Julia Kirchheiner

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

Codeine is an analgesic drug acting on μ-opiate receptors predominantly via its metabolite morphine, which is formed almost exclusively by the genetically polymorphic enzyme cytochrome P450 2D6 (CYP2D6). Whereas it is known that individuals lacking CYP2D6 activity (poor metabolizers, PM) suffer from poor analgesia from codeine, ultra-fast metabolizers (UM) due to the CYP2D6 gene duplication may experience exaggerated and even potentially dangerous opioidergic effects and no systematical study has been performed so far on this question. A single dose of 30 mg codeine was administered to 12 UM of CYP2D6 substrates carrying a CYP2D6 gene duplication, 11 extensive metabolizers (EM) and three PM. Genotyping was performed using polymerase chain reaction-restriction fragment length polymorphism methods and a single-base primer extension method for characterization of the gene-duplication alleles. Pharmacokinetics was measured over 24 h after drug intake and codeine and its metabolites in plasma and urine were analyzed by liquid chromatography with tandem mass spectrometry. Significant differences between the EM and UM groups were detected in areas under the plasma concentration versus time curves (AUCs) of morphine with a median (range) AUC of 11 (5–17) μg h l−1 in EMs and 16 (10–24) μg h l−1 in UM (P=0.02). In urine collected over 12 h, the metabolic ratios of the codeine+codeine-6-glucuronide divided by the sum of morphine+its glucuronides metabolites were 11 (6–17) in EMs and 9 (6–16) in UM (P=0.05). Ten of the 11 CYP2D6 UMs felt sedation (91%) compared to six (50%) of the 12 EMs (P=0.03). CYP2D6 genotypes predicting ultrarapid metabolism resulted in about 50% higher plasma concentrations of morphine and its glucuronides compared with the EM. No severe adverse effects were seen in the UMs in our study most likely because we used for safety reasons a low dose of only 30 mg. It might be good if physicians would know about the CYP2D6 duplication genotype of their patients before administering codeine.

Impact of CYP2C9 amino acid polymorphisms on glyburide kinetics and on the insulin and glucose response in healthy volunteers

Glyburide (INN, glibenclamide) is a second‐generation sulfonylurea antidiabetic agent with high potency. We hypothesized that glyburide may be a substrate of cytochrome P450 2C9 (CYP2C9), an enzyme that has two low‐activity amino acid variants‐Arg144Cys (CYP2C9*2) and Ile359Leu (CYP2C9*3). We explored the impact of these polymorphisms on glyburide pharmacokinetics and the effects on insulin and glucose concentrations.

The impact of the CYP2D6 polymorphism on haloperidol pharmacokinetics and on the outcome of haloperidol treatment

The genetically polymorphic enzyme cytochrome P450 (CYP) 2D6 contributes to the biotransformation of the antipsychotic drug haloperidol. The impact of the polymorphism on haloperidol pharmacokinetics, adverse events, and efficacy was prospectively evaluated under naturalistic conditions in 172 unselected psychiatric inpatients with acute psychotic symptoms.

Appropriate phenotyping procedures for drug metabolizing enzymes and transporters in humans and their simultaneous use in the cocktail approach

Phenotyping for drug metabolizing enzymes and transporters is used to assess quantitatively the effect of an intervention (e.g., drug therapy, diet) or a condition (e.g., genetic polymorphism, disease) on their activity. Appropriate selection of test drug and metric is essential to obtain results applicable for other substrates of the respective enzyme/transporter. The following phenotyping metrics are recommended based on the level of validation and on practicability: CYP1A2, paraxanthine/caffeine in plasma 6 h after 150 mg caffeine; CYP2C9, tolbutamide plasma concentration 24 h after 125 mg tolbutamide; CYP2C19, urinary excretion of 4′‐OH‐mephenytoin 0–12 h after 50 mg mephenytoin; CYP2D6, urinary molar ratio debrisoquine/4‐OH‐debrisoquine 0–8 h after 10 mg debrisoquine; and CYP3A4, plasma clearance of midazolam after 2 mg midazolam (all drugs given orally).

Genotype-guided dosing of coumarin derivatives: the European pharmacogenetics of anticoagulant therapy (EU-PACT) trial design

The narrow therapeutic range and wide interpatient variability in dose requirement make anticoagulation response to coumarin derivatives unpredictable. As a result, patients require frequent monitoring to avert adverse effects and maintain therapeutic efficacy. Polymorphisms in VKORC1 and CYP2C9 jointly account for about 40% of the interindividual variability in dose requirements. To date, several pharmacogenetic-guided dosing algorithms for coumarin derivatives, predominately for warfarin, have been developed. However, the potential benefit of these dosing algorithms in terms of their safety and clinical utility has not been adequately investigated in randomized settings. The European Pharmacogenetics of Anticoagulant Therapy (EU-PACT) trial will assess, in a single-blinded and randomized controlled trial with a follow-up period of 3 months, the safety and clinical utility of genotype-guided dosing in daily practice for the three main coumarin derivatives used in Europe. The primary outcome measure is the percentage time in the therapeutic range for international normalized ratio. This report describes the design and protocol for the trial.

Effects of the CYP2D6 gene duplication on the pharmacokinetics and pharmacodynamics of tramadol.

The analgesic drug tramadol is bioactivated by CYP2D6 to the opioid receptor agonist O-desmethyltramadol. Case reports indicated that carriers of the CYP2D6 gene duplication may be at high risk for opioid adverse events. However, the effects of the CYP2D6 duplication on kinetics and dynamics of tramadol have not been systematically studied. Pharmacokinetics and effects were monitored after a single dose of 100 mg racemic tramadol in 11 carriers of a CYP2D6 gene duplication allele (ultrarapid metabolizer [UM]) and compared with 11 carriers of 2 active CYP2D6 genes (extensive metabolizer [EM]). Pharmacodynamics was measured by cold pressure test, pupillometry, and standardized adverse event recording. The maximum plasma concentrations of the active metabolite (+)R,R-O-desmethyltramadol were significantly higher in the UM group compared with the EM group (P = 0.005; t test) with a mean difference of 14 ng/mL (95% confidence limit of difference, 2-26 ng/mL). Median (+)R,R-tramadol area under the curve was 786 and 587 μg·h·L−1 in EMs and UMs, and the corresponding median (+)R,R-O-desmethyltramadol area under thecurve was 416 and 448 μg·h·L−1 (P = 0.005, t test). There was an increased pain threshold and pain tolerance and a stronger miosis after tramadol in UMs compared with EMs. Almost 50% of the UM group experienced nausea compared with only 9% of the EM group. In conclusion, pharmacokinetic differences between EMs and UMs were smaller than expected; nevertheless, UMs were more sensitive to tramadol than EMs. Therefore, tramadol may frequently cause adverse effects in southern European and Northern African populations with a high proportion of UMs.

Enantiospecific effects of cytochrome P450 2C9 amino acid variants on ibuprofen pharmacokinetics and on the inhibition of cyclooxygenases 1 and 2

According to in vitro data, the polymorphic cytochrome P450 enzyme 2C9 (CYP2C9) may be the major S‐ibuprofen hydroxylase. In humans, there are 2 variants of CYP2C9 with a high population frequency. We studied their impact on ibuprofen pharmacokinetics and on the inhibition of cyclooxygenases 1 and 2.

Pharmacokinetics and pharmacodynamics of rosiglitazone in relation to CYP2C8 genotype.

Rosiglitazone is metabolically inactivated predominantly via the cytochrome P450 (CYP) enzyme CYP2C8. The functional impact of the CYP2C8*3 allele coding for the Arg139Lys and Lys399Arg amino acid substitutions is controversial. The purpose of this was to clarify the role of this polymorphism with regard to the pharmacokinetics and clinical effects of rosiglitazone.

Impact of the ultrarapid metabolizer genotype of cytochrome P450 2D6 on metoprolol Pharmacokinetics and Pharmacodynamics

In the treatment of heart failure and hypertension with metoprolol, ultrarapid metabolizers (UMs) may not achieve optimal target concentrations with recommended doses. We compared metoprolol pharmacokinetics and effects in UMs with extensive metabolizers (EMs) and with poor metabolizers (PM) as an additional reference group.

Characterization of a Functional Promoter Polymorphism of the Human Tryptophan Hydroxylase 2 Gene in Serotonergic Raphe Neurons

BACKGROUND Tryptophan hydroxylase 2 (TPH2) is the rate-limiting enzyme in brain serotonin (5-HT) biosynthesis. Although dysfunction of 5-HT neurotransmission has been implicated in a variety of neuropsychiatric conditions, the human TPH2 promoter has not been characterized in vitro. METHODS The functional relevance of TPH2 promoter polymorphisms was determined with luciferase assays in primary serotonergic neurons from rat raphe nuclei and in human small cell lung carcinoma cells (SHP-77 cells). We also investigated transcription factor binding to the variant promoter sequence with electrophoretic mobility shift assay (EMSA). RESULTS The polymorphism rs11178997 of the human TPH2 promoter significantly reduced TPH2 transcriptional activity by 22% and 7% in primary serotonergic neurons and in SHP-77 cells, respectively. In contrast, no significant differences in promoter activity were observed for the G- and T-alleles of rs4570625. The EMSA revealed reduced binding of the transcription factor POU3F2 (also known as Brn-2, N-Oct-3) to the A-allele of the polymorphism rs11178997. Overexpression of POU3F2 resulted in a robust activation of the TPH2 promoter (2.7-fold). CONCLUSIONS Our data suggest that the human TPH2 promoter polymorphism rs11178997 impacts on gene expression, which might have implications for the development and function of the serotonergic system in the brain.