OPRM1 and CYP2B6 gene variants as risk factors in methadone-related deaths

Pharmacogenetics of Lethal Opioid Overdose: Review of Current Evidence and Preliminary Results from a Pilot Study

There has been a worldwide substantial increase in accidental opioid-overdose deaths. The aim of this review, along with preliminary results from our pilot study, is to highlight the use of pharmacogenetics as a tool to predict causes of accidental opioid-overdose death. For this review, a systematic literature search of PubMed^® between the time period of January 2000 to March 2023 was carried out. We included study cohorts, case-controls, or case reports that investigated the frequency of genetic variants in opioid-related post-mortem samples and the association between these variants and opioid plasma concentrations. A total of 18 studies were included in our systematic review. The systematic review provides evidence of the use of CYP2D6, and to a lower extent, CYP2B6 and CYP3A4/5 genotyping in identifying unexpectedly high or low opioid and metabolite blood concentrations from post-mortem samples. Our own pilot study provides support for an enrichment of the CYP2B6*4-allele in our methadone-overdose sample (n = 41) compared to the anticipated frequency in the general population. The results from our systematic review and the pilot study highlight the potential of pharmacogenetics in determining vulnerability to overdose of opioids.

Adenosine N6-methylation upregulates the expression of human CYP2B6 by altering the chromatin status

N⁶-Methyladenosine (m⁶A) modification is the most prevalent RNA modification in mammals. We have recently demonstrated that inhibition of m⁶A modification by 3-deazaadenosine results in an increase in the expression of the cytochrome P450 (CYP) isoforms CYP1A2, CYP2B6, and CYP2C8 in human liver-derived cells. In the present study, we aimed to clarify the mechanism of m⁶A-mediated regulation of CYP2B6 expression. RNA immunoprecipitation using an anti-m⁶A antibody revealed that CYP2B6 mRNA in human liver and hepatocarcinoma-derived HepaRG cells was m⁶A-modified around the stop codon. In contrast to the treatment with 3-deazaadenosine, double knockdown of methyltransferase like (METTL) 3 and METTL14 (METTL3/14) resulted in a decrease in the levels of CYP2B6 mRNA in Huh-7 and HepaRG cells and a decrease in bupropion hydroxylase activity, a marker activity of CYP2B6, in HepaRG cells. The stability of CYP2B6 mRNA was not influenced by siMETTL3/14. Reporter assays using the plasmids containing the last exon or 5'-flanking region of CYP2B6 indicated that reporter activities were not influenced by knockdown of METTL3/14. The expression levels of the constitutive androstane receptor, pregnane X receptor, and retinoid X receptor, which are the nuclear receptors regulating the transcription of CYP2B6, were not influenced by siMETTL3/14. The chromatin immunoprecipitation and formaldehyde-assisted enrichment of regulatory elements assays revealed that H3K9me2, a repressive histone marker, was enriched in the vicinity of the upstream region of CYP2B6, and knockdown of METTL3/14 induced the condensation of the chromatin structure in this region. In conclusion, we demonstrated that METTL3/14 upregulated CYP2B6 expression by altering the chromatin status.

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.

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.

Haplotype-Based Association and In Silico Studies of OPRM1 Gene Variants with Susceptibility to Opioid Dependence Among Addicted Iranians Undergoing Methadone Treatment

The associations of OPRM1 gene variants with opioid dependence have been demonstrated. This study investigated the association of rs495491, rs1799971 (A118G), rs589046, and rs10457090 variants of OPRM1 gene with opium dependence and their haplotypes among addicted individuals undergoing methadone treatment. Moreover, we investigated whether any of these variants were associated with libido dysfunction or insomnia among addicted people. A total of 404 individuals were genotyped by amplification refractory mutation system (ARMS) PCR. In silico studies were designed through homology modeling of A118G structures (N40 and D40) and docked with 41 FDA-approved drugs of OPRM1 protein by SWISS-MODEL, COACH, MolProbity, ProSA, Errat, Glide XP, and Autodock 4. Results revealed that rs495491, A118G, rs589046, and rs10457090 were significantly associated with opium dependence under recessive (P = 6.66E-10), dominant (P = 0.017), co-dominant (P = 0.001), and recessive (P = 9.28E-6) models of inheritance, respectively. Further analyses indicated three significant haplotypes including A-A-A-C (P-permutation < 1E-9), G-G-A-C (P-permutation = 0.04), and G-A-G-C (P-permutation = 8.69E-4). Genotype-phenotype associations of OPRM1 variants with insomnia and libido dysfunction showed no significant association. Docking showed the higher binding affinity of N40 rather than D40 model; however, methadone and morphine were bonded with D40 structure more powerful. Consequently, rs495491, A118G, rs589046, and rs10457090 were associated with opioid dependence among Iranians; also, A118G might be the most remarkable marker of OPRM1 owing to its vital structural roles.

Impact of genetic polymorphisms on opioid misuse: a scoping review

We conducted a scoping review on genetic polymorphisms associated with opioid intake-related adverse patient outcomes including behavioral, physiological and clinical outcomes. We searched for studies on Medline®, EMBASE®, CINAHL®, Psychinfo® and SNPedia® from January 2006 to January 2018. Our study identified 33 genes and 71 SNPs associated with opioid-intake related adverse patient outcomes: four studies showing associations of nine SNPs with clinical events (e.g., arrhythmia, length of stay and deaths); six studies showing associations of 13 SNPs with respiratory depression and 25 studies showing associations of 50 SNPs with opioid misuse behaviors. Available pharmacogenetic-tests covered polymorphisms associated with opioids metabolism and ignored polymorphisms associated with opioids transport, receptor-binding and signaling that were linked with respiratory depression and misuse behaviors.

Genomics Testing and Personalized Medicine in the Preoperative Setting

Pharmacogenomics (PGx) is the study of how individuals' personal genotypes may affect their responses to various pharmacologic agents. The application of PGx principles in perioperative medicine is fairly novel. Challenges in executing PGx programs into health care systems include physician buy-in and integration into usual clinical workflow, including the electronic health record. This article discusses the current evidence highlighting the potential of PGx with various drug categories (including opioids, nonopioid analgesics, sedatives, β-blockers, antiemetics, and anticoagulants) used in the perioperative process and the challenges of integrating PGx into a health care system and relevant workflows.

Pharmacogenetics of Opioid Use Disorder Treatment

Opioid use disorder (OUD) is a significant health problem in the United States and many other countries. A combination of issues, most notably increased prescription of opioid analgesics, has resulted in climbing rates of opioid abuse and overdose over the last decade. This ongoing epidemic has produced a growing population of patients requiring treatment for OUD. Medications such as methadone and buprenorphine have well documented success rates in treating the disorder compared with placebo. However, significant percentages of the population still fail to maintain abstinence or reduce illicit opioid use while using such medications. Genetic variation may play a role in this variability in outcome through pharmacokinetic or pharmacodynamic effects on OUD medications, or by affecting the rate of negative side effects and adverse events. This review focuses on the existing literature on the pharmacogenetics of OUD treatment, with specific focus on medication metabolism, treatment outcomes, and adverse events.

The pharmacogenetics of medications used in general anesthesia

General anesthesia is a state of unconsciousness, amnesia, analgesia and akinesia induced by drugs including opioids, hypnotic-sedative agents, muscle relaxants and antiemetics. Clinical and genetic factors are reported to influence the efficacy and side effects of these agents. Based on the evidence, clinical action is needed to improve clinical outcomes. This review summarizes the latest knowledge with regards to the pharmacogenetics of anesthetics and general anesthesia related complications.

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.

Genome-Wide Pharmacogenomic Study on Methadone Maintenance Treatment Identifies SNP rs17180299 and Multiple Haplotypes on CYP2B6, SPON1, and GSG1L Associated with Plasma Concentrations of Methadone R- and S-enantiomers in Heroin-Dependent Patients

Methadone maintenance treatment (MMT) is commonly used for controlling opioid dependence, preventing withdrawal symptoms, and improving the quality of life of heroin-dependent patients. A steady-state plasma concentration of methadone enantiomers, a measure of methadone metabolism, is an index of treatment response and efficacy of MMT. Although the methadone metabolism pathway has been partially revealed, no genome-wide pharmacogenomic study has been performed to identify genetic determinants and characterize genetic mechanisms for the plasma concentrations of methadone R- and S-enantiomers. This study was the first genome-wide pharmacogenomic study to identify genes associated with the plasma concentrations of methadone R- and S-enantiomers and their respective metabolites in a methadone maintenance cohort. After data quality control was ensured, a dataset of 344 heroin-dependent patients in the Han Chinese population of Taiwan who underwent MMT was analyzed. Genome-wide single-locus and haplotype-based association tests were performed to analyze four quantitative traits: the plasma concentrations of methadone R- and S-enantiomers and their respective metabolites. A significant single nucleotide polymorphism (SNP), rs17180299 (raw p = 2.24 × 10⁻⁸), was identified, accounting for 9.541% of the variation in the plasma concentration of the methadone R-enantiomer. In addition, 17 haplotypes were identified on SPON1, GSG1L, and CYP450 genes associated with the plasma concentration of methadone S-enantiomer. These haplotypes accounted for approximately one-fourth of the variation of the overall S-methadone plasma concentration. The association between the S-methadone plasma concentration and CYP2B6, SPON1, and GSG1L were replicated in another independent study. A gene expression experiment revealed that CYP2B6, SPON1, and GSG1L can be activated concomitantly through a constitutive androstane receptor (CAR) activation pathway. In conclusion, this study revealed new genes associated with the plasma concentration of methadone, providing insight into the genetic foundation of methadone metabolism. The results can be applied to predict treatment responses and methadone-related deaths for individualized MMTs.

Methadone maintenance treatment (MMT), among the most effective therapies for heroin-dependent patients, reduces craving and withdrawal symptoms, increases treatment compliance, and improves the quality of life of patients. The plasma concentration of methadone is a primary index for quantifying and determining therapy responses to MMT. This study was the first whole-genome pharmacogenomic study on MMT to locate genomic regions associated with the plasma concentration of methadone. The analysis identified a single nucleotide polymorphism (SNP) marker (rs17180299) and 17 haplotypes on the SPON1, GSG1L, and CYP450 genes, including CYP2B6 significantly associated with the plasma concentrations of methadone enantiomers. The identified genetic variations accounted for approximately 10% and 25% of the variations in plasma concentrations of methadone R- and S-enantiomers, respectively. The identified genetic variations have afforded insight into the genetic mechanism of the metabolism of MMT, and have potential to pave the way towards individualized MMTs for heroin-dependent patients.

The AC/AG Diplotype for the 118A>G and IVS2 + 691G>C Polymorphisms of OPRM1 Gene is Associated with Sleep Quality Among Opioid-Dependent Patients on Methadone Maintenance Therapy

Introduction

Methadone is a full agonist of the opioid receptor mu 1 which is encoded by the OPRM1 gene. Sleep disorders were frequently reported by opioid-dependent patients during methadone maintenance therapy (MMT). It is possible, therefore, that genetic polymorphisms in OPRM1 influence sleep quality among patients on MMT. This study investigated the association of OPRM1 polymorphisms with sleep quality among opioid-dependent patients on MMT.

Methods

The sleep quality of 165 male opioid-dependent patients receiving MMT was evaluated using the Pittsburgh Sleep Quality Index (PSQI). DNA was extracted from whole blood and subjected to polymerase chain reaction (PCR) genotyping.

Results

Patients with IVS2 + 691 CC genotype had higher PSQI scores [mean (SD) = 5.73 (2.89)] compared to those without the IVS2 + 691 CC genotype (IVS2 + 691 GG/GC genotype) [4.92 (2.31)], but the difference did not reach statistical significance (p = 0.081). Patients with combined 118 AA genotype and IVS2 + 691 GC genotype (AC/AG diplotype) had significantly lower PSQI scores [mean (SD) = 4.25 (2.27)] compared to those without the diplotype [5.68 (2.77)] (p = 0.018).

Conclusion

Our study indicates that the AC/AG diplotype for the 118A>G and IVS2 + 691G>C polymorphisms of OPRM1 gene is associated with better sleep quality among males with opioid dependence on MMT.

The Opposing Roles of IVS2+691 CC Genotype and AC/AG Diplotype of 118A>G and IVS2+691G>C of OPRM1 Polymorphisms in Cold Pain Tolerance Among Opioid-Dependent Malay Males on Methadone Therapy

Introduction

We recently reported that a majority of opioid-dependent Malay males on methadone therapy are cold pain sensitive. It is postulated that common OPRM1 polymorphisms may be responsible. This study investigated the association between 118A>G (dbSNP rs1799971) and IVS2+691G>C (dbSNP rs2075572) variants on cold pain responses among opioid-dependent Malay males on methadone maintenance therapy.

Methods

Cold pain responses including pain threshold, pain tolerance, and pain intensity were measured using the cold pressor test. DNA was extracted from the venous blood before polymerase chain reaction genotyping. Repeated measures analysis of variance was used to compare the cold pain responses and OPRM1 polymorphisms (118A>G and IVS2+691G>C) using models including genotype dominant and recessive models, allelic additive models, and analysis of haplotypes and diplotypes.

Results

A total of 148 participants were recruited. With the recessive model, those with IVS2+691 homozygous CC genotype had a shorter cold pain tolerance time than those without CC genotype (i.e., GG/GC genotype; 29.81 vs. 43.08 s, respectively, P = 0.048). On the other hand, with diplotype analysis, participants with combined homozygous 118 AA genotype and heterozygous IVS2+691 GC genotype (i.e., AC/AG diplotype) had a longer cold pain tolerance time than those without this diplotype (49.34 vs. 31.48 s, respectively, P = 0.043). Cold pain threshold was not associated with any of the 118A>G and IVS2+691G>C variations despite being analyzed using various models (all P > 0.05).

Conclusion

The IVS2+691 CC genotype and AC/AG diplotype of 118A>G and IVS2+691G>C seem to have opposing roles in pain tolerance among opioid-dependent Malay males on methadone therapy. Haplotypes of OPRM1 may be associated with altered binding affinity.

Electronic supplementary material

The online version of this article (doi:10.1007/s40122-015-0041-y) contains supplementary material, which is available to authorized users.

Forensic Investigation of Methadone Concentrations in Deceased Breastfed Infants

There is a paucity of data to aid in assessing whether postmortem methadone findings in breastfed infants are clinically and/or toxicologically significant. Two cases are reported in which methadone was detected in deceased neonates whose mothers were enrolled in methadone maintenance programs and were breastfeeding. In addition to a complete autopsy and toxicological testing for alcohol, prescription medications, and drugs of abuse, pharmacogenetic analysis was performed for variants in genes related to methadone metabolism and response. In both cases, the postmortem methadone concentration measured in neonatal heart blood was higher than the maximum serum methadone concentration reported in living breastfed infants whose mothers were receiving methadone. However, additional analysis of antemortem blood indicated postmortem redistribution of methadone. Pharmacogenetic results were suggestive of a potential predisposition to methadone toxicity based on studies in adults; the significance of these findings in breastfed neonates requires further research. The medical cause of death was unascertained in both cases.

Contribution of BDNF and DRD2 genetic polymorphisms to continued opioid use in patients receiving methadone treatment for opioid use disorder: an observational study

Background

The heritability of opioid use disorder has been widely investigated; however, the influence of specific genes on methadone treatment outcomes is not well understood. The association between response to methadone treatment and genes that are involved in substance use behaviors and reward mechanisms is poorly understood, despite evidence suggesting their contribution to opioid use disorder. The aim of this study was to investigate the effect of brain-derived neurotrophic factor (BDNF) and dopamine receptor D2 (DRD2) polymorphisms on continued opioid use among patients on methadone treatment for opioid use disorder.

Methods

BDNF 196G>A (rs6265) and DRD2-241A>G (rs1799978) genetic variants were examined in patients with opioid use disorder who were recruited from methadone treatment clinics across Southern Ontario, Canada. We collected demographic information, substance use history, blood for genetic analysis, and urine to measure opioid use. We used regression analysis to examine the association between continued opioid use and genetic variants, adjusting for age, sex, ethnicity, methadone dose, duration in treatment, and number of urine screens.

Results

Among 240 patients treated with methadone for opioid use disorder, 36.3 percent (n = 87) and 11.3 percent (n = 27) had at least one risk allele for rs6265 and rs1799978, respectively. These genetic variants were not significantly associated with continued opioid use while on methadone maintenance treatment [rs6265: odds ratio (OR) = 1.37, 95 % confidence interval (CI) = 0.792, 2.371, p = 0.264; rs1799978: OR 1.27, 95 % CI 0.511, 3.182, p = 0.603].

Conclusions

Despite an association of BDNFrs6265 and DRD2rs1799978 with addictive behaviors, these variants were not associated with continued illicit opioid use in patients treated with methadone. Problematic use of opioids throughout treatment with methadone may be attributed to nongenetic factors or a polygenic effect requiring further exploration. Additional research should focus on investigating these findings in larger samples and different populations.

Risk Factors for Mortality and Endotracheal Intubation after Methadone Intoxication

This was a retrospective chart review to evaluate various risk factors associated with in-hospital mortality and intubation risk in acute methadone overdose. All patients admitted to an academic hospital in Tehran, Iran, during a 10-year period (2000-2009) constituted the study sample. Exclusion criteria were significant comorbidities and age under 18 years. Outcome variables were in-hospital mortality and being intubated during admission. A total of 802 patients were enrolled in the study. There were 15 (1.8%) deaths due to methadone overdose or its complications. The number of yearly admissions was 15 patients in 2000, 16 in 2001, 16 in 2002, 18 in 2003, 23 in 2004, 38 in 2005, 59 in 2006, 110 in 2007, 206 in 2008 and 301 in 2009. Based on logistic regression analysis, the most important independent variable predicting mortality was length of admission in toxicology ward [OR (95% CI): 1.6 (1.1-2.3)]. For the prediction of intubation, independent variables were Glasgow Coma Scale (GCS) score of 5-9 [OR (95% CI): 356.5 (9.8-12907.4)] in the emergency department (ED), miosis in the ED [356.9 (1.4-87872.5)] and respiratory rate in the ED [1.5 (1.1-2.1)]. Linear regression model for length of hospitalization showed patient age as the most important variable for prediction of this outcome. Despite a relatively low mortality rate, the increasing number of methadone-poisoned patients requires special attention to this common intoxication. Careful disposition of patients from ED to ordinary wards or intensive care units and also from higher to lower levels of care should be considered in methadone overdose.

Opioid Analgesics and Nicotine: More Than Blowing Smoke

Practitioners are highly likely to encounter patients with concurrent use of nicotine products and opioid analgesics. Smokers present with more severe and extended chronic pain outcomes and have a higher frequency of prescription opioid use. Current tobacco smoking is a strong predictor of risk for nonmedical use of prescription opioids. Opioid and nicotinic-cholinergic neurotransmitter systems interact in important ways to modulate opioid and nicotine effects: dopamine release induced by nicotine is dependent on facilitation by the opioid system, and the nicotinic-acetylcholine system modulates self-administration of several classes of abused drugs-including opioids. Nicotine can serve as a prime for the use of other drugs, which in the case of the opioid system may be bidirectional. Opioids and compounds in tobacco, including nicotine, are metabolized by the cytochrome P450 enzyme system, but the metabolism of opioids and tobacco products can be complicated. Accordingly, drug interactions are possible but not always clear. Because of these issues, asking about nicotine use in patients taking opioids for pain is recommended. When assessing patient tobacco use, practitioners should also obtain information on products other than cigarettes, such as cigars, pipes, smokeless tobacco, and electronic nicotine delivery systems (ENDS, or e-cigarettes). There are multiple forms of behavioral therapy and pharmacotherapy available to assist patients with smoking cessation, and opioid agonist maintenance and pain clinics represent underutilized opportunities for nicotine intervention programs.

The correlation between prolonged corrected QT interval with the frequency of respiratory arrest, endotracheal intubation, and mortality in acute methadone overdose

Corrected QT interval (QTc) prolongation is long considered as a predisposing factor for the occurrence of torsade de pointes (TdP) and sudden cardiac arrest in methadone maintenance treatment. We aimed to elucidate the correlation between QTc prolongation and in-hospital death, respiratory arrest, and endotracheal intubation in acute methadone-intoxicated patients presenting to the emergency department and to assess the value of QTc in predicting these outcomes. A prospective cross-sectional study with a convenience sample of patients with acute methadone overdose was done. Participants were 152 patients aged 15-65 with negative urinary dipstick test for cyclic antidepressants, no history of other QTc-prolonging conditions and co-ingestions, no severe comorbidities affecting the outcomes, and positive urinary dipstick results for methadone. QTc intervals were measured and calculated in triage-time electrocardiogram (ECG). Death was correlated with QTc (P = 0.014) and length of ICU admission (P < 0.001). In multivariable analysis, death was independently associated only with length of ICU admission [odds ratio (OR) 95 % confidence intervals (95 % CI) 1.36 (1.14-1.61)]. Intubation and respiratory arrest were independently associated with QTc interval [OR (95 % CI) 1.03 (1.02-1.04) and 1.02 (1.01-1.03), respectively]. The receiver operating characteristics curves drawn to show the ability of QTc to predict death, intubation, and respiratory arrest showed thresholds of 470, 447.5, and 450 ms with sensitivity (95 % CI) and specificity (95 % CI) of 87.5 (47.3-99.7), 86.8 (74.7-94.5), and 77.3 (62.2-88.5), respectively. Our study showed that QTc is a potential predictor for adverse outcomes related to acute methadone intoxication. The correlations shown in this study between triage-time QTc and in-hospital respiratory arrest or intubation in methadone overdose may be of clinical value, whether these outcomes are hypothesized to be a reflection of background TdP or intoxication severity.

CYP2B6 SNPs are associated with methadone dose required for effective treatment of opioid addiction

Adequate methadone dosing in methadone maintenance treatment (MMT) for opioid addiction is critical for therapeutic success. One of the challenges in dose determination is the inter-individual variability in dose-response. Methadone metabolism is attributed primarily to cytochrome P450 enzymes CYP3A4, CYP2B6 and CYP2D6. The CYP2B6*6 allele [single nucleotide polymorphisms (SNPs) 785A>G (rs2279343) and 516G>T (rs3745274)] was associated with slow methadone metabolism. To explore the effects of CYP2B6*6 allele on methadone dose requirement, it was genotyped in a well-characterized sample of 74 Israeli former heroin addicts in MMT. The sample is primarily of Middle Eastern/European ancestry, based on ancestry informative markers (AIMs). Only patients with no major co-medication that may affect methadone metabolism were included. The stabilizing daily methadone dose in this sample ranges between 13 and 260mg (mean 140±52mg). The mean methadone doses required by subjects homozygous for the variant alleles of the CYP2B6 SNPs 785A>G and 516G>T (88, 96mg, respectively) were significantly lower than those of the heterozygotes (133, 129mg, respectively) and the non-carriers (150, 151mg, respectively) (nominal P=0.012, 0.048, respectively). The results remain significant after controlling for age, sex and the ABCB1 SNP 1236C>T (rs1128503), which was previously shown to be associated with high methadone dose requirement in this population (P=0.006, 0.030, respectively). An additional 77 CYP2B6, CYP3A4 and CYP2D6 SNPs were genotyped. Of these, 24 SNPs were polymorphic and none showed significant association with methadone dose. Further studies are necessary to replicate these preliminary findings in additional subjects and other populations.

Contribution of CYP2B6 alleles in explaining extreme (S)-methadone plasma levels: a CYP2B6 gene resequencing study

BACKGROUND

(S)-Methadone, metabolized mainly by CYP2B6, shows a wide interindividual variability in its pharmacokinetics and pharmacodynamics.

METHODS

Resequencing of the CYP2B6 gene was performed in 12 and 35 selected individuals with high (S)-methadone plasma exposure and low (S)-methadone plasma exposure, respectively, from a previously described cohort of 276 patients undergoing methadone maintenance treatment. Selected genetic polymorphisms were then analyzed in the complete cohort.

RESULTS

The rs35303484 (*11; c136A>G; M46V) polymorphism was overrepresented in the high (S)-methadone level group, whereas the rs3745274 (*9; c516G>T; Q172H), rs2279344 (c822+183G>A), and rs8192719 (c1294+53C>T) polymorphisms were underrepresented in the low (S)-methadone level group, suggesting an association with decreased CYP2B6 activity. Conversely, the rs3211371 (*5; c1459C>T; R487C) polymorphism was overrepresented in the low-level group, indicating an increased CYP2B6 activity. A higher allele frequency was found in the high-level group compared with the low-level group for rs3745274 (*9; c516G>T; Q172H), rs2279343 (*4; c785A>G; K262R) (together representing CYP2B6*6), rs8192719 (c1294+53C>T), and rs2279344 (c822+183G>A), suggesting their involvement in decreased CYP2B6 activity. These results should be replicated in larger independent cohorts.

CONCLUSION

Known genetic polymorphisms in CYP2B6 contribute toward explaining extreme (S)-methadone plasma levels observed in a cohort of patients following methadone maintenance treatment.

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.

Genetic polymorphisms in the opioid receptor mu1 gene are associated with changes in libido and insomnia in methadone maintenance patients

Methadone, a synthetic racemic opioid that primarily works as a μ-opioid receptor (OPRM1) agonist, is commonly used for the treatment of heroin addiction. Genetic association studies have reported that the OPRM1 gene is involved in the physiology of heroin and alcohol addiction. Our current study is designed to test the hypothesis that genetic polymorphisms in the OPRM1 gene region are associated with methadone dosage, plasma concentrations, treatment responses, adverse reactions and withdrawal symptoms in a methadone maintenance treatment (MMT) cohort from Taiwan. Fifteen OPRM1 single nucleotide polymorphisms (SNPs) were selected and genotyped using DNA samples from 366 MMT patients. The plasma concentrations of methadone and its metabolite were measured by high performance liquid chromatography. The results obtained using dominant model analysis indicate that the OPRM1 SNPs rs1074287, rs6912029, rs12209447, rs510769, rs3798676, rs7748401, rs495491, rs10457090, rs589046, rs3778152, rs563649, and rs2075572 are significantly associated with change-in-libido side effects (adjusted p<0.042). Using recessive model analysis, these SNPs were also found to be significantly associated with insomnia side effects in this cohort (p<0.009). The significance of the insomnia findings was mainly contributed by a subgroup of patients who had a positive urine morphine test (p<0.022), and by individuals who did not use benzodiazepine hypnotics (p<0.034). Our current data thus suggest that genetic polymorphisms in OPRM1 may influence the change-in-libido and insomnia side effects sometimes found in MMT patients.

Pharmacogenomic considerations in opioid analgesia

Translating pharmacogenetics to clinical practice has been particularly challenging in the context of pain, due to the complexity of this multifaceted phenotype and the overall subjective nature of pain perception and response to analgesia. Overall, numerous genes involved with the pharmacokinetics and dynamics of opioids response are candidate genes in the context of opioid analgesia. The clinical relevance of CYP2D6 genotyping to predict analgesic outcomes is still relatively unknown; the two extremes in CYP2D6 genotype (ultrarapid and poor metabolism) seem to predict pain response and/or adverse effects. Overall, the level of evidence linking genetic variability (CYP2D6 and CYP3A4) to oxycodone response and phenotype (altered biotransformation of oxycodone into oxymorphone and overall clearance of oxycodone and oxymorphone) is strong; however, there has been no randomized clinical trial on the benefits of genetic testing prior to oxycodone therapy. On the other hand, predicting the analgesic response to morphine based on pharmacogenetic testing is more complex; though there was hope that simple genetic testing would allow tailoring morphine doses to provide optimal analgesia, this is unlikely to occur. A variety of polymorphisms clearly influence pain perception and behavior in response to pain. However, the response to analgesics also differs depending on the pain modality and the potential for repeated noxious stimuli, the opioid prescribed, and even its route of administration.

Cytochrome P450 CYP2B6 genotypes and haplotypes in a Colombian population: identification of novel variant CYP2B6 alleles

OBJECTIVE

Information on CYP2B6 allele frequencies and detrimental genotypes in mixed human populations is scarce. The aim of this study was to analyze the frequencies and haplotypes of nonsynonymous CYP2B6 single nucleotide polymorphisms (SNPs) in a Colombian population.

METHODS

One hundred and fifty-two healthy individuals were analyzed for five nonsynonymous CYP2B6 SNPs, namely rs8192709, rs3745274, rs2279343 rs28399499, and rs3211371.

RESULTS

Besides eight known variant alleles, we identified two as yet unknown variant alleles combining, respectively, the SNPs rs3745274 and rs3211371 and rs8192709 and rs3745274. Comparison of Colombian mestizo individuals with other mestizo population indicates statistically significant differences (P<0.001) for the gain-of-function CYP2B6*4 allele and for combined detrimental CYP2B6 alleles. In addition, we observed a low linkage between the SNPs rs3745274 and rs2279343, which are often assumed as linked.

CONCLUSION

In conclusion, large interethnic and intraethnic variability exists for CYP2B6 polymorphisms, thus reinforcing the need for tailored genotyping protocols for CYP2B6 testing as a biomarker of drug response.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Impact of genetic polymorphisms in ABCB1, CYP2B6, OPRM1, ANKK1 and DRD2 genes on methadone therapy in Han Chinese patients

Aim: The present study explored the integrative effect of genes encoding methadone pharmacokinetic and pharmacodynamic pathways on methadone maintenance doses in Han Chinese Patients. Materials & methods: Genomic DNA was extracted from 321 opioid-dependent patients and 202 healthy controls, and realtime-PCR and PCR-RFLP were conducted to determine the genotypes. Results: Pair-wise comparisons revealed that carriers of the variants ABCB1 3435C>T or CYP2B6 516G>T alleles were more likely to require a higher methadone dose than noncarriers (both p < 0.0001). On the other hand, carriers of the variant DRD2 -214A>G or 939C>T allele had a twofold chance of requiring a lower methadone dose than noncarriers (p = 0.001). Proportional odds regression with adjustment of cofactors demonstrated that ABCB1, CYP2B6, OPRM1, ANKK1 and DRD2 genetic variants were jointly correlated with optimal methadone dose (adjusted r2 = 53%). Conclusions: These findings provide new insight to the fact that the interindividual variability of methadone dosage requirement is polygenetic and cannot be explained by a single-gene effect.

Original submitted 4 May 2011; Revision submitted 8 July 2011

Profound changes in drug metabolism enzymes and possible effects on drug therapy in neonates and children

INTRODUCTION

There are profound changes that take place in drug metabolism enzymes during fetal and postnatal development. These changes may significantly impact drug therapy in children.

AREAS COVERED

A combination of focused and comprehensive literature searches using PubMed and reference lists (from inception to 7 November 2009) is undertaken to identify reports on in vitro and in vivo development of drug metabolism enzymes as well disposition of selected drugs and their effect in children. The article provides an update on development of drug metabolism enzymes and their impact on drug substrate disposition and disease, which may aid to improve clinical practice and optimally design clinical trials in children.

EXPERT OPINION

Drug metabolism enzyme activity changes profoundly throughout the continuum of postnatal development and often results in different disposition pathways than in adults. Genetics and co-morbidity interact significantly with these developmental changes. Translation of existing knowledge into age-adjusted dosing guidelines and clinical trial design is highly needed for there to be an improvement in drug therapy in children.

Genetic polymorphism and toxicology--with emphasis on cytochrome p450

Individual susceptibility to environmental, chemical, and drug toxicity is to some extent determined by polymorphism in drug-metabolizing enzymes, in particular the cytochromes P450 (CYPs). This polymorphism is in particular translated into risk differences concerning drugs metabolized by the highly polymorphic enzymes CYP2C9, CYP2C19, and CYP2D6, whereas CYP enzymes active in procarcinogen activation are relatively well conserved without important functional polymorphisms. Examples of drug toxicities that can be predicted by P450 polymorphism include those exerted by codeine, tramadol, warfarin, acenocoumarol, and clopidogrel. The polymorphic CYP2A6 has a role in nicotine metabolism and smoking behavior. Besides this genetic variation, genome-wide association studies now allow for the identification of an increasing number of predictive genetic biomarkers among, e.g., human leukocyte antigens and to some extent drug transporters that provide useful information regarding the choice of the drug and drug dosage in order to avoid toxicity. The translation of this information into the clinical practice has been slow; however, an increasing number of pharmacogenomic drug labels are assigned, where the predictive genotyping before drug treatment can be mandatory, recommended, or only for informational purposes. In this review, we provide an update of the field with emphasis on CYP polymorphism.