Codeine increases pain thresholds to copper vapor laser stimuli in extensive but not poor metabolizers of sparteine

Dutch Pharmacogenetics Working Group (DPWG) guideline for the gene-drug interaction between CYP2D6 and opioids (codeine, tramadol and oxycodone)

The current Dutch Pharmacogenetics Working Group (DPWG) guideline, describes the gene-drug interaction between CYP2D6 and the opioids codeine, tramadol and oxycodone. CYP2D6 genotype is translated into normal metaboliser (NM), intermediate metaboliser (IM), poor metaboliser (PM) or ultra-rapid metaboliser (UM). Codeine is contraindicated in UM adults if doses >20 mg every 6 h (q6h), in children ≥12 years if doses >10 mg q6h, or with additional risk factors. In PMs, an alternative analgesic should be given which is not or to a lesser extent metabolised by CYP2D6 (not tramadol). In IMs with insufficient analgesia, a higher dose or alternative analgesic should be given. For tramadol, the recommendations for IMs and PMs are the same as the recommendation for codeine and IMs. UMs should receive an alternative drug not or to a lesser extent metabolised by CYP2D6 or the dose should be decreased to 40% of the commonly prescribed dose. Due to the absence of effect on clinical outcomes of oxycodone in PMs, IMs and UMs no action is required. DPWG classifies CYP2D6 genotyping for codeine "beneficial" and recommends testing prior to, or shortly after initiation of treatment in case of higher doses or additional risk factors. CYP2D6 genotyping is classified as "potentially beneficial" for tramadol and can be considered on an individual patient basis.

Sex, estrous cycle, and hormone regulation of CYP2D in the brain alters oxycodone metabolism and analgesia

Opioids, and numerous centrally active drugs, are metabolized by cytochrome P450 2D (CYP2D). There are sex and estrous cycle differences in brain oxycodone analgesia. Here we investigated the mechanism examining the selective role of CYP2D in the brain on sex, estrous cycle, and hormonal regulation. Propranolol, CYP2D-specific mechanism-based inhibitor, or vehicle was delivered into cerebral ventricles 24 hours before administering oxycodone (or oxymorphone, negative control) orally to male and female (in estrus and diestrus) rats. Ovariectomized and sham-operated females received no treatment, estradiol, progesterone or vehicle. Analgesia was measured using tail-flick latency, and brain drug and metabolite concentrations were measured by microdialysis. Data were analyzed by two-way or mixed ANOVA. Following propranolol (versus vehicle) inhibition and oral oxycodone, there were greater increases in brain oxycodone concentrations and analgesia, and greater decreases in brain oxymorphone/oxycodone ratios (an in vivo phenotype of CYP2D in brain) in males and females in estrus, compared to females in diestrus; with no impact on plasma drug concentrations. There was no impact of propranolol pre-treatment, sex, or cycle after oral oxymorphone (non-CYP2D substrate) on brain oxymorphone concentrations or analgesia. There was no impact of propranolol pre-treatment following ovariectomy on brain oxycodone concentrations or analgesia, which was restored in ovariectomized females following estradiol, but not progesterone, treatment. Sex, cycle, and estradiol regulation of CYP2D in brain in turn altered brain oxycodone concentration and response, which may contribute to the large inter-individual variation in response to the numerous centrally acting CYP2D substrate drugs, including opioids.

Perspectives from the Society for Pediatric Research: pharmacogenetics for pediatricians

This review evaluates the pediatric evidence for pharmacogenetic associations for drugs that are commonly prescribed by or encountered by pediatric clinicians across multiple subspecialties, organized from most to least pediatric evidence. We begin with the pharmacogenetic research that led to the warning of increased risk of death in certain pediatric populations ("ultrarapid metabolizers") who are prescribed codeine after tonsillectomy or adenoidectomy. We review the evidence for genetic testing for thiopurine metabolism, which has become routine in multiple pediatric subspecialties. We discuss the pharmacogenetic research in proton pump inhibitors, for which clinical guidelines have recently been made available. With an increase in the prevalence of behavioral health disorders including attention deficit hyperactivity disorder (ADHD), we review the pharmacogenetic literature on selective serotonin reuptake inhibitors, selective norepinephrine reuptake inhibitors, and ADHD medications. We will conclude this section on the current pharmacogenetic data on ondansetron. We also provide our perspective on how to integrate the current research on pharmacogenetics into clinical care and what further research is needed. We discuss how institutions are managing pharmacogenetic test results and implementing them clinically, and how the electronic health record can be leveraged to ensure testing results are available and taken into consideration when prescribing medications. IMPACT: While many reviews of pharmacogenetics literature are available, there are few focused on pediatrics. Pediatricians across subspecialties will become more comfortable with pharmacogenetics terminology, know resources they can use to help inform their prescribing habits for drugs with known pharmacogenetic associations, and understand the limitations of testing and where further research is needed.

Oral and intravenous pharmacokinetics of metformin with and without oral codeine intake in healthy subjects: A cross-over study

The aim of the study was to investigate if there is a clinically relevant drug interaction between metformin and codeine. Volunteers were randomized to receive on four separate occasions: (A) orally administered metformin (1 g), (B) intravenously administered metformin (0.5 g), (C) five doses of tablet codeine 25 mg; the last dose was administered together with oral metformin (1 g), and (D) five doses of tablet codeine 25 mg; the last dose was administered together with metformin (0.5 g) intravenously. Blood samples were drawn for 24 h after administration of metformin, and for 6 h after administration of codeine and analyzed using liquid chromatography and tandem mass spectrometry. Healthy volunteers genotyped as CYP2D6 normal metabolizers (*1/*1) without known reduced function variants in the OCT1 gene (rs12208357, rs34130495, rs34059508, and rs72552763) were invited. The median absorption fraction of metformin was 0.31 and was not influenced by codeine intake. The median time to maximum concentration (Tmax) after oral intake of metformin was 2 h without, and 3 h with codeine (p = 0.06). The geometric mean ratios of the areas under the plasma concentration time‐curve (AUCs) for morphine and its metabolites M3G and M6G for oral intake of metformin‐to‐no metformin were 1.21, 1.31, and 1.27, respectively, and for i.v. metformin‐to‐no metformin 1.28, 1.34, and 1.30, respectively. Concomitant oral and i.v. metformin increased the plasma levels of morphine, M3G and M6G. These small pharmacokinetic changes may well contribute to an increased risk of early discontinuation of metformin. Hence, a clinically relevant drug‐drug interaction between metformin and codeine seems plausible.

Pharmacogenomic potential in advanced cancer patients

PURPOSE

The prevalence of pharmacogenetically actionable medications in advanced cancer patients whose therapy may be optimized with genotype data was determined.

METHODS

Patients enrolled in our institutional molecular tumor board observational cohort were included in this study. Collected data included demographics, type(s) of cancer, and outpatient medications. Medications were classified as "pharmacogenetically actionable" if there are Clinical Pharmacogenetics Implementation Consortium (CPIC) therapeutic recommendations for that medication based on the presence of germline variations. The prevalence of pharmacogenetically actionable medications in the study population was determined, and the frequency of opportunities for pharmacogenetic prescribing and adverse event (AE) mitigation were estimated.

RESULTS

In a cohort of 193 patients with advanced cancer, 65% of patients were taking a pharmacogenetically actionable medication. Approximately 10% of the outpatient medications taken by the study population had a pharmacogenetic association. The most common pharmacogenetically actionable medications being used were ondansetron (47%), capecitabine (10%), and sertraline (7%). Using published genetic variation frequencies and AE risk, we conservatively estimated that 7.1% of cancer patients would be eligible for genetic-based medication adjustment, and 101 AEs would be prevented in 10,000 patients genotyped.

CONCLUSION

Medications with pharmacogenetic associations are used commonly in the advanced cancer patient population. This widespread exposure supports the implementation of prospective genotyping in the treatment of these high-risk patients.

Is tramadol associated to bleeding peptic ulcer? A nationwide case-control study in hospitalized Swedish patients

AIMS

Tramadol, a widely used analgesic drug, inhibits the reuptake of noradrenaline and serotonin impairing the aggregation function of thrombocytes. However, the risk for severe bleeding has previously not been studied. The aim of the present study is to investigate the association between tramadol and bleeding peptic ulcer in the Swedish population.

METHODS

In this register based case-control study based on the Swedish national patient registry and prescription drug registry, we included 18 306 patients hospitalized with a first-time diagnosis of bleeding peptic ulcer. For every case, 4 matched controls were included. To investigate the temporal aspects of tramadol induced bleeding ulcer, exposure was divided into patients with newly initiated and ongoing treatment. To explore a possible confounding by indication, the effect of codeine, a drug also prescribed for the treatment of moderate pain, but not known to affect thrombocyte function, was investigated. Univariable and multivariable logistic regression was used to analyse the association between tramadol use and bleeding ulcer.

RESULTS

Tramadol was associated with an increased risk of bleeding ulcer (adjusted odds ratio (aOR) 2.1, 95% confidence interval: (2.0-2.3). The association was stronger for newly initiated treatment with tramadol 2.8 (2.5-3.2) and weaker for ongoing treatment 1.7 (1.6-1.9). Codeine was also associated with an increased risk of bleeding ulcer 1.9 (1.7-2.1) and this association was also stronger for newly initiated treatment with codeine 2.3 (2.0-2.6) and weaker for ongoing treatment 1.7 (1.5-1.9).

CONCLUSION

Treatment with tramadol was associated with an increased risk of bleeding peptic ulcer. Most of this association may be mediated by factors related to the pain condition rather than the pharmacologic effect per se.

Rat brain CYP2D activity alters in vivo central oxycodone metabolism, levels and resulting analgesia

Oxycodone is metabolized by CYP2D to oxymorphone. Despite oxymorphone being a more potent opioid-receptor agonist, its contribution to oxycodone analgesia may be minor because of low peripheral production, low blood-brain barrier permeability and central nervous system efflux. CYP2D metabolism within the brain may contribute to variation in central oxycodone and oxymorphone levels, thereby affecting analgesia. Brain CYP2D expression and activity are subject to exogenous regulation; nicotine induces rat brain, but not liver, CYP2D consistent with higher brain CYP2D in smokers. We assessed the role of rat brain CYP2D in orally administered oxycodone metabolism (in vivo brain microdialysis) and analgesia (tail-flick test) by inhibiting brain CYP2D selectively with intracerebroventricular propranolol (mechanism-based inhibitor) and inducing brain CYP2D with nicotine. Inhibiting brain CYP2D increased brain oxycodone levels (1.8-fold; P < 0.03) and analgesia (1.5-fold AUC0-60 ; P < 0.001) after oxycodone, while inducing brain CYP2D increased brain oxymorphone levels (4.6-fold; P < 0.001) and decreased analgesia (0.8-fold; P < 0.02). Inhibiting the induced brain CYP2D reversed the change in oxycodone levels (1.2-fold; P > 0.1) and analgesia (1.1-fold; P > 0.3). Brain, but not plasma, metabolic ratios were affected by pre-treatments. Peak analgesia was inversely correlated with ex vivo brain (P < 0.003), but not hepatic (P > 0.9), CYP2D activity. Altering brain CYP2D did not affect analgesia from oral oxymorphone (P > 0.9 for AUC0-60 across all groups), which is not a CYP2D substrate. Thus, brain CYP2D metabolism alters local oxycodone levels and response, suggesting that people with increased brain CYP2D activity may have reduced oxycodone response. Factors that alter individual oxycodone response may be useful for optimizing treatment and minimizing abuse liability.

Update of the Scientific Opinion on opium alkaloids in poppy seeds

Poppy seeds are obtained from the opium poppy (Papaver somniferum L.). They are used as food and to produce edible oil. The opium poppy plant contains narcotic alkaloids such as morphine and codeine. Poppy seeds do not contain the opium alkaloids, but can become contaminated with alkaloids as a result of pest damage and during harvesting. The European Commission asked EFSA to provide an update of the Scientific Opinion on opium alkaloids in poppy seeds. The assessment is based on data on morphine, codeine, thebaine, oripavine, noscapine and papaverine in poppy seed samples. The CONTAM Panel confirms the acute reference dose (ARfD) of 10 μg morphine/kg body weight (bw) and concluded that the concentration of codeine in the poppy seed samples should be taken into account by converting codeine to morphine equivalents, using a factor of 0.2. The ARfD is therefore a group ARfD for morphine and codeine, expressed in morphine equivalents. Mean and high levels of dietary exposure to morphine equivalents from poppy seeds considered to have high levels of opium alkaloids (i.e. poppy seeds from varieties primarily grown for pharmaceutical use) exceed the ARfD in most age groups. For poppy seeds considered to have relatively low concentrations of opium alkaloids (i.e. primarily varieties for food use), some exceedance of the ARfD is also seen at high levels of dietary exposure in most surveys. For noscapine and papaverine, the available data do not allow making a hazard characterisation. However, comparison of the dietary exposure to the recommended therapeutical doses does not suggest a health concern for these alkaloids. For thebaine and oripavine, no risk characterisation was done due to insufficient data. However, for thebaine, limited evidence indicates a higher acute lethality than for morphine and the estimated exposure could present a health risk.

Synbiological systems for complex natural products biosynthesis

Natural products (NPs) continue to play a pivotal role in drug discovery programs. The rapid development of synthetic biology has conferred the strategies of NPs production. Synthetic biology is a new engineering discipline that aims to produce desirable products by rationally programming the biological parts and manipulating the pathways. However, there is still a challenge for integrating a heterologous pathway in chassis cells for overproduction purpose due to the limited characterized parts, modules incompatibility, and cell tolerance towards product. Enormous endeavors have been taken for mentioned issues. Herein, in this review, the progresses in naturally discovering novel biological parts and rational design of synthetic biological parts are reviewed, combining with the advanced assembly technologies, pathway engineering, and pathway optimization in global network guidance. The future perspectives are also presented.

Pharmacogenetics of analgesic drugs

• Individual variability in pain perception and differences in the efficacy of analgesic drugs are complex phenomena and are partly genetically predetermined.

• Analgesics act in various ways on the peripheral and central pain pathways and are regarded as one of the most valuable but equally dangerous groups of medications.

• While pharmacokinetic properties of drugs, metabolism in particular, have been scrutinised by genotype–phenotype correlation studies, the clinical significance of inherited variants in genes governing pharmacodynamics of analgesics remains largely unexplored (apart from the µ-opioid receptor).

• Lack of replication of the findings from one study to another makes meaningful personalised analgesic regime still a distant future.

• This narrative review will focus on findings related to pharmacogenetics of commonly used analgesic medications and highlight authors’ views on future clinical implications of pharmacogenetics in the context of pharmacological treatment of chronic pain.

Pharmacogenetics of drug oxidation via cytochrome P450 (CYP) in the populations of Denmark, Faroe Islands and Greenland

Denmark, the Faroe Islands and Greenland are three population-wise small countries on the northern part of the Northern Hemisphere, and studies carried out here on the genetic control over drug metabolism via cytochrome P450 have led to several important discoveries. Thus, CYP2D6 catalyzes the 2-hydroxylation, and CYP2C19 in part catalyzes the N-demethylation of imipramine. The phenomenon of phenocopy with regard to CYP2D6 was first described when Danish patients changed phenotype from extensive to poor metabolizers during treatment with quinidine. It was a Danish extensive metabolizer patient that became a poor metabolizer during paroxetine treatment, and this was due to the potent inhibition of CYP2D6 by paroxetine, which is also is metabolized by this enzyme. Fluoxetine and norfluoxetine are also potent inhibitors of CYP2D6, and fluvoxamine is a potent inhibitor of both CYP1A2 and CYP2C19. The bioactivation of proguanil to cycloguanil is impaired in CYP2C19 poor metabolizers. The O-demethylation of codeine and tramadol to their respective my-opioid active metabolites, morphine and (+)-O-desmethyltramadol was markedly impaired in CYP2D6 poor metabolizers compared to extensive metabolizers, and this impairs the hypoalgesic effect of the two drugs in the poor metabolizers. The frequency of CYP2D6 poor metabolizers is 2%-3% in Greenlanders and nearly 15% in the Faroese population. The frequency of CYP2C19 poor metabolizers in East Greenlanders is approximately 10%. A study in Danish mono and dizygotic twins showed that the non-polymorphic 3-N-demethylation of caffeine catalyzed by CYP1A2 is subject to approximately 70% genetic control.

"Target-Site" Drug Metabolism and Transport

The recent symposium on "Target-Site" Drug Metabolism and Transport that was sponsored by the American Society for Pharmacology and Experimental Therapeutics at the 2014 Experimental Biology meeting in San Diego is summarized in this report. Emerging evidence has demonstrated that drug-metabolizing enzyme and transporter activity at the site of therapeutic action can affect the efficacy, safety, and metabolic properties of a given drug, with potential outcomes including altered dosing regimens, stricter exclusion criteria, or even the failure of a new chemical entity in clinical trials. Drug metabolism within the brain, for example, can contribute to metabolic activation of therapeutic drugs such as codeine as well as the elimination of potential neurotoxins in the brain. Similarly, the activity of oxidative and conjugative drug-metabolizing enzymes in the lung can have an effect on the efficacy of compounds such as resveratrol. In addition to metabolism, the active transport of compounds into or away from the site of action can also influence the outcome of a given therapeutic regimen or disease progression. For example, organic anion transporter 3 is involved in the initiation of pancreatic β-cell dysfunction and may have a role in how uremic toxins enter pancreatic β-cells and ultimately contribute to the pathogenesis of gestational diabetes. Finally, it is likely that a combination of target-specific metabolism and cellular internalization may have a significant role in determining the pharmacokinetics and efficacy of antibody-drug conjugates, a finding which has resulted in the development of a host of new analytical methods that are now used for characterizing the metabolism and disposition of antibody-drug conjugates. Taken together, the research summarized herein can provide for an increased understanding of potential barriers to drug efficacy and allow for a more rational approach for developing safe and effective therapeutics.

Nicotine Increases Codeine Analgesia Through the Induction of Brain CYP2D and Central Activation of Codeine to Morphine

CYP2D metabolically activates codeine to morphine, which is required for codeine analgesia. Permeability across the blood-brain barrier, and active efflux, suggests that initial morphine in the brain after codeine is due to brain CYP2D metabolism. Human CYP2D is higher in the brains, but not in the livers, of smokers and 7-day nicotine treatment induces rat brain, but not hepatic, CYP2D. The role of nicotine-induced rat brain CYP2D in the central metabolic activation of peripherally administered codeine and resulting analgesia was investigated. Rats received 7-day nicotine (1 mg/kg subcutaneously) and/or a single propranolol (CYP2D mechanism-based inhibitor; 20 μg intracerebroventricularly) pretreatment, and then were tested for analgesia and drug levels following codeine (20 mg/kg intraperitoneally) or morphine (3.5 mg/kg intraperitoneally), matched for peak analgesia. Nicotine increased codeine analgesia (1.59X AUC(0-30 min) vs vehicle; p<0.001), while propranolol decreased analgesia (0.56X; p<0.05); co-pretreatment was similar to vehicle controls (1.23X; p>0.1). Nicotine increased, while propranolol decreased, brain, but not plasma, morphine levels, and analgesia correlated with brain (p<0.02), but not plasma (p>0.4), morphine levels after codeine. Pretreatments did not alter baseline or morphine analgesia. Here we show that brain CYP2D alters drug response despite the presence of substantial first-pass metabolism of codeine and further that nicotine induction of brain CYP2D increases codeine response in vivo. Thus variation in brain CYP2D activity, due to genetics or environment, may contribute to individual differences in response to centrally acting substrates. Exposure to nicotine may increase central drug metabolism, not detected peripherally, contributing to altered drug efficacy, onset time, and/or abuse liability.

Codeine: An Under-Recognized and Easily Treated Cause of Acute Abdominal Pain

We present 2 cases of acute abdominal pain secondary to oral codeine that resolved after the administration of intravenous naloxone.

A validated method for simultaneous determination of codeine, codeine-6-glucuronide, norcodeine, morphine, morphine-3-glucuronide and morphine-6-glucuronide in post-mortem blood, vitreous fluid, muscle, fat and brain tissue by LC-MS

The toxicodynamics and, to a lesser degree, toxicokinetics of the widely used opiate codeine remain a matter of controversy. To address this issue, analytical methods capable of providing reliable quantification of codeine metabolites alongside codeine concentrations are required. This article presents a validated method for simultaneous determination of codeine, codeine metabolites codeine-6-glucuronide (C6G), norcodeine and morphine, and morphine metabolites morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) in post-mortem whole blood, vitreous fluid, muscle, fat and brain tissue by high-performance liquid chromatography mass spectrometry. Samples were prepared by solid-phase extraction. The validated ranges were 1.5-300 ng/mL for codeine, norcodeine and morphine, and 23-4,600 ng/mL for C6G, M3G and M6G, with exceptions for norcodeine in muscle (3-300 ng/mL), morphine in muscle, fat and brain (3-300 ng/mL) and M6G in fat (46-4,600 ng/mL). Within-run and between-run accuracy (88.1-114.1%) and precision (CV 0.6-12.7%), matrix effects (CV 0.3-13.5%) and recovery (57.8-94.1%) were validated at two concentration levels; 3 and 150 ng/mL for codeine, norcodeine and morphine, and 46 and 2,300 ng/mL for C6G, M3G and M6G. Freeze-thaw and long-term stability (6 months at -80°C) was assessed, showing no significant changes in analyte concentrations (-12 to +8%). The method was applied in two authentic forensic autopsy cases implicating codeine in both therapeutic and presumably lethal concentration levels.

Confronting the challenges of effective pain management in children following tonsillectomy

Tonsillectomy is an extremely common surgical procedure associated with significant morbidity and mortality. The post-operative challenges include: respiratory complications, post-tonsillectomy hemorrhage, nausea, vomiting and significant pain. The present model of care demands that most of these children are managed in an ambulatory setting. The recent Federal Drug Agency (FDA) warning contraindicating the use of codeine after tonsillectomy in children represents a significant change of practice for many pediatric otolaryngological surgeons. This introduces a number of other safety concerns when deciding on a safe alternative to codeine, especially since most tonsillectomy patients are managed by lay primary caregiver's at home. This review outlines the safety issues and proposes, based on currently available evidence, a preventative multi-modal strategy to manage pain, nausea and vomiting without increasing the risk of post-tonsillectomy bleeding.

A limited number of prescribed drugs account for the great majority of drug-drug interactions

PURPOSE

The purpose of this study was to investigate the prevalence of prescribed combinations of interacting drugs in the Swedish population.

METHODS

This study design was retrospective and cross-sectional, based on a national register of dispensed prescription drugs during the period from January 1 to April 30, 2010. Prescription data was linked to the drug-drug interaction database SFINX to yield the prevalence of interacting combinations dispensed in the population. The study focused in particular on C- (clinically relevant interactions that can be handled, e.g. by dose adjustments), and D-interactions (clinically relevant interactions that should be avoided).

RESULTS

Thirty-eight and 3.8 % of the population were dispensed combinations of drugs classified as C- or D- interactions, respectively, i.e. clinically relevant, involving all therapeutic areas. Half of the D-interactions were associated with increased risk of adverse drug reactions whereas the other half were considered interactions with a potential to cause therapeutic failure. We identified a top 15 list of D-interactions that included 80 % of the total number of interacting drug combinations. Regarding individual drugs, a group of only ten drugs was involved in as much as 94 % of all D-interactions.

CONCLUSIONS

This study reveals that the majority of prescribed interacting drug combinations in Sweden involve a limited number of drugs. The findings may increase the awareness among prescribers of these most common drug interactions in clinical practice and highlight an area for pharmacological education. It may also serve as an inventory of potential interactions within different therapeutic areas for further research.

Precise genetic mapping and integrative bioinformatics in Diversity Outbred mice reveals Hydin as a novel pain gene

Mouse genetics is a powerful approach for discovering genes and other genome features influencing human pain sensitivity. Genetic mapping studies have historically been limited by low mapping resolution of conventional mouse crosses, resulting in pain-related quantitative trait loci (QTL) spanning several megabases and containing hundreds of candidate genes. The recently developed Diversity Outbred (DO) population is derived from the same eight inbred founder strains as the Collaborative Cross, including three wild-derived strains. DO mice offer increased genetic heterozygosity and allelic diversity compared to crosses involving standard mouse strains. The high rate of recombinatorial precision afforded by DO mice makes them an ideal resource for high-resolution genetic mapping, allowing the circumvention of costly fine-mapping studies. We utilized a cohort of ~300 DO mice to map a 3.8 Mbp QTL on chromosome 8 associated with acute thermal pain sensitivity, which we have tentatively named Tpnr6. We used haplotype block partitioning to narrow Tpnr6 to a width of ~230 Kbp, reducing the number of putative candidate genes from 44 to 3. The plausibility of each candidate gene's role in pain response was assessed using an integrative bioinformatics approach, combining data related to protein domain, biological annotation, gene expression pattern, and protein functional interaction. Our results reveal a novel, putative role for the protein-coding gene, Hydin, in thermal pain response, possibly through the gene's role in ciliary motility in the choroid plexus-cerebrospinal fluid system of the brain. Real-time quantitative-PCR analysis showed no expression differences in Hydin transcript levels between pain-sensitive and pain-resistant mice, suggesting that Hydin may influence hot-plate behavior through other biological mechanisms.

Codeine-related adverse drug reactions in children following tonsillectomy: a prospective study

OBJECTIVES/HYPOTHESIS

To prospectively determine factors associated with codeine's adverse drug reactions (ADRs) at home in a large homogenous population of children undergoing outpatient tonsillectomy.

STUDY DESIGN

Prospective, genotype blinded, observational study with a single group and repeated ADR measures documented by parents at home.

METHODS

A total of 249 children 6 to 15 years of age scheduled for tonsillectomy were enrolled. The primary outcome was number of daily codeine-related ADRs. We examined the number and type of ADR by race and by days and further modeled factors potentially associated with ADR risk in a subcohort of white children. Sedation following a dose of codeine was a secondary outcome measure. Parents recorded their children's daily ADRs and sedation scores during postoperative days (POD) 0 to 3 at home.

RESULTS

Diaries were returned for 134 children, who were given codeine. A total of 106 (79%) reported at least one ADR. The most common ADRs were nausea, lightheadedness/dizziness for white children and nausea, and vomiting for African American children. In a subcohort of white children ≤ 45 kg, increased ADR risk was associated with the presence of one or more full function CYP2D6 alleles (P < 0.001), POD (P < 0.001), and sex (P = 0.027). Increased pain intensity (P = 0.009) and PODs 0 and 1 (P = 0.001) contributed to a higher sedation risk. Neither obstructive apnea nor predicted CYP2D6 phenotype were associated with sedation risk.

CONCLUSIONS

Our results provide evidence that multiple factors are associated with codeine-related ADRs and support the FDA recommendation to avoid codeine's routine use following tonsillectomy in children.

First demonstration that brain CYP2D-mediated opiate metabolic activation alters analgesia in vivo

The response to centrally acting drugs is highly variable between individuals and does not always correlate with plasma drug levels. Drug-metabolizing CYP enzymes in the brain may contribute to this variability by affecting local drug and metabolite concentrations. CYP2D metabolizes codeine to the active morphine metabolite. We investigated the effect of inhibiting brain, and not liver, CYP2D activity on codeine-induced analgesia. Rats received intracerebroventricular injections of CYP2D inhibitors (20 μg propranolol or 40 μg propafenone) or vehicle controls. Compared to vehicle-pretreated rats, inhibitor-pretreated rats had: (a) lower analgesia in the tail-flick test (p<0.05) and lower areas under the analgesia-time curve (p<0.02) within the first hour after 30 mg/kg subcutaneous codeine, (b) lower morphine concentrations and morphine to codeine ratios in the brain (p<0.02 and p<0.05, respectively), but not in plasma (p>0.6 and p>0.7, respectively), tested at 30 min after 30 mg/kg subcutaneous codeine, and (c) lower morphine formation from codeine ex vivo by brain membranes (p<0.04), but not by liver microsomes (p>0.9). Analgesia trended toward a correlation with brain morphine concentrations (p=0.07) and correlated with brain morphine to codeine ratios (p<0.005), but not with plasma morphine concentrations (p>0.8) or plasma morphine to codeine ratios (p>0.8). Our findings suggest that brain CYP2D affects brain morphine levels after peripheral codeine administration, and may thereby alter codeine's therapeutic efficacy, side-effect profile and abuse liability. Brain CYPs are highly variable due to genetics, environmental factors and age, and may therefore contribute to interindividual variation in the response to centrally acting drugs.

CYP2D6 poor metabolizer genotype and smoking predict severe postoperative pain in female patients on arrival to the recovery room

Recent studies have shown that CYP2D6 acts at critical steps for endogenous morphine biosynthesis. The present study assessed the contribution of CYP2D6 genetic polymorphisms, smoking, and other factors on acute severe postoperative pain (linear analog pain scores ≥8).

METHODS

Two hundred thirty-six female patients were found to have adequate information in a previously developed female surgical patient database to be included in this current analysis. Multiple logistic regression analysis was used to assess the predictors for acute severe postoperative pain. DNA had been previously extracted from blood samples in all patients and was genotyped by the Amplichip to determine the specific CYP2D6 genotypes.

RESULTS

It was noted that the incidence of acute severe postoperative pain (linear analog pain scores ≥8) was more frequent in patients with the CYP2D6 poor metabolizer (PM) genotype, 71%, compared with 28% in intermediate metabolizers (IMs), 26% in extensive metabolizers (EMs), and 27% in ultrarapid metabolizers (UMs). The overall association between metabolizer groups and severe postoperative pain was significant (P=0.023). PMs were significantly more likely to suffer from severe postoperative pain than IMs, EMs, and UMs (P=0.007, 0.002, and 0.050, respectively). There were no significant differences among IMs, EMs, and UMs. Additionally, it was noted that there was an increased frequency of acute severe postoperative pain in smokers vs nonsmokers (P=0.014).

CONCLUSION

This study demonstrated that female patients possessing the PM genotype of CYP2D6 and patients who smoke had a higher incidence of acute severe postoperative pain.

Analgesics in patients with hepatic impairment: pharmacology and clinical implications

The physiological changes that accompany hepatic impairment alter drug disposition. Porto-systemic shunting might decrease the first-pass metabolism of a drug and lead to increased oral bioavailability of highly extracted drugs. Distribution can also be altered as a result of impaired production of drug-binding proteins or changes in body composition. Furthermore, the activity and capacity of hepatic drug metabolizing enzymes might be affected to various degrees in patients with chronic liver disease. These changes would result in increased concentrations and reduced plasma clearance of drugs, which is often difficult to predict. The pharmacology of analgesics is also altered in liver disease. Pain management in hepatically impaired patients is challenging owing to a lack of evidence-based guidelines for the use of analgesics in this population. Complications such as bleeding due to antiplatelet activity, gastrointestinal irritation, and renal failure are more likely to occur with nonsteroidal anti-inflammatory drugs in patients with severe hepatic impairment. Thus, this analgesic class should be avoided in this population. The pharmacokinetic parameters of paracetamol (acetaminophen) are altered in patients with severe liver disease, but the short-term use of this drug at reduced doses (2 grams daily) appears to be safe in patients with non-alcoholic liver disease. The disposition of a large number of opioid drugs is affected in the presence of hepatic impairment. Certain opioids such as codeine or tramadol, for instance, rely on hepatic biotransformation to active metabolites. A possible reduction of their analgesic effect would be the expected pharmacodynamic consequence of hepatic impairment. Some opioids, such as pethidine (meperidine), have toxic metabolites. The slower elimination of these metabolites can result in an increased risk of toxicity in patients with liver disease, and these drugs should be avoided in this population. The drug clearance of a number of opioids, such as morphine, oxycodone, tramadol and alfentanil, might be decreased in moderate or severe hepatic impairment. For the highly excreted morphine, hydromorphone and oxycodone, an important increase in bioavailability occurs after oral administration in patients with hepatic impairment. Lower doses and/or longer administration intervals should be used when these opioids are administered to patients with liver disease to avoid the risk of accumulation and the potential increase of adverse effects. Finally, the pharmacokinetics of phenylpiperidine opioids such as fentanyl, sufentanil and remifentanil appear to be unaffected in hepatic disease. All opioid drugs can precipitate or aggravate hepatic encephalopathy in patients with severe liver disease, thus requiring cautious use and careful monitoring.

Opioid genetics: the key to personalized pain control?

There are now several strong opioids available to choose from for the relief of moderate to severe pain. On a population level, there is no difference in terms of analgesic efficacy or adverse reactions between these drugs; however, on an individual level there is marked variation in response to a given opioid. The genetic influences to this variation are complex, and although current research has shown some promising results, these have not been replicated across larger studies and as such the ultimate aim of personalized prescribing remains elusive. If personalized prescribing could be achieved this would have a major impact at an individual level to facilitate safe, effective and rapid symptom control. This review presents some of the recent positive advances in opioid pharmacogenetic studies, focusing on associations between candidate genes and the three main elements of opioid response: analgesic, upper gastrointestinal and central adverse reactions.

Clinical Application of Pharmacogenomics

The purpose of this review is to discuss the clinical application of pharmacogenomics for select drug therapies (eg, proton pump inhibitors [PPIs], codeine, and carbamazepine) and to highlight limitations and challenges that preclude implementation of pharmacogenomics into clinical practice. Genetic polymorphisms of cytochrome P450 (CYP) enzymes and the presence of the human leukocyte antigen ( HLA) -B*1502 allele influence drug disposition and/or response. A portion of PPI pharmacokinetic and pharmacodynamic variability can be explained by CYP2C19 genotype. However, conflicting evidence exists related to Helicobacter pylori cure rates based on CYP2C19 genotype. For codeine, adverse drug reactions in neonates through breast-feeding from CYP2D6 ultra-rapid metabolizers have been reported. However, there is lack of conclusive evidence regarding the overall influence of CYP2D6 polymorphisms on codeine efficacy and toxicity. Although CYP2C19 and CYP2D6 genotyping tests are available, clinical utility remains low. The presence of the HLA-B*1502 allele is associated with carbamazepine-induced Stevens-Johnson syndrome (SJS) and/or toxic epidermal necrolysis (TEN). Pharmacogenomic testing is required prior to initiating carbamazepine in high-risk patients. Lack of sufficient resources, provider knowledge, and ethical, legal, and social issues are several limitations and challenges to implementing pharmacogenomic testing in clinical practice.

Prediction of codeine toxicity in infants and their mothers using a novel combination of maternal genetic markers

Substantial variation exists in response to standard doses of codeine ranging from poor analgesia to life-threatening central nervous system (CNS) depression. We aimed to discover the genetic markers predictive of codeine toxicity by evaluating the associations between polymorphisms in cytochrome P450 2D6 (CYP2D6), UDP-glucuronosyltransferase 2B7 (UGT2B7), P-glycoprotein (ABCB1), mu-opioid receptor (OPRM1), and catechol O-methyltransferase (COMT) genes, which are involved in the codeine pathway, and the symptoms of CNS depression in 111 breastfeeding mothers using codeine and their infants. A genetic model combining the maternal risk genotypes in CYP2D6 and ABCB1 was significantly associated with the adverse outcomes in infants (odds ratio (OR) 2.68; 95% confidence interval (CI) 1.61-4.48; P(trend) = 0.0002) and their mothers (OR 2.74; 95% CI 1.55-4.84; P(trend) = 0.0005). A novel combination of the genetic and clinical factors predicted 87% of the infant and maternal CNS depression cases with a sensitivity of 80% and a specificity of 87%. Genetic markers can be used to improve the outcome of codeine therapy and are also probably important for other opioids sharing common biotransformation pathways.

CYP2D6 polymorphisms and codeine analgesia in postpartum pain management: a pilot study

BACKGROUND

Codeine, a common opiate prescribed for pain postcesarean section (c-section), is biotransformed by the highly polymorphic Cytochrome P450 enzyme 2D6 (CYP2D6). Ultrarapid metabolizers (UMs), individuals with multiple active copies of CYP2D6, can biotranform up to 50% more codeine into morphine than normal individuals can. In contrast, poor metabolizers (PMs), individuals who have no active CYP2D6 genes, convert almost no codeine into morphine and as a result may take multiple doses of codeine without attaining analgesia.

OBJECTIVE

The aim was to study the relationship between CYP2D6 genotype and codeine analgesia among women recovering from c-section.

METHODS

Forty-five mothers prescribed codeine provided a blood sample for CYP2D6 genotyping and recorded their pain level 4 times a day for 3 days immediately after a c-section. Codeine was used on an as-needed basis; doses and times were recorded. The relationship between CYP2D6 genotype, pain scores, need for codeine, and adverse events was studied. Theoretical morphine dose, based on CYP2D6 genotype, was estimated.

RESULTS

Women at the genotypic extremes reported codeine effects consistent with their genotype: the 2 PMs of codeine reported no analgesia as a result of taking codeine, whereas 2 of the 3 UMs reported immediate pain relief from codeine but stopped taking it due to dizziness and constipation. Much larger numbers are needed to study similar correlations among extensive and intermediate metabolizers.

CONCLUSIONS

In this pilot study, the extreme CYP2D6 genotypes (PMs and UMs) seemed to predict pain response and adverse events. Larger sample sizes are needed to correlate the range of genotypes with pain response.

Role of cytochrome P450 genotype in the steps toward personalized drug therapy

Genetic polymorphism for cytochrome 450 (P450) enzymes leads to interindividual variability in the plasma concentrations of many drugs. In some cases, P450 genotype results in decreased enzyme activity and an increased risk for adverse drug effects. For example, individuals with the CYP2D6 loss-of-function genotype are at increased risk for ventricular arrhythmia if treated with usual does of thioridazine. In other cases, P450 genotype may influence the dose of a drug required to achieve a desired effect. This is the case with warfarin, with lower doses often necessary in carriers of a variant CYP2C9*2 or *3 allele to avoid supratherapeutic anticoagulation. When a prodrug, such as clopidogrel or codeine, must undergo hepatic biotransformation to its active form, a loss-of-function P450 genotype leads to reduced concentrations of the active drug and decreased drug efficacy. In contrast, patients with multiple CYP2D6 gene copies are at risk for opioid-related toxicity if treated with usual doses of codeine-containing analgesics. At least 25 drugs contain information in their US Food and Drug Administration-approved labeling regarding P450 genotype. The CYP2C9, CYP2C19, and CYP2D6 genes are the P450 genes most often cited. To date, integration of P450 genetic information into clinical decision making is limited. However, some institutions are beginning to embrace routine P450 genotyping to assist in the treatment of their patients. Genotyping for P450 variants may carry less risk for discrimination compared with genotyping for disease-associated variants. As such, P450 genotyping is likely to lead the way in the clinical implementation of pharmacogenomics. This review discusses variability in the CYP2C9, CYP2C19, and CYP2D6 genes and the implications of this for drug efficacy and safety.

Absorption, distribution, metabolism and excretion pharmacogenomics of drugs of abuse

Pharmacologic and toxic effects of xenobiotics, such as drugs of abuse, depend on the genotype and phenotype of an individual, and conversely on the isoenzymes involved in their metabolism and transport. The current knowledge of such isoenzymes of frequently abused therapeutics such as opioids (oxycodone, hydrocodone, methadone, fentanyl, buprenorphine, tramadol, heroin, morphine and codeine), anesthetics (γ-hydroxybutyric acid, propofol, ketamine and phencyclidine) and cognitive enhancers (methylphenidate and modafinil), and some important plant-derived hallucinogens (lysergide, salvinorin A, psilocybin and psilocin), as well as of nicotine in humans are summarized in this article. The isoenzymes (e.g., cytochrome P450, glucuronyltransferases, esterases and reductases) involved in the metabolism of drugs and some pharmacokinetic data are discussed. The relevance of such data is discussed for predicting possible interactions with other xenobiotics, understanding pharmacokinetic behavior and pharmacogenomic variations, assessing toxic risks, developing suitable toxicological analysis procedures, and finally for interpretating drug testing results.

Drug-drug interactions that reduce the formation of pharmacologically active metabolites: a poorly understood problem in clinical practice

Drug-drug interactions can lead to reduced efficacy of medical treatment. Therapeutic failure may for instance result from combined treatment with an inhibitor of the specific pathway that is responsible for the generation of pharmacologically active drug metabolites. This problem may be overlooked in clinical practice. Several examples of drugs will be discussed -clopidogrel, losartan, tamoxifen and codeine - to illustrate differences in the potential impact on drug treatment in clinical practice. We conclude that the combined use of cytochrome P450-blocking serotonin reuptake inhibitors and tamoxifen or codeine should be avoided, whereas the situation is much more complex regarding the use of proton pump inhibitors together with clopidogrel, and the evidence regarding cytochrome P450 inhibitor-dependent activation of losartan is inconclusive.

Nationwide drug-dispensing data reveal important differences in adherence to drug label recommendations on CYP2D6-dependent drug interactions

AIMS

The study aimed to investigate the clinical adherence to drug label recommendations on important drug-drug interactions (DDIs). Dispensing data on drug combinations involving selective serotonin reuptake inhibitor (SSRI) antidepressants could help to identify areas for intensified medical education.

METHODS

This was a retrospective, cross-sectional analysis of individual dispensing data regarding all individuals > or =15 years old in Sweden. The study analysed the prescribing and dispensing of CYP2D6 drugs (metoprolol, donepezil, galantamine, codeine, tamoxifen) together with CYP2D6-blocking SSRIs (paroxetine/fluoxetine) or SSRIs without significant CYP2D6 inhibition (citalopram/escitalopram/sertraline), and the related prescribing of CYP2D6-independent comparator drugs (atenolol, rivastigmine, propoxyphene, anastrozole). Odds were calculated between each CYP2D6 drug and the corresponding comparator drug in patients on fluoxetine/paroxetine and citalopram/escitalopram/sertraline, respectively. The odds ratio (OR) was calculated by dividing the obtained odds in patients on fluoxetine/paroxetine by the corresponding odds in patients on citalopram/escitalopram/sertraline.

RESULTS

Compared with patients that were dispensed citalopram/escitalopram/sertraline, patients dispensed fluoxetine/paroxetine had lower prescribing rates of metoprolol (adjusted OR 0.80; 95% confidence interval 0.76, 0.85), donepezil (0.65; 0.49, 0.86) and galantamine (0.58; 0.41, 0.81). In contrast, the use of prodrugs codeine (compared woth propoxyphene) or tamoxifen (compared with anastrozole) was similar among patients on fluoxetine/paroxetine and citalopram/escitalopram/sertraline (adjusted OR 1.03; 0.94, 1.12 and 1.29; 0.96, 1.73, respectively).

CONCLUSIONS

Clinically important DDIs that are associated with impaired bioactivation of prodrugs might be more easily neglected in clinical practice compared with DDIs that cause drug accumulation and symptomatic adverse drug reactions.

Pain management of musculoskeletal injuries in children: current state and future directions

BACKGROUND

Pain is the most common reason for seeking health care in the Western world and is a contributing factor in up to 80% of all emergency department (ED) visits. In the pediatric emergency setting, musculoskeletal injuries are one of the most common painful presentations. Inadequate pain management during medical care, especially among very young children, can have numerous detrimental effects. No standard of care exists for the management of acute musculoskeletal injury-related pain in children. Within the ED setting, pain from such injuries has been repeatedly shown to be undertreated.

OBJECTIVES

Upon completion of this CME article, the reader should be better able to (1) distinguish multiple nonpharmacological techniques for minimizing and treating pain and anxiety in children with musculoskeletal injuries, (2) apply recent medical literature in deciding pharmacological strategies for the treatment of children with musculoskeletal injuries, and (3) interpret the basic principles of pharmacogenomics and how they relate to analgesic efficacy.

RESULTS

Pediatric musculoskeletal injuries are both common and painful. There is growing evidence that, in addition to pharmacological therapy, nonpharmacological methods can be introduced to improve analgesia in the ED and after discharge. Traditionally, acetaminophen with codeine has been used to treat moderate orthopedic injury-related pain in children. Other oral opioids (hydrocodone, oxycodone) are gaining popularity, as well. Current data suggest that ibuprofen is at least as effective as acetaminophen-codeine and codeine alone. Medication compliance might be improved if adverse effects were minimized, and ibuprofen has been shown to have a similar or better adverse effect profile than the oral opioids to which it has been compared. Pharmacogenomic data show that nearly 50% of individuals have at least 1 reduced functioning allele resulting in suboptimal conversion of codeine to active analgesic, so it is not surprising that codeine analgesic efficacy is not optimal. At the same time, nonpharmacological therapies are emerging as commonly used treatment options by parents and adjuncts to analgesic medication. The efficacy and role of techniques (massage, music therapy, transcutaneous electrical nerve stimulation), although promising, require further clarification in the treatment of orthopedic injury pain.

CONCLUSIONS

There is a need to optimize the measurement, documentation, and treatment of pain in children. There is growing evidence that nonpharmacological methods can be introduced to improve analgesia in the ED, and efforts to help parents implement these methods at home might be advantageous to optimize outpatient treatment plans. In pharmacotherapy, ibuprofen has emerged as an appropriate first-line choice for mild-moderate orthopedic pain. Other oral opioids (hydrocodone, oxycodone) are gaining popularity over codeine, because of the current understanding of the pharmacogenomics of such medications.

Recent advances in the use of opioids for cancer pain

Opioids are the mainstay of treatment for moderate to severe cancer pain. In recent years there have been many advances in the use of opioids for cancer pain. Availability and consumption of opioids have increased and opioids other than morphine (including methadone, fentanyl, oxycodone) have become more widely used. Inter-individual variation in response to opioids has been identified as a significant challenge in the management of cancer pain. Many studies have been published demonstrating the benefits of opioid switching as a clinical maneuver to improve tolerability. Constipation has been recognized as a significant burden in cancer patients on opioids. Peripherally restricted opioid antagonists have been developed for the prevention and management of opioid induced constipation. The phenomenon of breakthrough pain has been characterized and novel modes of opioid administration (transmucosal, intranasal, sublingual) have been explored to facilitate improved management of breakthrough cancer pain. Advances have also been made in the realm of molecular biology. Pharmacogenetic studies have explored associations between clinical response to opioids and genetic variation at a DNA level. To date these studies have been small but future research may facilitate prospective prediction of response to individual drugs.

Pediatric palliative care: use of opioids for the management of pain

Pediatric palliative care (PPC) is provided to children experiencing life-limiting diseases (LLD) or life-threatening diseases (LTD). Sixty to 90% of children with LLD/LTD undergoing PPC receive opioids at the end of life. Analgesia is often insufficient. Reasons include a lack of knowledge concerning opioid prescribing and adjustment of opioid dose to changing requirements. The choice of first-line opioid is based on scientific evidence, pain pathophysiology, and available administration modes. Doses are calculated on a bodyweight basis up to a maximum absolute starting dose. Morphine remains the gold standard starting opioid in PPC. Long-term opioid choice and dose administration is determined by the pathology, analgesic effectiveness, and adverse effect profile. Slow-release oral morphine remains the dominant formulation for long-term use in PPC with hydromorphone slow-release preparations being the first rotation opioid when morphine shows severe adverse effects. The recently introduced fentanyl transdermal therapeutic system with a drug-release rate of 12.5 microg/hour matches the lower dose requirements of pediatric cancer pain control. Its use may be associated with less constipation compared with morphine use. Though oral transmucosal fentanyl citrate has reduced bioavailability (25%), it inherits potential for breakthrough pain management. However, the gold standard breakthrough opioid remains immediate-release morphine. Buprenorphine is of special clinical interest as a result of its different administration routes, long duration of action, and metabolism largely independent of renal function. Antihyperalgesic effects, induced through antagonism at the kappa-receptor, may contribute to its effectiveness in neuropathic pain. Methadone also has a long elimination half-life (19 [SD 14] hours) and NMDA receptor activity although dose administration is complicated by highly variable morphine equianalgesic equivalence (1 : 2.5-20). Opioid rotation to methadone requires special protocols that take this into account. Strategies to minimize adverse effects of long-term opioid treatment include dose reduction, symptomatic therapy, opioid rotation, and administration route change. Patient- or nurse-controlled analgesia devices are useful when pain is rapidly changing, or in terminal care where analgesic requirements may escalate. In this article, we present detailed pediatric pharmacokinetic and pharmacodynamic data for opioids, their indications and contraindications, as well as dose-administration regimens that include practical strategies for opioid switching and dose reduction. Additionally, we discuss the problem of hyperalgesia and the use of adjuvant drugs to support opioid therapy.

The impact of CYP2D6 genetic polymorphisms on postoperative morphine consumption

BACKGROUND

Endogenous morphine-like compounds have been identified in humans and are released in response to stress. Human monocytes and granulocytes express the micro opiate receptor, micro3, which is morphine selective but opiate peptide insensitive. Recent studies have shown that CYP2D6 acts at critical steps for endogenous morphine biosynthesis. We theorized that ultrarapid (UM) CYP2D6 metabolizers may have an enhancement of their endogenous pain modulating mechanisms.

METHODS

After institutional review board approval, a previously developed surgical patient database was evaluated for information concerning CYP2D6 genotypes and morphine consumption. One hundred forty-two patients were found to have adequate information to be included in this current analysis. The study group was divided, based on morphine consumption, into two subgroups: low morphine consumers (LMC) (< or =10 mg/4 h, N = 80) and high morphine consumers (HMC) (>10 mg/4 h, N = 62). DNA was extracted from blood in all patients and was genotyped by the Amplichip (Roche, Pleasanton, CA) to determine the specific CYP2D6 genotypes.

RESULTS

CYP2D6 UM were found to occur more frequently in the LMC group than in the HMC group (8/80 vs 0/62, P = 0.0091). No significant differences were noted for the poor, intermediate, or extensive metabolizers.

CONCLUSIONS

Our current results suggest that CYP2D6 UM appear to require less morphine in the acute postoperative period compared with other CYP2D6 metabolizer groups. One possible mechanism for this observation is that CYP2D6 UM may have higher efficiency in synthesizing endogenous morphine compared with other metabolizers, thus increasing endogenous pain modulation and reducing the need for exogenous morphine.

Neuroimaging the genomics of pain processing--a perspective

The significant inter-individual variability in pain behavior and its contributing factors remains a pervasive clinical challenge and has generated intense scientific investigations in this field. Recent studies indicate that our genes considerably influence nociceptive information processes, how our nociceptive system copes with peripheral injury, and the individual response to analgesic treatments. Neuroimaging studies of pain are beginning to link such genetic influences with physiological processes in the human brain. This review presents the pioneering endeavors of the hybrid approach of neuroimaging pain genes in humans and identifies potential future directions.

Can extremely low or high morphine formation from codeine be predicted prior to therapy initiation?

Activation of codeine by O-demethylation into morphine is a prerequisite for its analgesic effects and severe toxicity. Identifying patients in whom morphine is formed either at extremely low or at extremely high amounts may improve efficacy and safety of codeine therapy. To assess how well this identification is possible, we compared the performance of current CYP2D6 phenotype association systems (traditional genotype-based classification, a recently proposed CYP2D6 activity score, and the plasma dextromethorphan metabolic ratio) in 57 healthy Caucasians after oral administration of 30 mg dextromethorphan hydrobromide or 50 mg codeine. Most subjects (87.5%) at the lower 15% of morphine formation from codeine and thus likely to not to respond to codeine therapy were correctly identified by CYP2D6 genotype- or phenotype-based systems. In contrast, in subjects at the upper 15% of morphine formation being at risk for opioid toxicity, CYP2D6 genotyping predicted only the 50% who carried gene duplication, whereas dextromethorphan-based phenotyping identified 67.5% of the subjects with high morphine formation. However, satisfactory prediction (87.5%) of high morphine formation was only achieved when combining genotyping with phenotyping. In conclusion, insufficient morphine formation from codeine and thus likely failure of analgesia can currently be well predicted. However, to make codeine therapy safe, extremely high morphine formation has to be predicted as well, which has to be obtained at the effort of combining genotyping with phenotyping.