Showing papers in "Clinical Pharmacology & Therapeutics in 2005"

Safety, pharmacodynamics, and pharmacokinetics of single doses of BAY 59-7939, an oral, direct factor Xa inhibitor.

Abstract: Background and Objective There is a clinical need for new oral anticoagulants to prevent and treat thromboembolic diseases. Given its integral role in the coagulation cascade, factor Xa is a particularly promising target for new anticoagulation therapies. The aim of this study was to investigate the safety, pharmacodynamics, and pharmacokinetics of BAY 59–7939, an oral, direct factor Xa inhibitor. Methods This single–center, randomized, single–blinded, placebo–controlled, dose–escalation study included 108 healthy white male subjects aged 19 to 45 years. Subjects received single oral doses of either BAY 59–7939 (1.25–80 mg) or placebo; in addition, 1 group received 2 doses of BAY 59–7939 (5–mg tablet and oral solution) or placebo in a crossover design. Results Oral BAY 59–7939 in single doses up to 80 mg was safe and well tolerated and was not associated with an increased risk of bleeding compared with placebo. Pharmacodynamic effects (inhibition of factor Xa activity, prothrombin time, activated partial thromboplastin time, and HepTest) and plasma concentration profiles were dose–dependent. Maximum inhibition of factor Xa activity was achieved 1 to 4 hours after administration of BAY 59–7939 and ranged from 20% to 61% for the 5– to 80–mg doses. BAY 59–7939 selectively inhibited factor Xa activity; thrombin (factor IIa) and antithrombin were unaffected. Inhibition of factor Xa activity and prolongation of prothrombin time correlated well with BAY 59–7939 plasma concentrations (r = 0.949 and 0.935, respectively). Conclusions BAY 59–7939 was well tolerated with predictable pharmacodynamics and pharmacokinetics across a wide range of doses in healthy male subjects. BAY 59–7939 was shown to be an effective and specific factor Xa inhibitor. Clinical Pharmacology & Therapeutics (2005) 78, 412–421; doi: 10.1016/j.clpt.2005.06.011

Transporters and drug therapy: implications for drug disposition and disease.

Abstract: The efficacy of drug therapy results from the complex interplay of multiple processes that govern drug disposition and response. Most studies to date have focused on the contribution of drug-metabolizing enzymes to the drug disposition process. However, over the past decade, it has become increasingly apparent that carrier-mediated processes, or transporters, also play critical roles in the overall disposition of numerous drugs in clinical use. In addition to their roles in xenobiotic transport, drug transporters often mediate important physiologic functions via transport of endogenous substrates such as amino acids, bile acids, and hormones that are critical for maintenance of normal homeostasis. In this review we focus on the emerging field of transporter proteins in relation to the drug disposition process, with particular emphasis on clinical implications of transporters to drug-drug interactions and subsequent development of adverse effects, interindividual variability in drug response, and human disease.

Pharmacokinetics and pharmacodynamics of sitagliptin, an inhibitor of dipeptidyl peptidase IV, in healthy subjects: Results from two randomized, double-blind, placebo-controlled studies with single oral doses

Abstract: Background Sitagliptin (MK-0431 [(2R)-4-oxo-4-(3-[trifluoromethyl]-5,6-dihydro[1,2,4]triazolo[4,3-a]pyrazin-7[8H]-yl)-1-(2,4,5-trifluorophenyl)butan-2-amine]) is an orally active, potent, and selective inhibitor of dipeptidyl peptidase IV (DPP-IV) currently in phase III development for the treatment of type 2 diabetes. Methods Two double-blind, randomized, placebo-controlled, alternating-panel studies evaluated the safety, tolerability, pharmacokinetics, and pharmacodynamics of single oral doses of sitagliptin (1.5–600 mg) in healthy male volunteers. Results Sitagliptin was well absorbed (approximately 80% excreted unchanged in the urine) with an apparent terminal half-life ranging from 8 to 14 hours. Renal clearance of sitagliptin averaged 388 mL/min and was largely uninfluenced by the dose administered. The area under the plasma concentration–time curve for sitagliptin increased in an approximately dose-dependent manner and was not meaningfully influenced by food. Single doses of sitagliptin markedly and dose-dependently inhibited plasma DPP-IV activity, with approximately 80% or greater inhibition of DPP-IV activity occurring at 50 mg or greater over a 12-hour period and at 100 mg or greater over a 24-hour period. Compared with placebo, sitagliptin produced an approximately 2-fold increase in postmeal active glucagon-like peptide 1 levels. Sitagliptin was well tolerated and was not associated with hypoglycemia. Conclusions This study provides proof of pharmacologic characteristics for sitagliptin in humans. By inhibiting plasma DPP-IV activity, sitagliptin increases the postprandial rise in active glucagon-like peptide 1 concentrations without causing hypoglycemia in normoglycemic healthy male volunteers. Sitagliptin possesses pharmacokinetic and pharmacodynamic characteristics that support a once-daily dosing regimen. Clinical Pharmacology & Therapeutics (2005) 78, 675–688; doi: 10.1016/j.clpt.2005.09.002

Clinical consequences of cytochrome P450 2C9 polymorphisms.

Abstract: The gene coding for the cytochrome P450 (CYP) enzyme 2C9 (CYP2C9) carries numerous inherited polymorphisms. Those coding for R144C (*2) and I359L (*3) amino acid substitutions have both significant functional effects and appreciable high population frequencies, and their in vivo consequences have been studied in humans with regard to drug metabolism. This review summarizes present knowledge about the pharmacokinetics, drug responses, and outcomes of clinical studies in individuals with different CYP2C9 genotypes. Tentative estimates of how CYP2C9 genotyping might be applied to dose adjustments in clinical therapy were based on dose-related pharmacokinetic parameters such as clearance or trough drug concentrations. Mean clearances in homozygous carriers of the *3 allele were below 25% of that of the wild type for S -warfarin, tolbutamide, glipizide, celecoxib, and fluvastatin. In the more frequent heterozygous carriers (genotype *1/*3), the clearances were between 40% and 75%. In these cases in which individual dosages are derived from clinical drug effects, such as for the oral anticoagulants, the pharmacogenetics-based dose adjustments showed a good correlation with the genotype-specific empirically derived doses. In addition to its role in pharmacokinetics, CYP2C9 contributes to the metabolism of fatty acids, prostanoids, and steroid hormones, and it may catalyze potentially toxic bioactivation reactions. However, our current understanding of the role of CYP2C9 in biotransformation of endogenous signaling molecules and in drug toxicity is relatively meager.

Rosuvastatin pharmacokinetics and pharmacogenetics in white and Asian subjects residing in the same environment.

Abstract: Background Systemic exposure to rosuvastatin had been observed to be approximately 2-fold higher in Japanese subjects living in Japan compared with white subjects in Western Europe or the United States. The organic anion transporting polypeptide 1B1 contributes to the hepatic uptake of rosuvastatin. Polymorphisms in the SLCO1B1 gene can lead to reduced transport function in vitro (T521>C). This study was conducted to determine whether the pharmacokinetic differences between Japanese and white subjects extended to other Asian ethnic groups and to determine whether polymorphisms in the SLCO1B1 gene contribute to any pharmacokinetic differences observed. Methods Rosuvastatin pharmacokinetics was studied in an open-label, parallel-group, single–oral dose (40 mg) study in 36 white, 36 Chinese, 35 Malay, and 35 Asian-Indian subjects living in Singapore, Singapore. Plasma concentrations of rosuvastatin and metabolites were determined by HPLC–mass spectrophotometry. Two SLCO1B1 polymorphisms (A388>G and T521>C) were genotyped. Results Ratios for rosuvastatin area under the plasma concentration–time curve from time 0 to the time of the last quantifiable concentration were 2.31, 1.91, and 1.63 and ratios for maximum plasma concentration were 2.36, 2.00, and 1.68 in Chinese, Malay, and Asian-Indian subjects, respectively, compared with white subjects. Similar increases in exposure to N-desmethyl rosuvastatin and rosuvastatin-lactone were observed. SLCO1B1 genotypes did not account for the observed pharmacokinetic differences between Asians and white subjects. Conclusions Plasma exposure to rosuvastatin and its metabolites was significantly higher in Chinese, Malay, and Asian-Indian subjects compared with white subjects living in the same environment. Clinical Pharmacology & Therapeutics (2005) 78, 330–341; doi: 10.1016/j.clpt.2005.06.013

Polymorphic organic anion transporting polypeptide 1B1 is a major determinant of repaglinide pharmacokinetics.

Abstract: Background and Objective A large interindividual variability exists in the plasma concentrations of repaglinide. Our aim was to investigate possible associations between the pharmacokinetics of repaglinide and single nucleotide polymorphisms (SNPs) in the genes encoding for the drug transporters organic anion transporting polypeptide 1B1 (OATP1B1) (SLCO1B1) and P-glycoprotein (MDR1, ABCB1) and the drug-metabolizing enzymes cytochrome P450 (CYP) 2C8 and CYP3A5. Methods A total of 56 healthy volunteers ingested a single 0.25-mg dose of repaglinide. Plasma repaglinide and blood glucose concentrations were measured for up to 7 hours. All subjects were genotyped for the −11187G>A and 521T>C SNPs in SLCO1B1 and the 3435C>T and 2677G>T/A SNPs in ABCB1, as well as for the CYP2C8*3 (416G>A, 1196A>G), CYP2C8*4 (792C>G), and CYP3A5*3 (6986A>G) alleles. Results The area under the plasma concentration-time curve from time 0 to infinity [AUC(0-∞)] and peak concentration in plasma (Cmax) of repaglinide varied 16.9-fold and 10.7-fold, respectively, between individual subjects. Multiple regression analyses indicated that the SLCO1B1 521T>C SNP and the CYP2C8*3 allele were independent predictors of the AUC(0-∞) and Cmax of repaglinide (adjusted multiple R2 = 45% and 36%, respectively). In subjects with the SLCO1B1 521CC genotype, the AUC(0-∞) of repaglinide was 107% and 188% higher, respectively, than in subjects with the SLCO1B1 521TC or 521TT (reference) genotype (P < .0001). In subjects with the CYP2C8*1/*3 genotype, the AUC(0-∞) and Cmax of repaglinide were 48% and 44% lower, respectively, than in those with the CYP2C8*1/*1 genotype (P < .05). The pharmacokinetics of repaglinide was not associated with the studied ABCB1 SNPs or the CYP3A5*3 allele. The elimination half-life of repaglinide was not associated with any SNP. Only the SLCO1B1 −11187GA genotype was significantly associated with an enhanced effect of repaglinide on blood glucose. Conclusions Genetic polymorphism in SLCO1B1 is a major determinant of interindividual variability in the pharmacokinetics of repaglinide. The effect of SLCO1B1 polymorphism on the pharmacokinetics of repaglinide may be clinically important. Clinical Pharmacology & Therapeutics (2005) 77, 468–478; doi: 10.1016/j.clpt.2005.01.018

High‐dose statins and skeletal muscle metabolism in humans: A randomized, controlled trial

Abstract: Background Myopathy, probably caused by 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibition in skeletal muscle, rarely occurs in patients taking statins. This study was designed to assess the effect of high-dose statin treatment on cholesterol and ubiquinone metabolism and mitochondrial function in human skeletal muscle. Methods Forty-eight patients with hypercholesterolemia (33 men and 15 women) were randomly assigned to receive 80 mg/d of simvastatin (n = 16), 40 mg/d of atorvastatin (n = 16), or placebo (n = 16) for 8 weeks. Plasma samples and muscle biopsy specimens were obtained at baseline and at the end of the follow-up. Results The ratio of plasma lathosterol to cholesterol, a marker of endogenous cholesterol synthesis, decreased significantly by 66% in both statin groups. Muscle campesterol concentrations increased from 21.1 ± 7.1 nmol/g to 41.2 ± 27.0 nmol/g in the simvastatin group and from 22.6 ± 8.6 nmol/g to 40.0 ± 18.7 nmol/g in the atorvastatin group (P = .005, repeated-measurements ANOVA). The muscle ubiquinone concentration was reduced significantly from 39.7 ± 13.6 nmol/g to 26.4 ± 7.9 nmol/g (P = .031, repeated-measurements ANOVA) in the simvastatin group, but no reduction was observed in the atorvastatin or placebo group. Respiratory chain enzyme activities were assessed in 6 patients taking simvastatin with markedly reduced muscle ubiquinone and in matched subjects selected from the atorvastatin (n = 6) and placebo (n = 6) groups. Respiratory chain enzyme and citrate synthase activities were reduced in the patients taking simvastatin. Conclusions High-dose statin treatment leads to changes in the skeletal muscle sterol metabolism. Furthermore, aggressive statin treatment may affect mitochondrial volume. Clinical Pharmacology & Therapeutics (2005) 78, 60–68; doi: 10.1016/j.clpt.2005.03.006

Imaging P-glycoprotein transport activity at the human blood-brain barrier with positron emission tomography.

Abstract: Background Numerous knockout mouse studies have revealed that P-glycoprotein (P-gp) significantly limits drug distribution across the mouse blood-brain barrier (BBB). To determine the importance of P-gp at the human BBB, we developed a state-of-the-art, noninvasive, quantitative imaging technique to measure P-gp activity by use of carbon 11-labeled verapamil as the P-gp substrate and cyclosporine (INN, ciclosporin) as the P-gp inhibitor. Methods In brief, 11C-verapamil (approximately 0.2 mCi/kg) was administered to healthy volunteers (n = 12 [6 women and 6 men]) as an intravenous infusion over a period of approximately 1 minute before and after at least a 1-hour infusion of cyclosporine (2.5 mg · kg−1 · h−1). Arterial blood samples and brain positron emission tomography images were obtained at frequent intervals for 45 minutes. Both blood and plasma radioactivity contents were determined in each verapamil sample. The content of verapamil and its metabolites in the 20- and 45-minute plasma samples was determined by a rapid solid-phase extraction method. The brain uptake of 11C-radioactivity (brain area under the curve [AUCbrain]/blood area under the curve [AUCblood]) was determined in the presence and absence of cyclosporine. Results The AUCbrain/AUCblood ratio of 11C-radioactivity was increased by 88% ± 20% (1.02 ± 0.18 versus 0.55 ± 0.10, P < .001) in the presence of cyclosporine (mean blood concentration, 2.8 ± 0.4 μmol/L) without affecting 11C-verapamil metabolism or plasma protein binding. The corresponding increases for the brain white and gray matter were 84% ± 13% and 84% ± 18%, respectively. Conclusions This is the first time that P-gp activity at the human BBB has been measured. The modest inhibition of human BBB P-gp by cyclosporine has implications for P-gp-based drug interactions at the human BBB. Our method for imaging P-gp activity can be used to identify multidrug-resistant tumors or to determine the contribution of P-gp polymorphism, inhibition, or induction to interindividual variability in drug response. Clinical Pharmacology & Therapeutics (2005) 77, 503–514; doi: 10.1016/j.clpt.2005.01.022

In Vivo Effects of Goldenseal, Kava Kava, Black Cohosh, and Valerian on Human Cytochrome P450 1A2, 2D6, 2E1, and 3A4/5 Phenotypes

Abstract: Objectives Phytochemical-mediated modulation of cytochrome P-450 activity may underlie many herb-drug interactions Single time-point, phenotypic metabolic ratios were used to determine whether long-term supplementation of goldenseal (Hydrastis canadensis), black cohosh (Cimicifuga racemosa), kava kava (Piper methysticum), or valerian (Valeriana officinalis) extracts affected CYP1A2, CYP2D6, CYP2E1, or CYP3A4/5 activity

Implications of CYP2A6 genetic variation for smoking behaviors and nicotine dependence.

Abstract: Nicotine is the primary addictive compound in tobacco smoke. In this review we summarize nicotine dependence and the genetics of smoking in brief before focusing on cytochrome P450 (CYP) 2A6. In humans nicotine is mainly inactivated to cotinine and CYP2A6 mediates approximately 90% of this conversion. Some, but not all, studies suggest that genetic variation in CYP2A6 may play a role in smoking. We review some of the recent findings on the influence of CYP2A6 genetic polymorphisms on nicotine kinetics, smoking behaviors, and how the gene appears to exert differential effects during various stages of smoking (eg, initiation, conversion to dependence, amount smoked during dependence, and quitting). These new findings will be put in the context of the discrepancies found in the literature. Implications of these recent findings on current and novel treatment approaches for smoking cessation and tobacco-related lung cancer will also be discussed.

Methadone enantiomer plasma levels, CYP2B6, CYP2C19, and CYP2C9 genotypes, and response to treatment

Abstract: Background and Objective Recent in vitro studies have suggested an important role of cytochrome P450 (CYP) 2B6 and CYP2C19 in methadone metabolism. We aimed to determine the influence of CYP2B6, CYP2C9, and CYP2C19 genetic polymorphism on methadone pharmacokinetics and on the response to treatment. Methods We included 209 patients in methadone maintenance treatment on the basis of their response to treatment and their daily methadone dose. Patients were genotyped for CYP2B6, CYP2C9, and CYP2C19. Steady-state trough and peak (R)-, (S)-, and (R,S)-plasma levels and peak-to-trough plasma level ratios were measured. Results CYP2B6 genotype influences (S)-methadone and, to a lesser extent, (R)-methadone plasma levels, with the median trough (S)-methadone plasma levels being 105, 122, and 209 ng · kg/mL · mg for the noncarriers of allele *6, heterozygous carriers, and homozygous carriers (*6/*6), respectively (P = .0004). CYP2C9 and CYP2C19 genotypes do not influence methadone plasma levels. Lower peak and trough plasma levels of methadone and higher peak-to-trough ratios were measured in patients considered as nonresponders [median (R,S)-methadone trough plasma levels of 183 and 249 ng · kg/mL · mg (P = .0004) and median peak-to-trough ratios of 1.82 and 1.58 for high-dose nonresponders and high-dose responders, respectively (P = .0003)]. Conclusion Although CYP2B6 influences (S)-methadone plasma levels, given that only (R)-methadone contributes to the opioid effect of this drug, a major influence of CYP2B6 genotype on response to treatment is unlikely and has not been shown in this study. Lower plasma levels of methadone in nonresponders, suggesting a higher clearance, and higher peak-to-trough ratios, suggesting a shorter elimination half-life, are in agreement with the usual clinical measures taken for such patients, which are to increase methadone dosages and to split the daily dose into several intakes. Clinical Pharmacology & Therapeutics (2005) 78, 593–604; doi: 10.1016/j.clpt.2005.08.011

Cytochrome P450 2C8: substrates, inhibitors, pharmacogenetics, and clinical relevance.

Abstract: Cytochrome P450 (CYP) 2C8 [corrected] has been a relatively neglected member of the human CYP2C family. Over the period from 2000 through 2003, PubMed searches with the key word CYP2C8 returned only 10% to 15% of the citations obtained for all of the CYP2C enzymes combined. However, in the past year a crystal structure for CYP2C8 has been described, new inhibitors and probe substrates for the enzyme have been in development, the first case study was published linking CYP2C8 genetic polymorphisms to a disease state, and there has been an increasing awareness of the role that CYP2C8 plays in the disposition of therapeutic agents, especially from the pharmacogenetic and drug-drug interaction perspectives. This report discusses baseline characteristics of the enzyme and summarizes recent developments in these areas and their clinical relevance.

Cyclosporine markedly raises the plasma concentrations of repaglinide.

Abstract: Background and Objective Repaglinide is an antidiabetic drug metabolized by cytochrome P450 (CYP) 2C8 and 3A4, and it appears to be a substrate of the hepatic uptake transporter organic anion transporting polypeptide 1B1 (OATP1B1). We studied the effects of cyclosporine (INN, ciclosporin), an inhibitor of CYP3A4 and OATP1B1, on the pharmacokinetics and pharmacodynamics of repaglinide. Methods In a randomized crossover study, 12 healthy volunteers took 100 mg cyclosporine or placebo orally at 8 PM on day 1 and at 8 AM on day 2. At 9 AM on day 2, they ingested a single 0.25-mg dose of repaglinide. Concentrations of plasma and urine repaglinide and its metabolites (M), blood cyclosporine, and blood glucose were measured for 12 hours. The subjects were genotyped for single-nucleotide polymorphisms in CYP2C8, CYP3A5, SLCO1B1 (encoding OATP1B1), and ABCB1 (P-glycoprotein). The effect of cyclosporine on repaglinide metabolism was studied in human liver microsomes in vitro. Results During the cyclosporine phase, the mean peak repaglinide plasma concentration was 175% (range, 56%-365%; P = .013) and the total area under the plasma concentration–time curve [AUC(0-∞)] was 244% (range, 119%-533%; P < .001) of that in the placebo phase. The amount of unchanged repaglinide and its metabolites M2 and M4 excreted in urine were raised 2.7-fold, 7.5-fold, and 5.0-fold, respectively, by cyclosporine (P < .001). The amount of M1 excreted in urine remained unchanged, but cyclosporine reduced the ratio of M1 to repaglinide by 62% (P < .001). Cyclosporine had no significant effect on the elimination half-life or renal clearance of repaglinide. Although the mean blood glucose–lowering effect of repaglinide was unaffected in this low-dose study with frequent carbohydrate intake, the subject with the greatest pharmacokinetic interaction had the greatest increase in blood glucose–lowering effect. The effect of cyclosporine on repaglinide AUC(0-∞) was 42% lower in subjects with the SLCO1B1 521TC genotype than in subjects with the 521TT (reference) genotype (P = .047). In vitro, cyclosporine inhibited the formation of M1 (IC50 [concentration of inhibitor to cause 50% inhibition of original enzyme activity], 0.2 μmol/L) and M2 (IC50, 4.3 μmol/L) but had no effect on M4. Conclusions Cyclosporine raised the plasma concentrations of repaglinide, probably by inhibiting its CYP3A4-catalyzed biotransformation and OATP1B1-mediated hepatic uptake. Coadministration of cyclosporine may enhance the blood glucose–lowering effect of repaglinide and increase the risk of hypoglycemia. Clinical Pharmacology & Therapeutics (2005) 78, 388–399; doi: 10.1016/j.clpt.2005.07.005

Safety and immunogenicity of a nicotine conjugate vaccine in current smokers.

Abstract: Immunotherapy is a novel potential treatment for nicotine addiction. The aim of this study was to assess the safety and immunogenicity of a nicotine conjugate vaccine, NicVAX, and its effects on smoking behavior. Smokers (N = 68) were recruited for a noncessation treatment study and assigned to 1 of 3 doses of the nicotine vaccine (50, 100, or 200 μg) or placebo. They were injected on days 0, 28, 56, and 182 and monitored for a period of 38 weeks. Results showed that the nicotine vaccine was safe and well tolerated. Vaccine immunogenicity was dose-related (P<.001), with the highest dose eliciting antibody concentrations within the anticipated range of efficacy. There was no evidence of compensatory smoking or precipitation of nicotine withdrawal with the nicotine vaccine. The 30-day abstinence rate was significantly different across the 4 doses (P = .02), with the highest rate of abstinence occurring with 200 μg. The nicotine vaccine appears to be a promising medication for tobacco dependence. Clinical Pharmacology & Therapeutics (2005) 78, 456–467; doi: 10.1016/j.clpt.2005.08.007

The role of common variants of ABCB1, CYP3A4, and CYP3A5 genes in lipid-lowering efficacy and safety of simvastatin treatment.

Abstract: Objective Our objective was to investigate the interactions between common polymorphisms in ABCB1, CYP3A4, and CYP3A5 genes and the lipid-lowering efficacy and safety of the 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor simvastatin. Methods One hundred sixteen hypercholesterolemic patients were prospectively screened by physical examination, medical history, and clinical laboratory evaluation and were included in this study. Subjects entering the study were treated with 20 mg/d simvastatin. Plasma lipid and lipoprotein levels were measured before treatment, after 2 months of treatment, and after 6 months of treatment. Ninety-nine patients completed the 6-month follow-up and were included in the association analysis for treatment efficacy. Seventeen subjects who had adverse drug reactions (ADRs) to simvastatin (ADR group) could not complete the 6-month follow-up and were included in the association analyses for safety. Myalgia was observed in 15 of 17 subjects and was the only ADR included in the association analyses, but other common ADRs were also observed. Myalgia was defined as proximal or diffuse muscle pain, tenderness, or weakness, or both pain and weakness, with normal or slightly increased serum creatine phosphokinase levels. ABCB1 (1236C>T, 2677G>A/T, and 3435C>T), CYP3A4 (−392A>G), and CYP3A5 (6986A>G) allele variants were determined by polymerase chain reaction and restriction mapping. Results After adjustment for covariates, carriers of the ABCB1 1236T variant allele had a greater reduction in total cholesterol and low-density lipoprotein cholesterol with simvastatin treatment, as compared with homozygotes with the wild-type allele (−29.0% [95% confidence interval (CI), −25.9 to −32.5] versus −24.2% [95% CI, −19.0 to −29.3] [P = .042] and −39.6% [95% CI, −35.8 to −44.0] versus −33.8% [95% CI, −27.4 to −40.2] [P = .042], respectively). Similar results were observed for the 2677G>A/T polymorphism and haplotype data. The 1236T, 2677non-G, and 3435T alleles were less frequent in ADR cases than in the non-ADR group (P G and CYP3A5*3 allele variants and the efficacy or tolerability of simvastatin. Conclusions Our data suggest an association of ABCB1 gene polymorphisms and the efficacy and safety of simvastatin. Clinical Pharmacology & Therapeutics (2005) 78, 551–558; doi: 10.1016/j.clpt.2005.08.003

The impact of uridine diphosphate-glucuronosyltransferase 1A9 (UGT1A9) gene promoter region single-nucleotide polymorphisms T-275A and C-2152T on early mycophenolic acid dose-interval exposure in de novo renal allograft recipients.

Abstract: Background Mycophenolic acid (MPA), an effective immunosuppressive drug used in renal transplantation, is extensively glucuronidated by several uridine diphosphate–glucuronosyltransferases (UGTs) into an inactive 7-O-glucuronide and, to a lesser extent, into a pharmacologically active acyl-glucuronide. Experiments using human liver microsomes have shown that T—275A and C—2152T single-nucleotide polymorphisms (SNPs) of the UGT1A9 promoter region are associated with higher hepatic expression of UGT1A9 and increased in vitro glucuronidation activity for MPA. Methods The distribution of UGT1A9 promoter region T—275A and C—2152T SNPs and the less frequent UGT1A9*3 coding region mutation, which results in decreased in vitro activity, was determined in 95 de novo renal recipients. The impact of these UGT1A9 SNPs on early clinical MPA pharmacokinetics was evaluated. Results Only in patients taking 2 g mycophenolate mofetil daily was a decreased MPA exposure observed in those who carried either the T—275A or the C—2152T polymorphism (or both) compared with those who did not (area under concentration–time curve [AUC] from 0 to 12 hours, 31.7 ± 17.6 mg · h/L versus 63.6 ± 30.9 mg · h/L [P = .009]; predose trough plasma concentration, 1.23 ± 1.25 mg/L versus 2.84 ± 1.64 mg/L [P = .04]). The partial MPA AUC from 6 to 12 hours (AUC6–12)—an estimate of MPA enterohepatic recirculation—and the ratio between partial MPA AUC6–12 and dose–interval AUC from 0 to 12 hours decreased when either or both UGT1A9 promoter region SNPs were present (AUC6–12, 6.2 ± 5.4 mg · h/L versus 21.5 ± 14.9 mg · h/L [P = .002]; ratio, 18.4% ± 7.8% versus 31.7% ± 8.8% [P = .002]). Conclusion The T—275A and C—2152T SNPs of the UGT1A9 gene promoter are associated with significantly lower MPA exposure in renal recipients treated with 2 g mycophenolate mofetil daily, and part of this effect is caused by interruption of enterohepatic recirculation of MPA. Clinical Pharmacology & Therapeutics (2005) 78, 351–361; doi: 10.1016/j.clpt.2005.06.007

Effect of grapefruit juice volume on the reduction of fexofenadine bioavailability: possible role of organic anion transporting polypeptides.

Abstract: Objectives The purpose of this study was to elucidate the potential clinical relevance and mechanism(s) of action of 2 different volumes of grapefruit juice on the reduction of bioavailability of fexofenadine, a substrate of organic anion transporting polypeptides. Methods Grapefruit juice or water at normal (300 mL) or high (1200 mL) volume was ingested concomitantly with 120 mg fexofenadine by 12 healthy volunteers in a randomized 4-way crossover study, and fexofenadine pharmacokinetics were determined over a period of 8 hours. Results The 300-mL volume of grapefruit juice decreased the mean area under the plasma drug concentration-time curve (AUC) and the peak plasma drug concentration of fexofenadine to 58% (P < .001) and 53% (P < .001), respectively, of those with the corresponding volume of water, and 1200 mL grapefruit juice reduced these parameters to 36% (P < .001) and 33% (P < .001), respectively, of those with the corresponding volume of water. The 300-mL volume of grapefruit juice diminished the AUC of fexofenadine variably among individuals. This decline correlated with baseline AUC of fexofenadine with water at equivalent volume (r2 = 0.97, P < .0001). The 1200-mL volume of grapefruit juice decreased the AUC of fexofenadine more than the 300-mL volume of grapefruit juice compared with the corresponding volume of water in each subject by a constant amount. Grapefruit juice, 300 mL and 1200 mL, reduced the coefficient of variation of the AUC of fexofenadine by 2-fold compared with that with a matching volume of water. Conclusions Grapefruit juice at a commonly consumed volume diminished the oral bioavailability of fexofenadine sufficiently to be pertinent clinically, likely by direct inhibition of uptake by intestinal organic anion transporting polypeptide A (OATP-A; new nomenclature, OATP1A2). A much higher volume caused an additional modest effect, possibly from reduced intestinal concentration and transit time of fexofenadine. This food-drug interaction appears to be novel and may be relevant to other fruit juices and drugs. Clinical Pharmacology & Therapeutics (2005) 77, 170–177; doi: 10.1016/j.clpt.2004.10.005

BIBN4096BS antagonizes human α-calcitonin gene related peptide-induced headache and extracerebral artery dilatation

Abstract: Background and Objective Calcitonin gene-related peptide (CGRP) plays a pivotal role in migraine pathogenesis. BIBN4096BS is the first CGRP receptor antagonist available for human studies, and its efficacy in the acute treatment of migraine has been demonstrated. We investigated the ability of BIBN4096BS to inhibit human αCGRP (h-αCGRP)–induced headache and cerebral hemodynamic changes in healthy volunteers. Methods Ten healthy volunteers completed this double-blind, placebo-controlled crossover study with 2.5 mg BIBN4096BS and placebo as pretreatments before a 20-minute intravenous infusion of h-αCGRP (1.5 Μg/min). Transcranial Doppler ultrasonography was used to measure blood flow velocity in the middle cerebral artery (MCA); regional and global cerebral blood flow (CBF) was measured by xenon 133 inhalation single-photon emission computed tomography. The temporal and radial artery diameter was measured by high-frequency ultrasound. Systemic hemodynamics, end-tidal partial pressure of carbon dioxide (PETCO2), and headache were monitored. Results Of the 10 volunteers, 6 had a CGRP-induced headache during the in-hospital phase after placebo pretreatment but none after BIBN4096BS (P = .031). BIBN4096BS did not affect changes in the diameter of the MCA or changes in CBF induced by h-αCGRP. Vasodilatation of the extracranial arteries was, however, significantly inhibited (P < .001 for temporal artery and P = .001 for radial artery). Conclusions These results show that BIBN4096BS effectively prevents CGRP-induced headache and extracerebral vasodilatation but does not significantly affect the induced cerebral hemodynamic changes. Clinical Pharmacology & Therapeutics (2005) 77, 202–213; doi: 10.1016/j.clpt.2004.10.001

Effect of OATP1B1 (SLCO1B1) variant alleles on the pharmacokinetics of pitavastatin in healthy volunteers

Abstract: Background Pitavastatin is a potent, newly developed 3-hydroxy-3-methylglutaryl–coenzyme A reductase inhibitor for the treatment of hyperlipidemia. We characterized the effects of organic anion transporting polypeptide 1B1 (OATP1B1) alleles *1a, *1b, and *15 on the pharmacokinetics of pitavastatin. Methods Twenty–four healthy Korean volunteers who had previously participated in a pharmacokinetic study of pitavastatin (single oral dose, 1–8 mg) were further investigated. Subjects were grouped according to OATP1B1 genotype. Dose–normalized area under the plasma concentration–time curve (AUC) and peak plasma concentration (Cmax) values were analyzed, because different dosages were administered to subjects, whereas the pharmacokinetics showed linear characteristics. Results Dose-normalized pitavastatin AUCs for *1b/*1b (group 1), *1a/*1a or *1a/*1b (group 2), and *1a/*15 or *1b/*15 (group 3) were 38.8 ± 13.3, 54.4 ± 12.4, and 68.1 ± 16.3 ng · h · mL−1 · mg−1 (mean ± SD), respectively, with significant differences between all 3 groups (P = .008) and between subjects carrying and those not carrying the *15 allele (P = .004). Dose-normalized pitavastatin Cmax values were 13.2 ± 3.3, 18.2 ± 5.7, and 29.4 ± 9.6 ng · mL−1 · mg−1 in groups 1, 2, and 3, respectively, and also showed significant differences (P = .003) in a manner similar to that shown by AUC. No significant differences were found between the genotype groups in terms of dose-normalized AUC or Cmax values of pitavastatin lactone. Conclusion OATP1B1 variant haplotypes were found to have a significant effect on the pharmacokinetics of pitavastatin. These results suggest that the *15 allele is associated with decreased pitavastatin uptake from blood into hepatocytes and that OATP1B1 genetic polymorphisms have no effect on the pharmacokinetics of pitavastatin lactone. Clinical Pharmacology & Therapeutics (2005) 78, 342–350; doi: 10.1016/j.clpt.2005.07.003

Effect of clopidogrel and ticlopidine on cytochrome P450 2B6 activity as measured by bupropion hydroxylation.

Abstract: Objective Our objective was to study the effect of the antiplatelet agents clopidogrel and ticlopidine on bupropion (INN, amfebutamone) hydroxylation, a probe reaction for cytochrome P450 (CYP) 2B6 activity. Methods Twelve healthy male volunteers took a single 150-mg oral dose of bupropion either alone or after pretreatment with 75 mg clopidogrel once daily or 250 mg ticlopidine twice daily for 4 days. On day 4, a single 150-mg oral dose of bupropion was administered. Plasma concentrations of bupropion and its CYP2B6-catalyzed metabolite, hydroxybupropion, were measured for up to 72 hours. Results The mean area under the plasma concentration-time curve (AUC) of hydroxybupropion calculated from time 0 to infinity was reduced by 52% (P = .001; 95% confidence interval [CI], 39% to 66%) by clopidogrel and by 84% (P < .0001; 95% CI, 73% to 94%) by ticlopidine. Clopidogrel reduced the AUC ratio of hydroxybupropion over bupropion by 68% (P = .002; 95% CI, 58% to 77%) and ticlopidine by 90% (P = .001; 95% CI, 85% to 96%). The AUC of bupropion was increased by 60% (P = .02; 95% CI, 21% to 98%) and by 85% (P < .0001; 95% CI, 48% to 85%) with clopidogrel and ticlopidine, respectively. Conclusions Both clopidogrel and ticlopidine significantly inhibited the CYP2B6-catalyzed bupropion hydroxylation. Patients receiving either clopidogrel or ticlopidine are likely to require dose adjustments when treated with drugs primarily metabolized by CYP2B6. Clinical Pharmacology & Therapeutics (2005) 77, 553–559; doi: 10.1016/j.clpt.2005.02.010

Different effects of three transporting inhibitors, verapamil, cimetidine, and probenecid, on fexofenadine pharmacokinetics.

Abstract: Objective Fexofenadine is a substrate of P-glycoprotein and organic anion transporting polypeptides. The aim of this study was to compare the inhibitory effects of different transporting inhibitors on fexofenadine pharmacokinetics. Methods Twelve male volunteers took a single oral 120-mg dose of fexofenadine. Thereafter three 6-day courses of either 240 mg verapamil, an inhibitor of P-glycoprotein, 800 mg cimetidine, an inhibitor of organic cation transporters, or 2000 mg probenecid, an inhibitor of organic anion transporting polypeptides, were administered on a daily basis in a randomized fashion with the same dose of fexofenadine on day 6. Plasma and urine concentrations of fexofenadine were monitored up to 48 hours after dosing. Results Verapamil treatment significantly increased the peak plasma concentration by 2.9-fold (95% confidence interval [CI], 2.4- to 4.0-fold) and the area under the plasma concentration-time curve from time 0 to infinity [AUC(0-∞)] of fexofenadine by 2.5-fold (95% CI, 2.0- to 3.3-fold). No changes in any plasma pharmacokinetic parameters of fexofenadine were found during cimetidine treatment. AUC(0-∞) was slightly but significantly increased during probenecid treatment by 1.5-fold (95% CI, 1.1- to 2.4-fold). Renal clearance of fexofenadine was significantly decreased during cimetidine treatment to 61% (95% CI, 50%-98%) and during probenecid treatment to 27% (95% CI, 20%-58%) but not during verapamil treatment. Conclusion This study suggests that verapamil increases fexofenadine exposure probably because of an increase in bioavailability through P-glycoprotein inhibition and that probenecid slightly increases the area under the plasma concentration-time curve of fexofenadine as a result of a pronounced reduction in renal clearance. However, it may be difficult to explain these interactions by simple inhibitory mechanisms on target transporters. Clinical Pharmacology & Therapeutics (2005) 77, 17–23; doi: 10.1016/j.clpt.2004.08.026

Imatinib induces hypothyroidism in patients receiving levothyroxine.

Abstract: Interactions of imatinib with other drugs have been scarcely reported We report a previously unknown effect of imatinib on levothyroxine therapy Eleven patients (1 with gastrointestinal stromal tumor and 10 with medullary thyroid carcinoma) received imatinib Eight had undergone thyroidectomy and used levothyroxine, and 3 had the thyroid in situ Thyroid function was measured before, during, and within 2 weeks after any change in either imatinib or levothyroxine dosage We observed symptoms of hypothyroidism in all patients who had undergone thyroidectomy, whereas patients with the thyroid in situ remained clinically and biochemically euthyroid On average, thyrotropin (INN, thyrotrophin) levels increased to 384% +/- 228% of the upper limit in patients after thyroidectomy, whereas free thyroxine (fT4) and free tri-iodothyronine (fT3) values remained within the reference range (59% +/- 17% of the upper limit for fT4 and 63% +/- 4% of the upper limit for fT3) Clinicians should be aware that hypothyroid subjects receiving imatinib have a high likelihood for increased levothyroxine replacement and should be closely monitored for elevations in thyrotropin indicating worsening hypothyroidism

Rifampin markedly decreases and gemfibrozil increases the plasma concentrations of atorvastatin and its metabolites.

Abstract: Background The pharmacokinetic interactions of the widely used statin atorvastatin with fibrates and enzyme inducers are not known. Therefore we studied the effects of rifampin (INN, rifampicin) and gemfibrozil on the pharmacokinetics of atorvastatin. Methods Two randomized crossover studies were conducted. In study 1, 10 healthy volunteers took 600 mg rifampin or placebo once daily for 5 days. On day 6, they ingested a single 40-mg dose of atorvastatin. In study 2, 10 healthy volunteers took 600 mg gemfibrozil or placebo twice daily for 5 days. On day 3, they ingested a single 20-mg dose of atorvastatin. Plasma concentrations of atorvastatin (in nanograms per milliliter) and its metabolites (in arbitrary units) were measured by liquid chromatography-tandem mass spectrometry up to 48 to 72 hours after dosing. Results Rifampin reduced the total area under the plasma concentration-time curve (AUC) of unchanged atorvastatin (acid) by 80% (95% confidence interval [CI], 73% to 84%; P < .001), that of the active metabolites 2-hydroxyatorvastatin acid by 43% (95% CI, 29% to 51%; P < .001) and 4-hydroxyatorvastatin acid by 81% (95% CI, 74% to 84%; P < .001), and that of their lactones by 93% (95% CI, 90% to 95%), by 61% (95% CI, 50% to 69%), and by 76% (95% CI, 70% to 81%), respectively (P < .001). The peak plasma concentration of 2-hydroxyatorvastatin acid was increased by 68% (95% CI, 21% to 127%; P = .005) by rifampin. Rifampin shortened (P < .001) the half-lives of atorvastatin (by 74%; 95% CI, 67% to 81%) and its metabolites, for example, atorvastatin lactone (by 82%; 95% CI, 80% to 85%) and 2-hydroxyatorvastatin acid (by 70%; 95% CI, 64% to 78%). Gemfibrozil increased the AUC of atorvastatin (by 24%; 95% CI, −1% to 50%; P = .059), 2-hydroxyatorvastatin acid (by 51%; 95% CI, 28% to 70%; P < .001) and its lactone (by 29%; 95% CI, 13% to 53%; P = .003), and 4-hydroxyatorvastatin acid (by 82%; 95% CI, 60% to 126%; P < .001) and its lactone (by 28%; 95% CI, 15% to 51%; P = .001). The half-lives of atorvastatin and its lactone metabolites were slightly shortened by gemfibrozil (P < .05). Conclusions Rifampin markedly decreases and gemfibrozil moderately increases the plasma concentrations of atorvastatin and its metabolites. It is advisable to increase the dosage of atorvastatin and preferable to administer it in the evening to guarantee adequate concentrations during the nighttime rapid cholesterol synthesis when rifampin or other potent inducers of cytochrome P450 3A4 are coadministered. Care is warranted, and only low doses of atorvastatin should be used if coadministration with gemfibrozil is needed. Clinical Pharmacology & Therapeutics (2005) 78, 154–167; doi: 10.1016/j.clpt.2005.04.007

Grapefruit juice ingestion significantly reduces talinolol bioavailability.

Abstract: Objectives Our objectives were to evaluate the effect of single and repeated grapefruit juice ingestion relative to water on the oral pharmacokinetics of the nonmetabolized and P-glycoprotein-transported drug talinolol in humans and to assess the potential impact of grapefruit juice ingestion on P-glycoprotein and intestinal uptake transporters. Methods The oral pharmacokinetics of 50 mg talinolol was determined with water, with 1 glass of grapefruit juice (300 mL), and after 6 days of repeated grapefruit juice ingestion (900 mL/d) in 24 healthy white volunteers. MDR1 messenger ribonucleic acid and P-glycoprotein levels were measured in duodenal biopsy specimens obtained from 3 individuals before and after ingestion of grapefruit juice. Three commonly occurring polymorphisms in the MDR1 gene were also assessed. Results A single glass of grapefruit juice decreased the talinolol area under the serum concentration-time curve (AUC), peak serum drug concentration (Cmax), and urinary excretion values to 56% (P < .001), 57% (P < .001), and 56% (P < .001), respectively, of those with water. Repeated ingestion of grapefruit juice had a similar effect (44% to 65% reduction; P < .01). Single or repeated juice ingestion did not affect renal clearance, elimination half-life, or time to reach Cmax (tmax). MDR1 messenger ribonucleic acid and P-glycoprotein levels in duodenal biopsy specimens were not affected by grapefruit juice. MDR1 genotypes (C1236T, G2677T/A, and C3435T) were not associated with altered talinolol pharmacokinetics. Conclusion Because both single and repeated ingestion of grapefruit juice lowered rather than increased talinolol AUC, our findings suggest that constituents in grapefruit juice preferentially inhibited an intestinal uptake process rather than P-glycoprotein. Moreover, grapefruit juice did not alter intestinal P-glycoprotein expression. Clinical Pharmacology & Therapeutics (2005) 77, 291–301; doi: 10.1016/j.clpt.2004.11.111

Drug-metabolizing enzymes: evidence for clinical utility of pharmacogenomic tests.

Abstract: D h n a p F s i ( t m s p t n t n d f r C i i l rom Clinical Pharmacology, AstraZeneca, Mölndal; Division of Clinical Pharmacology, Indiana University School of Medicine, Indianapolis; Department of Biology (Galton Lab), University College London, London; Center for Drug Evaluation and Research, Food and Drug Administration, Rockville; School of Pharmacy, University of California San Francisco, San Francisco; Pharmacogenomics, Pfizer Global Research and Development, Groton; University of Chicago, Chicago; and University of Washington, Seattle. his commentary was based on presentations made at a Food and Drug Administration/Johns Hopkins University/Pharmaceutical Research and Manufacturers of America educational workshop, September 13, 2004, Rockville, Md. he views presented in this article do not necessarily reflect those of the Food and Drug Administration. eceived for publication June 6, 2005; accepted Aug 12, 2005. eprint requests: Shiew-Mei Huang, PhD, FCP, Deputy Office Director for Science, Office of Clinical Pharmacology and Biopharmaceutics, Center for Drug Evaluation and Research, Food and Drug Administration, 10903 New Hampshire Ave, Silver Spring, MD 20993-0002. -mail: huangs@cder.fda.gov lin Pharmacol Ther 2005;78:559-81. 009-9236/$30.00 opyright © 2005 by the American Society for Clinical Pharmacology and Therapeutics. u oi:10.1016/j.clpt.2005.08.013 rug research and development have recently been ampered by high costs, notably high investigational ew drug (IND) failure rates and multiple new drug pplication (NDA) review cycles. The number of aplications for new molecular entities submitted to the ood and Drug Administration (FDA) has declined teadily. As part of the FDA’s strategic plan, the FDA s developing standards to apply emerging technologies eg, pharmacogenomics) to provide effective translaion of new scientific discoveries into safe and effective edical products. A recent document by the FDA tressed the following: “The product development roblems we are seeing today can be addressed, in part, hrough an aggressive, collaborative effort to create a ew generation of performance standards and predicive tools. The new tools will match and move forward ew scientific innovations and will build on knowledge elivered by recent advances in science, such as bioinormatics, genomics, imaging technologies, and mateials science.” There are various initiatives within the enter for Drug Evaluation and Research to address ssues in the area of pharmacogenomics. A guidance for ndustry on genomic data submission has been pubished. The guidance was intended to encourage volntary genomic data submission by sponsors using

Opposite effects of short-term and long-term St John's wort intake on voriconazole pharmacokinetics.

Abstract: Objectives Constituents of St John's wort (SJW) in vivo induce the cytochrome P450 (CYP) isozymes 3A4, 2C9, and 2C19 but in vitro were shown to inhibit them. This study investigates both short- and long-term effects of SJW on the antifungal voriconazole, which is metabolized by these enzymes. Methods In a controlled, open-label study, single oral doses of 400 mg voriconazole were administered to 16 healthy men stratified for CYP2C19 genotype before and on day 1 and day 15 of concomitant SJW intake (300 mg LI 160 3 times daily). Plasma and urine concentrations of voriconazole were determined by liquid chromatography with mass-spectrometric detection. Results During the initial 10 hours of the first day of SJW administration, the area under the voriconazole plasma concentration–time curve was increased by 22% compared with control (15.5 ± 6.84 h · μg/mL versus 12.7 ± 4.16 h · μg/mL, P = .02). After 15 days of SJW intake, the area under the plasma concentration–time curve from hour 0 to infinity was reduced by 59% compared with control (9.63 ± 6.03 h · μg/mL versus 23.5 ± 15.6 h · μg/mL, P = .0004), with a corresponding increase in oral voriconazole clearance (CL/F) from 390 ± 192 to 952 ± 524 mL/min (P = .0004). The baseline CL/F of voriconazole and the absolute increase in CL/F were smaller in carriers of 1 or 2 deficient CYP2C19*2 alleles compared with wild-type individuals (P < .03). Conclusions Coadministration of SJW leads to a short-term but clinically irrelevant increase followed by a prolonged extensive reduction in voriconazole exposure. SJW might put CYP2C19 wild-type individuals at highest risk for potential voriconazole treatment failure. Clinical Pharmacology & Therapeutics (2005) 78, 25–33; doi: 10.1016/j.clpt.2005.01.024

In vitro cytochrome P450 inhibition data and the prediction of drug‐drug interactions: Qualitative relationships, quantitative predictions, and the rank‐order approach

Abstract: q s Drug-drug interactions (DDIs) represent a serious roblem in clinical practice. However, with knowledge ained over the past 15 years on the human drugetabolizing enzymes, a better understanding of the nderlying mechanisms behind many of the pharmacoinetic DDIs has been obtained. Previously, studies of DIs for new drugs were carried out empirically or ere gained through random clinical observations. In he past combinations of drugs chosen for investigation f DDIs were selected on the basis of the potential for ntroduction of toxicity of a drug with a narrow theraeutic index (eg, digoxin, theophylline, and warfarin) r if there was frequent coprescription with another gent for a given condition. However, with an increased nderstanding of drug-metabolizing enzymes and their oles in the metabolism of specific drugs, it is possible o apply a more mechanistic approach to assessing DIs. In particular, the possibility of extrapolation of esults of clinical DDI studies with 1 drug known to be leared by a particular drug-metabolizing enzyme to ther drugs that are cleared by that same enzyme is ttractive.

Functional role of P-glycoprotein in the human blood-placental barrier.

Abstract: Objective In vitro and animal experiments suggest that P-glycoprotein forms a functional barrier between maternal and fetal blood circulation in the placenta, thus protecting the fetus from exposure to xenobiotics during pregnancy. In this study we aimed to characterize the role of P-glycoprotein in the blood-placental barrier by use of dually perfused human placenta. Methods Twenty-eight human placentas were obtained after delivery, and both the maternal side and the fetal side were perfused for 2 hours. Saquinavir was used as a probe drug for P-glycoprotein-dependent active transfer, and PSC833 (valspodar) or GG918 was used as an inhibitor of P-glycoprotein function in a maternal-to-fetal and fetal-to-maternal perfusion setting. Genotyping for ABCB1 (C3435T and G2677A/T) polymorphism and quantification of P-glycoprotein expression were done for each placenta. Results The fetal-to-maternal transfer of saquinavir was 108-fold higher (P = .003) compared with transfer from the maternal to the fetal direction. Preperfusion with PSC833 increased the placental transfer of saquinavir by 7.9-fold (P < .001), and preperfusion with GG918 increased it by 6.2-fold (P < .001). The end-perfusion transfer (percentage) of saquinavir at 120 minutes was 11-fold (P < .001) and 6-fold (P < .001) higher in placentas preperfused with PSC833 and GG918, respectively, compared with control. However, PSC833 had no effect on the transfer of saquinavir from the fetal to the maternal direction (P = .79). P-glycoprotein expression was correlated with the PSC833-induced change in the saquinavir transfer (r = 0.75, P = .086). ABCB1 polymorphism did not affect the PSC833- or GG918-induced change in the saquinavir transfer. Conclusions P-glycoprotein has a major functional role in the human blood-placental barrier but a negligible role in the removal of substances from the fetal circulation to maternal blood. Pharmacologic blockade of P-glycoprotein function can lead to disruption of the blood-placental barrier and increase the transfer of P-glycoprotein substrates to the fetal side by several-fold, which may be a noteworthy mechanism for teratogenicity. Clinical Pharmacology & Therapeutics (2005) 78, 123–131; doi: 10.1016/j.clpt.2005.04.014

Genotype-phenotype associations of cytochrome P450 3A4 and 3A5 polymorphism with midazolam clearance in vivo.

Abstract: The molecular basis for the wide interindividual variability of cytochrome P450 (CYP) 3A metabolic activity was studied in vivo at a genetic level. A single oral dose of midazolam was administered to 26 healthy subjects. The variability in midazolam oral clearance was 11-fold. No differences in midazolam oral clearance related to gender or ethnicity were observed. Selective sequencing of CYP3A4 and CYP3A5 genes revealed 18 single nucleotide polymorphisms (SNPs), including 8 novel CYP3A4 SNPs. Thirteen novel CYP3A4 haplotypes, 2 novel CYP3A5 haplotypes, and 1 major novel multigene haplotype ( CYP3A4*VI - CYP3A5*3A ) were also identified. No significant genotype-phenotype or haplotype-phenotype associations were found for any of the SNPs or haplotypes studied, including CYP3A4*1B , CYP3A5*3 , and CYP3A5*6 , even when ethnicity was considered. The only exceptions were the haplotype CYP3A4*VI and the multigene haplotype CYP3A4*VI - CYP3A5*3A . The carriers of the haplotype CYP3A4*VI had a 1.8-fold higher clearance of midazolam in black subjects (ANOVA on ranks, P = .028) compared with other individuals, and the carriers of the multigene haplotype CYP3A4*VI - CYP3A5*3A had a 1.7-fold higher clearance in the entire population (ANOVA on ranks, P = .012). In conclusion, these results indicate that the genetic variants identified so far in the CYP3A4 and CYP3A5 genes have only a limited impact on CYP3A-mediated drug metabolism in vivo.

Impact of CYP2D6 genotype on adverse effects during treatment with metoprolol: A prospective clinical study

Abstract: Objective 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. Methods 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 α-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. Results 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/α–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]). Conclusions 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. Clinical Pharmacology & Therapeutics (2005) 78, 378–387; doi: 10.1016/j.clpt.2005.07.004