Showing papers in "Clinical Pharmacology & Therapeutics in 2004"

Polymorphisms in human MDR1 (P-glycoprotein): recent advances and clinical relevance.

Abstract: Drug transporters are increasingly recognized to be important to drug disposition and response. P-glycoprotein, the encoded product of the human MDR1 (ABCB1) gene, is of particular clinical relevance in that this transporter has broad substrate specificity, including a variety of structurally divergent drugs in clinical use today. Moreover, expression of this efflux transporter in certain tissue compartments such as the gastrointestinal tract and brain capillary endothelial cells limits oral absorption and central nervous system entry of many drugs. Recently, a number of single-nucleotide polymorphisms (SNPs) in MDR1 have been identified. An increasing number of studies have also implicated certain commonly occurring SNPs in MDR1 in problems including altered drug levels and host susceptibility to diseases such as Parkinson's disease, inflammatory bowel disease, refractory seizures, and CD4 cell recovery during human immunodeficiency virus therapy. However, in many such cases, the reported effects of MDR1 SNPs have been inconsistent and, in some cases, conflicting. In this review SNPs in MDR1 in relation to population frequencies, drug levels, and phenotypes are outlined. In addition, issues relating to MDR1 haplotypes, environmental factors, and study design, as potential confounding factors of the observed MDR1 polymorphism effect in vivo, are also discussed.

Nicotine metabolite ratio as an index of cytochrome P450 2A6 metabolic activity.

Abstract: Background Nicotine and a variety of other drugs and toxins are metabolized by cytochrome P450 (CYP) 2A6. Our objective was to evaluate the use of oral nicotine with measurement of the trans-3′-hydroxycotinine (3HC)/cotinine (COT) metabolite ratio as a noninvasive probe of CYP2A6 activity. Methods Sixty-two healthy volunteers received an oral solution of deuterium-labeled nicotine (2 mg) and its metabolite cotinine (10 mg). Plasma nicotine and plasma and saliva cotinine and 3HC concentrations were measured over time. Results The 3HC/COT ratio derived from deuterium–labeled cotinine, measured in either plasma (2–8 hours after administration) or saliva (at 6 hours), was strongly correlated with the oral clearance of nicotine (r = 0.76–0.83, depending on the time of measurement). The 6–hour 3HC/COT ratio from nicotine derived from tobacco in 14 smokers was highly correlated with the ratio derived from deuterium–labeled nicotine (r = 0.88) and was also highly correlated with the oral clearance of nicotine (r = 0.90). Two subjects homozygous for inactive CYP2A6 alleles produced no 3HC, confirming the specificity of the metabolite ratio. The 3HC/COT ratio was also highly correlated with the clearance and half–life of cotinine, consistent with the fact that cotinine is also primarily metabolized by CYP2A6. Conclusions The 3HC/COT ratio derived from nicotine either administered as a probe drug or from tobacco use, measured in either plasma or saliva, is highly correlated with the oral clearance of nicotine. The ratio appears to be a useful noninvasive marker of the rate of nicotine metabolism (which is important in studying nicotine addiction and smoking behavior), as well as a general marker of CYP2A6 activity (which is important in studying drug and toxin metabolism). Clinical Pharmacology & Therapeutics (2004) 76, 64–72; doi: 10.1016/j.clpt.2004.02.011

Rosuvastatin pharmacokinetics in heart transplant recipients administered an antirejection regimen including cyclosporine

Abstract: Background Cyclosporine (INN, ciclosporin) increases the systemic exposure of all statins. Therefore rosuvastatin pharmacokinetic parameters were assessed in an open-label trial involving stable heart transplant recipients (> or =6 months after transplant) on an antirejection regimen including cyclosporine. Rosuvastatin has been shown to be a substrate for the human liver transporter organic anion transporting polypeptide C (OATP-C). Inhibition of this transporter could increase plasma concentrations of rosuvastatin. Therefore the effect of cyclosporine on rosuvastatin uptake by cells expressing OATP-C was also examined. Methods Ten subjects were assessed while taking 10 mg rosuvastatin for 10 days; 5 of these were then assessed while taking 20 mg rosuvastatin for 10 days. Rosuvastatin steady-state area under the plasma concentration-time curve from time 0 to 24 hours [AUC(0-24)] and maximum observed plasma concentration (Cmax) were compared with values in controls (historical data from 21 healthy volunteers taking 10 mg rosuvastatin). Rosuvastatin uptake by OATP-C-transfected Xenopus oocytes was also studied by use of radiolabeled rosuvastatin with and without cyclosporine. Results In transplant recipients taking 10 mg rosuvastatin, geometric mean values and percent coefficient of variation for steady-state AUC(0-24) and Cmax were 284 ng. h/mL (31.3%) and 48.7 ng/mL (47.2%), respectively. In controls, these values were 40.1 ng. h/mL (39.4%) and 4.58 ng/mL (46.9%), respectively. Compared with control values, AUC(0-24) and Cmax were increased 7.1-fold and 10.6-fold, respectively, in transplant recipients. In transplant recipients taking 20 mg rosuvastatin, these parameters increased less than dose-proportionally. Rosuvastatin had no effect on cyclosporine blood concentrations. The in vitro results demonstrate that rosuvastatin is a good substrate for OATP-C-mediated hepatic uptake (association constant, 8.5 +/- 1.1 micromol/L) and that cyclosporine is an effective inhibitor of this process (50% inhibition constant, 2.2 +/- 0.4 micromol/L when the rosuvastatin concentration was 5 micromol/L). Conclusions Rosuvastatin exposure was significantly increased in transplant recipients on an antirejection regimen including cyclosporine. Cyclosporine inhibition of OATP-C-mediated rosuvastatin hepatic uptake may be the mechanism of the drug-drug interaction. Coadministration of rosuvastatin with cyclosporine needs to be undertaken with caution.

The Effect of Echinacea (Echinacea purpurea Root) on Cytochrome P450 Activity in Vivo

Abstract: Background Echinacea is a widely available over-the-counter herbal remedy. Tinctures of echinacea have been shown to inhibit cytochrome P450 (CYP) in vitro. The effect of echinacea (Echinacea purpurea root) on CYP activity in vivo was assessed by use of the CYP probe drugs caffeine (CYP1A2), tolbutamide (CYP2C9), dextromethorphan (CYP2D6), and midazolam (hepatic and intestinal CYP3A). Methods Twelve healthy subjects (6 men) completed this 2-period, open-label, fixed-schedule study. Caffeine, tolbutamide, dextromethorphan, and oral and intravenous midazolam were administered before and after a short course of echinacea (400 mg 4 times a day for 8 days) to determine in vivo CYP activities. Results Echinacea administration significantly increased the systemic clearance of midazolam by 34%, from 32 ± 7 L/h to 43 ± 16 L/h (P = .003; 90% confidence interval [CI], 116%-150%), and significantly reduced the midazolam area under the concentration-time curve by 23%, from 127 ± 36 μg · h/L to 102 ± 43 μg · h/L (P = .024; 90% CI, 63%-88%). In contrast, the oral clearance of midazolam was not significantly altered (P = .655; 90% CI, 75%-124%), 137 ± 19 L/h compared with 146 ± 71 L/h. The oral availability of midazolam after echinacea dosing was significantly increased (P = .028; 90% CI, 108%-153%), from 0.23 ± 0.06 to 0.33 ± 0.13. Hepatic availability (0.72 ± 0.08 versus 0.61 ± 0.16; P = .006; 90% CI, 73%-90%) and intestinal availability (0.33 ± 0.11 versus 0.61 ± 0.38; P = .015; 90% CI, 125%-203%) were significantly altered in opposite directions. Echinacea dosing significantly reduced the oral clearance of caffeine, from 6.6 ± 3.8 L/h to 4.9 ± 2.3 L/h (P = .049; 90% CI, 58%-96%). The oral clearance of tolbutamide was reduced by 11%, from 0.81 ± 0.18 L/h to 0.72 ± 0.19 L/h, but this change was not considered to be clinically relevant because the 90% CIs were within the 80% to 125% range. The oral clearance of dextromethorphan in 11 CYP2D6 extensive metabolizers was not affected by echinacea dosing (1289 ± 414 L/h compared with 1281 ± 483 L/h; P = .732; 90% CI, 89%-108%). Conclusions Echinacea (E purpurea root) reduced the oral clearance of substrates of CYP1A2 but not the oral clearance of substrates of CYP2C9 and CYP2D6. Echinacea selectively modulates the catalytic activity of CYP3A at hepatic and intestinal sites. The type of drug interaction observed between echinacea and other CYP3A substrates will be dependent on the relative extraction of drugs at hepatic and intestinal sites. Caution should be used when echinacea is coadministered with drugs dependent on CYP3A or CYP1A2 for their elimination. Clinical Pharmacology & Therapeutics (2004) 75, 89–100; doi: 10.1016/j.clpt.2003.09.013

Evidence for Inverse Effects of OATP-C (SLC21A6) *5 and *1b Haplotypes on Pravastatin Kinetics

Abstract: Objective We compared the pharmacogenetic effects of OATP-C (organic anion transporting polypeptide C) *1a, *1b (A388G), and *5 (T521C) haplotypes on single-dose pharmacokinetics of pravastatin in white subjects. Methods Thirty healthy white male subjects were grouped according to their OATP-C haplotype. Each group contained 10 individuals who were either homozygous or heterozygous carriers of the *1a, *1b, or *5 haplotype. After a single oral dose of 40 mg pravastatin, we analyzed kinetic parameters of pravastatin disposition. Results Values for the area under the plasma concentration–time curve from time 0 to 6 hours [AUC(0–6)] in *1a/*1a, *1a/*1b or *1b/*1b, and *1a/*5 individuals were 114.5 ± 68.6 μg · L−1 · h, 74.8 ± 35.6 μg · L−1 · h, and 163.0 ± 64.6 μg · L−1 · h, respectively, with highly significant differences across all 3 study groups (P = .006) and between subjects carrying the *1b and *5 haplotype (P = .002). Strikingly, values of AUC(0–6) from the OATP-C *1b group were more than 60% lower than those derived from carriers of the wild-type OATP-C *1a haplotype, although this difference failed to reach statistical significance. However, the amount of pravastatin excreted into the urine from time 0 to 12 hours [Ae(0–12)] was significantly diminished in the OATP-C *1b haplotype group (1729 ± 907 μg) compared with *1a wild-type control subjects (2974 ± 1590 μg) (P = .049). Conclusion There was a significant effect of tested OATP-C variant haplotypes on pravastatin disposition. Whereas *5 expression delayed the hepatocellular uptake of pravastatin, *1b expression seemed to accelerate OATP-C–dependent uptake of the drug. Clinical Pharmacology & Therapeutics (2004) 75, 415–421; doi: 10.1016/j.clpt.2003.12.016

The effect of gemfibrozil on the pharmacokinetics of rosuvastatin

Abstract: Background Coadministration of statins and gemfibrozil is associated with an increased risk for myopathy, which may be due in part to a pharmacokinetic interaction. Therefore the effect of gemfibrozil on rosuvastatin pharmacokinetics was assessed in healthy volunteers. Rosuvastatin has been shown to be a substrate for the human hepatic uptake transporter organic anion transporter 2 (OATP2). Inhibition of this transporter could increase plasma concentrations of rosuvastatin. The effect of gemfibrozil on rosuvastatin uptake by cells expressing OATP2 was also examined. Methods In a randomized, double-blind, 2-period crossover trial, 20 healthy volunteers were given oral doses of gemfibrozil, 600 mg, or placebo twice daily for 7 days. On the fourth morning of each dosing period, a single oral dose of rosuvastatin, 80 mg, was coadministered. Plasma concentrations of rosuvastatin, N-desmethyl rosuvastatin, and rosuvastatin-lactone were measured. In addition, the effect of gemfibrozil on the uptake of radiolabeled rosuvastatin by OATP2-transfected Xenopus oocytes was studied. Results Gemfibrozil increased the rosuvastatin area under the plasma concentration–time curve from time 0 to the time of the last quantifiable concentration [AUC(0-t)] 1.88-fold (90% confidence interval, 1.60–2.21) and the maximum observed rosuvastatin plasma concentration (Cmax) 2.21-fold (90% confidence interval, 1.81–2.69) compared with placebo. N-desmethyl rosuvastatin AUC(0-t) and Cmax decreased by 48% and 39%, respectively. Pharmacokinetics of rosuvastatin-lactone was unchanged. The in vitro results indicate that the maximum gemfibrozil inhibition of rosuvastatin OATP2-mediated uptake was 50%; the inhibition constant for the inhibitory process was 4.0 ± 1.3 μmol/L. Conclusions Gemfibrozil increased rosuvastatin plasma concentrations approximately 2-fold, which is similar to the effect of gemfibrozil on pravastatin, simvastatin acid, and lovastatin acid plasma concentrations and substantially less than the effect observed for cerivastatin. Gemfibrozil inhibition of OATP2-mediated rosuvastatin hepatic uptake may contribute to the mechanism of the drug-drug interaction. Care is warranted when gemfibrozil is coadministered with rosuvastatin and other statins. Clinical Pharmacology & Therapeutics (2004) 75, 455–463; doi: 10.1016/j.clpt.2003.12.014

Pharmacodynamics and pharmacokinetics of AMG 531, a novel thrombopoietin receptor ligand

Abstract: Objective The objective of this study was to evaluate the tolerability, pharmacodynamics, and pharmacokinetics of AMG 531, a novel thrombopoietin receptor ligand, after a single intravenous or subcutaneous injection in healthy subjects. Methods This was a first-in-human randomized, double-blind, placebo-controlled study with 48 subjects. Six subjects in each cohort were sequentially randomized in a 2:1 ratio to receive a single injection of AMG 531 or placebo. The dose ranges investigated were 0.3 to 10.0 μg/kg and 0.1 to 2.0 μg/kg via the initravenous and subcutaneous dosing routes, respectively. The pharmacodynamic response of AMG 531 was measured as the elevation in platelet counts. AMG 531 serum levels were determined by use of a validated enzyme-linked immunosorbent assay. Results Single intravenous or subcutaneous administration of AMG 531 induced a dose-dependent increase in platelet count in healthy subjects, with peak platelet count being achieved on days 12 to 16. The highest intravenous dose, 10.0 μg/kg, caused a nearly 6-fold increase in platelet count. The maximum increase for this cohort occurred on day 15, and the mean platelet count was 1380 × 109/L (range, 923–1790 × 109/L). Of 8 subjects receiving the 2.0-μg/kg subcutaneous dose, 6 had peak platelet levels that were double the baseline value. Platelet count was elevated to a similar extent after single intravenous or subcutaneous administration of AMG 531 at the same dose level (1.0 μg/kg), even though the subcutaneous serum levels were barely detectable. Platelet counts were close to the baseline value by day 28. After a single intravenous administration, the pharmacokinetics of AMG 531 was nonlinear in the 0.3- to 10.0-μg/kg dose range. Most AMG 531 serum levels fell below the assay's lower quantitation limit of 18 pg/mL after a single subcutaneous dose. There were no serious or life-threatening adverse events reported in this study. The most frequently reported events were mild to moderate headache and sore throat. Conclusion AMG 531 was well tolerated in this study and was effective at raising platelet counts in healthy volunteers after single intravenous or subcutaneous administration. Clinical Pharmacology & Therapeutics (2004) 76, 628–638; doi: 10.1016/j.clpt.2004.08.010

UGT1A1 haplotypes associated with reduced glucuronidation and increased serum bilirubin in irinotecan-administered Japanese patients with cancer.

Abstract: Purpose A comprehensive haplotype analysis of UGT1A1 in the Japanese population was conducted, and the effects of these haplotypes were investigated with respect to UGT1A1-related phenotypic parameters in patients with cancer who received irinotecan. Methods The UGT1A1 gene, including the enhancer, the promoter, and all 5 exons and their flanking regions, was sequenced from 195 Japanese subjects. The gene was divided into 2 blocks, and the haplotypes of each block were assigned. The association of these haplotypes with area under the concentration-time curve (AUC) ratios (7-ethyl-10-hydroxycamptothecin glucuronide [SN-38G]/7-ethyl-10-hydroxycamptothecin [SN-38]) and pretreatment levels of serum total bilirubin was investigated in 85 cancer patients who received irinotecan. Results Four haplotype groups (*1, *60, *28, and *6) were assigned in block 1, and 2 haplotype groups (*IA and *IB) were in block 2. The majority of the *IB haplotypes in block 2 were linked to either the *1 or the *60 haplotype but not to *28 in block 1. Highly significant associations were obtained between the *28 haplotypes and both a reduced AUC ratio (P = .0014, Jonckheere-Terpstra [JT] test) and an increased total bilirubin level (P = .0007, JT test). Increased total bilirubin levels in the *60 (P = .0048, JT test) and *IB groups (P = .0224, JT test) were also observed. The reduction in the AUC ratio by the *6 group was moderate (P = .0372, JT test) but was remarkable in combination with *60 (*6/*60) or *28 (*6/*28) as compared with the *1 group (*1/*1) (P = .049 and P = .0071, respectively; nonparametric Dunnett test). Conclusion This study identified several UGT1A1 haplotypes significantly associated with the reduced AUC ratio (*28 and *6) and with the increased total bilirubin level (*28, *60, and *IB) and suggested that the novel haplotype *IB might be functionally important. These findings will be useful for further pharmacogenetic studies on adverse reactions to irinotecan. Clinical Pharmacology & Therapeutics (2004) 75, 501–515; doi: 10.1016/j.clpt.2004.01.010

Diflomotecan pharmacokinetics in relation to ABCG2 421C>A genotype

Abstract: Objective The adenosine triphosphate–binding cassette transporter ABCG2 (breast cancer resistance protein [BCRP]) functions as an efflux transporter for many drugs, including the topoisomerase I inhibitor diflomotecan, and is expressed at high levels in the intestine and liver. We performed an exploratory analysis to evaluate the effects of the natural allelic variant ABCG2 421C>A on the pharmacokinetics of diflomotecan. Methods The drug was administered to 22 adult white patients with cancer as a 20–minute infusion (dose, 0.10–0.27 mg), followed 2 weeks later by an oral solution (dose, 0.10–0.35 mg). Results The ABCG2 421C>A genotype significantly affected the pharmacokinetics of diflomotecan; in 5 patients heterozygous for this allele, plasma levels after intravenous drug administration were 299% (P = .015) of those in 15 patients with wild–type alleles, at mean values of 138 ng · h/mL · mg−1 (95% confidence interval, 11.3–264 ng · h/mL · mg−1) versus 46.1 ng · h/mL · mg−1 (95% confidence interval, 25.6–66.7 ng · h/mL · mg−1), respectively. Diflomotecan levels were not significantly influenced by 11 known variants in the ABCB1, ABCC2, cytochrome P450 (CYP) 3A4, and CYP3A5 genes. Conclusion These findings provide the first evidence linking variant ABCG2 alleles to altered drug exposure and suggest that interindividual variability in substrate drug effects might be influenced, in part, by ABCG2 genotype. Clinical Pharmacology & Therapeutics (2004) 76, 38–44; doi: 10.1016/j.clpt.2004.03.003

Cyclic antidepressants and the risk of sudden cardiac death.

Abstract: Background Tricyclic and other related cyclic antidepressants (TCAs), used frequently for the treatment of depression and several other indications, have cardiovascular effects that may increase the risk of sudden cardiac death. We thus sought to quantify the risk of sudden cardiac death among TCA users, according to dose, as well as among users of selective serotonin reuptake inhibitors (SSRIs). Methods We conducted a retrospective cohort study in Tennessee Medicaid, from Jan 1, 1988, through Dec 31, 1993, which included large numbers of antidepressant users and computer files describing medication use and comorbidity. The cohort included 1,282,091 person-years of follow-up for persons aged 15 to 84 years who were not in a nursing home and were free of life-threatening noncardiac illness. This included 58,956 person-years for current use of TCAs alone, 6291 person-years for SSRIs only, and 96,220 person-years for former use. Results The cohort included 1487 confirmed sudden cardiac deaths occurring in the community. When compared with nonusers of antidepressants, current users of TCAs had a dose-related increase in the risk of sudden cardiac death. Rate ratios increased from 0.97 (95% confidence interval [CI], 0.72–1.29) for doses lower than 100 mg (amitriptyline or its equivalent) to 2.53 (95% CI, 1.04–6.12) for doses of 300 mg or more (P = .03, test for dose-response). The rate ratio for SSRIs was 0.95 (95% CI, 0.42–2.15). There was no evidence that TCA doses lower than 100 mg increased the risk of sudden cardiac death in subgroups defined by pre-existing cardiovascular disease, female sex, age 65 years or older, or use of amitriptyline. Conclusions Our data suggest that SSRI antidepressants and TCAs in doses of less than 100 mg (amitriptyline equivalents) did not increase the risk of sudden cardiac death. However, higher doses of TCAs were associated with increased relative risk, which suggests that such doses should be used cautiously, particularly in patients with an elevated baseline risk of sudden death. Clinical Pharmacology & Therapeutics (2004) 75, 234–241; doi: 10.1016/j.clpt.2003.09.019

Cytochrome P450 3A4 and P-glycoprotein Expression in Human Small Intestinal Enterocytes and Hepatocytes: A Comparative Analysis in Paired Tissue Specimens

Abstract: Objectives Our objectives were to determine the content of cytochrome P450 (CYP) 3A4, CYP3A5, and P-glycoprotein and to measure CYP3A4-dependent catalytic activity in paired human small intestinal and liver specimens. Methods Samples of duodenum or proximal jejunum and liver wedge biopsy specimens were obtained from 15 patients undergoing a gastrointestinal operation. Enterocytes were isolated from the intestinal samples. The contents of CYP3A4, CYP3A5, and P-glycoprotein and CYP3A4-mediated catalytic activities were determined in homogenized enterocyte and liver samples. Results The CYP3A4 protein content was about 3 times (P < .01) and the P-glycoprotein content about 7 times (P < .0001) higher in the enterocyte homogenates than in the liver homogenates. CYP3A5 protein was detected in all samples, but the levels were too low in most cases to allow quantification. The 2 cases with a quantifiable hepatic CYP3A5 content had the CYP3A5*1/*3 genotype; all other cases were homozygous for the CYP3A5*3 allele. No intraindividual correlations between the intestine and liver with respect to CYP3A4 content, P-glycoprotein content, or the measured catalytic activities were present. Values for the maximum rate of metabolism (Vmax) of verapamil N-dealkylation (formation of D-617) and N-demethylation (formation of norverapamil) activities correlated with the CYP3A4 protein content in both organs. Conclusions This work demonstrated a much higher content of both CYP3A4 protein and P-glycoprotein in enterocytes isolated from human duodenal or jejunal mucosa than in paired specimens of liver tissue. These results lend support to the view that biotransformation in the gut wall substantially contributes to the overall first-pass metabolism of many CYP3A4 substrates. Furthermore, the high content of P-glycoprotein on the apical surface of enterocytes supports the theory that this efflux transporter may act in concert with CYP3A4 to limit oral drug bioavailability. Finally, these results indicate that neither CYP3A4 nor MDR1 (P-glycoprotein) is coordinately regulated in the liver and intestine. Clinical Pharmacology & Therapeutics (2004) 75, 172–183; doi: 10.1016/j.clpt.2003.10.008

Effect of St John's Wort on imatinib mesylate pharmacokinetics

Abstract: Objective Imatinib is a potent inhibitor of the Bcr-Abl and c-kit tyrosine kinases and is approved for the treatment of Philadelphia chromosome–positive chronic myelogenous leukemia and gastrointestinal stromal tumors. Because imatinib is predominantly metabolized by cytochrome P450 (CYP) 3A4, its pharmacokinetics may be altered when it is coadministered with drugs or herbs (eg, St John's wort) that modulate CYP3A4 activity. Thus we examined the effects of St John's wort on imatinib pharmacokinetics. Methods This 2-period, open-label, fixed-sequence study was completed by 12 healthy subjects (6 men and 6 women) aged between 20 and 51 years. Each subject received 400 mg imatinib orally on study day 1, St John's wort (300 mg 3 times daily) on days 4 to 17, and 400 mg imatinib again on day 15. Serial blood samples were obtained over a 72-hour period after each imatinib dose. Imatinib and N-desmethyl-imatinib (CGP 74588) were quantified in plasma by liquid chromatography–mass spectrometry. Results St John's wort administration increased imatinib clearance by 43% (P < .001), from 12.5 ± 3.6 L/h to 17.9 ± 5.6 L/h; imatinib area under the concentration versus time curve (AUC) extrapolated to infinity was decreased by 30%, from 34.5 ± 9.5 μg · h/mL to 24.2 ± 7.0 μg · h/mL (P < .001). Imatinib half-life (12.8 hours versus 9.0 hours) and maximum concentration (Cmax) (2.2 μg/mL versus 1.8 μg/mL) were also significantly decreased (P < .005). N-desmethyl-imatinib Cmax was increased from 285 ± 95 ng/mL to 318 ± 95 ng/mL during St John's wort dosing, but the AUC from 0 to 72 hours was not altered. Conclusions These data indicate that St John's wort increases imatinib clearance. Thus patients taking imatinib should avoid taking St John's wort. Concomitant use of enzyme inducers, including St John's wort, may necessitate an increase in the imatinib dose to maintain clinical effectiveness. Clinical Pharmacology & Therapeutics (2004) 76, 323–329; doi:10.1016/j.clpt.2004.06.007

Role of hepatic and intestinal cytochrome P450 3A and 2B6 in the metabolism, disposition, and miotic effects of methadone

Abstract: Background The disposition of the long-acting opioid methadone, used to prevent opiate withdrawal and treat short- and long-lasting pain, is highly variable. Methadone undergoes N-demethylation to the primary metabolite 2-ethyl-1,5-dimethyl-3,3-diphenylpyrrolinium (EDDP), catalyzed in vitro by intestinal, hepatic, and expressed cytochrome P450 (CYP) 3A4. However, the role of CYP3A4 in human methadone disposition in vivo is unclear. This investigation tested the hypothesis that CYP3A induction (or inhibition) would increase (or decrease) methadone metabolism and clearance in humans. Methods Healthy volunteers were studied in a randomized, balanced, 4-way crossover study. They received intravenous (IV) midazolam (to assess CYP3A4 activity) and then simultaneous oral deuterium-labeled and IV unlabeled methadone after pretreatment with rifampin (INN, rifampicin) (hepatic/intestinal CYP3A induction), troleandomycin (hepatic/intestinal CYP3A inhibition), grapefruit juice (selective intestinal CYP3A inhibition), or nothing. Methadone effects were measured by dark-adapted pupil diameter. CYP isoforms catalyzing methadone metabolism by human liver microsomes and expressed CYPs in vitro were also evaluated. Results Methadone had high oral bioavailability (70%) and low intestinal (22%) and hepatic (9%) extraction, and there was a significant correlation (r = 0.94, P < .001) between oral bioavailability and intestinal (but not hepatic) availability. Rifampin decreased bioavailability and oral and IV methadone plasma concentrations and increased IV clearance (4.42 ± 1.00 mL · kg−1 · min−1 versus 1.61 ± 0.67 mL · kg−1 · min−1, P < .05) and oral clearance (8.50 ± 3.68 mL · kg−1 · min−1 versus 2.05 ± 0.92 mL · kg−1 · min−1, P < .05), EDDP/methadone area under the curve (AUC) ratios, EDDP formation clearances, and hepatic extraction (0.27 ± 0.06 versus 0.09 ± 0.04, P < .05). Troleandomycin and grapefruit juice decreased the EDDP/methadone AUC ratio after oral methadone (0.17 ± 0.10 and 0.14 ± 0.06 versus 0.27 ± 0.20, P < .05) but not IV methadone and had no effect on methadone plasma concentrations, IV clearance (1.29 ± 0.41 mL · kg−1 · min−1 and 1.48 ± 0.55 mL · kg−1 · min−1) or oral clearance (2.05 ± 1.52 mL · kg−1 · min−1 and 1.89 ± 1.07 mL · kg−1 · min−1), or other kinetic parameters. There was no correlation between methadone clearance and hepatic CYP3A4 activity. Pupil diameter changes reflected plasma methadone concentrations. In vitro experiments showed a predominant role for both CYP3A4 and CYP2B6 in liver microsomal methadone N-demethylation. Conclusion First-pass intestinal metabolism is a determinant of methadone bioavailability. Intestinal and hepatic CYP3A activity only slightly affects human methadone N-demethylation but has no significant effect on methadone concentrations, clearance, or clinical effects. Greater rifampin effects, compared with troleandomycin and grapefruit juice, on methadone disposition suggest a major role for intestinal transporters and for other CYPs, such as CYP2B6. Interindividual variability and drug interactions affecting intestinal transporter and hepatic CYP3A and CYP2B6 activity may alter methadone disposition. Clinical Pharmacology & Therapeutics (2004) 76, 250–269; doi: 10.1016/j.clpt.2004.05.003

Contribution of age, body size, and CYP2C9 genotype to anticoagulant response to warfarin.

Abstract: Objective Our objective was to assess the contribution of CYP2C9 genotype, age, body size, and vitamin K and lipid status to warfarin dose requirements. Methods Patients with stable warfarin dose requirements and an international normalized ratio (INR) of the prothrombin time within the target range of 2.0 to 3.0 were recruited to the study. On arrival at the clinic in the morning, after an overnight fast, a blood sample was taken from each patient for CYP2C9 genotyping and for determination of venous INR and plasma vitamin K, R- and S-warfarin, and triglyceride concentrations. Results A total of 121 patients were recruited to the study. CYP2C9 genotyping showed that 74 patients were homozygous wild-type (*1/*1), 30 were heterozygous *1/*2, and 15 were heterozygous *1/*3 genotype. One patient was found to have the genotype *2/*3, and another was found to have the genotype *3/*3. The mean warfarin daily dose requirement in milligrams fell from 4.06 ± 1.72 mg in homozygous wild-type patients to 3.63 ± 1.78 mg for *1/*2-positive patients and 2.70 ± 1.36 mg for *1/*3-positive patients. The multiple linear regression model for warfarin dose indicated significant contributions from age (r = 0.41, P < .001), genotype (r = 0.24, P < .005), and age and genotype together (r = 0.45, P < .005). Although there were significant linear correlations between warfarin dose and body surface area (r = 0.21, P = .02), body weight (r = 0.25, P = .005), and plasma vitamin K concentration (r = 0.18, P < .05), none of these variables made a significant contribution to the regression model for warfarin dose. CYP2C9 genotype had a significant effect on S-warfarin clearance (r = 0.34, P < .0001) but none on R-warfarin clearance. Conclusion This study showed that age and CYP2C9 polymorphism affect warfarin dose requirements in patients receiving long-term therapy and having stable control of anticoagulation. It is anticipated that using dosing regimens modified to take into account the contribution of age and CYP2C9 genotype has the potential to improve the safety of warfarin therapy. Clinical Pharmacology & Therapeutics (2004) 75, 204–212; doi: 10.1016/j.clpt.2003.10.001

In vivo assessment of botanical supplementation on human cytochrome P450 phenotypes: Citrus aurantium, Echinacea purpurea, milk thistle, and saw palmetto

Abstract: Objectives Phytochemical-mediated modulation of cytochrome P450 (CYP) activity may underlie many herb-drug interactions. Single-time point phenotypic metabolic ratios were used to determine whether long-term supplementation of Citrus aurantium, Echinacea purpurea, milk thistle (Silybum marianum), or saw palmetto (Serenoa repens) extracts affected CYP1A2, CYP2D6, CYP2E1, or CYP3A4 activity. Methods Twelve healthy volunteers (6 women, 6 men) were randomly assigned to receive C aurantium, E purpurea, milk thistle, or saw palmetto for 28 days. For each subject, a 30-day washout period was interposed between each supplementation phase. Probe drug cocktails of midazolam and caffeine, followed 24 hours later by chlorzoxazone and debrisoquin (INN, debrisoquine), were administered before (baseline) and at the end of supplementation. Presupplementation and postsupplementation phenotypic trait measurements were determined for CYP3A4, CYP1A2, CYP2E1, and CYP2D6 by use of 1-hydroxymidazolam/midazolam serum ratios (1-hour sample), paraxanthine/caffeine serum ratios (6-hour sample), 6-hydroxychlorzoxazone/chlorzoxazone serum ratios (2-hour sample), and debrisoquin urinary recovery ratios (8-hour collection), respectively. The content of purported “active” phytochemicals was determined for each supplement. Results Comparisons of presupplementation and postsupplementation phenotypic ratios suggested that these particular supplements had no significant effect on CYP1A2, CYP2D6, CYP2E1, or CYP3A4 activity. Phytochemical profiles indicated that C aurantium was devoid of the CYP3A4 inhibitor 6′,7′-dihydroxybergamottin. Quantities of fatty acids, flavonolignans, and cichoric acid were consistent with label claims for saw palmetto, milk thistle, and E purpurea, respectively. Conclusions Botanical supplements containing C aurantium, milk thistle, or saw palmetto extracts appear to pose a minimal risk for CYP-mediated herb-drug interactions in humans. Although the effects of E purpurea on CYP activity were minor, further study into the interaction potential of this botanical is merited. Clinical Pharmacology & Therapeutics (2004) 76, 428–440; doi: 10.1016/j.clpt.2004.07.007

Time response of cytochrome P450 1A2 activity on cessation of heavy smoking.

Abstract: Background and objective Cytochrome P450 (CYP) 1A2 activity is induced by cigarette smoking. Thus smoking cessation in patients while they are undergoing therapy with a CYP1A2 substrate such as theophylline or clozapine increases its concentrations and may cause adverse effects. Our objective was to determine the time course of CYP1A2 activity changes after smoking cessation in heavy smokers as the basis for dosing adaptation schemes. Methods The study was conducted in 8 men and 4 women (all white) who smoked 20 cigarettes or more per day. Sudden smoking cessation was carried out after a 14-day run-in period. Subjects were phenotyped for CYP1A2 activity at 6, 4, and 1 day before smoking cessation and at 0, 1, 2, 3, 6, 8, 10, and 13 days thereafter by use of the paraxanthine-to-caffeine ratio in plasma 6 hours after a 148-mg caffeine test dose. A monoexponential decay of CYP1A2 activity to a residual value was fitted to the data by nonlinear regression analysis. Results On cessation of smoking, initial caffeine clearance (estimated geometric means and 95% confidence intervals) decreased significantly (P < .01), by 36.1% (30.9%-42.2%), from 2.47 mL · min−1 · kg−1 body weight (2.03–3.00 mL · min−1 · kg−1 body weight) to a new steady state of 1.53 mL · min−1 · kg−1 body weight (1.24–1.89 mL · min−1 · kg−1 body weight). The apparent half-life of CYP1A2 activity decrease was 38.6 hours (27.4–54.4 hours). Conclusion Doses of CYP1A2 substrates with a narrow therapeutic range should be decreased immediately on cessation of heavy smoking. As a rule of thumb, a stepwise daily dose reduction of approximately 10% until the fourth day after smoking cessation is proposed, which should be accompanied by therapeutic drug monitoring. Clinical Pharmacology & Therapeutics (2004) 76, 178–184; doi: 10.1016/j.clpt.2004.04.003

Simvastatin treatment for inflammatory attacks of the hyperimmunoglobulinemia D and periodic fever syndrome.

Abstract: Hyperimmunoglobulinemia D (hyper-IgD) and periodic fever syndrome, a hereditary autoinflammatory syndrome, is characterized by lifelong recurrent episodes of fever and inflammation. No effective treatment is known. It is caused by a defect of mevalonate kinase, an enzyme that follows 3'-hydroxy-3'-methylglutaryl-coenzyme A (HMG-CoA) reductase in the isoprenoid pathway. We wanted to test the hypothesis that inhibition of HMG-CoA reductase would ameliorate the inflammatory attacks. Six patients with hyper-IgD syndrome and proven mevalonate kinase deficiency were followed up for 2 treatment periods with either simvastatin, 80 mg/d, or placebo for 24 weeks, separated by a 4-week washout period in a double-blind fashion. Simvastatin resulted in a drop in urinary mevalonic acid concentration in all patients and decreased the number of febrile days in 5 of 6 patients. No side effects were observed. These data offer preliminary evidence for the hypothesis that simvastatin may improve inflammatory attacks in the hyper-IgD syndrome. This highlights the anti-inflammatory properties of HMG-CoA reductase inhibition.

CYP2D6 Genotype: Impact on Adverse Effects and Nonresponse During Treatment with Antidepressants—a Pilot Study

Abstract: Objective Treatment with antidepressants is frequently associated with adverse effects or insufficient clinical response. Several antidepressants are metabolized by cytochrome P450 (CYP) 2D6. The activity of this enzyme markedly varies among individuals from poor to ultrarapid metabolism on the basis of the polymorphism of the CYP2D6 gene. This association study investigated whether the CYP2D6 genotype distribution differs from that of the German white population either in patients with marked adverse effects or in nonresponders during treatment with antidepressants metabolized by CYP2D6. Methods By use of a retrospective, naturalistic approach, outpatient practices and hospitals in southern Germany were asked to report on patients who either had had adverse drug effects or were nonresponsive during treatment with CYP2D6-dependent antidepressants. CYP2D6 genotyping was performed by a panel of polymerase chain reaction techniques. Poor and intermediate metabolizer alleles, as well as allelic duplications of CYP2D6, were detected. Results Of 28 patients with adverse effects during treatment with a CYP2D6-dependent antidepressant, 8 (29%) had 2 inactive alleles and thus were poor metabolizers. This is a 4-fold increase as compared with the German population (P < .0001). Amplification of fully functional alleles (associated with ultrarapid metabolism) was found in 3 (19%) of the 16 nonresponders (approximately 5.0-fold higher in nonresponders than in the population) (P = .0012). Conclusion The results suggest that the CYP2D6 genotype is associated with the occurrence of adverse effects and clinical nonresponse in psychiatric patients treated with CYP2D6-dependent antidepressants. Clinical Pharmacology & Therapeutics (2004) 75, 386–393; doi: 10.1016/j.clpt.2003.12.015

CYP3A5 and MDR1 genetic polymorphisms and cyclosporine pharmacokinetics after renal transplantation.

Abstract: Background The immunosuppressive drug cyclosporine (INN, ciclosporin), whose pharmacokinetic characteristics vary greatly among individuals, is a substrate for cytochrome P450 (CYP) 3A and P-glycoprotein, the product of the multidrug resistance 1 (MDR1) gene. Some of the single nucleotide polymorphisms (SNPs) in these genes are associated with deficient protein expression and reduced in vivo activity. We postulated that, in renal transplant recipients, these SNPs could be associated with interindividual variations in cyclosporine pharmacokinetics. Purpose In 106 renal transplant patients, we evaluated retrospectively the effects of 4 MDR1 SNPs [T−129C, C1236T, G2677(T,A), and C3435T] and of the CYP3A5*1/*3 SNP on cyclosporine pharmacokinetic parameters and exposure indices. Results The CYP3A5*1 allele was present in 8.5% of patients. The MDR1 C1236T, G2677(T,A), and C3435T SNPs were frequent (17.9%, 18.9%, and 33%, respectively, for the variant homozygous genotype) and exhibited incomplete linkage disequilibrium. None of the cyclosporine pharmacokinetic parameters were associated with the CYP3A5 genetic polymorphism. Patients with the wild-type genotype in MDR1 C1236T SNP had slightly but significantly lower dose-adjusted peak drug concentrations (−16%) (P < .02) and dose-adjusted area under the concentration-time curve (AUC) values over the first 4 hours (−14%) (P < .05) as compared with mutated allele carriers. Haplotype analysis including MDR1 C1236T, G2677(T,A), and C3435T SNPs showed no significant association between haplotypes and cyclosporine pharmacokinetics or systemic exposure, although there was a nonsignificant trend toward higher dose-adjusted AUC values over the first 4 hours and AUC over the 12-hour administration interval for the T-T-T haplotype. Conclusion The presence of the CYP3A5 SNP does not explain the high variability of cyclosporine pharmacokinetics in stable renal transplant patients. Despite the weak association found for the MDR1 C1236T SNP, MDR1 SNPs are unlikely to be useful for cyclosporine dose optimization in clinical practice. Clinical Pharmacology & Therapeutics (2004) 75, 422–433; doi: 10.1016/j.clpt.2004.01.009

Clinical Efficacy and Toxicity Profile of Tacrolimus and Mycophenolic Acid in Relation to Combined Long-term Pharmacokinetics in de Novo Renal Allograft Recipients

Abstract: Introduction Tacrolimus and mycophenolate mofetil are effective drugs characterized by specific toxicity profiles that may compromise their long-term use in renal transplant recipients. Clinicians, therefore, need reliable drug monitoring tools for relating efficacy and toxicity to drug exposure. Study design We conducted a prospective 12-month pharmacokinetic study of tacrolimus and mycophenolic acid in 100 de novo recipients. The aim was to examine whether tacrolimus and mycophenolic acid exposure parameters (predose trough blood concentration [C0], area under the concentration curve from 0 to 12 hours [AUC0–12], maximum blood or plasma concentration [Cmax], and dose) would reflect clinical efficacy and toxicity at different time points after transplantation (7 days, 6 weeks, and 3, 6, and 12 months). Results Initially, after grafting, the tacrolimus AUC0–12 was higher in recipients with infection (P = .01 on day 7, P = .02 at week 6), whereas the mycophenolic acid AUC0–12 was not different. There was no difference in tacrolimus exposure between patients who had arterial hypertension or hyperlipidemia and those who did not. Patients with tacrolimus nephrotoxicity received a higher drug dose (P = .03) and had higher drug clearance (P = .02). From 3 months, recipients with anemia or leukopenia had higher mycophenolic acid AUC0–12 (anemia, P = .03 at month 3 and P = .01 at month 12; leukopenia, P = .01 at month 3 and P = .04 at 1 year) and C0 (anemia, P = .001 at month 3 and P = .001 at month 12; leukopenia, P = .01 at month 3 and P = .04 at 1 year). Finally, for recipients who did not simultaneously have a target tacrolimus AUC0–12 of 150 ng · h/mL and a mycophenolic acid AUC0–12 of 45 mg · h/L by day 7, the incidence of acute rejection tended to be higher (26.3%) compared with patients who reached both target values (7.7%) (P = .07). Conclusions Pharmacokinetic exposure parameters of tacrolimus and mycophenolic acid are related to specific drug-induced side effects in a time-dependent fashion. In addition, this study has provided a conceptual basis for defining a combined target therapeutic window for tacrolimus and mycophenolic acid based on sparse AUC0–12 measurements. Clinical Pharmacology & Therapeutics (2004) 75, 434–447; doi: 10.1016/j.clpt.2003.12.009

Population pharmacokinetics of cyclosporine in kidney and heart transplant recipients and the influence of ethnicity and genetic polymorphisms in the MDR-1, CYP3A4, and CYP3A5 genes

Abstract: Objective Our objective was to determine the relationship between single nucleotide polymorphisms (SNPs) in the multidrug resistance 1 (MDR-1) gene and the cytochrome P450 (CYP) genes CYP3A4 and CYP3A5 and the pharmacokinetics of cyclosporine (INN, ciclosporin). Methods Cyclosporine pharmacokinetics of 151 kidney and heart transplant recipients undergoing maintenance therapy was described by use of nonlinear mixed-effects modeling (NONMEM) according to a 2-compartment pharmacokinetic model with first-order absorption and elimination. All patients were genotyped for the CYP3A4*1B and *3, CYP3A5*3 and *6, and MDR-1 3435C→T SNPs. Results For a typical 70-kg white patient, the following parameters were estimated: absorption rate constant, 1.27 h−1; absorption time lag, 0.47 hour; oral volume of distribution of the central and peripheral compartment, 56.3 and 185.0 L, respectively; oral clearance (Cl/F), 30.7 L/h; and oral intercompartmental clearance, 31.7 L/h. Estimated interpatient variability of Cl/F was 28%. Cl/F was significantly correlated with weight and ethnicity; Cl/F was 13% higher (95% confidence interval, 8%-18%; P < .005) in white patients than in black and Asian patients. In carriers of a CYP3A4*1B variant allele, Cl/F was 9% (95% confidence interval, 1%-17%; P < .05) higher compared with CYP3A4*1 homozygotes, and this effect was independent of ethnicity or weight. Incorporation of these covariates into the NONMEM model did not markedly reduce interpatient variability of Cl/F. None of the other SNPs studied significantly influenced any of the pharmacokinetic parameters. Conclusion Patients carrying a CYP3A4*1B variant allele have a significantly higher oral cyclosporine clearance compared with patients homozygous for CYP3A4*1. However, this genetic effect on cyclosporine disposition was small, and genotyping of transplant recipients for CYP3A4 is thus unlikely to assist in planning initial cyclosporine dosing. Clinical Pharmacology & Therapeutics (2004) 76, 545–556; doi: 10.1016/j.clpt.2004.08.022

A novel intronic mutation, 2988G>A, with high predictivity for impaired function of cytochrome P450 2D6 in white subjects.

Abstract: Background Individuals with the cytochrome P450 (CYP) 2D6 intermediate metabolizer (IM) phenotype have low residual enzyme activity and compose about 10% to 15% of white populations. Their identification is clinically relevant but remains unsatisfactory because of incomplete characterization of the major allele involved, termed CYP2D6*41 (−1584C, R296C, S486T). Methods To search for novel mutations, resequencing of the entire CYP2D6 gene was performed in selected individuals. Genotype-phenotype correlation analysis was done in a population sample of 308 white subjects phenotyped with sparteine and previously genotyped for all major alleles. Results A total of 16 novel polymorphic positions were identified, of which 7 were located within 2.4 kilobases of previously uncharacterized 2D7–2D6 intergenic sequence and 9 were located within intronic regions. The novel mutation 2988G>A in intron 6 appeared to be specifically associated with the IM phenotype. Further analysis in the population sample demonstrated that 2988G>A was strongly linked to allele *41 but not to any other alleles including *1, *2, *2×N, *4, *6, *7, *8, *9, *10, and *35. The overall frequency of the novel polymorphism was 8.4% in the normal white population. Compared with conventionally determined *41, 2988G>A was shown to have improved predictivity for the IM phenotype. With 2988G>A being taken into account, alleles *1, *2, and *35 (−1584G, V11M, R296C, S486T) were found to be phenotypically equivalent. Conclusions CYP2D6 genotyping can be considerably simplified by using 2988G>A as a marker for *41 and by omitting genotyping for the functionally equivalent alleles *2 and *35. Clinical Pharmacology & Therapeutics (2004) 76, 128–138; doi: 10.1016/j.clpt.2004.04.009

Substantial pharmacokinetic interaction between digoxin and ritonavir in healthy volunteers

Abstract: Background Ritonavir is a potent in vitro inhibitor of several cytochrome P450 isozymes and ABC transporters including the efflux pump P-glycoprotein (P-gp). This study assessed the effect of repetitive ritonavir administration on digoxin distribution and total and renal digoxin clearance as a marker for P-gp activity in vivo. Methods In a randomized, placebo-controlled crossover study, 12 healthy male participants received oral ritonavir (300 mg twice daily) for 11 days. With the assumption that ritonavir steady state had been reached, 0.5 mg digoxin was given intravenously on day 3. Digoxin concentrations were determined in plasma and urine by radioimmunoassay, and plasma ritonavir concentrations were determined by liquid chromatography–tandem mass spectrometry. Digoxin kinetics was estimated by compartmental and noncompartmental analyses, by use of the area under the plasma concentration–time curve, and the corresponding digoxin amount excreted into urine was used for digoxin clearance calculations. Results Ritonavir significantly (P < .01) increased digoxin area under the plasma concentration–time curve from time 0 to infinity by 86% and its volume of distribution by 77% and decreased nonrenal and renal digoxin clearance by 48% and 35%, respectively. Digoxin terminal half-life in plasma increased by 156% (P < .01). Conclusion This inhibition of renal digoxin clearance is likely caused by ritonavir inhibition of P-gp. Its extent is considerable and similar to the effect of other potent P-gp inhibitors on digoxin disposition such as quinidine. These findings may, therefore, indicate that the pharmacokinetics of P-gp substrates sharing the renal tubular elimination pathway will be affected when combined with therapeutic doses of ritonavir in antiretroviral treatment regimens. In addition and contrarily to quinidine, these data indicate that ritonavir promotes digoxin distribution in the body. Clinical Pharmacology & Therapeutics (2004) 76, 73–84; doi: 10.1016/j.clpt.2004.02.008

Interindividual variability in ibuprofen pharmacokinetics is related to interaction of cytochrome P450 2C8 and 2C9 amino acid polymorphisms.

Abstract: Objective Our objective was to identify genetic factors related to interindividual variability in the pharmacokinetics of ibuprofen and its enantiomers. Methods The time course for ibuprofen plasma concentration was measured by HPLC in 130 healthy individuals who received a single oral dose of 400 mg racemic ibuprofen. Genomic deoxyribonucleic acid was analyzed for common mutations at CYP2C8 and CYP2C9 genes that cause amino acid substitutions. Results Ibuprofen clearance values were 4.04 L/h (95% confidence interval [CI], 3.61–4.47 L/h), 2.79 L/h (95% CI, 2.07–3.52 L/h), and 0.40 L/h (95% CI, 0.37–0.43 L/h) for carriers of CYP2C8 genotypes *1/*1, *1/*3, and *3/*3, respectively, and 4.43 L/h (95% CI, 3.94–4.92 L/h), 3.26 L/h (95% CI, 2.53–3.99 L/h), 2.91 L/h (95% CI, 1.52–4.30 L/h), 2.05 L/h (95% CI, 0–6.37 L/h), 1.83 L/h (95% CI, 1.24–2.41 L/h), and 1.13 L/h (95% CI, 0.58–1.66 L/h) for carriers of the CYP2C9 genotypes *1/*1, *1/*2, *1/*3, *2/*2, *2/*3, and *3/*3, respectively. The P values for comparison across nonmutated, heterozygous, and homozygous genotypes were as follows: P < .001 for CYP2C8*3, P < .005 for CYP2C9*2, and P < .001 for CYP2C9*3. The main genetic factor for reduced clearance of R-(−)-ibuprofen is the CYP2C8*3 allele, whereas the clearance for S-(+)-ibuprofen is influenced by CYP2C8*3 and CYP2C9*3 alleles to a similar extent. The CYP2C9*2 allele was associated with low clearance only when it was present in combination with the CYP2C8*3 allele. As compared with individuals with no mutations, individuals with the common genotype CYP2C8*1/*3 plus CYP2C9*1/*2 (19% of the population) displayed decreased ibuprofen clearance (mean, 65% [95% CI, 42%–89%]; P < .001). Individuals homozygous or double-heterozygous for CYP2C8*3 and CYP2C9*3 variant alleles (8% of the population) had extremely low ibuprofen clearance rates, with values ranging from 7% to 27% of the mean clearance rates among noncarriers of mutations (P < .001). No enantiospecific reduction of ibuprofen clearance was observed. Conclusion Low ibuprofen clearance occurs in a substantial proportion of healthy subjects, is not enantiospecific, and is strongly linked to CYP2C8 and CYP2C9 polymorphisms. Clinical Pharmacology & Therapeutics (2004) 76, 119–127; doi: 10.1016/j.clpt.2004.04.006

Carbamazepine regulates intestinal P‐glycoprotein and multidrug resistance protein MRP2 and influences disposition of talinolol in humans

Abstract: Background and methods The antiepileptic drug carbamazepine is known to be an inducer of cytochrome P450 (CYP) 3A4 after binding to the nuclear pregnane X receptor. To evaluate whether it also regulates the multidrug transporter proteins P-glycoprotein (P-gp) and multidrug resistance protein MRP2 in humans, duodenal expression of multidrug resistance gene MDR1 messenger ribonucleic acid (mRNA) and MRP2 mRNA, content of P-gp and MRP2, and disposition of the nonmetabolized P-gp substrate talinolol after intravenous (30 mg) and long-term oral administration (100 mg for 19 days) were assessed in 7 healthy subjects (age, 23–35 years; body weight, 64–93 kg) before and after comedication of carbamazepine (600 mg for 14–18 days). Results Carbamazepine medication was associated with increased urinary excretion of D-glucaric acid and induction of carbamazepine elimination. Creatinine clearance was not affected. Duodenal expression of both MDR1 mRNA and MRP2 mRNA and the MPR2 protein was significantly induced, whereas the P-gp content was not affected. MDR1 mRNA expression and MPR2 mRNA expression were correlated (r = 0.873, P < .001). After carbamazepine, metabolic clearance of intravenous talinolol was significantly increased. Residual clearance was significantly decreased in dependence on MDR1 mRNA expression (r = −0.647, P = .012) and MRP2 mRNA expression (r = −0.613, P = .020). Oral absorption of talinolol was significantly lower after carbamazepine comedication (53.2% ± 15.5% versus 62.1% ± 13.0%, P = .018), and renal clearance and metabolic clearance were significantly increased, correlated in each case with MDR1 mRNA (r = 0.612, P = .020, and r = 0.554, P = .040, respectively) and MRP2 mRNA (r = 0.596, P = .025, and r = 0.565, P = .035, respectively). Conclusions Aside from induction of CYP3A4, carbamazepine acts as an inducer of intestinal MDR1 mRNA, MRP2 mRNA, and MRP2 protein content. Clinical Pharmacology & Therapeutics (2004) 76, 192–200; doi: 10.1016/j.clpt.2004.04.011

Pharmacokinetics of Voriconazole and Cytochrome p450 2C19 Genetic Status

Abstract: gested and that possibly could be suitable for drugs eliminated by biliary excretion. Even though a specific classification is used for including subjects in the study, evaluation of data and dose recommendations may very well be based on relevant individual laboratory parameters (albumin levels, prothrombin time, bilirubin levels, alkaline phosphatase levels, fibrinogen levels, and so on). Sponsors and regulatory authorities also need to consider that the characteristics of the classification systems are not well known by the prescribers. In conclusion, we have presented several arguments disqualifying the Maddrey function as a general marker for hepatic function in pharmacokinetic trials. An inappropriate and routinelike classification of hepatic impairment may result in the selection of a study population that does not reflect the effects of reduced elimination capacity and a study with a low predictive value for patients at risk. Clearly, there is a need for further development with respect to what hepatic marker or classification systems to use in pharmacokinetic hepatic impairment studies. The pharmaceutical industry is encouraged to implement its knowledge of the mechanism of drug elimination into the design of the studies and to further explore hepatic markers that may be used for characterizing the pharmacokinetic profile of drugs dependent on hepatic function for their elimination.

Randomized trial of buprenorphine for treatment of concurrent opiate and cocaine dependence

Abstract: Buprenorphine is a partial μ-opiate agonist and κ-opiate antagonist marketed in the United States and worldwide as a parenteral or sublingual analgesic1 and as maintenance treatment for opiate dependence.2 However, its efficacy for treatment of dually (cocaine and heroin) dependent individuals has not been established. For the treatment of opiate dependence, sublingual doses of 8 to 16 mg daily of buprenorphine are as effective as 55 to 80 mg of oral methadone in reducing opiate use.3,4 There is some evidence that sublingual doses as low as 2 mg daily may reduce opiate use.5 Because buprenorphine has a relatively long duration of action, every-other-day and 3-times-weekly dosing have also shown efficacy.6-9 A large proportion of opiate users use cocaine, even during opiate agonist substitution treatment for opiate dependence. Cocaine use during opiate agonist maintenance treatment is associated with increased opiate use, poorer treatment outcomes, and premature dropout from treatment.10 Methadone and levomethadyl acetate (INN, levacetylmethadol) do not seem efficacious for the treatment of cocaine dependence in opiate-dependent individuals. Thus a medication that effectively reduced both opiate and cocaine use would offer a therapeutic advantage over methadone or levomethadyl acetate alone for the many patients who are using or are dependent on both opiates and cocaine. Buprenorphine was found to have such potential in preclinical studies. It significantly reduced cocaine self-administration in monkeys without interfering with appetitive behaviors such as eating.11-14 However, attempted replications of these effects in humans have yielded inconsistent results. In some human laboratory studies, buprenorphine treatment reduced self-reported cocaine craving and cocaine self-administration; other studies have failed to replicate those results.15-18 Currently, there is no consensus in the literature about the clinical efficacy of buprenorphine for treatment of concurrent cocaine and heroin dependence. In some clinical trials with opiate-dependent subjects, buprenorphine treatment has been associated with reductions in cocaine use,19-21 whereas other trials have found no evidence of efficacy.18,22 One negative trial used buprenorphine doses of 4 mg and 12 mg/d22; the other used a mean dose of 11.2 mg.18 The positive studies have used buprenorphine doses up to 16 mg daily, with more reduction of cocaine use at higher doses. The purpose of this study was to evaluate the efficacy of sublingual buprenorphine maintenance (10 weeks) in reducing cocaine and opiate use in dually (cocaine and opiate) dependent outpatients.

A novel functional polymorphism in the uridine diphosphate–glucuronosyltransferase 2B7 promoter with significant impact on promoter activity

Abstract: To clarify the molecular determinants of the metabolic variability of morphine, we searched for genetic polymorphisms in the gene for uridine diphosphate-glucuronosyltransferase 2B7 (UGT2B7) and evaluated their functional impact in vitro and in patients with cancer receiving long-term morphine therapy. Genetic analysis revealed the existence of 8 single-nucleotide polymorphisms (SNPs), 6 of which are tightly linked and are at positions -1248, -1241, -1054, -842, -268, and -102 relative to the hepatic start site. In contrast, an SNP at position -66 occurs independently, whereas a novel variation at position -79 appears to be in linkage disequilibrium with the codon 268 SNP (UGT2B7*2). At least 4 haplotypes were observed in white subjects included in the initial SNP screening. On functional in vitro characterization, promoter-reporter gene constructs with the -79 variation displayed 2.5- to 7-fold less activity compared with the wild-type construct in Caco-2 colon cells and HepG2 hepatoma cells, respectively (P =.015 and P <.001, respectively). To investigate a possible effect of the -79 variation in vivo, serum morphine and morphine glucuronide concentrations were measured by liquid chromatography-mass spectrometry in patients with cancer who received long-term oral morphine therapy, and subjects were then genotyped for the -79 polymorphism. Among 175 patients with normal hepatic and renal function, 6 were heterozygous for the -79 variation, and the morphine-6-glucuronide (M6G)/morphine and morphine-3-glucuronide (M3G)/morphine ratios versus those in the 169 noncarriers were 5.9 +/- 3.5 versus 7.1 +/- 7.0 for M6G/morphine (P =.96) and 31.2 +/- 17.1 versus 42.9 +/- 31.2 for M3G/morphine (P =.53), respectively. Further studies in larger samples are needed to make conclusions about the possible clinical relevance of the -79 polymorphism in the UGT2B7 gene.

Bioequivalence revisited: Influence of age and sex on CYP enzymes

Abstract: Background and objective The activity of cytochrome P450 (CYP) enzymes, which determine the rate of elimination of lipid-soluble drugs, demonstrates considerable interindividual variability. The extent to which age and sex influence CYP activity remains unclear in humans. Our objectives were to determine whether in vivo activity of selected CYP enzymes is affected by age or sex and to evaluate sex bioequivalence in a large sample size. Methods We have assessed in vivo activity of the CYP1A2, 2C19, 2D6, 2E1, and 3A4 enzymes in 161 normal subjects (51% female subjects and 40% aged >50 years). After simultaneous administration of a cocktail of selective probes (caffeine, mephenytoin, debrisoquin [INN, debrisoquine], chlorzoxazone, and dapsone, respectively), phenotypic indices for metabolism of these drugs were used as measures of individual CYP activity. Sex bioequivalence analysis used the bootstrap method. Results There were no sex differences associated with CYP1A2 activity. A significant negative correlation (r = −0.572, P < .01) between enzyme activity and age was observed for CYP2C19, but there were no sex differences. CYP2D6 activity showed no dependence on age or sex. In contrast, CYP2E1 activity showed an age-associated increase (r = 0.393, P < .01), which developed earlier in life in male subjects compared with female subjects. These results were further supported by the sex bioequivalence analysis of CYP phenotypic activity, which revealed that sexes were equivalent with respect to CYP2C19 (90% confidence interval [CI], 0.874–1.04), CYP3A4 (90% CI, 0.95–1.176), and CYP2D6 (90% CI, 0.928–1.09) phenotype and just exceeded the 0.8 to 1.25 limits to be equivalent with respect to CYP2E1 (90% CI, 0.785–1.08) and CYP1A2 (90% CI, 0.736–1.03) phenotype. Conclusion These observations suggest that the presence of selective mechanisms of regulation for individual CYP enzymes can be influenced by age and sex. However, we suggest that sex has a limited ability to explain intersubject variation of activity for these phenotypic measures of CYP enzyme activity. Clinical Pharmacology & Therapeutics (2004) 76, 618–627; doi: 10.1016/j.clpt.2004.08.021