterize its pharmacokinetic profile in healthy opioid-naive volunteers.

RESULTS

D-Methadone exhibits linear pharmacokinetics with dose proportionality for most single-dose and multiple-dose parameters. Single doses up to 150 mg and daily doses up to 75 mg for 10 days were well tolerated with mostly mild treatment-emergent adverse events and no severe or serious adverse events. Dose-related somnolence and nausea occurred and were mostly present at the higher dose level. There was no evidence of respiratory depression, dissociative and psychotomimetic effects, or withdrawal signs and symptoms upon abrupt discontinuation. An overall dose-response effect was observed, with higher doses resulting in larger QTcF (QT interval corrected using Fridericia formula) changes from baseline, but none of the changes were considered clinically significant by the investigators. Mild, dose-dependent pupillary constriction of brief duration occurred particularly at the 60-mg dose or above in the single-ascending-dose study and at the dose of 75 mg in the multiple-ascending-dose study. No detectable conversion of D-methadone to L-methadone occurred in vivo.

CONCLUSIONS

These results support the safety and continued clinical development of D-methadone as an NMDAR antagonist for the treatment of depression and other central nervous system disorders.

Efavirenz Metabolism: Influence of Polymorphic CYP2B6 Variants and Stereochemistry

Efavirenz (more specifically the S-enantiomer) is a cornerstone antiretroviral therapy for treatment of HIV infection. The major primary metabolite is S-8-hydroxyefavirenz, which does not have antiretroviral activity but is neurotoxic. Cytochrome P450 2B6 (CYP2B6) is the major enzyme catalyzing S-8-hydroxyefavirenz formation. CYP2B6 genetics and drug interactions are major determinants of clinical efavirenz disposition and dose adjustment. In addition, as a prototypic CYP2B6 substrate, S-efavirenz and analogs can inform on the structure, activity, catalytic mechanisms, and stereoselectivity of CYP2B6. Metabolism of R-efavirenz by CYP2B6 remains unexplored. This investigation assessed S-efavirenz metabolism by clinically relevant CYP2B6 genetic variants. This investigation also evaluated R-efavirenz hydroxylation by wild-type CYP2B6.1 and CYP2B6 variants. S-Efavirenz 8-hydroxylation by wild-type CYP2B6.1 and variants exhibited positive cooperativity and apparent cooperative substrate inhibition. On the basis of Cl_max values, relative activities for S-efavirenz 8-hydroxylation were in the order CYP2B6.4 > CYP2B6.1 ≈ CYP2B6.5 ≈ CYP2B6.17 > CYP2B6.6 ≈ CYP2B6.7 ≈ CYP2B6.9 ≈ CYP2B6.19 ≈ CYP2B6.26; CYP2B6.16 and CYP2B6.18 showed minimal activity. Rates of R-efavirenz metabolism were approximately 1/10 those of S-efavirenz for wild-type CYP2B6.1 and variants. On the basis of Cl_max values, there was 14-fold enantioselectivity (S > R-efavirenz) for wild-type CYP2B6.1, and 5- to 22-fold differences for other CYP2B6 variants. These results show that both CYP2B6 516G > T (CYP2B6*6 and CYP2B6*9) and 983T > C (CYP2B6*16 and CYP2B6*18) polymorphisms cause canonical diminishment or loss-of-function variants for S-efavirenz 8-hydroxylation, provide a mechanistic basis for known clinical pharmacogenetic differences in efavirenz disposition, and may predict additional clinically important variant alleles. Efavirenz is the most stereoselective CYP2B6 drug substrate yet identified and may be a useful probe for the CYP2B6 active site and catalytic mechanisms. SIGNIFICANCE STATEMENT: Clinical disposition of the antiretroviral S-efavirenz is affected by CYP2B6 polymorphisms. Expressed CYP2B6 with 516G>T (CYP2B6*6 and CYP2B6*9), and 983T>C (CYP2B6*16 and CYP2B6*18) polymorphisms had a diminishment or loss of function for efavirenz 8-hydroxylation. This provides a mechanistic basis for efavirenz clinical pharmacogenetics and may predict additional clinically important variant alleles. Efavirenz metabolism showed both cooperativity and cooperative substrate inhibition. With greater than 10-fold enantioselectivity (S- vs. R- metabolism), efavirenz is the most stereoselective CYP2B6 drug substrate yet identified. These findings may provide mechanistic insights.

The optimization of methadone dosing whilst treating with rifampicin: A pharmacokinetic modeling study

BACKGROUND

The use of oral methadone in opioid substitution treatment (OST) for the management of opioid use disorder is established clinical practice. Confounding treatment is the increased risks of contracting Mycobacterium tuberculosis, the mainstay treatment of which incorporates the potent CYP 2B6 inducer rifampicin.

METHODS

This study applied pharmacokinetic modelling using virtual clinical trials, to pharmacokinetically quantify the extent and impact of rifampicin-mediated drug-drug interactions (DDI) on methadone plasma concentrations. An R-methadone model was developed and validated against 11 retrospective clinical studies prior to use in all subsequent studies. The aims were to investigate: (i) the impact of the DDI on daily methadone doses of 60 mg, 90 mg and 120 mg; (ii) dose escalation during rifampicin and (iii) dose reduction following rifampicin cessation.

RESULTS

A dose increase to 160 mg daily during rifampicin treatment phases was required to maintain peak methadone plasma concentrations within a derived therapeutic window of 80-700 ng/mL. Dose escalation prior to rifampicin initiation was not required and resulted in an increase in subjects with supra-therapeutic concentrations. However, during rifampicin cessation, a dose reduction of 10 mg every 2 days commencing prior to rifampicin cessation, ensured that most patients possessed a peak methadone plasma concentration within an optimal therapeutic window.

IMPLICATIONS

Rifampicin significantly alters methadone plasma concentrations and necessitates dose adjustments. Daily doses of almost double those used perhaps more commonly in clinical practice are required for optimal plasma concentration and careful consideration of dose reduction strategies would be required during the deinduction phase.

Nicotine oxidation by genetic variants of CYP2B6 and in human brain microsomes

Common variation in the CYP2B6 gene, encoding the cytochrome P450 2B6 enzyme, is associated with substrate-specific altered clearance of multiple drugs. CYP2B6 is a minor contributor to hepatic nicotine metabolism, but the enzyme has been proposed as relevant to nicotine-related behaviors because of reported CYP2B6 mRNA expression in human brain tissue. Therefore, we hypothesized that CYP2B6 variants would be associated with altered nicotine oxidation, and that nicotine metabolism by CYP2B6 would be detected in human brain microsomes. We generated recombinant enzymes in insect cells corresponding to nine common CYP2B6 haplotypes and demonstrate genetically determined differences in nicotine oxidation to nicotine iminium ion and nornicotine for both (S) and (R)-nicotine. Notably, the CYP2B6.6 and CYP2B6.9 variants demonstrated lower intrinsic clearance relative to the reference enzyme, CYP2B6.1. In the presence of human brain microsomes, along with nicotine-N-oxidation, we also detect nicotine oxidation to nicotine iminium ion. However, unlike N-oxidation, this activity is NADPH independent, does not follow Michaelis-Menten kinetics, and is not inhibited by NADP or carbon monoxide. Furthermore, metabolism of common CYP2B6 probe substrates, methadone and ketamine, is not detected in the presence of brain microsomes. We conclude that CYP2B6 metabolizes nicotine stereoselectively and common CYP2B6 variants differ in nicotine metabolism activity, but did not find evidence of CYP2B6 activity in human brain.

Safe and Appropriate Use of Methadone in Hospice and Palliative Care: Expert Consensus White Paper

Methadone has several unique characteristics that make it an attractive option for pain relief in serious illness, but the safety of methadone has been called into question after reports of a disproportionate increase in opioid-induced deaths in recent years. The American Pain Society, College on Problems of Drug Dependence, and the Heart Rhythm Society collaborated to issue guidelines on best practices to maximize methadone safety and efficacy, but guidelines for the end-of-life scenario have not yet been developed. A panel of 15 interprofessional hospice and palliative care experts from the U.S. and Canada convened in February 2015 to evaluate the American Pain Society methadone recommendations for applicability in the hospice and palliative care setting. The goal was to develop guidelines for safe and effective management of methadone therapy in hospice and palliative care. This article represents the consensus opinion of the hospice and palliative care experts for methadone use at end of life, including guidance on appropriate candidates for methadone, detail in dosing, titration, and monitoring of patients' response to methadone therapy.

Common Polymorphisms of CYP2B6 Influence Stereoselective Bupropion Disposition

Bupropion hydroxylation is a bioactivation and metabolic pathway, and the standard clinical CYP2B6 probe. This investigation determined the influence of CYP2B6 allelic variants on clinical concentrations and metabolism of bupropion enantiomers. Secondary objectives evaluated the influence of CYP2C19 and P450 oxidoreductase variants. Healthy volunteers in specific cohorts (CYP2B6*1/*1, CYP2B6*1/*6, CYP2B6*6/*6, and also CYP2B6*4 carriers) received single-dose oral bupropion. Plasma and urine bupropion and hydroxybupropion was quantified. Subjects were also genotyped for CYP2C19 and P450 oxidoreductase variants. Hydroxylation of both bupropion enantiomers, assessed by plasma hydroxybupropion/bupropion AUC ratios and urine hydroxybupropion formation clearances, was lower in CYP2B6*6/*6 but not CYP2B6*1/*6 compared with CYP2B6*1/*1 genotypes, and numerically greater in CYP2B6*4 carriers. CYP2C19 and P450 oxidoreductase variants did not influence bupropion enantiomers hydroxylation or plasma concentrations. The results show that clinical hydroxylation of both bupropion enantiomers was equivalently influenced by CYP2B6 allelic variation. CYP2B6 polymorphisms affect S-bupropion bioactivation, which may affect therapeutic outcomes.

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.

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.

Nicotine dependence is associated with functional variation in FMO3, an enzyme that metabolizes nicotine in the brain

A common haplotype of the flavin-containing monooxygenase gene FMO3 is associated with aberrant mRNA splicing, a twofold reduction in in vivo nicotine N-oxidation and reduced nicotine dependence. Tobacco remains the largest cause of preventable mortality worldwide. CYP2A6, the primary hepatic nicotine metabolism gene, is robustly associated with cigarette consumption but other enzymes contribute to nicotine metabolism. We determined the effects of common variants in FMO3 on plasma levels of nicotine-N-oxide in 170 European Americans administered deuterated nicotine. The polymorphism rs2266780 (E308G) was associated with N-oxidation of both orally administered and ad libitum smoked nicotine (P⩽3.3 × 10-5 controlling for CYP2A6 genotype). In vitro, the FMO3 G308 variant was not associated with reduced activity, but rs2266780 was strongly associated with aberrant FMO3 mRNA splicing in both liver and brain (P⩽6.5 × 10-9). Surprisingly, in treatment-seeking European American smokers (n=1558) this allele was associated with reduced nicotine dependence, specifically with a longer time to first cigarette (P=9.0 × 10-4), but not with reduced cigarette consumption. As N-oxidation accounts for only a small percentage of hepatic nicotine metabolism we hypothesized that FMO3 genotype affects nicotine metabolism in the brain (unlike CYP2A6, FMO3 is expressed in human brain) or that nicotine-N-oxide itself has pharmacological activity. We demonstrate for the first time nicotine N-oxidation in human brain, mediated by FMO3 and FMO1, and show that nicotine-N-oxide modulates human α4β2 nicotinic receptor activity in vitro. These results indicate possible mechanisms for associations between FMO3 genotype and smoking behaviors, and suggest nicotine N-oxidation as a novel target to enhance smoking cessation.

Effect of genetic polymorphism of human CYP2B6 on the metabolic activation of chlorpyrifos

Chlorpyrifos (CPS) is a broad-spectrum organophosphate insecticide that is neurotoxic in humans. Chlorpyrifos oxon (CPO) is a toxic metabolite of CPS that is produced by CYP2B6. In this study, we examined the variability of CPS metabolism resulting from single-nucleotide polymorphisms in CYP2B6. Wild-type CYP2B6 (CYP2B6.1) and two variants each with a single amino acid substitution: CYP2B6.5 (R487C) and CYP2B6.8 (K139E) were co-expressed together with human NADPH-dependent cytochrome P450 reductase in Escherichia coli (E. coli). Both of the CYP2B6 variants were successfully expressed in E. coli. The conversion of CPS to CPO by the CYP2B6 variants was analyzed with high-performance liquid chromatography. K_m and V_max of the reaction by CYP2B6.1 were 18.50±2.94μM and 17.07±1.15mol/min/mol P450, respectively. The CYP2B6 variants produced CPO with the following kinetic parameters: K_m for CYP2B6.5 and CYP2B6.8 were 20.44±6.43 and 44.69±9.97μM, respectively; and V_max were 1.10±0.10 and 1.77±0.26mol/min/mol P450, respectively. These results indicate that the amino acid substitutions in the CYP2B6 variants suppressed the metabolic activation of CPS. CYP2B6 variants have altered capacity to bioactivate CPF and may affect individual susceptibility of CPF.

Distinctiveness of the Roma population within CYP2B6 worldwide variation

AIM

To determine variation of CYP2B6 gene within the genetically specific Croatian Roma (Gypsy) population originating from India and to examine it in the worldwide perspective.

MATERIALS & METHODS

Seven SNP loci (rs12721655, rs2279343, rs28399499, rs34097093, rs3745274, rs7260329 and rs8192709) were genotyped in 439 subjects using Kompetitive Allele Specific PCR (KASP) method.

RESULTS

The Croatian Roma took an outlying position in CYP2B6 variation from the worldwide perspective mainly due to their exceptionally high minor allele frequency (MAF) for rs8192709 (12.8%), and lower for rs2279343 (21.1%) compared with south Asian populations.

CONCLUSION

This study provides the first data of several CYP2B6 polymorphisms in Roma population and indicates the need for systematic investigation of the most important pharmacogenes' variants in this large, transnationally isolated population worldwide.

Population Genetic-Based Pharmacokinetic Modeling of Methadone and its Relationship with the QTc Interval in Opioid-Dependent Patients

BACKGROUND AND OBJECTIVES

Methadone is a μ-opioid agonist widely used for the treatment of pain, and for detoxification or maintenance treatment in opioid addiction. It has been shown to exhibit large pharmacokinetic variability and concentration-QTc relationships. In this study we investigated the relative influence of genetic polymorphism and other variables on the dose concentration-QTc relationship.

PATIENTS AND METHODS

A population model for methadone enantiomers in 251 opioid-dependent patients was developed using non-linear mixed effect modeling (NONMEM^®). Various models were tested to characterize the pharmacokinetics of (R)- and (S)-methadone and the pharmacokinetic-pharmacodynamic relationship, while including demographics, physiological conditions, co-medications, and genetic variants as covariates. Model-based simulations were performed to assess the relative increase in QTc with dose upon stratification according to genetic polymorphisms involved in methadone disposition.

RESULTS

A two-compartment model with first-order absorption and lag time provided the best model fit for (R)- and (S)-methadone pharmacokinetics. (S)-methadone clearance was influenced by cytochrome P450 (CYP) 2B6 activity, ABCB1 3435C>T, and α-1 acid glycoprotein level, while (R)-methadone clearance was influenced by CYP2B6 activity, POR*28, and CYP3A4*22. A linear model described the methadone concentration-QTc relationship, with a mean QTc increase of 9.9 ms and 19.2 ms per 1000 ng/ml of (R)- and (S)-methadone, respectively. Simulations with different methadone doses up to 240 mg/day showed that <8 % of patients presented with a QTc interval above 450 ms; however, this might reach 12 to 18 % for (R)- and (S)-methadone, respectively, in patients with a genetic status associated with a decreased methadone elimination at doses exceeding 240 mg/day.

CONCLUSION

Risk factor assessment, electrocardiogram monitoring, and therapeutic drug monitoring are beneficial to optimize treatment in methadone patients, especially for those who have low levels despite high methadone doses, or who are at risk of overdosing.

Relationship between CYP2B6*6 and cold pressor pain sensitivity in opioid dependent patients on methadone maintenance therapy (MMT)

BACKGROUND

CYP2B6 polymorphisms contribute to inter-individual variations in pharmacokinetics of methadone. Increased pain sensitivity is frequently reported by opioid dependent patients on methadone maintenance therapy (MMT). It is possible, therefore, that genetic polymorphisms in CYP2B6, which affects the metabolism of methadone, influence pain sensitivity among patients on MMT. This study investigated CYP2B6 polymorphisms and pain sensitivity in this group.

METHODS

The cold pressor pain responses of 148 opioid dependent patients receiving MMT were evaluated using the cold pressor test (CPT). DNA was extracted from whole blood and subjected to polymerase chain reaction (PCR)-genotyping.

RESULTS

Of the 148 subjects, 77 (52.0%) were carriers of CYP2B6*6 allele. CYP2B6*6 allele carriers had shorter cold pain threshold and pain tolerance times than non-carriers of CYP2B6*6 allele (21.05s vs 33.69s, p=0.036 and 27.15s vs 44.51s, p=0.020, respectively). Pain intensity scores of the CYP2B6*6 allele carriers was 67.55, whereas that of the CYP2B6*6 allele non-carriers was 64.86 (p=0.352).

CONCLUSION

Our study indicates that the CYP2B6*6 allele is associated with a lower pain threshold and lower pain tolerance among males with opioid dependence on MMT. The CYP2B6*6 allele may provide a mechanistic explanation for clinical observations of heightened pain sensitivity among opioid dependent patients receiving MMT.

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.