Pharmacokinetics of codeine and its metabolite morphine in ultra-rapid metabolizers due to CYP2D6 duplication

Advanced urine toxicology testing

Urine toxicology screening testing is an important standard of care in the addiction and pain treatment setting, offering a reproducible, unbiased, and accurate laboratory test to monitor patients and provide objective support for clinical observations. It has been shown that physicians do not have proficiency in the ordering or interpretation of these tests. This article is an attempt to respond to that need. Current antibody-based enzymatic immunoassays (EIAs) used for urine toxicology screening are useful to detect classes of drugs (ex., opiate) but cannot determine which specific drug (ex., morphine) is present. Gas chromatography and mass spectroscopy can determine exactly which drugs are present, allowing prescribed (or illicit) opiates and benzodiazepines to be identified. This article will discuss principles and details of opiate and benzodiazepine EIA and gas chromatography and mass spectroscopy urine toxicology testing. The approach to detecting patients attributing positive opiate EIAs to prescription opiates who are using heroin or other opioids will be reviewed. Cases of controlled prescription drugs that do not produce the expected positive urine tests (ex., oxycodone producing negative opiate screening tests) will be discussed. How to differentiate codeine from heroin and the role of poppy seeds in toxicology will be examined. The case of an anti-depressant drug that produces false-positive benzodiazepine results and antibiotics that cause positive opiate urine toxicology results will be reviewed. Common benzodiazepines (ex., clonazepam and lorazepam) that do not reliably produce positive benzodiazepine EIAs will be discussed. The approach to detection and management of all these types of toxicology cases will be reviewed, and it is hoped that the analyses presented will impart an adequate information base to medical providers and staff members of drug treatment and pain centers, enabling them to order and interpret these tests in the clinic more effectively as an integrated part of whole patient care.

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.

Pharmacogenomics and adverse drug reactions

Adverse drug reactions (ADRs) observed during drug development have been the cause for discontinuing development of many drugs. In addition, serious but rare ADRs observed after marketing have led to withdrawal of some drugs. A priori identification of individuals at risk of developing ADRs for a given drug will help develop strategies to reduce the risk for ADRs in these patients. US FDA initiatives and efforts at reducing ADRs to make drugs safer are described, including updating of drug labels to include genomic information intended to reduce ADRs. Pharmacogenomics can also be harnessed to identify individuals at risk of developing serious ADRs and to treat these individuals with alternative therapy, thus converting ADRs that are traditionally considered unavoidable to avoidable ADRs.

Assessment of activity levels for CYP2D6*1, CYP2D6*2, and CYP2D6*41 genes by population pharmacokinetics of dextromethorphan

The pharmacokinetics of dextromethorphan (DM) is markedly influenced by cytochrome P450 2D6 (CYP2D6) enzyme polymorphisms. The aim of this study was to quantify the effects of the CYP2D6*1, *2, and *41 variants on DM metabolism in vivo and to identify other sources of pharmacokinetic variability. Concentrations of DM and dextrorphan (DO) in plasma and urine were evaluated in 36 healthy Caucasian men. These volunteers participated in three clinical studies and received a single oral dose of 30 mg DM-HBr. Data were modeled simultaneously using the population pharmacokinetics NONMEM software. A five-compartment model adequately described the data. The activity levels of the alleles assessed differed significantly. The clearance attributable to an individual CYP2D6*1 copy was 2.5-fold higher as compared with CYP2D6*2 (5,010 vs. 2,020 l/h), whereas the metabolic activity of CYP2D6*41 was very low (85 l/h). Urinary pH was confirmed as a significant covariate for DM renal clearance. These results refine genotype-based predictions of pharmacokinetics for DM and presumably for other CYP2D6 substrates as well.

Effect of the inhibition of CYP3A4 or CYP2D6 on the pharmacokinetics and pharmacodynamics of oxycodone

PURPOSE

The main metabolic pathways of oxycodone, a potent opioid analgetic, are N-demethylation (CYP3A4) to inactive noroxycodone and O-demethylation (CYP2D6) to active oxymorphone. We performed a three-way, placebo-controlled, double-blind cross-over study to assess the pharmacokinetic and pharmacodynamic consequences of drug interactions with oxycodone.

METHODS

The 12 participants (CYP2D6 extensive metabolizers) were pre-treated with placebo, ketoconazole or paroxetine before oral oxycodone ingestion (0.2 mg/kg).

RESULTS

Pre-treatment with ketoconazole increased the AUC for oxycodone 2- to 3-fold compared with placebo or paroxetine. In combination with placebo, oxycodone induced the expected decrease in pupil diameter. This decrease was accentuated in the presence of ketoconazole, but blunted by paroxetine. In comparison to pre-treatment with placebo, ketoconazole increased nausea, drowsiness, and pruritus associated with oxycodone. In contrast, the effect of pre-treatment with paroxetine on the above-mentioned adverse events was not different from that of placebo. Ketoconazole increased the analgetic effect of oxycodone, whereas paroxetine was not different from placebo.

CONCLUSIONS

Inhibition of CYP3A4 by ketoconazole increases the exposure and some pharmacodynamic effects of oxycodone. Paroxetine pretreatment inhibits CYP2D6 without inducing relevant changes in oxycodone exposure, and partially blunts the pharmacodynamic effects of oxycodone due to intrinsic pharmacological activities. Pharmacodynamic changes associated with CYP3A4 inhibition may be clinically important in patients treated with oxycodone.

Pharmacokinetics of acetaminophen, codeine, and the codeine metabolites morphine and codeine-6-glucuronide in healthy Greyhound dogs

The purpose of this study was to determine the pharmacokinetics of codeine and the active metabolites morphine and codeine-6-glucuronide after i.v. codeine administration and the pharmacokinetics of acetaminophen (APAP), codeine, morphine, and codeine-6-glucuronide after oral administration of combination product containing acetaminophen and codeine to dogs. Six healthy Greyhound dogs were administered 0.734 mg/kg codeine i.v. and acetaminophen (10.46 mg/kg mean dose) with codeine (1.43 mg/kg mean dose) orally. Blood samples were collected at predetermined time points for the determination of codeine, morphine, and codeine-6-glucuronide plasma concentrations by LC/MS and acetaminophen by HPLC with UV detection. Codeine was rapidly eliminated after i.v. administration (T(1/2) = 1.22 h; clearance = 29.94 mL/min/kg; volume of distribution = 3.17 L/kg) with negligible amounts of morphine present, but large amounts of codeine-6-glucuronide (C(max) = 735.75 ng/mL) were detected. The oral bioavailability of codeine was 4%, morphine concentrations were negligible, but large amounts of codeine-6-glucuronide (C(max) = 1952.86 ng/mL) were detected suggesting substantial first pass metabolism. Acetaminophen was rapidly absorbed (C(max) = 6.74 microg/mL; T(max) = 0.85 h) and eliminated (T(1/2) = 0.96 h). In conclusion, the pharmacokinetics of codeine was similar to other opioids in dogs with a short half-life, rapid clearance, large volume of distribution, and poor oral bioavailability. High concentrations of codeine-6-glucuronide were detected after i.v. and oral administration.

Communicating pharmacogenetic research results to breastfeeding mothers taking codeine: a pilot study of perceptions and benefits

Sixty-two codeine-prescribed breastfeeding mothers from a pharmacogenetic study were interviewed regarding the communication of individual CYP2D6 genotype results and overall research findings. All participants wanted to receive the results of their individual genetic tests; however, individuals placed different values on the usefulness of this information toward future medical decisions. Receiving one's pharmacogenetic test results was not associated with a negative psychosocial impact. Thirty-three percent of the participants wished to withhold these results from their physicians. Participants' expectations seem to dictate the extent of transparency of pharmacogenetic research results.

Pharmacogenetics and human genetic polymorphisms

The term pharmacogenetics was first used in the late 1950s and can be defined as the study of genetic factors affecting drug response. Prior to formal use of this term, there was already clinical data available in relation to variable patient responses to the drugs isoniazid, primaquine and succinylcholine. The subject area developed rapidly, particularly with regard to genetic factors affecting drug disposition. There is now comprehensive understanding of the molecular basis for variable drug metabolism by the cytochromes P450 and also for variable glucuronidation, acetylation and methylation of certain drugs. Some of this knowledge has already been translated to the clinic. The molecular basis of variation in drug targets, such as receptors and enzymes, is generally less well understood, although there is consistent evidence that polymorphisms in the genes encoding the β-adrenergic receptors and the enzyme vitamin K epoxide reductase is of clinical importance. The genetic basis of rare idiosyncratic adverse drug reactions had also been examined. Susceptibility to reactions affecting skin and liver appears to be determined in part by the HLA (human leucocyte antigen) genotype, whereas reactions affecting the heart and muscle may be determined by polymorphisms in genes encoding ion channels and transporters respectively. Genome-wide association studies are increasingly being used to study drug response and susceptibility to adverse drug reactions, resulting in identification of some novel pharmacogenetic associations.

Personalized therapy in pain management: where do we stand?

Genomic variations influencing response to pharmacotherapy of pain are currently under investigation. Drug-metabolizing enzymes represent a major target of ongoing research in order to identify associations between an individual’s drug response and genetic profile. Polymorphisms of the cytochrome P450 enzymes (CYP2D6) influence metabolism of codeine, tramadol, hydrocodone, oxycodone and tricyclic antidepressants. Blood concentrations of some NSAIDs depend on CYP2C9 and/or CYP2C8 activity. Genomic variants of these genes associate well with NSAIDs’ side effect profile. Other candidate genes, such as those encoding (opioid) receptors, transporters and other molecules important for pharmacotherapy in pain management, are discussed; however, study results are often equivocal. Besides genetic variants, further variables, for example, age, disease, comorbidity, concomitant medication, organ function as well as patients’ compliance, may have an impact on pharmacotherapy and need to be addressed when pain therapists prescribe medication. Although pharmacogenetics as a diagnostic tool has the potential to improve patient therapy, well-designed studies are needed to demonstrate superiority to conventional dosing regimes.

Incorporating evidence from pharmacologic and pharmacogenetic studies of atypical antipsychotic drugs into advanced psychiatric nursing practice

PURPOSE

To present a conceptual framework for incorporating pharmacologic findings and pharmacogenetic evidence related to atypical antipsychotic drugs (AADs) into advanced psychiatric nursing practice.

CONCLUSIONS

Three evidence domains lend important information about differential AAD response. These include the pharmacology of AADs, the molecular genetics of metabolizing enzymes, and the molecular genetics of neurotransmitter receptor drug targets.

PRACTICE IMPLICATIONS

These evidence domains can be incorporated into nursing practice decisions related to medication planning, patient and family education, and medication monitoring processes. The central focus of the framework is patient outcomes, which include medication adherence, tolerability of the AADs, and demonstrated clinical effectiveness.

Composite functional genetic and comedication CYP2D6 activity score in predicting tamoxifen drug exposure among breast cancer patients

Accurate assessment of CYP2D6 phenotypes from genotype is inadequate in patients taking CYP2D6 substrate together with CYP2D6 inhibitors. A novel CYP2D6 scoring system is proposed that incorporates the impact of concomitant medications with the genotype in calculating the CYP2D6 activity score. Training (n = 159) and validation (n = 81) data sets were obtained from a prospective cohort tamoxifen pharmacogenetics registry. Two inhibitor factors were defined: 1 genotype independent and 1 genotype based. Three CYP2D6 gene scoring systems, and their combination with the inhibitor factors, were compared. These 3 scores were based on Zineh, Zanger, and Gaedigk's approaches. Endoxifen/NDM-Tam plasma ratio was used as the phenotype. The overall performance of the 3 gene scoring systems without consideration of CYP2D6-inhibiting medications in predicting CYP2D6 phenotype was poor in both the training set (R(2) = 0.24, 0.22, and 0.18) and the validation set (R(2) = 0.30, 0.24, and 0.15). Once the CYP2D6 genotype-independent inhibitor factor was integrated into the score calculation, the R(2) values in the training and validation data sets were nearly twice as high as the genotype-only scoring model: (0.44, 0.43, 0.38) and (0.53, 0.50, 0.41), respectively. The integration of the inhibitory effect of concomitant medications with the CYP2D6 genotype into the composite CYP2D6 activity score doubled the ability to predict the CYP2D6 phenotype. However, endoxifen phenotypes still varied substantially, even with incorporation of CYD2D6 genotype and inhibiting factors, suggesting that other, as yet unidentified factors must be involved in tamoxifen activation.

Pharmacogenomics: a systems approach

Pharmacogenetics and pharmacogenomics involve the study of the role of inheritance in individual variation in drug response, a phenotype that varies from potentially life-threatening adverse drug reactions to equally serious lack of therapeutic efficacy. Pharmacogenetics-pharmacogenomics represents a major component of the movement to 'individualized medicine'. Pharmacogenetic studies originally focused on monogenic traits, often involving genetic variation in drug metabolism. However, contemporary studies increasingly involve entire 'pathways' that include both pharmacokinetics (PKs)--factors that influence the concentration of a drug reaching its target(s)--and pharmacodynamics (PDs), factors associated with the drug target(s), as well as genome-wide approaches. The convergence of advances in pharmacogenetics with rapid developments in human genomics has resulted in the evolution of pharmacogenetics into pharmacogenomics. At the same time, studies of drug response are expanding beyond genomics to encompass pharmacotranscriptomics and pharmacometabolomics to become a systems-based discipline. This discipline is also increasingly moving across the 'translational interface' into the clinic and is being incorporated into the drug development process and governmental regulation of that process. The article will provide an overview of the development of pharmacogenetics-pharmacogenomics, the scientific advances that have contributed to the continuing evolution of this discipline, the incorporation of transcriptomic and metabolomic data into attempts to understand and predict variation in drug response phenotypes as well as challenges associated with the 'translation' of this important aspect of biomedical science into the clinic.

Pharmacogenetics in reproductive and perinatal medicine

The clinical application of pharmacogenetics has been well accepted by some medical specialties, but not all. The aim of this review is to discuss the current use of pharmacogenetics in reproductive and perinatal medicine and to highlight areas where pharmacogenetics may be able to help in the future to predict response to medicines in terms of efficacy and safety. This applies to drugs that are specific to pregnancy and reproduction, as well as drugs prescribed for the treatment of medical disorders in pregnancy. Our review points out the need for well-designed clinical studies on the efficacy and safety of medicines used in women of childbearing age in order to define the additional utility provided by pharmacogenetic testing.

Targeting protein kinases in central nervous system disorders

Protein kinases are a growing drug target class in disorders in peripheral tissues, but the development of kinase-targeted therapies for central nervous system (CNS) diseases remains a challenge, largely owing to issues associated specifically with CNS drug discovery. However, several candidate therapeutics that target CNS protein kinases are now in various stages of preclinical and clinical development. We review candidate compounds and discuss selected CNS protein kinases that are emerging as important therapeutic targets. In addition, we analyse trends in small-molecule properties that correlate with key challenges in CNS drug discovery, such as blood-brain barrier penetrance and cytochrome P450-mediated metabolism, and discuss the potential of future approaches that will integrate molecular-fragment expansion with pharmacoinformatics to address these challenges.

An analysis of renal dysfunction in 1511 patients with fractured neck of femur: the implications for peri-operative analgesia

SUMMARY

Following two deaths from respiratory failure secondary to opioid toxicity in patients admitted for surgical repair of fractured neck of femur, we retrospectively studied the serum urea and electrolyte concentrations of 1511 consecutive patients requiring surgery for proximal femoral fracture, and calculated their glomerular filtration rate. Five hundred and forty-five (36.1%) patients had renal dysfunction on admission (glomerular filtration rate < 60 ml x min(-1).1.73 m(-2)); 435 (28.8%) had grade 3 chronic kidney disease (moderate; glomerular filtration rate 30-59 ml x min(-1).1.73 m(-2)), 82 (5.4%) had grade 4 disease (severe; glomerular filtration rate 15-29 ml x min(-1).1.73 m(-2)) and 28 (1.9%) had grade 5 (renal failure; glomerular filtration rate < 15 ml min(-1).1.73 m(-2)). The 30-day mortality for patients with renal dysfunction (62/536; 11.6%) was significantly greater (p = 0.004) than for patients with normal renal function (68/958; 7.1%), although median (IQR [range]) postoperative lengths of stay were similar 15 (10-22 [1-125]) vs 14 (9-22 [1-120]) days respectively; p = 0.06). Renal impairment is common in patients admitted for fixation of fractured neck of femur, who are consequently at risk of opioid toxicity.

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.

Molecular properties and CYP2D6 substrates: central nervous system therapeutics case study and pattern analysis of a substrate database

CYP2D6 substrate status is a critical Go/No Go decision criteria in central nervous system (CNS) drug discovery efforts because the polymorphic nature of CYP2D6 can lead to variable patient safety and drug efficacy. In addition, CYP2D6 is disproportionately involved in the metabolism of CNS drugs compared with other drug classes. Therefore, identifying trends in small molecule properties of CNS-penetrant compounds that can help discriminate potential CYP2D6 substrates from nonsubstrates would allow additional prioritization in the synthesis and biological evaluation of new therapeutic candidates. We report here the conversion of the CNS drug minaprine from substrate to nonsubstrate, as well as the conversion of the related CNS drug minozac from nonsubstrate to substrate, through the use of analog synthesis and CYP2D6 enzyme kinetic analyses. No single molecular property strongly correlated with substrate status for this 3-amino-4-methyl-6-phenylpyridazine scaffold, although molecular volume and charge appeared to be indirectly related. A parsed database of CYP2D6 substrates across diverse chemical structures was assembled and analyzed for physical property trends correlating with substrate status. We found that a complex interplay of properties influenced CYP2D6 substrate status and that the particular chemical scaffold affects which properties are most prominent. The results also identified an unexpected issue in CNS drug discovery, in that some property trends correlative with CYP2D6 substrates overlap previously reported properties that correlate with CNS penetrance. These results suggest the need for a careful balance in the design and synthesis of new CNS therapeutic candidates to avoid CYP2D6 substrate status while maintaining CNS penetrance.

CNS penetration of small molecules following local inflammation, widespread systemic inflammation or direct injury to the nervous system

AIMS

We sought to investigate effects of local and systemic inflammation on CNS permeability of small molecules and compare these to effects of direct injury to the nervous system.

MAIN METHODS

Evans blue was used to determine the integrity of the blood-brain barrier (BBB) following local inflammation, systemic inflammation, injury to the L5 spinal nerve or transient occlusion of the middle cerebral artery. In addition, three compounds having low, medium and high brain permeability (atenolol, morphine and oxycodone, respectively) were used. Following model establishment (4-hr post-carrageenan, 24-hr post-FCA, 2-, 4- and 24-hr post-LPS, 21 days post-nerve injury) compounds were administered and 30 min later the brain, spinal cord and blood removed. The plasma and tissue concentrations of compounds were quantified by LC/MS/MS.

KEY FINDINGS

Localized inflammation did not affect Evans blue penetration into the CNS but significantly increased morphine penetration into the spinal cord. Systemic inflammation increased the quantity of Evans blue in the CNS but also decreased the penetration of atenolol, morphine and oxycodone into the brain 4-hr post-insult. Nerve injury had no effect on Evans blue or compound penetration, while middle cerebral artery occlusion resulted in a large but short lived increase in Evans blue penetration into both the cortex and striatum.

SIGNIFICANCE

The presence of inflammation may affect the CNS penetration of some compounds but is unlikely to lead to a large non-selective BBB breakdown. As a result, it is appropriate to test for side-effects, and conduct brain pharmacokinetic determinations, in naïve rats.

Drug dosing error with drops: severe clinical course of codeine intoxication in twins

In spite of the lack of evidence for its efficacy, and of sporadic reports of severe adverse events, codeine is still widely used as an antitussive agent in children. A 3-year-old boy (twin 1) was found lying in vomit and apnoeic at night; he was resuscitated and immediately transferred to our paediatric intensive care unit (PICU). Two and a half hours later, his twin brother (twin 2) was found dead in his bed at home. Twin 1 required mechanical ventilation for 3 days, but he eventually made a full recovery; autopsy in twin 2 showed massive aspiration of gastric content. History revealed that the monozygotic twins had an upper respiratory tract infection for several days and had both been given codeine at a dose of "10 drops per day" by their mother. The blood of both twins was found to contain high levels of codeine and its metabolites. The weight of "10 drops" was determined experimentally and was found to range from 494 to 940 mg. Thus, the highest possible dose given by mother was 23.5 mg of codeine instead of the recommended 10 mg. The twins had identical CYP2D6 gene polymorphisms corresponding to the "extensive metaboliser" type.

CONCLUSIONS

Because of the variability of drop size drug dosage, dosage "by drops" is unprecise and may result in accidental overdose. The combination of repeated overdosing and extensive metabolism to morphine is likely to have caused apnoea in these twins. These cases illustrate the danger of codeine as an antitussive in young children.

Opioid metabolism

Clinicians understand that individual patients differ in their response to specific opioid analgesics and that patients may require trials of several opioids before finding an agent that provides effective analgesia with acceptable tolerability. Reasons for this variability include factors that are not clearly understood, such as allelic variants that dictate the complement of opioid receptors and subtle differences in the receptor-binding profiles of opioids. However, altered opioid metabolism may also influence response in terms of efficacy and tolerability, and several factors contributing to this metabolic variability have been identified. For example, the risk of drug interactions with an opioid is determined largely by which enzyme systems metabolize the opioid. The rate and pathways of opioid metabolism may also be influenced by genetic factors, race, and medical conditions (most notably liver or kidney disease). This review describes the basics of opioid metabolism as well as the factors influencing it and provides recommendations for addressing metabolic issues that may compromise effective pain management. Articles cited in this review were identified via a search of MEDLINE, EMBASE, and PubMed. Articles selected for inclusion discussed general physiologic aspects of opioid metabolism, metabolic characteristics of specific opioids, patient-specific factors influencing drug metabolism, drug interactions, and adverse events.

CYP2D6 genotyping for functional-gene dosage analysis by allele copy number detection

BACKGROUND

Cytochrome P450 2D6 (CYP2D6), one of the most important drug-metabolizing enzymes, has been reported to possess variation in the encoding CYP2D6 gene (cytochrome P450, family 2, subfamily D, polypeptide 6) that affects enzymatic activity. For the pharmacogenetic study of CYP2D6, accurate measurement of the dosage of the functional gene is essential; however, current genotyping techniques are insufficient because of their inability to provide the exact copy number of functional CYP2D6 genes.

METHODS

We developed 3 quantitative real-time PCR (qPCR) assays for estimating the total copy number of the CYP2D6 gene, as well as 24-multiplex PCR-based real-time Invader assays (mPCR-RETINAs) for estimating the allele ratio at each variation locus. After determining the allele copy number at each locus, we estimated the frequencies of CYP2D6 alleles in a population and the diplotype in each individual by a CNVphaser (copy number variation phaser). The qPCR assays and RETINAs used for HapMap Japanese and Chinese samples were applied to 455 Japanese individuals.

RESULTS

Forty-two individuals (9.2%) had one CYP2D6 gene copy, 207 (45.5%) had 2 copies, 161 (35.4%) had 3 copies, 40 (8.8%) had 4 copies, and 5 (1.1%) had 5 copies of the CYP2D6 gene. We found 16 different CYP2D6 alleles, with frequencies similar to those described in previous reports. In the diplotype analysis, we observed that CYP2D6*1/*1 and *1/*10-*36 were the most common diplotypes (approximately 20%) in our population.

CONCLUSIONS

Our method is the first to determine the exact number of functional CYP2D6 gene copies. We believe our method will facilitate and accelerate the detailed pharmacogenetic analysis of CYP2D6.

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.

The pharmacokinetics of codeine and its metabolites in Blacks with sickle cell disease

PURPOSE

We conducted a prospective, open-label study in 54 adult subjects with sickle cell disease to determine the relationship between morphine concentrations, cytochrome P450 (CYP) 2D6 genotype, and clinical outcomes.

METHODS

A blood sample was obtained for genotyping and serial blood samples were drawn to measure codeine and its metabolites in the plasma before and after oral codeine sulfate 30 mg. Codeine and its metabolites were measured by liquid chromatography-tandem mass spectrometry (LC-MS). CYP2D6 genetic testing included four single nucleotide polymorphisms (SNP) indicative of three variant alleles: *17 (1023T); *29 (1659A, 3183A); and *41 (2988A) alleles.

RESULTS

Thirty subjects (group I) had a mean (standard deviation) maximal morphine concentration of 2.0 (1.0) ng/ml. Morphine was not measurable in the remaining 24 subjects (group II). Nine (30%) subjects in group I and 11 (46%) subjects in group II carried a variant *17, *29, or *41 allele (p = 0.23); one (3%) subject in group I and 5 (21%) subjects in group II were homozygous for *17 or *29 allele (p = 0.07). Emergency room visits (group I 1.5 +/- 1.8 vs. group II 2.1 +/- 4.3, p = NS) did not differ based on metabolic status, but more hospital admissions (0.9 +/- 1.4 vs. 2.2 +/- 4.1, p = 0.05) were documented in patients with no measurable morphine concentrations.

CONCLUSIONS

We conclude that Blacks with sickle cell disease without measurable plasma morphine levels after a single dose of codeine were not more likely to be a carrier of a single variant allele commonly associated with reduced CYP2D6 metabolic capacity; however, homozygosity for a variant CYP2D6 allele may result in reduced metabolic capacity. Furthermore, it appears that subjects without measurable morphine concentrations were more likely to be admitted to the hospital for an acute pain crisis.

Fatal and severe codeine intoxication in 3-year-old twins--interpretation of drug and metabolite concentrations

This work presents two cases of codeine intoxication in 3-year-old monozygotic twin brothers while treated with a codeine slow-release formulation. One child had to be admitted to the hospital, whereas the other one died at home after aspiration of gastric content. The concentrations of codeine and major metabolites including morphine and corresponding glucuronide conjugates were measured by liquid chromatography-tandem mass spectrometry in serum, urine, cerebrospinal fluid, and brain tissue, respectively. A genetic polymorphism study was carried out in order to determine the ability of the children to metabolize codeine by O-demethylation. A pharmacokinetic calculation was also performed to estimate the administered dose of codeine in question. High concentrations of all substances were found in samples of both children. The pharmacokinetic estimate suggests an overdose of codeine, and the possible reasons for the high opiate concentrations are discussed. Furthermore, the postmortem distribution--during and after resuscitation--might play a major role in the interpretation of postmortem concentration levels.

Pharmacogenetic testing in psychiatry: a review of features and clinical realities

This article focuses on the first generation of pharmacogenetic tests that are potentially useful in psychiatry. All pharmacogenetic tests currently on the market, or soon to be marketed in psychiatry, for which some information has been published in peer-reviewed journal articles (or abstracts), were selected. Five pharmacogenetic tests are reviewed in detail: the Roche AmpliChip CYP450 Test, the Luminex Tag-It Mutation Detection Kit, the LGC clozapine response test, the PGxPredict: Clozapine test, and the Genomas PhyzioType system. After reviewing these tests, three practical aspects of implementing pharmacogenetic testing in psychiatric clinical practice are briefly reviewed: (1) the evaluation of these tests in clinical practice, (2) cost-effectiveness, and (3) regulatory oversight. Finally, the future of these and other pharmacogenetic tests in psychiatry is discussed.

The value of CYP2D6 and OPRM1 pharmacogenetic testing for opioid therapy

In managing pain, clinicians working with the more than 80 million people in the United States who suffer annually from serious pain face decisions about choosing the most appropriate pharmacologic agent, to contemplating nonpharmacologic modalities. This article focuses on opioid use for pain management, their risks of toxicity and addiction, adverse reactions, undertreatment for fear of addiction, and integration of novel diagnostics, such as the pharmacogenetic biomarkers CYP2D6 and OPRM1 as holding promise for assessing a patient's risk of adverse events or likelihood of efficacy. Incorporation of such biomarkers is emerging on the forefront of personalized medicine, and has the potential to dramatically improve the utility and efficacy of both current and future pain management strategies.

DNA microarray technology in the clinical environment: the AmpliChip CYP450 test for CYP2D6 and CYP2C19 genotyping

INTRODUCTION

An important technological advance in genetic testing is the DNA microarray, which allows for the simultaneous testing of thousands of DNA sequences. The AmpliChip CYP450 Test employs this microarray technology for cytochrome P450 (CYP) 2D6 and CYP2C19 genotyping. Isoenzymes encoded by these genes are responsible for the metabolism of many widely prescribed drugs. The objectives of this study were to identify CYP2D6 and CYP2C19 alleles and phenotypes in a psychiatric patient population in Kentucky, and to describe practical issues associated with DNA microarray technology.

METHODS

A total of 4,532 psychiatric patients were recruited from three state hospitals in Kentucky. Whole blood, buccal swabs, or saliva samples were genotyped with the AmpliChip CYP450 Test to derive a predicted phenotype.

RESULTS

In this cohort, the overall prevalence of CYP2D6 poor metabolizers was 7.6% (95% CI 7%, 8.3%), 8.2% in the Caucasians (95% CI 7.4%, 9.%) and 1.8% in the African Americans (95% CI 0.9%, 3.5%). The overall prevalence of CYP2D6 ultrarapid metabolizers was 1.5% (95% CI 1.2%, 1.9%), 1.5% in the Caucasians (95% CI 1.1%, 1.9%) and 2.0% in the African Americans (95% CI 1.1%, 3.7%). The overall prevalence of CYP2C19 poor metabolizers was 2.0% (95% CI 1.8%, 2.7%), 2.2% in Caucasians (95% CI 1.6%, 2.5%) and 4.0% in African Americans (95% CI 2.6%, 6.1%).

CONCLUSION

We also propose a numeric system for expression of CYP2D6 and CYP2C19 enzyme activity to aid clinicians in determining treatment strategy for patients receiving therapeutics that are metabolized by the CYP2D6 or CYP2C19 gene products.

Genetic polymorphism in cytochrome P450 2D6 (CYP2D6): Population distribution of CYP2D6 activity

Cytochrome P-450 2D6 (CYP2D6) is involved in the metabolism of many therapeutic drugs even though the enzyme represents a small proportion of the total CYP content of human liver. In vivo phenotyping with probe drug substrates such as debrisoquine and dextromethorphan showed a clear separation between poor metabolizers (PM) and extensive metabolizers (EM). This polymorphism may affect susceptibility to environmental disease, as suggested by molecular epidemiologic studies that found an association between CYP2D6 metabolizer phenotype and cancer risk; however, this association is not consistent. There are only a few examples of CYP2D6 involvement in toxicant mechanism of action, but this has not been extensively studied. Gene probe studies documented a number of genetic polymorphisms that underlie CYP2D6 metabolizer phenotypes. The EM group carries the wild-type (*1) or active (*2) variant alleles, while the PM group carries the *3, *4, *5, or *6 alleles, all of which code for a protein that has lower or null CYP2D6 activity. The current analysis characterizes (a) influence of genotype on phenotype based upon in vivo metabolism studies of probe drugs and (b) frequency of the major genotypes in different population groups is also characterized. These data were then incorporated into Monte Carlo modeling to simulate population distributions of CYP2D6 activity. This analysis reproduced the bimodal distributions commonly seen in phenotyping studies of Caucasians and found extensive population variability in enzyme activity, as indicated by the 9- to 56-fold difference between the PM modal median and the total population median CYP2D6 activity. This substantial degree of interindividual variability in CYP function indicates that assessments involving CYP2D6 substrates need to consider the full distribution of enzyme activity in refining estimates of internal dose in health assessments of xenobiotics.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

The efficacy and tolerability of multiple-dose tapentadol immediate release for the relief of acute pain following orthopedic (bunionectomy) surgery

OBJECTIVE

Tapentadol is a new, centrally acting analgesic with two mechanisms of action, combining μ-opioid agonism and norepinephrine reuptake inhibition in a single molecule. This study assessed tapentadol immediate release (IR) in patients with postsurgical orthopedic pain.

METHODS

This randomized, double-blind, phase II study involved patients with moderate-to-severe pain after bunionectomy surgery (first metatarsal with osteotomy). Patients (N = 269) were randomly assigned to receive tapentadol IR 50 or 100 mg, oxycodone HCl IR 10 mg, or placebo; the study drug was taken every 4-6 h, over a 72-h period starting 1 day after surgery (Evaluation Day 2). The primary endpoint was the sum of pain intensity over 24 h (SPI-24) on the second day after randomization (Evaluation Day 3). Potential limitations of this study included the use of rescue medication and the fact that it was not powered to statistically compare between-group differences in tolerability measures.

RESULTS

Mean (standard deviation [SD]) SPI-24 values on Evaluation Day 3 were significantly lower for tapentadol IR (50 mg: 33.6 [19.7], p = 0.0133; 100 mg: 29.2 [15.2], p = 0.0001) and oxycodone HCl IR 10 mg (35.7 [17.2]; nominal p = 0.0365) compared with placebo (41.9 [17.7]). The most common treatment-emergent adverse events for all treatment groups were characteristic of drugs with μ-opioid agonist activity. While providing similar analgesic efficacy, tapentadol IR 50 mg was associated with lower rates of nausea (46.3% vs. 71.6% for oxycodone HCl IR 10 mg), dizziness (32.8% vs. 56.7%), vomiting (16.4% vs. 38.8%), and constipation (6.0% vs. 17.9%), and a similar rate of somnolence (28.4% vs. 26.9%) compared with oxycodone HCl IR 10 mg.

CONCLUSIONS

Tapentadol IR 50 and 100 mg and oxycodone HCl IR 10 mg were effective in this study for the relief of acute postoperative pain following bunionectomy. The study results suggest improved gastrointestinal tolerability of tapentadol IR 50 mg compared with oxycodone at a dose showing comparable efficacy.

Role of active metabolites in the use of opioids

The opioid class of drugs, a large group, is mainly used for the treatment of acute and chronic persistent pain. All are eliminated from the body via metabolism involving principally CYP3A4 and the highly polymorphic CYP2D6, which markedly affects the drug's function, and by conjugation reactions mainly by UGT2B7. In many cases, the resultant metabolites have the same pharmacological activity as the parent opioid; however in many cases, plasma metabolite concentrations are too low to make a meaningful contribution to the overall clinical effects of the parent drug. These metabolites are invariably more water soluble and require renal clearance as an important overall elimination pathway. Such metabolites have the potential to accumulate in the elderly and in those with declining renal function with resultant accumulation to a much greater extent than the parent opioid. The best known example is the accumulation of morphine-6-glucuronide from morphine. Some opioids have active metabolites but at different target sites. These are norpethidine, a neurotoxic agent, and nordextropropoxyphene, a cardiotoxic agent. Clinicians need to be aware that many opioids have active metabolites that will become therapeutically important, for example in cases of altered pathology, drug interactions and genetic polymorphisms of drug-metabolizing enzymes. Thus, dose individualisation and the avoidance of adverse effects of opioids due to the accumulation of active metabolites or lack of formation of active metabolites are important considerations when opioids are used.

Respiratory depression with tramadol in a patient with renal impairment and CYP2D6 gene duplication

We observed opioid-related respiratory depression in a patient receiving tramadol via patient-controlled analgesia. Predisposing factors were the patient's genetic background and renal impairment. Complete recovery occurred after naloxone administration, thus confirming opioid intoxication. Analysis of the patient's genotype revealed a CYP2D6 gene duplication resulting in ultra-rapid metabolism of tramadol to its active metabolite (+)O-desmethyltramadol. Concomitant renal impairment resulting in decreased metabolite clearance enhanced opioid toxicity. This genetic CYP2D6 variant is particularly common in specific ethnic populations and should be a future diagnostic target whenever administration of tramadol or codeine is anticipated, as both drugs are subject to a comparable CYP2D6-dependent metabolism.

Pharmacogenetic insights into codeine analgesia: implications to pediatric codeine use

Codeine has been used medicinally since the 1800s as an analgesic and antitussive agent. Although very few studies have methodically examined the safety of codeine use in the pediatric age group, it is nonetheless commonly prescribed to children and breastfeeding mothers. Empirical evidence over the last century has suggested variability in the efficacy of codeine, and recent genomic advancements have shed important light on the mechanisms leading to such variability. Aside from evaluating the role of genetic variability in drug-metabolizing enzymes, receptors and transporters, the development of the blood–brain-barrier and the ontogeny of drug-metabolizing enzymes must also be considered in newborns and young children.

Functional pharmacogenetics/genomics of human cytochromes P450 involved in drug biotransformation

We investigated the elimination routes for the 200 drugs that are sold most often by prescription count in the United States. The majority (78%) of the hepatically cleared drugs were found to be subject to oxidative metabolism via cytochromes P450 of the families 1, 2 and 3, with major contributions from CYP3A4/5 (37% of drugs) followed by CYP2C9 (17%), CYP2D6 (15%), CYP2C19 (10%), CYP1A2 (9%), CYP2C8 (6%), and CYP2B6 (4%). Clinically well-established polymorphic CYPs (i.e., CYP2C9, CYP2C19, and CYP2D6) were involved in the metabolism of approximately half of those drugs, including (in particular) NSAIDs metabolized mainly by CYP2C9, proton-pump inhibitors metabolized by CYP2C19, and beta blockers and several antipsychotics and antidepressants metabolized by CYP2D6. In this review, we provide an up-to-date summary of the functional polymorphisms and aspects of the functional genomics of the major human drug-metabolizing cytochrome P450s, as well as their clinical significance.

Pharmacogenetics of analgesics: toward the individualization of prescription

The use of analgesics is based on the empiric administration of a given drug with clinical monitoring for efficacy and toxicity. However, individual responses to drugs are influenced by a combination of pharmacokinetic and pharmacodynamic factors that can sometimes be regulated by genetic factors. Whereas polymorphic drug-metabolizing enzymes and drug transporters may affect the pharmacokinetics of drugs, polymorphic drug targets and disease-related pathways may influence the pharmacodynamic action of drugs. After a usual dose, variations in drug toxicity and inefficacy can be observed depending on the polymorphism, the analgesic considered and the presence or absence of active metabolites. For opioids, the most studied being morphine, mutations in the ABCB1 gene, coding for P-glycoprotein (P-gp), and in the µ-opioid receptor reduce morphine potency. Cytochrome P450 (CYP) 2D6 mutations influence the analgesic effect of codeine and tramadol, and polymorphism of CYP2C9 is potentially linked to an increase in nonsteroidal anti-inflammatory drug-induced adverse events. Furthermore, drug interactions can mimic genetic deficiency and contribute to the variability in response to analgesics. This review summarizes the available data on the pharmacokinetic and pharmacodynamic consequences of known polymorphisms of drug-metabolizing enzymes, drug transporters, drug targets and other nonopioid biological systems on central and peripheral analgesics.

[Genetic polymorphisms and opioid therapies]

Interindividual variability in pain perception and response to opioids in terms of efficacy and side effects has been long noted. Numerous genes have been proposed as ideal candidate genes for the study of the genetic component of pain and pharmacogenetics of opioids. Despite the inherent complexity in studying pain, it is obvious that several genetic polymorphisms contribute to modulate nociception and the antinociceptive effects of opioids; specifically those involved in pharmacokinetics and the metabolism of opioids (cytochrome P450) and neurotransmitters (catechol-O-methyltransferase), as well as those affecting pharmacodynamics or the drug targets such as the mu-opioid receptor or the unexpected melanocortin 1-receptor.

CYP2D6 phenotype prediction from genotype: which system is the best?

Genotyping of the polymorphic cytochrome P450 (CYP) 2D6 gene is used increasingly in clinical practice. Several psychiatric hospitals already use CYP2D6 testing before treating a patient with antidepressant or antipsychotic drug therapy. In other fields of drug therapy, such as for breast cancer, CYP2D6 status has been reported to be an independent predictor for the outcome with tamoxifen. Thus, a more favorable tamoxifen treatment seems to be feasible through a priori genetic assessment of CYP2D6.