Cytochrome P450 2D6 Phenotype Research Articles

Polymorphism of Human Cytochrome P450 2D6 and Its Clinical Significance

Part I of this article discussed the potential functional importance of genetic mutations and alleles of the human cytochrome P450 2D6 (CYP2D6) gene. The impact of CYP2D6 polymorphisms on the clearance of and response to a series of cardiovascular drugs was addressed. Since CYP2D6 plays a major role in the metabolism of a large number of other drugs, Part II of the article highlights the impact of CYP2D6 polymorphisms on the response to other groups of clinically used drugs. Although clinical studies have observed a gene-dose effect for some tricyclic antidepressants, it is difficult to establish clear relationships of their pharmacokinetics and pharmacodynamic parameters to genetic variations of CYP2D6; therefore, dosage adjustment based on the CYP2D6 phenotype cannot be recommended at present. There is initial evidence for a gene-dose effect on commonly used selective serotonin reuptake inhibitors (SSRIs), but data on the effect of the CYP2D6 genotype/phenotype on the response to SSRIs and their adverse effects are scanty. Therefore, recommendations for dose adjustment of prescribed SSRIs based on the CYP2D6 genotype/phenotype may be premature. A number of clinical studies have indicated that there are significant relationships between the CYP2D6 genotype and steady-state concentrations of perphenazine, zuclopenthixol, risperidone and haloperidol. However, findings on the relationships between the CYP2D6 genotype and parkinsonism or tardive dyskinesia treatment with traditional antipsychotics are conflicting, probably because of small sample size, inclusion of antipsychotics with variable CYP2D6 metabolism, and co-medication. CYP2D6 phenotyping and genotyping appear to be useful in predicting steady-state concentrations of some classical antipsychotic drugs, but their usefulness in predicting clinical effects must be explored. Therapeutic drug monitoring has been strongly recommended for many antipsychotics, including haloperidol, chlorpromazine, fluphenazine, perphenazine, risperidone and thioridazine, which are all metabolized by CYP2D6. It is possible to merge therapeutic drug monitoring and pharmacogenetic testing for CYP2D6 into clinical practice. There is a clear gene-dose effect on the formation of O-demethylated metabolites from multiple opioids, but the clinical significance of this may be minimal, as the analgesic effect is not altered in poor metabolizers (PMs). Genetically caused inactivity of CYP2D6 renders codeine ineffective owing to lack of morphine formation, decreases the efficacy of tramadol owing to reduced formation of the active O-desmethyl-tramadol and reduces the clearance of methadone. Genetically precipitated drug interactions might render a standard opioid dose toxic. Because of the important role of CYP2D6 in tamoxifen metabolism and activation, PMs are likely to exhibit therapeutic failure, and ultrarapid metabolizers (UMs) are likely to experience adverse effects and toxicities. There is a clear gene-concentration effect for the formation of endoxifen and 4-OH-tamoxifen. Tamoxifen-treated cancer patients carrying CYP2D6*4, *5, *10, or *41 associated with significantly decreased formation of antiestrogenic metabolites had significantly more recurrences of breast cancer and shorter relapse-free periods. Many studies have identified the genetic CYP2D6 status as an independent predictor of the outcome of tamoxifen treatment in women with breast cancer, but others have not observed this relationship. Thus, more favourable tamoxifen treatment seems to be feasible through a priori genetic assessment of CYP2D6, and proper dose adjustment may be needed when the CYP2D6 genotype is determined in a patient. Dolasetron, ondansetron and tropisetron, all in part metabolized by CYP2D6, are less effective in UMs than in other patients. Overall, there is a strong gene-concentration relationship only for tropisetron. CYP2D6 genotype screening prior to antiemetic treatment may allow for modification of antiemetic dosing. An alternative is to use a serotonin agent that is metabolized independently of CYP2D6, such as granisetron, which would obviate the need for genotyping and may lead to an improved drug response. To date, the functional impact of most CYP2D6 alleles has not been systematically assessed for most clinically important drugs that are mainly metabolized by CYP2D6, though some initial evidence has been identified for a very limited number of drugs. The majority of reported in vivo pharmacogenetic data on CYP2D6 are from single-dose and steady-state pharmacokinetic studies of a small number of drugs. Pharmacodynamic data on CYP2D6 polymorphisms are scanty for most drug studies. Given that genotype testing for CYP2D6 is not routinely performed in clinical practice and there is uncertainty regarding genotype-phenotype, gene-concentration and gene-dose relationships, further prospective studies on the clinical impact of CYP2D6-dependent metabolism of drugs are warranted in large cohorts.

Rapid and reliable genotyping procedure for detection of alleles with mutations, deletion, or/and duplication of the CYP2D6 gene

Background Polymorphisms of cytochrome P450 2D6 (CYP2D6) have a significant effect on the pharmacokinetics of most tricyclic antidepressants. More than 150 alleles lead to four distinct phenotypes of drug metabolism. The phenotypes are described as ultrarapid, extensive, intermediate, and poor metabolizers. Therapeutic plasma levels of CYP2D6 substrates may be difficult to achieve. Here we describe a rapid and reliable procedure for CYP2D6*4, *3, *6, and *9 genotyping. Design and methods Serum concentrations of venlafaxine and its pharmacologically active metabolite, O-desmethylvenlafaxine, were measured in patients treated with the antidepressant venlafaxine, a substrate of CYP2D6. The ODV/V ratio was used as an indicator of the CYP2D6 phenotype, with a higher ratio reflecting more rapid metabolism. Real-time PCR with fluorometric melting point analysis of the PCR products (LightCycler ®) is used to identify SNPs. Using quantitative PCR, gene deletion and gene duplication or multiplication are investigated by measurement of the fluorescence intensity quotient ( q, N) of the CYP2D6 gene relative to that of the albumin gene as an internal standard. Results Melting curves are verified using DNA samples of known genotypes and by sequencing the PCR products. The genotypes and phenotypes that were detected correspond to each other. Conclusion A PCR procedure for the detection of CYP2D6 SNPs, deletions, and duplications is described and is rapid and reliable.

Functional Characterization of 17CYP2D6Allelic Variants (CYP2D6.2, 10, 14A–B, 18, 27, 36, 39, 47–51, 53–55, and 57)

Cytochrome P450 2D6 (CYP2D6) is an enzyme of potential importance for the metabolism of drugs used clinically, and it exhibits genetic polymorphism with interindividual differences in metabolic activity. To date, 21 CYP2D6 allelic variants have been identified in the Japanese population. The aim of this study was to investigate the functional characterization of CYP2D6 variants identified in Japanese subjects. Wild-type CYP2D6 and its variants, namely, CYP2D6.2, CYP2D6.10, CYP2D6.14A, CYP2D6.14B, CYP2D6.18, CYP2D6.27, CYP2D6.36, CYP2D6.39, CYP2D6.47, CYP2D6.48, CYP2D6.49, CYP2D6.50, CYP2D6.51, CYP2D6.53, CYP2D6.54, CYP2D6.55, and CYP2D6.57 were transiently expressed in COS-7 cells, and enzymatic activities of the CYP2D6 variant proteins were characterized using bufuralol and dextromethorphan. Functional characterization of 17 CYP2D6 variants revealed an absence of enzyme activity in four (CYP2D6.14A, CYP2D6.36, CYP2D6.47, and CYP2D6.57), low activity in eight (CYP2D6.10, CYP2D6.14B, CYP2D6.18, CYP2D6.49, CYP2D6.50, CYP2D6.51, CYP2D6.54, and CYP2D6.55), and high activity in one (CYP2D6.53) compared with the wild type. Analysis of CYP2D6 variant proteins can be useful for predicting CYP2D6 phenotypes and could be applied to personalized drug therapy.

The AmpliChip CYP450 test: cytochrome P450 2D6 genotype assessment and phenotype prediction

Polymorphisms of the cytochrome P450 2D6 (CYP2D6) gene affecting enzyme activity are involved in interindividual variability in drug efficiency/toxicity. Four phenotypic groups are found in the general population: ultra rapid (UM), extensive (EM), intermediate (IM) and poor (PM) metabolizers. The AmpliChip CYP450 test is the first genotyping array allowing simultaneous analysis of 33 CYP2D6 alleles. The main aim of this study was to evaluate the performance of this test in CYP2D6 phenotype prediction. We first verified the AmpliChip CYP450 test genotyping accuracy for five CYP2D6 alleles routinely analysed in our laboratory (alleles 3,4,5,6, x N; n=100). Results confirmed those obtained by real-time PCR. Major improvements using the array are the detection of CYP2D6 intermediate alleles and identification of the duplicated alleles. CYP2D6 phenotype was determined by assessing urinary elimination of dextromethorphan and its metabolite dextrorphan and compared to the array prediction (n=165). Although a low sensitivity of UM prediction by genotyping was observed, phenotype prediction was optimal for PM and satisfying for EM and IM.

The influence of CYP2D6 phenotype on the clinical response of nebivolol in patients with essential hypertension

* The variability in drug metabolism has been recognized as an important factor in the occurrence of adverse effects or lack of therapeutic efficacy. * The metabolism of the third-generation beta(1)-receptor antagonist nebivolol has been shown to be highly dependent on cytochrome P450 2D6 enzymatic activity in preclinical studies. * This paper assesses the role of a cytochrome P450 2D6 gene defect on the antihypertensive response to nebivolol in a clinical setting. * Despite significant differences in drug disposition, the chronic administration of nebivolol produced similar efficacy and tolerability in hypertensive patients either characterized as poor or extensive metabolizers of the drug. * The study offers insight into the relative contribution of nebivolol enantiomers in systemic blood pressure control. Nebivolol is a beta(1)-adrenergic receptor antagonist with vasodilating properties used in the treatment of hypertension. It is administered as a racemic mixture (D- and L-nebivolol) and is highly metabolized by the cytochrome P-450 2D6 (CYP2D6). The purpose of this study was to determine the role of CYP2D6 phenotypes on the efficacy and tolerability of nebivolol during chronic administration to patients with essential hypertension. Two hundred and eighteen patients were genotyped and phenotyped for CYP2D6 activity, allowing to find and match 14 poor metabolizers (PMs) with 23 extensive metabolizers (EMs). Patients took rac-nebivolol 5 mg daily for 12 weeks. Blood pressure (BP), heart rate, adverse events, plasma levels of the two enantiomers D- and L-nebivolol and their corresponding hydroxymetabolites were assessed. The metabolic disposition of nebivolol was enantioselective and highly influenced by CYP2D6 phenotypes. Mean steady-state plasma concentrations of D- and L-nebivolol were 10- and 15-fold greater in PMs than in EMs, respectively (P < 0.0001). Despite these differences in the pharmacokinetics of nebivolol, EMs and PMs displayed similar BP responses. Mean reductions in sitting systolic and diastolic BPs were -11/-10 +/- 9/4 mmHg in EMs and -11/-9 +/- 10/5 mmHg in PMs. Side-effects were mild to moderate and not different between groups. Polymorphisms in the gene encoding CYP2D6 significantly influenced the metabolism of nebivolol, but not its antihypertensive efficacy and tolerability. The similar clinical response between EMs and PMs could be explained by the contribution of active hydroxylated metabolites of nebivolol to its antihypertensive actions in EMs.

Evaluation of probe drugs and pharmacokinetic metrics for CYP2D6 phenotyping

Cytochrome P450 2D6 (CYP2D6) is one of the most important enzymes catalyzing biotransformation of xenobiotics in the human liver. This enzyme's activity shows a high degree of interindividual variability caused in part by its genetic polymorphism, the so-called debrisoquine/sparteine polymorphism. The genetic component influencing CYP2D6 activity can be determined by genotyping. However, genotyping alone is not sufficient to accurately predict an individual's actual CYP2D6 activity, as this is also influenced by other factors. For the determination of the exact actual enzymatic activity ("phenotyping"), adequate probe drugs have to be administered prior to measurements of these compounds and/or their metabolites in body fluids. PROBE DRUGS: Debrisoquine, sparteine, metoprolol or dextromethorphan represent well-established probe drugs while tramadol has been recently investigated for this purpose. The enzymatic activity is reflected by various pharmacokinetic metrics such as the partial clearance of a parent compound to the respective CYP2D6-mediated metabolite or metabolic ratios. Appropriate metrics need to fulfill pre-defined validation criteria. In this review, we have compiled a list of such criteria useful to select the best metrics to reflect CYP2D6 activity. A comprehensive Medline search for reports on CYP2D6 phenotyping trials with the above mentioned probe drugs was carried out. Application of the validation criteria suggests that dextromethorphan and debrisoquine are the best CYP2D6 phenotyping drugs, with debrisoquine having the problem of very limited availability as a therapeutic drug. However, the assessment of the best dextromethorphan CYP2D6 phenotyping metric/procedure is still ongoing.

Clinical Guidelines for Psychiatrists for the Use of Pharmacogenetic Testing for CYP450 2D6 and CYP450 2C19

Pharmacogenetics has arrived in clinical psychiatric practice with the FDA approval of the AmpliChip CYP450 Test that genotypes for two cytochrome P450 2D6 (CYP2D6) and 2C19 (CYP2C19) genes. Other pharmacogenetic tests, including those focused on pharmacodynamic genes, are far from ready for clinical application. CYP2D6 is important for the metabolism of many antidepressants and antipsychotics, and CY2C19 is important for some antidepressant metabolism. Poor metabolizers (PMs), lacking the enzyme, account for up to 7% of Caucasians for CYP2D6 and up to 25% of East Asians for CYP2C19. Patients having three or more active CYP2D6 alleles (up to 29% in North Africa and the Middle East), are called CYP2D6 ultra-rapid metabolizers (UMs). CYP2D6 phenotypes (particularly PMs) are probably important in patients taking tricyclic antidepressants (TCAs), venlafaxine, typical antipsychotics, and risperidone. The CYP2C19 PM phenotype is probably important in patients taking TCAs and perhaps citalopram, escitalopram, and sertraline. On the basis of the literature and the authors' clinical experience, the authors provide provisional recommendations for identifying and treating CYP2D6 PMs, CYP2C19 PMs, and CYP2D6 UMs. The next few years will determine whether CYP2D6 genotyping is beneficial for patients taking the new drugs aripiprazole, duloxetine, and atomoxetine. Practical recommendations for dealing with laboratories offering CYP2D6 and CYP2C29 genotyping are provided.

Tramadol as a new probe for cytochrome P450 2D6 phenotyping: A population study

Polymorphic cytochrome P450 (CYP) 2D6 activity has been shown to be a determinant of the pharmacokinetics and pharmacodynamics of tramadol via hepatic phase I O -demethylation of (+)-tramadol to (+)-O-desmethyltramadol. Our objective was to investigate whether tramadol can be used as a probe for CYP2D6 phenotyping by determining the concordance between the 8-hour tramadol and 12-hour sparteine metabolic urinary ratios. Sparteine phenotyping test was carried out in 278 healthy, white subjects. At a minimum of 2 weeks later, each subject took 50 mg tramadol hydrochloride followed by 8-hour urine collection, and a venous blood sample was drawn from 276 subjects. Urine and plasma concentrations of (+/-)-tramadol and (+/-)-O-desmethyltramadol were determined. CYP2D6 genotyping was performed with regard to *3, *4, *6, and *9 alleles. There were 28 poor metabolizers of sparteine (10.1% [confidence interval, 6.8%-14.2%]). Very low recoveries of (+)-M1 were found in poor metabolizers (0.53% [range, 0.1%-1.1%]) compared with extensive metabolizers (8.7% [range, 1.7%-23.2%]). A bimodal distribution of the metabolic ratio of (-)-M1/(+)-M1 was found. The visual antimode was 2.0. This new phenotype test had only 1 misclassified subject compared with sparteine phenotyping (sensitivity and negative predictive value, 100%; specificity, 99.6%; positive predictive value, 96.6%). Of the 28 sparteine poor metabolizers, 26 were found to be genotypically poor metabolizers with regard to the inactivating mutations *3, *4, and *6. Fifty milligrams of tramadol is an alternative CYP2D6 phenotype probe by use of the 8-hour urinary ratio of (-)-M1/(+)-M1. The poor metabolizers have a metabolic ratio of 2.0 or higher.

Identification and characterization of novel sequence variations in the cytochrome P4502D6 (CYP2D6) gene in African Americans

Cytochrome P4502D6 (CYP2D6) genotyping reliably predicts poor metabolizer phenotype in Caucasians, but is less accurate in African Americans. To evaluate discordance we have observed in phenotype to genotype correlation studies, select African American subjects were chosen for complete resequencing of the CYP2D6 gene including 4.2 kb of the CYP2D7-2D6 intergenic region. Comparisons were made to a CYP2D6(*)1 reference sequence revealing novel SNPs in the upstream, coding and intervening sequences. These sequence variations, defining four functional alleles (CYP2D6(*)41B, (*)45A and B and (*)46), were characterized for their ability to influence splice site strength, transcription level or catalytic protein activity. Furthermore, their frequency was determined in a population of 251 African Americans. A -692(TGTG) deletion (CYP2D6(*)45B) did not significantly decrease gene expression, nor could any other upstream SNP explain a genotype-discordant case. CYP2D6(*)45 and (*)46 have a combined frequency of 4% and can be identified by a common SNP. Carriers are predicted to exhibit an extensive or intermediate CYP2D6 phenotype.

The CYP2D6 Poor Metabolizer Phenotype May Be Associated With Risperidone Adverse Drug Reactions and Discontinuation

The cytochrome P450 2D6 (CYP2D6) enzyme metabolizes risperidone. CYP2D6 poor metabolizers have no CYP2D6 activity (7% of whites and 1%-2% of other races). This study tested whether the CYP2D6 poor metabolizer phenotype was associated with adverse drug reactions (ADRs) and discontinuation due to ADRs. Adult inpatients and outpatients were recruited from July 2000 to March 2003 including (1) 325 who were stabilized on risperidone therapy and classified as either expressing moderate-to-marked ADRs (22%, 73/325) or not (78%, 252/325) and (2) 212 who discontinued risperidone and were classified as discontinued due to ADRs (38%, 81/212) or for other reasons (62%, 131/212). Genetic tests were performed by allele-specific polymerase chain reaction and/or by the AmpliChip CYP450 microarray system for up to 34 separate CYP2D6 alleles. Two logistic regression models with dependent variables (moderate-to-marked ADRs while taking risperidone and risperidone discontinuation due to ADRs) were evaluated with respect to the CYP2D6 phenotype. The odds ratios (ORs) and 95% confidence intervals (CIs) for the CYP2D6 poor metabolizer phenotype in the univariate analyses and after correcting for clinical variables were (1) OR = 3.1 (CI = 1.4 to 7.0) and 3.4 (CI = 1.5 to 8.0) for moderate-to-marked ADRs on risperidone and (2) OR = 3.0 (CI = 0.85 to 10.6) and 6.0 (CI = 1.4 to 25.4) for discontinuation due to ADRs. The CYP2D6 poor metabolizer phenotype appears to be associated with risperidone ADRs and discontinuation due to ADRs; however, this finding requires further study in larger patient populations. The CYP3A5 and p-glycoprotein exon 21 and 26 genotypes were not significantly associated with risperidone response.

Inhibition of cytochrome P4502D6 activity with paroxetine normalizes the ultrarapid metabolizer phenotype as measured by nortriptyline pharmacokinetics and the debrisoquin test

The ultrarapid metabolizer phenotype of the cytochrome P4502D6 (CYP2D6) enzyme has been considered a relevant cause of nonresponse to antidepressant drug therapy. Prescribing high doses of antidepressants to such patients leads to high concentrations of potentially toxic metabolites and an increased risk for adverse reactions. Normalization of the metabolic status of ultrarapid metabolizers by inhibition of CYP2D6 activity could offer a clinically acceptable method to successfully treat such patients with antidepressants. Five ultrarapid metabolizers with a CYP2D6 gene duplication or triplication were treated with 25 mg nortriptyline twice a day for 3 consecutive weeks, alone during the first week and concomitantly with the CYP2D6 inhibitor paroxetine 10 mg or 20 mg twice a day, respectively, during the second and third weeks. After the third week, nortriptyline was discontinued and the subjects were treated with paroxetine 20 mg twice a day during the fourth study week. At the end of each study week, the steady-state pharmacokinetic parameters of nortriptyline or paroxetine were determined within the dose interval. In addition, the CYP2D6 phenotype was determined by debrisoquin (INN, debrisoquine) test at baseline and at the end of each study phase. Treatment-related adverse events were recorded during drug administration and for 1 week thereafter. All 5 subjects had very low (subtherapeutic) nortriptyline concentrations after 7 days' treatment with nortriptyline only. Addition of paroxetine 10 mg twice a day to the nortriptyline regimen resulted in a change in all individuals to the "normal" extensive debrisoquine metabolizer phenotype, and therapeutic plasma nortriptyline concentrations were achieved in 4 of 5 subjects after a 3 times mean increase in nortriptyline trough concentration (P =.0011). Doubling the paroxetine dose caused a 15 times mean increase in paroxetine trough concentration (P <.001), indicating strong inhibition by paroxetine of its own metabolism. The high paroxetine concentrations in 2 subjects caused them to have the poor debrisoquine metabolizer phenotype and resulted in a further increase in plasma nortriptyline trough concentration (P =.0099). A strong correlation (rank correlation coefficient [r(s)] = 0.89; P <.0001) was observed between paroxetine and nortriptyline trough concentrations. Paroxetine also significantly decreased the fluctuation of nortriptyline concentrations within the dose interval. One subject discontinued the study after the second study week because of adverse effects; otherwise, the study drugs were well tolerated. Paroxetine, with a daily dosage from 20 to 40 mg, is an effective tool in normalizing the metabolic status of CYP2D6 ultrarapid metabolizers.

Clinical efficacy of nebivolol in hypertensive patients based on cytochrome p-450 2d6 phenotype

Clinical efficacy of nebivolol in hypertensive patients based on cytochrome p-450 2d6 phenotype

Cytochrome P450 2D6 and glutathione S-transferase genotype in sudden infant death syndrome.

The risk of sudden infant death syndrome (SIDS) has been linked with xenobiotic exposures, race and inheritance. Because cytochrome P450 2D6 (CYP2D6) and glutathione S-transferases (GSTM1 and GSTT1) are genetically regulated, polymorphically distributed, and responsible for detoxification of many centrally acting exogenous and endogenous bioactive compounds, our objective was to determine whether the prevalences of deficiencies in CYP2D6, GSTM1, and GSTT1 differ in SIDS victims compared to healthy controls. CYP2D6 mutations (deletion, A, B, and T alleles) and GSTM1 and GSTT1 null genotypes were assessed in DNA from 50 SIDS victims. CYP2D6 phenotype, assigned using dextromethorphan urinary ratios, was assessed in 25 unrelated parents of SIDS victims. The CYP2D6B mutation was the only mutant CYP2D6 allele found in SIDS victims, present in 26.2% of patients (11/42) and 13.1% (11/84) of alleles. Adjusting for race, the prevalence of wild-type CYP2D6 alleles and of homozygous wild-type CYP2D6 phenotype was not different in SIDS victims compared to controls (P = 0.585 and 0.224, respectively). Among the 25 parents of SIDS victims, all subjects were extensive metabolizers, a prevalence not different from controls (P = 0.243). The prevalence of the null genotype for GSTM1, GSTT1 and double-null for GSTM1 and GSTT1 was 33.3%, 21.4% and 9.5%, respectively, among SIDS victims, and was not different than controls (P = 0.61, 0.1, 0.28, respectively). The combination of CYP2D6 homozygous wild-type genotype and the null genotype for GSTM1, GSTT1, or both GSTM1 + GSTT1 also did not differ in SIDS victims and controls. The frequencies of CYP2D6 mutant genotypes and the null genotypes for GSTM1 and GSTT1 were not different among SIDS victims compared to normal controls, and thus these polymorphisms are unlikely to identify families with a high risk of SIDS.

Pharmacokinetics of Dextromethorphan and Metabolites in Humans

Dextromethorphan is primarily metabolized to dextrorphan by cytochrome P450 2D6 (CYP2D6), a genetically polymorphic enzyme in humans. Dextrorphan is an active metabolite that produces phencyclidine-like behavioral effects in animals and exhibits anticonvulsant and neuroprotective properties in a variety of experimental models. In these studies, we examined the effects of CYP2D6 phenotype and quinidine inhibition on the pharmacokinetics of dextromethorphan and its metabolites in humans. After a single oral dose of dextromethorphan HBr (30 mg), the major metabolites in the plasma of extensive metabolizers (N = 5) were conjugated dextrorphan and conjugated 3-hydroxymorphinan. Free dextrorphan concentrations were about 100-fold less than the conjugated dextrorphan, and dextromethorphan was not detectable. Pretreatment of these subjects with 100 mg of quinidine, a selective inhibitor of CYP2D6, significantly suppressed the formation of dextrorphan and elevated the concentrations of dextromethorphan (t1/2, 16.4 hours). In poor metabolizers (N = 4) given the same dose, dextromethorphan was the major component in the plasma with a t1/2 of 29.5 hours. Present at concentrations 5- to 10-fold less were conjugated dextrorphan and the other two metabolites. Urinary recovery studies indicated that the inhibition by quinidine was reversible and that the elimination of dextromethorphan primarily depends on CYP2D6 activity rather than renal elimination. These data demonstrated that the CYP2D6 phenotype and the concurrent administration of quinidine significantly affect the disposition of dextromethorphan and the formation of the active metabolite dextrorphan and are important factors to be considered in studies of the pharmacologic and behavioral effects of dextromethorphan.

Variable disposition kinetics and electrocardiographic effects of flecainide during repeated dosing in humans: Contribution of genetic factors, dose-dependent clearance, and interaction with amiodarone

We studied the influence of cytochrome P450 2D6 (CYP2D6) on the steady-state disposition kinetics and the electrocardiographic effects of flecainide at two doses and during combination with amiodarone. Seven extensive and five poor metabolizers of dextromethorphan were studied during a three-period crossover study. All subjects received 50 mg flecainide every 12 hours, alone or together with 200 mg amiodarone every 12 hours, and 100 mg flecainide every 12 hours for 5 days. Mean steady-state plasma concentration of flecainide and QRS change from predrug value did not differ significantly among extensive and poor metabolizer subjects during each study period. Except for a shortened elimination half-life and nonlinear kinetics in extensive metabolizer subjects, phenotype had no significant influence on flecainide pharmacokinetics. Combination with amiodarone resulted in an increase in mean flecainide plasma concentration and effect in subjects with both phenotypes. Our findings indicate that CYP2D6 phenotype predicts flecainide nonlinear kinetics and flecainide half-life but has no influence on electrocardiographic effects during repeated administration of flecainide or on the extent of the amiodarone-flecainide interaction.

Disposition of lupanine and 13-hydroxylupanine in man.

1. The in vivo disposition of lupanine and 13-hydroxylupanine was studied in subjects identified as poor metabolizers (PM, n = 4) and extensive metabolizers (EM, n = 7) phenotypes for cytochrome P4502D6 (CYP2D6). 2. After oral administration (40.26 mumol), the half-life (t1/2) of lupanine determined from urinary excretion rate studies in EM subjects was 6.2 +/- 0.5 h (mean +/- SEM) with 95.5 +/- 6.0% of the dose recovered unchanged within 72 h. Similarly, in PM subjects t1/2 = 6.5 +/- 0.9 h and recovery 89.9 +/- 4.5%. 3. For orally administered 13-hydroxylupanine (37.83 mumol) the t1/2 in EM subjects was 6.8 +/- 1.0 h with a recovery of 100.5 +/- 5.3%, and in PM subjects t1/2 = 5.9 +/- 1.6 h with a recovery of 102.5 +/- 4.8%. 4. The t1/2s of both lupanine and 13-hydroxylupanine respectively did not differ significantly between EM and PM phenotypes. In addition, total recovery of dose for both alkaloids was similar between phenotypes. 5. In most subjects, > 76% of lupanine and > 85% of 13-hydroxylupanine was recovered as the unchanged compound. Significant apparent partial dehydroxylation of 13-hydroxy-lupanine was observed in one EM (14% of dose) and one PM (34% of dose) subject. 6. Overall, the finding of a high urinary recovery of unchanged lupanine or 13-hydroxylupanine together with similar t1/2s for both alkaloids in EM and PM CYP2D6 phenotypes suggests that clinical toxicity is unlikely to result from the use of lupin seed in footstuffs.

CYP2D6 phenotype determines the metabolic conversion of hydrocodone to hydromorphone

The contribution of cytochrome P450 2D6 (CYP2D6) to the formation of hydrocodone's active metabolite, hydromorphone, was examined in vitro and in vivo. Human liver microsomes prepared from an individual homozygous for the D6-B mutation of the CYP2D6 gene catalyzed this reaction at a negligible rate. Urinary metabolic ratios of hydrocodone/hydromorphone were highly correlated with O-demethylation ratios for dextromethorphan, an established marker drug of CYP2D6 activity (rs = 0.85; n = 18). The kinetics of hydrocodone after a single oral dose and its partial metabolic clearance to hydromorphone were investigated in five extensive metabolizers of dextromethorphan, six poor metabolizers, and four extensive metabolizers after pretreatment with quinidine, a selective inhibitor of CYP2D6 activity. The mean values for partial metabolic clearance by O-demethylation in the three groups were 28.1 +/- 10.3, 3.4 +/- 2.4, and 5.0 +/- 3.6 ml/hr/kg, respectively. No statistically significant phenotypic differences in physiologic measures were observed. However, over the first hour after dosing, the extensive metabolizers reported more "good opiate effects" and fewer "bad opiate effects" than poor metabolizers and extensive metabolizers in whom CYP2D6 was inhibited by quinidine. These data establish the importance of CYP2D6 in the formation of hydromorphone from hydrocodone and suggest that the activity of this enzyme may limit the abuse liability of hydrocodone.

No Time-Dependent Change in Cytochrome P-450 2D6 Phenotyping Results During HPLC Analysis

No Time-Dependent Change in Cytochrome P-450 2D6 Phenotyping Results During HPLC Analysis