Andrea Gaedigk

Pharmacogenetics of morphine poisoning in a breastfed neonate of a codeine-prescribed mother

In April, 2005, a full-term healthy male infant, delivered vaginally, showed intermittent periods of diffi culty in breastfeeding and lethargy starting on day 7. During a well-baby paediatric visit on day 11, the paediatrician noted that the baby had regained his birthweight. On day 12, however, he had grey skin and his milk intake had fallen. He was found dead on day 13. Postmortem analysis showed no anatomical anomalies. Blood concentration of morphine (the active metabolite of codeine) was 70 ng/mL by gas chromatography-mass spectrometry (GC-MS)— neonates breastfed by mothers receiving codeine typically have morphine serum concentrations of 0–2·2 ng/mL. The mother had been prescribed a combination preparation of codeine 30 mg and paracetamol 500 mg after birth for episiotomy pain (initially two tablets every 12 h, reduced to half that dose from day 2 because of somnolence and constipation). She continued the tablets for 2 weeks. Because of poor neonatal feeding, she stored milk on day 10, which was later assayed for morphine by GC-MS. A morphine concentration of 87 ng/mL was found—the typical range of milk concentrations after repeated maternal codeine is 1·9–20·5 ng/mL at doses of 60 mg every 6 h. Genotype analysis was done for cytochrome P450 2D6 (CYP2D6), the enzyme catalysing the O-demethylation of codeine to morphine. The mother was heterozygous for a CYP2D6*2A allele with CYP2D6*2×2 gene duplication, classifi ed as an ultra-rapid metaboliser. This genotype leads to increased formation of morphine from codeine, consistent with the somnolence and constipation she experienced. The maternal grandfather, the father, and the infant had two functional CYP2D6 alleles (CYP2D6*1/ *2 genotypes), classifi ed as extensive metabolisers. The maternal grandmother was an ultra-rapid metaboliser. The clinical and laboratory picture is consistent with opioid toxicity leading to neonatal death. Most of the analgesic and central-nervous-system depressant eff ects of codeine are secondary to its metabolism to morphine by CYP2D6. Neonates invariably have impaired capacity to metabolise and eliminate morphine. Codeine is a commonly used analgesic after labour for pain associated with episiotomy and caesarean section. The American Academy of Pediatrics lists codeine as compatible with breastfeeding, despite lack of suffi cient published data to support this recommendation. This case shows that polymorphism of CYP2D6 can be life threatening for some breastfed babies. Given that the frequency of CYP2D6 ultra-rapid metaboliser genotypes ranges from 1% in Finland and Denmark to 10% in Greece and Portugal, and 29% in Ethiopia, this polymorphism is clinically important. Several strategies can be considered to prevent life-threatening neonatal toxicity (table). Careful follow-up of breastfeeding mothers using codeine, and their infants, may be a useful approach. Testing of mother–child pairs when the mother or neonate is experiencing symptoms consistent with opioid toxicity may be necessary—eg somnolence, or poor milk intake. The facilities to measure morphine concentrations are not routine in most hospitals; in any suspicious case, naloxone can reverse, and, therefore, corroborate opioid toxicity. Above all, avoidance of codeine use during breastfeeding, with its use being retained as second or third line for uncontrolled pain, could also avert this situation. Whatever clinical approach is taken, codeine cannot be considered as a safe drug for all infants during breastfeeding.

The CYP2D6 Activity Score: Translating Genotype Information into a Qualitative Measure of Phenotype

Inferring CYP2D6 phenotype from genotype is increasingly challenging, considering the growing number of alleles and their range of activity. This complexity poses a challenge in translational research where genotyping is being considered as a tool to personalize drug therapy. To simplify genotype interpretation and improve phenotype prediction, we evaluated the utility of an “activity score” (AS) system. Over 25 CYP2D6 allelic variants were genotyped in 672 subjects of primarily Caucasian and African‐American heritage. The ability of genotype and AS to accurately predict phenotype using the CYP2D6 probe substrate dextromethorphan was evaluated using linear regression and clustering methods. Phenotype prediction, given as a probability for each AS group, was most accurate if ethnicity was considered; among subjects with genotypes containing a CYP2D6*2 allele, CYP2D6 activity was significantly slower in African Americans compared to Caucasians. The AS tool warrants further prospective evaluation for CYP2D6 substrates and in additional ethnic populations.

Assessment of the predictive power of genotypes for the in-vivo catalytic function of CYP2D6 in a German population

The polymorphic cytochrome P450 CYP2D6 catalyses the biotransformation of at least 40 drugs. The CYP2D6 genetic polymorphism is responsible for pronounced interindividual differences in plasma concentrations and, hence, in drug action and side-effects after administration of the same dose. Provided there is a close relationship between CYP2D6 genotypes and catalytic function, genotyping could be used in the clinical setting for individualization of drug dose. In the present study, we evaluated the relationship between the in-vivo enzyme activity and 35 different genotypes in order to determine whether genotyping can be used to predict a persons metabolic capacity for CYP2D6-catalysed drug oxidation using sparteine as a probe drug. One hundred and ninety-five Caucasian individuals were genotyped for seven nonfunctional (CYP2D6 x 3, x 4, x 5, x 6, x 7, x 8, x 16) and eight functional alleles (CYP2D6 x 1, x 2, x 2 x 2, x 2B, x 2B x 2, x 9, x 10, x 17). The metabolic ratio distribution for sparteine showed trimodality, with 15 poor metabolizers, 21 intermediate metabolizers, and 1.59 extensive and ultrarapid metabolizers. All poor metabolizers were unambiguously identified as carriers of two nonfunctional alleles. In contrast, the most frequent functional genotypes extensively overlapped and, with few exceptions, genotype was not a useful predictor of function. Gene dose effects among homozygotes and heterozygotes of the major functional alleles were not significant and could not explain the wide variations. Only a minor fraction of phenotypical ultrarapid metabolizers, arbitrarily defined as individuals with a metabolic ratio < 0.2, could be identified as carriers of three functional gene copies, including duplicated CYP2D6 x 2 x 2 alleles. Similarly, only a minor fraction of the intermediate metabolizers had predictive genotypes involving alleles coding for enzyme with impaired function. Thus, genotyping correctly identifies poor metabolizers, but quantitative prediction of drug metabolism capacity among extensive metabolizers is not possible.

Clinical Pharmacogenetics Implementation Consortium Guideline for CYP2D6 and CYP2C19 Genotypes and Dosing of Tricyclic Antidepressants

Polymorphisms in CYP2D6 and CYP2C19 affect the efficacy and safety of tricyclics, with some drugs being affected by CYP2D6 only, and others by both polymorphic enzymes. Amitriptyline, clomipramine, doxepin, imipramine, and trimipramine are demethylated by CYP2C19 to pharmacologically active metabolites. These drugs and their metabolites, along with desipramine and nortriptyline, undergo hydroxylation by CYP2D6 to less active metabolites. Evidence from published literature is presented for CYP2D6 and CYP2C19 genotype–directed dosing of tricyclic antidepressants.

Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines for Codeine Therapy in the Context of Cytochrome P450 2D6 (CYP2D6) Genotype

Codeine is bioactivated to morphine, a strong opioid agonist, by the hepatic cytochrome P450 2D6 (CYP2D6); hence, the efficacy and safety of codeine as an analgesic are governed by CYP2D6 polymorphisms. Codeine has little therapeutic effect in patients who are CYP2D6 poor metabolizers, whereas the risk of morphine toxicity is higher in ultrarapid metabolizers. The purpose of this guideline (periodically updated at http://www.pharmgkb.org) is to provide information relating to the interpretation of CYP2D6 genotype test results to guide the dosing of codeine.

Pharmacogenetics of Neonatal Opioid Toxicity Following Maternal Use of Codeine During Breastfeeding: A Case–Control Study

A large number of women receive codeine for obstetric pain while breastfeeding. Following a case of fatal opioid poisoning in a breastfed neonate whose codeine prescribed mother was a CYP2D6 ultrarapid metabolizer (UM), we examined characteristics of mothers and infants with or without signs of central nervous system (CNS) depression following codeine exposure while breastfeeding in a case–control study. Mothers of symptomatic infants (n = 17) consumed a mean 59% higher codeine dose than mothers of asymptomatic infants (n = 55) (1.62 (0.79) mg/kg/day vs. 1.02 (0.54) mg/kg/day; P = 0.004). There was 71% concordance between maternal and neonatal CNS depression. Two mothers whose infants exhibited severe neonatal toxicity were CYP2D6 UMs and of the UGT2B7*2/*2 genotype. There may be a dose–response relationship between maternal codeine use and neonatal toxicity, and strong concordance between maternal‐infant CNS depressive symptoms. Breastfed infants of mothers who are CYP2D6 UMs combined with the UGT2B7*2/*2 are at increased risk of potentially life‐threatening CNS depression.

Systematic genetic and genomic analysis of cytochrome P450 enzyme activities in human liver

Liver cytochrome P450s (P450s) play critical roles in drug metabolism, toxicology, and metabolic processes. Despite rapid progress in the understanding of these enzymes, a systematic investigation of the full spectrum of functionality of individual P450s, the interrelationship or networks connecting them, and the genetic control of each gene/enzyme is lacking. To this end, we genotyped, expression-profiled, and measured P450 activities of 466 human liver samples and applied a systems biology approach via the integration of genetics, gene expression, and enzyme activity measurements. We found that most P450s were positively correlated among themselves and were highly correlated with known regulators as well as thousands of other genes enriched for pathways relevant to the metabolism of drugs, fatty acids, amino acids, and steroids. Genome-wide association analyses between genetic polymorphisms and P450 expression or enzyme activities revealed sets of SNPs associated with P450 traits, and suggested the existence of both cis-regulation of P450 expression (especially for CYP2D6) and more complex trans-regulation of P450 activity. Several novel SNPs associated with CYP2D6 expression and enzyme activity were validated in an independent human cohort. By constructing a weighted coexpression network and a Bayesian regulatory network, we defined the human liver transcriptional network structure, uncovered subnetworks representative of the P450 regulatory system, and identified novel candidate regulatory genes, namely, EHHADH, SLC10A1, and AKR1D1. The P450 subnetworks were then validated using gene signatures responsive to ligands of known P450 regulators in mouse and rat. This systematic survey provides a comprehensive view of the functionality, genetic control, and interactions of P450s.

Characterization of the microsomal epoxide hydrolase gene in patients with anticonvulsant adverse drug reactions.

Therapy with the aromatic anticonvulsants phenytoin, phenobarbital and carbamazepine has been associated with the occurrence of rare idiosyncratic hypersensitivity reactions. These drugs are thought to be activated to potentially reactive arene oxide (epoxide) metabolites by cytochrome P450-dependent monooxygenation, while liver microsomal epoxide hydrolase (mEH) plays a detoxifying role by converting such reactive intermediates to non-toxic dihydrodiols. Evidence from in vitro lymphocyte toxicity tests and enzyme inhibitor studies has suggested that an inherited defect in mEH function may be responsible for the enhanced drug toxicity observed in affected individuals. To test this hypothesis we designed methods to directly compare mEH gene structure in patients presenting with anticonvulsant adverse reactions and in control subjects in which no in vivo or in vitro toxicity to anticonvulsants could be demonstrated. Southern analysis of peripheral lymphocyte DNA using a full-length mEH cDNA as hybridization probe revealed no gross differences in mEH gene structure in selected patients when compared with DNA samples from unaffected control subjects. The human mEH gene was then cloned and characterized from a control individual. Nine exons were identified within a 22 kb region and sequences of selected regions, including all exons, were determined. Single strand conformation polymorphism (SSCP) analysis was performed on all exonic regions in genomic DNA from each of 26 subjects, including six unrelated patients with previous toxicity to anticonvulsant therapy and seven siblings (three of whom had displayed toxicity). Several distinct SSCP patterns could be observed among the subjects tested, each corresponding to a specific point mutation within one of the amplified fragments of the mEH gene. However, none of the SSCP patterns reflecting point mutations was correlated with the occurrence of anticonvulsant toxicity. From these observations we conclude that a genetic defect altering the structure and function of the mEH protein is unlikely to be responsible for predisposing patients to anticonvulsant adverse reactions.

Ontogeny of dextromethorphan O- and N-demethylation in the first year of life.

The exponential increase in the number of drugs used to treat infant and childhood illnesses necessitates an understanding of the ontogeny of drug biotransformation for the development of safe and effective therapies. Healthy infants received an oral dose (0.3 mg/kg) of dextromethorphan (DM) at 0.5, 1, 2, 4, 6, and 12 months of age. DM and its major metabolites were measured in urine. CYP2D6 genotype was determined by polymerase chain reaction‐restriction fragment length polymorphism. Genotyping data indicated a strong correlation between CYP2D6 genotype and DM O‐demethylation (β=−0.638; 95% CI: −0.745, −0.532; P<0.001). CYP2D6 activity was detectable and concordant with genotype by 2 weeks of age, showed no relationship with gestational age, and did not change with post natal age up to 1 year. In contrast, DM N‐demethylation developed significantly more slowly over the first year of life. Genotype and the temporal acquisition of drug biotransformation are critical determinants of a drug response in infants.

The role of xenobiotic metabolizing enzymes in arylamine toxicity and carcinogenesis : Functional and localization studies

In both animal models and humans, the first and obligatory step in the activation of arylamines is N-hydroxylation. This pathway is primarily mediated by the phase-I enzymes CYP1A1, CYP1A2 and CYP4B1. In the presence of flavonoids such as alpha-naphthoflavone and flavone, both CYP3A4 and CYP3A5 have also been shown to play a minor role in the activation of food-derived heterocyclic amines. The further activation of N-hydroxyarylamines by phase-II metabolism can involve both N, O-acetylation and N, O-sulfonation catalyzed by N-acetyltransferases (NAT1 and NAT2) and sulfotransferases, respectively. Using an array of techniques, we have been unable to detect constitutive CYP1A expression in any segments of the human gastrointestinal tract. This is in contrast to the rabbit where CYP1A1 protein was readily detectable on immunoblots in microsomes prepared from the small intestine. In humans, CYP3A3/3A4 expression was detectable in the esophagus and all segments of the small intestine. Northern blot analysis of eleven human colons showed considerable heterogeneity in CYP3A mRNA between individuals, with the presence of two mRNA species in some subjects. Employing the technique of hybridization histochemistry (also known as in situ hybridization), CYP4B1 expression was observed in some human colons but not in the liver or the small intestine. Hybridization histochemistry studies have also demonstrated variable NAT1 and NAT2 expression in the human gastrointestinal tract. NAT1 and NAT2 mRNA expression was detected in the human liver, small intestine, colon, esophagus, bladder, ureter, stomach and lung. Using a general aryl sulfotransferase riboprobe (HAST1), we have demonstrated marked sulfotransferase expression in the human colon, small intestine, lung, stomach and liver. These studies demonstrate that considerable variability exists in the expression of enzymes involved in the activation of aromatic amines in human tissues. The significance of these results in relation to a role for heterocyclic amines in colon cancer is discussed.