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The International Tamoxifen Pharmacogenomics Consortium was established to address the controversy regarding cytochrome P450 2D6 (CYP2D6) status and clinical outcomes in tamoxifen therapy. We performed a meta-analysis on data from 4,973 tamoxifen-treated patients (12 globally distributed sites). Using strict eligibility requirements (postmenopausal women with estrogen receptor-positive breast cancer, receiving 20 mg/day tamoxifen for 5 years, criterion 1); CYP2D6 poor metabolizer status was associated with poorer invasive disease-free survival (IDFS: hazard ratio = 1.25; 95% confidence interval = 1.06, 1.47; P = 0.009). However, CYP2D6 status was not statistically significant when tamoxifen duration, menopausal status, and annual follow-up were not specified (criterion 2, n = 2,443; P = 0.25) or when no exclusions were applied (criterion 3, n = 4,935; P = 0.38). Although CYP2D6 is a strong predictor of IDFS using strict inclusion criteria, because the results are not robust to inclusion criteria (these were not defined a priori), prospective studies are necessary to fully establish the value of CYP2D6 genotyping in tamoxifen therapy.

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 amiodaroneflecainide interaction. Clinical Pharmacology and Therapeutics (1994) 55, 256–269; doi:10.1038/clpt.1994.26

The expression and function of the drug transporter P-glycoprotein are highly variable. Environmental and genetic factors contribute to this variation. We studied the disposition of digoxin, a frequently used probe drug for P-glycoprotein function in humans, in monozygotic (MZ) twins and found that digoxin pharmacokinetics after oral and intravenous administration are highly correlated within MZ twins, supporting the hypothesis of a robust contribution from genetic variance. Our study suggests that studies involving twins could be more widely applied to elucidate pharmacogenetics.

The pyrazolone drug metamizole is a widely used analgesic. Analysis of liver microsomes from patients treated with metamizole revealed selectively higher expression of cytochromes P450, CYP2B6 and CYP3A4 (3.8- and 2.8-fold, respectively), and 2.9-fold higher bupropion hydroxylase activity compared with untreated subjects. Further investigation of metamizole and various derivatives on different potential target genes in human primary hepatocytes demonstrated time- and concentration-dependent induction by metamizole of CYP2B6 (7.8- and 3.1-fold for mRNA and protein, respectively, at 100 muM) and CYP3A4 (2.4- and 2.9-fold, respectively), whereas other genes (CYP2C9, CYP2C19, CYP2D6, NADPH:cytochrome P450 reductase, ABCB1, constitutive androstane receptor (CAR), pregnane X receptor (PXR)) were not substantially altered. Using reporter gene assays, we show that metamizole is not acting as a direct ligand to either PXR or CAR, suggesting a phenobarbital-like mechanism of induction. These data warrant further studies to elucidate the drug-interaction potential of metamizole, especially in patients with long-term treatment.

To assess the association of CYP2B6 allelic diversity with efavirenz (EFV) pharmacokinetics, we performed extensive genotyping of 15 relevant single nucleotide polymorphism in 169 study participants, and full resequencing of CYP2B6 in individuals with abnormal EFV plasma levels. Seventy-seven (45.5%) individuals carried a known (CYP2B6*6, *11, *15, or *18) or new loss/diminished-function alleles. Resequencing defined two new loss-of-function alleles: allele *27 (marked by 593T>C [M198T]), that results in 85% decrease in enzyme activity and allele *28 (marked by 1132C>T), that results in protein truncation at arginine 378. Median AUC levels were 188.5 microg h/ml for individuals homozygous for a loss/diminished-function allele, 58.6 microg h/ml for carriers, and 43.7 microg h/ml for noncarriers (P

Background The antiepileptic drug carbamazepine (CBZ) is a potent inducer of human drug metabolism, resulting in serious interactions with many commonly prescribed drugs. The molecular mechanisms underlying this response are not well understood, however, and the spectrum of CBZ-inducible genes in human liver has not been thoroughly investigated. Methods The availability of liver ribonucleic acid from 2 epileptic patients treated with CBZ and from 7 control subjects enabled us to study the global induction response of drug-metabolizing enzymes, drug transporters, and nuclear receptors in vivo. Results Using expression profiling, we identified 64 significantly up-regulated transcripts but only 1 significantly down-regulated transcript (SLC22A5). We confirmed the induction of several genes that previously have been shown to be inducible by drugs in vitro, including multiple cytochrome P450 (CYP) genes in the CYP1A, CYP2A, CYP2B, CYP2C, and CYP3A subfamilies, as well as glutathione S-transferase A1, uridine diphosphate-glucuronosyltransferase 1As, the drug transporter ABCC2, and the nuclear receptors CAR (constitutive androstane receptor) and PXR (pregnane X receptor). Moreover, we identified a number of additional genes not previously known to be induced by CBZ, including CYP39A1, sulfotransferase 1A1, glutathione S-transferase Z1, and the drug transporters SLCO1A2, ABCG2, and ABCB7, as well as the glucocorticoid and aldosterone receptors. In transactivation studies in CV-1 cells, we demonstrated that both CBZ and its major metabolite, CBZ-10,11-epoxide, activate the nuclear receptor PXR in a concentration-dependent fashion and at therapeutic concentrations with 50% inhibitory concentration values of approximately 50 μmol/L. Conclusions CBZ is a potent inducer of a broad spectrum of drug-metabolizing enzymes and drug transporters in the human liver, and these effects are mediated at least in part by activation of PXR. Clinical Pharmacology & Therapeutics (2006) 80, 440–456; doi: 10.1016/j.clpt.2006.08.013

Our objective was to study the impact of the cytochrome P450 (CYP) 2D6 polymorphism on the tolerability of metoprolol in a real-life primary care setting. The adverse effects studied comprised effects related to the central nervous system, cardiovascular effects, and sexual dysfunction.Patients in whom treatment with metoprolol was considered were enrolled into this prospective, 6-week multicenter study. The dosage of metoprolol was determined on an individual basis and could be freely adjusted on clinical grounds. The indication for treatment was hypertension in about 90% of cases. Systolic and diastolic blood pressure, resting heart rate, and plasma metoprolol and alpha-hydroxymetoprolol concentrations were measured. CYP2D6 genotyping covered alleles *3 to *10 and *41 and the duplications. Possible adverse effects of metoprolol were systematically assessed over a 6-week period by means of standardized rating scales and questionnaires.The final study population comprised 121 evaluable patients (all white patients); among them, there were 5 ultrarapid metabolizers (UMs) (4.1%), 91 extensive metabolizers (EMs) (75%), 21 intermediate metabolizers (IMs) (17%), and 4 poor metabolizers (PMs) (3.3%). Plasma metoprolol concentrations normalized for the daily dose and metoprolol/alpha-hydroxymetoprolol ratios at steady state were markedly influenced by CYP2D6 genotype and displayed a gene-dose effect. The median of the dose-normalized metoprolol concentration was 0.0088 ng/mL, 0.047 ng/mL, 0.34 ng/mL, and 1.34 ng/mL among UMs, EMs, IMs, and PMs, respectively (P.0001). There was no significant association between CYP2D6 genotype-derived phenotype (EMs and UMs combined versus PMs and IMs combined) and adverse effects during treatment with metoprolol. There was a tendency toward a more frequent occurrence of cold extremities in the PM plus IM group as compared with the EM plus UM group (16.0% versus 4.2%, P=.056; relative risk, 3.8 [95% confidence interval, 1.03--14.3]).CYP2D6 genotype-derived phenotype was not significantly associated with a propensity for adverse effects to develop during treatment with metoprolol. However, the results concerning tolerability of metoprolol in PMs were inconclusive because of the small number of PMs enrolled.

Background and aims In humans gut wall metabolism can be quantitatively as important as hepatic drug metabolism in limiting the systemic exposure to drugs after oral administration. However, it has been proposed that the role of gut wall metabolism might be overemphasized, because high luminal drug concentrations would lead to a saturation of gut wall metabolism. Therefore we investigated the impact of concentration and rate of intraluminal drug delivery on absorption (Fabs) and gastrointestinal extraction (EGI) of a luminally administered cytochrome P450 (CYP) 3A4 substrate (verapamil) using a multilumen perfusion catheter in combination with a stable isotope technique. Methods Two 20-cm-long, adjacent jejunal segments were isolated with the multilumen perfusion catheter in 7 subjects. In this study 80 mg of unlabeled verapamil (d0-verapamil15 min) was infused into one segment over a 15-minute period, 80 mg of 3-fold deuterated verapamil (d3-verapamil240 min) was administered over a 240-minute period into the other segment, and simultaneously, 5 mg of 7-fold deuterated verapamil (d7-verapamil) was injected intravenously over a 15-minute period. Results The rate of intraluminal drug delivery had only a modest effect on bioavailability of the verapamil isotopes (after correction for Fabs) (F/Fabsd3-verapamil240 min versus d0-verapamil15 min, 0.24 ± 0.10 versus 0.20 ± 0.09; P < .05). Accordingly, the EGI value for d3-verapamil240 min was 0.50 ± 0.18 compared with 0.59 ± 0.14 for d0-verapamil15 min (P < .05). In vivo, EGI(d0-verapamil15 min) correlated strongly with EGI(d3-verapamil240 min) (r = 0.94, P < .005). Moreover, intrinsic clearance of CYP3A4-mediated verapamil metabolism in homogenates of simultaneously collected shed enterocytes correlated with in vivo EGI of d0-verapamil15 min/d3-verapamil240 min (r = 0.62, P = .03). Conclusions Substantial gut wall metabolism of verapamil occurs in humans and can be predicted from ex vivo data by use of shed enterocytes. The different intraluminal concentrations and rates of intraluminal drug delivery did not lead to a pronounced saturation of intestinal drug metabolism. Clinical Pharmacology & Therapeutics (2004) 76, 230–238; doi: 10.1016/j.clpt.2004.04.013

Background High-dose busulfan is widely used as part of conditioning regimens for patients who are undergoing hematopoietic stem cell or bone marrow transplantation. High plasma concentrations of busulfan have been linked to the occurrence of hepatic venoocclusive disease (VOD), a severe complication associated with a high mortality. Because conjugation with glutathione, the major route of biotransformation of busulfan, is predominantly catalyzed by the isozyme glutathione S-transferase A1 (GSTA1), we hypothesized that low expression or function of GSTA1 in liver caused by genetic polymorphisms may be the mechanism underlying VOD. Methods Immunoblot analysis of GSTA and measurement of busulfan-glutathione conjugation by liquid chromatography-mass spectrometry were performed in 48 normal human liver samples. To search for polymorphisms, the complete GSTA1 coding regions and the promoter fragment were sequenced. All results were compared by multivariate analysis. Results Absolute levels of GSTA protein and formation rates of busulfan-glutathione conjugate displayed a 7- and 8-fold range, from 240 to 1600 pmol/mg and 25 to 205 pmol/min per milligram of total cytosolic protein, respectively, and correlate (r2 = 0.49, P < .0001). A total of 8 single nucleotide polymorphisms (SNPs) of GSTA1 were identified, 1 of which was a silent mutation in exon 5 (A375G); all others were found in the promoter region. Haplotype analysis revealed the existence of 5 defined alleles. There was no significant relationship between any of the GSTA1 SNPs or haplotypes and either hepatic glutathione S-transferase A (GSTA) expression or GSTA1 function. Conclusions The identified GSTA1 polymorphisms are not likely to be related to the VOD because they do not appear to be associated with changes in GSTA expression or function. Compared with other members of the GST family, GSTA1 displays surprisingly little variation. Clinical Pharmacology & Therapeutics (2002) 71, 479–487; doi: 10.1067/mcp.2002.124518

Intestinal metabolism and transport are now recognized as protective barriers against orally ingested xenobiotics, including drugs. However, in vitro studies of the expression and function of intestinal proteins are hampered by the limited availability of human intestinal tissues. Because enterocytes are constantly shed in large numbers into the gut lumen, this study investigated whether these cells could be collected with a multilumen perfusion catheter and whether they are functionally active.In healthy volunteers, a 20-cm isolated jejunal segment was generated with the perfusion catheter by inflating 2 balloons with air. Shed cells were characterized by fluorescence-activated cell sorting analysis for leukocyte-specific CD45 and enterocyte-specific villin, as well as for apoptosis. Homogenates of the cells were used for reverse transcriptase polymerase chain reaction and Western blotting. Cytochrome P450 enzyme activity was determined with the calcium channel blocker verapamil as a substrate.On average, 4.83 mg protein and 56.23 million cells were collected from a 20-cm segment during 2 hours. A total of 84.2% of the cells were positive for enterocyte-specific villin, and only 1.6% of the collected cells were positive for CD45. The majority of cells (65.3%) were not in early or late apoptosis or necrosis. In all volunteers, drug-metabolizing enzymes (such as members of the cytochrome P450 family) could be detected as both messenger ribonucleic acid and proteins. Consistent with expression data, formation of verapamil metabolites catalyzed by CYP3A4 and CYP2C was shown.The majority of shed human enterocytes collected with a multilumen perfusion catheter were still functionally active and not apoptotic. Harvesting of spontaneously shed enterocytes provides a new tool for studies on expression and function of intestinal proteins.

Background Codeine is widely used as an analgesic and antitussive drug. The analgesic effect of codeine is mediated by its metabolite morphine, which is formed by the polymorphically expressed enzyme CYP2D6; therefore poor metabolizers have no analgesia after administration of codeine. Like other opiates, codeine causes a delay of gastric emptying and spastic constipation. It is not yet known whether the effect on gastrointestinal motility is mediated by codeine or its metabolite morphine. Methods To test the hypothesis that the metabolite morphine is responsible for the effects of codeine on gastrointestinal motility, a randomized, double-blind, two-way crossover study was performed. The orocecal transit time was studied in five extensive and five poor metabolizers of sparteine with the sulfasalazine-sulfapyridine method, assuming that no effects are observed in poor metabolizers because negligible amounts of morphine are formed. Results No differences of orocecal transit times were observed between extensive metabolizers and poor metabolizers after oral placebo administration. However, after oral codeine administration orocecal transit time was significantly prolonged in extensive metabolizer but not poor metabolizer subjects. All pharmacokinetic parameters of codeine showed no differences between extensive metabolizers and poor metabolizers. The pharmacokinetic parameters (mean ± SD) of the metabolite morphine were significantly different between extensive metabolizer and poor metabolizer subjects (peak serum concentration, 13.9 ± 10.5 versus 0.68 ± 0.15 pmol/ml; area under the serum concentration-time curve, 27.8 ± 16.0 versus 1.9 ± 0.7 hr · pmol/ml; total amount of morphine excreted in urine, 0.160 ± 0.036 versus 0.015 ± 0.007 μmol). Conclusions Because the orocecal transit time prolongation after codeine administration was observed only in extensive metabolizers, the effect of codeine on gastrointestinal motility, like the analgesia, is mediated by its metabolite morphine. Clinical Pharmacology & Therapeutics (1997) 61, 459–466; doi

Objectives To examine whether the variability of CYP2D6 activity in patients with chronic renal failure can be assessed, particularly among subjects with the extensive metabolizer phenotype, by use of standard in vivo indexes of CYP2D6 activity derived from oral administration of dextromethorphan and sparteine. Methods A single 100 mg oral dose of sparteine and a single 40 mg oral dose of dextromethorphan were administered on two occasions to 12 patients with chronic renal failure (creatinine clearance ranging from 20 to 70 ml/min) and 12 age- and sex-matched healthy subjects. Sparteine clearances, sparteine metabolic ratio, and urinary recovery of dextrorphan were calculated. Patients and healthy control subjects were not selected on the basis of their CYP2D6 phenotypes. Results Chronic renal failure was associated with a decrease in sparteine partial metabolic clearance to dehydrosparteine (median of 322 ml/min and range of 62 to 670 ml/min in patients with renal failure versus median of 635 ml/min and range of 77 to 1276 ml/min in normal subjects; p < 0.02). Sparteine apparent oral clearance (p < 0.03) and renal clearance (p < 0.001) decreased in patients with renal failure. However, sparteine metabolic ratio was not significantly altered in patients with renal failure and showed that all patients were extensive metabolizers of sparteine. Although fractional urinary excretion of dextrorphan decreased in patients with renal failure (median, 24.4%; range, 9.7% to 55.9%) compared with control (median, 47.5%; range, 24.1% to 72.1%) (p = 0.02), it also showed that all subjects were extensive metabolizers of dextromethorphan. The amount of dextrorphan excreted in urine correlated with creatinine clearance independently from CYP2D6 activity measured as sparteine partial metabolic clearance. However, it did not correlate with sparteine metabolic ratio or with fractional urinary excretion of dehydrosparteine. Conclusion Assessment of CYP2D6 activity by use of dextromethorphan and sparteine is possible in extensive metabolizer patients with chronic renal failure. However, in these subjects, dextromethorphan and sparteine do not reflect CYP2D6 activity in the same way. Clinical Pharmacology & Therapeutics (1996) 59, 583–592; doi

Sparteine metabolism was studied in human liver microsomes from nine extensive ([EM] urinary metabolic ratio [MR] less than 20) and four poor metabolizers ([PM] MR greater than 20). The formation of 2- and 5-dehydrospartein displayed monophasic Michaelis-Menten kinetics. In livers from PMs the formation of the major sparteine metabolite 2-dehydrosparteine was characterized by more than a thirtyfold increase in Michaelis-Menten constant (Km)(1880 +/- 1044 mumol/L) as compared with EM subjects with an MR less than 1 (58.3 +/- 38.8 mumol/L). EM subjects with an MR greater than 3 who may constitute heterozygous metabolizers showed Km values in between (658 +/- 301 mumol/L). There was no difference in maximum rate of metabolism (Vmax) of 2-dehydrosparteine formation between EM and PM subjects (101 +/- 39 vs. 86 +/- 52 pmol/min/mg). The formation of 5-dehydrosparteine exhibited a Km of 100 +/- 123 mumol/L similar to the Km of 2-dehydrosparteine formation. The urinary MR correlated positively with the Km for 2-dehydrosparteine formation. The intrinsic clearance for 2-dehydrosparteine showed a highly significant negative correlation with the MR. The pronounced differences in Km together with the significant correlation between Km and MR with no marked differences in Vmax between phenotypes suggest that the impaired oxidation capacity in PM subjects is more likely the result of a P-450 isozyme with altered catalytic properties rather than a decreased amount of enzyme.

Theophylline plasma clearance (Clp) and clearance to its metabolites (Clm), as well as antipyrine saliva clearance (Clsal) and its Clm were compared in a crossover study in 25 healthy subjects. They were selected with regard to smoking status (nine smokers, 16 nonsmokers) and oxidation phenotype of debrisoquine and sparteine (six poor metabolizers [PMs] and 19 extensive metaboUzers [EMs]). Clm of theophylline (1,3-dimethyluric acid, 1-methyluric acid, and 3-methylxanthine) correlated (r ≥ 0.92) to each other and to total theophylline Clp (r ≥ 0.97). Smokers had higher Clm to all metabolites, particularly by the N-demethylation pathways. After correction for the effect of smoking, there was no difference between EMs and PMs with regard to theophylline Clp or Clm. Antipyrine clearances by EMs and PMs (Clsal and Clm of 4-OH-antipyrine, 3-OH-methylantipyrine, or norantipyrine) also did not differ. Antipyrine Clsal and Clm correlated to theophylline Clp (r between 0.50 and 0.69). It is concluded that theophylline metabolism (N-demethylations and C-oxidation) is not under the same genetic control as sparteine and debrisoquine oxidations, and that there may be a partial overlap in factors that regulate the metabolism of theophylline and antipyrine. Clinical Pharmacology and Therapeutics (1984) 35, 815–821; doi:10.1038/clpt.1984.118

Thirty-eight healthy subjects were given single oral doses of debrisoquine and sparteine in a crossover study. The close correlation between urinary metabolic ratios of the two drugs (rs = 0.91; P less than 0.001) demonstrates that the polymorphic N-oxidation of sparteine and 4-hydroxylation of debrisoquine are related pharmacogenetic entities; the metabolism of the two drugs is regulated by identical or closely related genetic factors.

After the oral administration of 160 mg pseudoracemic verapamil (80 mg dideuterodextro (d) isomer and 80 mg levo (l) isomer), the prolongation of the PR interval was assessed in relation to d- and l-verapamil plasma concentrations. Concentration-effect curves were analyzed with the sigmoidal Emax model. Because of stereoselective first-pass metabolism, the mean plasma d- to l-verapamil concentration ratio of 4.5 ±1.2 was substantially greater than that of 2.1 ± 0.3 after intravenous dosing. Compared with the concentration after intravenous injection, the total verapamil concentration after oral dosing consisted of a substantially smaller proportion of the more potent l-isomer. These differences in isomer composition of the total verapamil plasma concentration as a result of the route of administration explain the diminished negative dromotropic potency of racemic verapamil after oral dosing. The concentration required to reach 50% of the maximum effect (EC50) for total verapamil concentration was 129.0 ± 22.9 ng/ml, which was more than three times higher than that after intravenous injection. To assess the relative contributions of the d- and l-isomers to overall dromotropic potency, changes in the PR interval were measured after separate oral dosing with 250 mg d-verapamil and 100 mg l-verapamil. The EC50 showed an 11-fold difference between the l-(36.9 ± 14.7 ng/ml) and d- (363.1 ± 64.2 ng/ml) isomers. The EC50 for the l-isomer concentration after oral pseudoracemic verapamil (20.2 ± 6.3 ng/ml) did not differ significantly from that after l-verapamil alone (36.9 ± 14.7 ng/ml). We conclude that the l-isomer determines the negative dromotropic effects of verapamil and that the d-isomer is of minor importance. Clinical Pharmacology and Therapeutics (1985) 38, 71–76; doi:10.1038/clpt.1985.137

Propafenone is a new class 1 antiarrhythmic agent. The drug is extensively metabolized. 5-Hydroxylation and N-dealkylation constitute major metabolic pathways. Recently it has been demonstrated that the in vivo metabolism of propafenone is controlled by the debrisoquin/sparteine polymorphism. To elucidate which of the above metabolic reactions is catalyzed by cytochrome P-450db1, the formation of 5-hydroxypropafenone and N-desalkylpropafenone was studied in the microsomal fraction of four human kidney donor livers previously characterized with regard to their ability to hydroxylate the β-adrenergic antagonist bufuralol. The l'hydroxylation of bufuralol is catalyzed by the P-450db1 responsible for polymorphic debrisoquin/sparteine oxidation. The formation of 5-hydroxypropafenone but not N-desalkylpropafenone was closely related to bufuralol l'hydroxylation. Incubation with LKM1 antibodies, which selectively recognize P-450db1, inhibited 5-hydroxypropafenone formation completely whereas N-dealkylation was unimpaired. Propafenone was a strong competitive inhibitor of bufuralol l'hydroxylation. Thus it can be concluded that 5-hydroxypropafenone is formed by the cytochrome P-450 isozyme involved in polymorphic bufuralol oxidation. Clinical Pharmacology and Therapeutics (1989) 45, 28–33; doi:10.1038/clpt.1989.5

The kinetics of carbamazepine using 15N-carbamazepine were investigated in epileptic patients during combined anticonvulsant therapy. The 15N-carbamazepine plasma half-lives ranged from 5.0 to 13.6 hr with a mean of 8.2 hr. These half-lives are appreciably shorter than reported during chronic carbamazepine monotherapy. Predicted steady-state plasma levels and observed plasma levels of carbamazepine were in excellent agreement. Between 32% and 61% of the dose administered is excreted in the urine as carbamazepine-trans-diol, 5.2% to 8.8% as 9-hydroxymethyl-10-carbamoyl acridane, 1% to 1.4% as 10,-11-carbamazepine epoxide, and 0.5% as carbamazepine. The data indicate that it is the epoxide-diol pathway which is induced during long-term treatment. Concomitant therapy with primidone, phenytoin, phenobarbital, ethosuximide, or methsuximide further induces carbamazepine metabolism.