Clinical inhibition of CYP2D6-catalysed metabolism by the antianginal agent perhexiline

AIMS: Perhexiline is an antianginal agent that displays both saturable and polymorphic metabolism via CYP2D6. The aim of this study was to determine whether perhexiline produces clinically significant inhibition of CYP2D6-catalysed metabolism in angina patients. METHODS: The effects of perhexiline on CYP2D6-catalysed metabolism were investigated by comparing urinary total dextrorphan/dextromethorphan metabolic ratios following a single dose of dextromethorphan (16.4 mg) in eight matched control patients not taking perhexiline and 24 patients taking perhexiline. All of the patients taking perhexiline had blood drawn for CYP2D6 genotyping as well as to measure plasma perhexiline and cis-OH-perhexiline concentrations. RESULTS: Median (range) dextrorphan/dextromethorphan metabolic ratios were significantly higher (P < 0.0001) in control patients, 271.1 (40.3-686.1), compared with perhexiline-treated patients, 5.0 (0.3-107.9). In the perhexiline-treated group 10/24 patients had metabolic ratios consistent with poor metabolizer phenotypes; however, none was a genotypic poor metabolizer. Interestingly, 89% of patients who had phenocopied to poor metabolizers had only one functional CYP2D6 gene. There was a significant negative linear correlation between the log of the dextrorphan/dextromethorphan metabolic ratio and plasma perhexiline concentrations (r(2) = 0.69, P < 0.0001). Compared with patients with at least two functional CYP2D6 genes, those with one functional gene were on similar perhexiline dosage regimens but had significantly higher plasma perhexiline concentrations, 0.73 (0.21-1.00) vs. 0.36 (0.04-0.69) mg l(-1) (P = 0.04), lower cis-OH-perhexiline/perhexiline ratios, 2.85 (0.35-6.10) vs. 6.51 (1.84-11.67) (P = 0.03), and lower dextrorphan/dextromethorphan metabolic ratios, 2.51 (0.33-39.56) vs. 11.80 (2.90-36.93) (P = 0.005). CONCLUSIONS: Perhexiline significantly inhibits CYP2D6-catalysed metabolism in angina patients. The plasma cis-OH-perhexiline/perhexiline ratio may help to both phenotype patients and predict those in whom perhexiline may be most likely to cause clinically significant metabolic inhibition.     

The influence of CYP2D6 genotype on trough plasma perhexiline and cis-OH-perhexiline concentrations following a standard loading regimen in patients with myocardial ischaemia

AIMS: CYP2D6 protein expression is determined by the number of functional CYP2D6 alleles. It is also higher in individuals with at least one CYP2D6*2 allele. This study has investigated the effect of the number of functional CYP2D6 alleles and the influence of CYP2D6*2 alleles on plasma perhexiline concentrations in patients administered a standard loading regimen over 3 days. METHODS: Eighteen patients with myocardial ischaemia who were not taking any drugs known to inhibit CYP2D6 metabolism in vivo commenced treatment with 200 mg of perhexiline twice per day. On the fourth day, blood was drawn for genotyping and the measurement of trough plasma concentrations of perhexiline and its major metabolite, cis-OH-perhexiline. RESULTS: The only genotypic CYP2D6 poor metabolizer had a trough plasma perhexiline concentration of 2.70 mg l-1 and no detectable cis-OH-perhexiline. The mean+/-SD trough plasma perhexiline concentration in patients with one functional allele was significantly higher (0.63+/-0.31 mg l-1, n=8, P=0.05) than in patients with two functional alleles (0.37+/-0.17 mg l-1, n=9). Conversely, the mean metabolic ratio was significantly lower in patients with one functional allele (2.90+/-1.76, P<0.01) compared with patients with two functional alleles (6.52+/-3.26). Patients with at least one CYP2D6*2 allele had a lower plasma perhexiline concentration (0.20+/-0.09 mg l-1, n=5, P<0.001) and a higher metabolic ratio (7.86+/-2.51, P<0.01) than the non-poor metabolizer patients with no CYP2D6*2 alleles (0.62+/-0.23 mg l-1 and 3.55+/-2.54, respectively, n=12). CONCLUSION: Patients with only one functional allele and not CYP2D6*2 have diminished CYP2D6 metabolic capacity for perhexiline.     

CYP2B6, CYP2D6, and CYP3A4 catalyze the primary oxidative metabolism of perhexiline enantiomers by human liver microsomes.

The cytochrome P450 (P450)-mediated 4-monohydroxylations of the individual enantiomers of the racemic antianginal agent perhexiline (PHX) were investigated in human liver microsomes (HLMs) to identify stereoselective differences in metabolism and to determine the contribution of the polymorphic enzyme CYP2D6 and other P450s to the intrinsic clearance of each enantiomer. The cis-, trans1-, and trans2-4-monohydroxylation rates of (+)- and (-)-PHX by human liver microsomes from three extensive metabolizers (EMs), two intermediate metabolizers (IMs), and two poor metabolizers (PMs) of CYP2D6 were measured with a high-performance liquid chromatography assay. P450 isoform-specific inhibitors, monoclonal antibodies directed against P450 isoforms, and recombinantly expressed human P450 enzymes were used to define the P450 isoform profile of PHX 4-monohydroxylations. The total in vitro intrinsic clearance values (mean +/- S.D.) of (+)- and (-)-PHX were 1376 +/- 330 and 2475 +/- 321, 230 +/- 225 and 482 +/- 437, and 63.4 +/- 1.6 and 54.6 +/- 1.2 microl/min/mg for the EM, IM, and PM HLMs, respectively. CYP2D6 catalyzes the formation of cis-OH-(+)-PHX and trans1-OH-(+)-PHX from (+)-PHX and cis-OH-(-)-PHX from (-)-PHX with high affinity. CYP2B6 and CYP3A4 each catalyze the trans1- and trans2-4-monohydroxylation of both (+)- and (-)-PHX with low affinity. Both enantiomers of PHX are subject to significant polymorphic metabolism by CYP2D6, although this enzyme exhibits distinct stereoselectivity with respect to the conformation of metabolites and the rate at which they are formed. CYP2B6 and CYP3A4 are minor contributors to the intrinsic P450-mediated hepatic clearance of both enantiomers of PHX, except in CYP2D6 PMs.     

Polymorphic hydroxylation of perhexiline in vitro

AIMS: The aims of this study were to examine the in vitro enzyme kinetics and CYP isoform selectivity of perhexiline monohydroxylation using human liver microsomes. METHODS: Conversion of rac-perhexiline to monohydroxyperhexiline by human liver microsomes was assessed using a high-performance liquid chromatography assay with precolumn derivatization to measure the formation rate of the product. Isoform selective inhibitors were used to define the CYP isoform profile of perhexiline monohydroxylation. RESULTS: The rate of perhexiline monohydroxylation with microsomes from 20 livers varied 50-fold. The activity in 18 phenotypic perhexiline extensive metabolizer (PEM) livers varied about five-fold. The apparent Km was 3.3 +/- 1.5 micro m, the Vmax was 9.1 +/- 3.1 pmol min-1 mg-1 microsomal protein and the in vitro intrinsic clearance (Vmax/Km) was 2.9 +/- 0.5 micro l min-1 mg-1 microsomal protein in the extensive metabolizer livers. The corresponding values in the poor metabolizer livers were: apparent Km 124 +/- 141 micro m; Vmax 1.4 +/- 0.6 pmol min-1 mg-1 microsomal protein; and intrinsic clearance 0.026 micro l min-1 mg-1 microsomal protein. Quinidine almost completely inhibited perhexiline monohydroxylation activity, but inhibitors selective for other CYP isoforms had little effect. CONCLUSIONS: Perhexiline monohydroxylation is almost exclusively catalysed by CYP2D6 with activities being about 100-fold lower in CYP2D6 poor metabolizers than in extensive metabolizers. The in vitro data predict the in vivo saturable metabolism and pharmacogenetics of perhexiline.     

Consequences of rifampicin treatment on propafenone disposition in extensive and poor metabolizers of CYP2D6

Propafenone undergoes extensive metabolism both by phase I and phase II enzymes: cytochrome P4502D6 (CYP2D6) dependent polymorphic hydroxylation to its main metabolite 5-OH-propafenone, CYP3A4/1A2 dependent N-dealkylation and further glucuronidation and sulfation. Since CYP2D6 is not inducible by rifampicin, an important drug interaction between rifampicin and propafenone is not to be expected a priori. However, non-CYP2D6-dependent pathways may be induced as a case report described dramatically lowered plasma concentrations of propafenone with loss of dysrhythmia control associated with rifampicin treatment. Therefore, this study aimed to investigate induction properties of rifampicin on propafenone disposition in extensive metabolizers and poor metabolizers of CYP2D6. Six extensive metabolizers and six poor metabolizers ingested 600 mg rifampicin once daily for nine consecutive days. The day before the first rifampicin dose and on the day of the last rifampicin dose each individual received a single intravenous (i.v.) infusion of 140 mg unlabelled propafenone and 2 h later a single dose of 300 mg deuterated propafenone orally (p.o.). During enzyme induction maximum QRS prolongation decreased significantly after propafenone p.o. (21 +/- 7% versus 13 +/- 6% in extensive metabolizers, P < 0.01; 15 +/- 6% versus 9 +/- 6% in poor metabolizers, P < 0.01) and not after propafenone i.v. In parallel, there were no substantial differences in pharmacokinetics of propafenone i.v. by rifampicin. However, bioavailability of propafenone dropped from 30 +/- 15% to 10 +/- 8% in extensive metabolizers (P < 0.01) and from 81 +/- 6% to 48 +/- 8% in poor metabolizers (P < 0.001). Following propafenone p.o. clearances through N-dealkylation (4.1 +/- 2.1 ml/min versus 23.5 +/- 12.6 ml/min in extensive metabolizers, P < 0.01; 3.4 +/- 1.3 ml/min versus 16.0 +/- 5.5 ml/min in poor metabolizers, P < 0.001) and glucuronidation (123 +/- 48 ml/min versus 457 +/- 267 ml/min in extensive metabolizers, P < 0.05; 43 +/- 9 ml/min versus 112 +/- 34 ml/min in poor metabolizers, P < 0.01), but not 5-hydroxylation increased regardless of phenotype indicating substantial enzyme induction. Clearances to propafenone sulfate and conjugates of 5-OH-propafenone were significantly enhanced by rifampicin treatment in poor metabolizers (P < 0.01). Thus, induction of both phase I pathways (CYP3A4/1A2) and phase II pathways (glucuronidation, sulfation) of propafenone by rifampicin resulted in a clinically relevant metabolic drug interaction which was more pronounced in extensive metabolizers than in poor metabolizers with regard to percentage decrease in bioavailability of propafenone.     

Time-dependent amplified pharmacokinetic and pharmacodynamic responses of rabeprazole in cytochrome P450 2C19 poor metabolizers

STUDY OBJECTIVES: To determine the pharmacokinetic and pharmacodynamic rationale for the optimum regimen of rabeprazole in the treatment of Helicobacter pylori infection in patients who are cytochrome P450 (CYP) 2C19 poor metabolizers or extensive metabolizers. DESIGN: Prospective, multiple-dose pharmacokinetic and pharmacodynamic study. SETTING: University-affiliated medical center in Taiwan. SUBJECTS: Twelve healthy volunteers (aged 20-30 yrs) who were identified as CYP2C19 poor metabolizers (six subjects) or extensive metabolizers (six). INTERVENTION: Each subject received rabeprazole 20 mg twice/day for 3 consecutive days and once/day on the fourth day. MEASUREMENTS AND MAIN RESULTS: Pharmacokinetic and pharmacodynamic parameters were compared between CYP2C19 poor and extensive metabolizers on day 1 and day 4 of dosing. The mean +/- SD values of area under the concentration-time curve of rabeprazole and rabeprazole thioether were significantly higher in poor metabolizers than in extensive metabolizers on day 1 (5357 +/- 883 vs 1131 +/- 512 ng x hr/ml and 1703 +/- 432 vs 561 +/- 358 ng x hr/ml, respectively; p<0.001) and on day 4 (5601 +/- 669 vs 1619 +/- 778 ng x hr/ml and 1914 +/- 378 vs 511 +/- 360 ng x hr/ml, respectively; p<0.001). However, no significant difference was noted between day 1 and day 4 of dosing within the same genotype groups. Only CYP2C19 poor metabolizers had significantly higher plasma gastrin levels on day 4 compared with those levels on day 1 (p<0.05). The pharmacokinetic-pharmacodynamic relationship of rabeprazole appears to be time dependent. CONCLUSION: The pharmacokinetic and pharmacodynamic data suggest that CYP2C19 poor metabolizers might be subject to advantageous conditions, especially after day 4, for treating H. pylori infection with rabeprazole.     

Population estimation regarding the effects of cytochrome P450 2C19 and 3A5 polymorphisms on zonisamide clearance

We aimed to evaluate the effects of cytochrome P450 (CYP) 2C19 and CYP3A5 polymorphisms on zonisamide (ZNS) clearance. The pharmacokinetics of the 282 ZNS concentrations at a steady state obtained from 99 Japanese epileptic patients was performed with a nonlinear mixed-effect modeling program, using a one-compartment open pharmacokinetic model with first-order elimination. The covariates screened included the total body weight, gender, ZNS daily dose, CYP2C19 and CYP3A5 genotypes, and the coadministered antiepileptic drugs. The final model of ZNS apparent clearance was as follows: CL = 1.22 x (BW/44)0.77 x DOSE(-0.17 x 0.84CYP2C19 hetero EM x 0.70CYP2C19 PM x 1.24CBZ x 1.28PHT x 1.29PB x eetaCL where CL is the apparent oral clearance of ZNS, DOSE is ZNS daily dose, and CYP2C19 heterozygous extensive metabolizer (EM) or CYP2C19 poor metabolizer (PM) is equal to 1 if one or two CYP2C19-defective alleles are carried, respectively; otherwise, it is 0. Carbamazepine (CBZ), phenytoin (PHT), or phenobarbital (PB) is equal to 1 if carbamazepine, phenytoin, or phenobarbital is coadministered, respectively; otherwise, it is 0. etaCL is the independent random error distributed normally with the mean zero and variance equal to omegaCL. The CL of ZNS was lower in the CYP2C19 heterozygous extensive metabolizers and poor metabolizers than in the homozygous extensive metabolizers by 16% and 30%, respectively (P < 0.001). An effect of CYP3A5 polymorphisms was not identified. The coadministration of carbamazepine, phenytoin, or phenobarbital increased the CL of ZNS by 24% to 29%. This report demonstrates that the CYP2C19 genotype affects the ZNS metabolism in Japanese epileptic subjects. The clinical relevance of these changes remains to be explored in future studies.     

Lornoxicam pharmacokinetics in relation to cytochrome P450 2C9 genotype

AIMS: To investigate the pharmacokinetics of lornoxicam and the relationship with CYP2C9 polymorphism in healthy Chinese subjects. METHODS: A single oral dose of 8 mg lornoxicam was administered to 18 healthy Chinese male subjects. Plasma was sampled for 24 h post dose, and plasma concentrations of lornoxicam were measured using a validated LC/MS/MS method. CYP2C9 genotype was determined by polymerase chain reaction-based restriction fragment length polymorphism or by direct sequencing of the coding region of the CYP2C9 gene. RESULTS: Of the 18 subjects, one subject was found to be a very poor metabolizer of lornoxicam with a long t(1/2) of 106 h, a low CL/F of 0.71 ml min(-1), and a high AUC(0-infinity) of 187.6 microg ml(-1) h. Genotyping studies revealed that this subject was heterozygous for CYP2C9*3 and a new variant CYP2C9 allele. Of the other 17 subjects, 13 were *1/*1 carriers, three were *1/*3 carriers, and one was a *1/*2 carrier. Mean AUC(0-infinity) values (95% confidence intervals) of lornoxicam were 9.25 (6.55, 11.95) vs. 4.75 (3.55, 5.95) microg ml(-1) h in *1 heterozygotes vs.*1 homozygotes, and mean CL/F values were 14.8 (10.2, 19.4) vs. 32.9 (24.5, 41.3) ml min(-1), respectively (P < 0.05 for both AUC and CL/F). CONCLUSIONS: The results show that the pharmacokinetics of lornoxicam are dependent on CYP2C9 polymorphism. In particular, the presence of the CYP2C9*3 allele impairs the oral clearance of lornoxicam.     

Enantiospecific pharmacokinetics of metoprolol in CYP2D6 ultra-rapid metabolizers and correlation with exercise-induced heart rate

OBJECTIVE: Our objective was to study the enantioselective pharmacokinetics of metoprolol in CYP2D6 ultra-rapid metabolizers (UM) compared with extensive (EM) and poor (PM) metabolizers to quantify differential effects of metoprolol enantiomers on the beta1-adrenoreceptor blockade. METHODS: Twenty-nine healthy individuals were selected based on their CYP2D6 genotype, and 100 mg racemic metoprolol was administered. Plasma concentrations of R- and S-metoprolol and the metabolites SS-, SR-, RS-, and RR-hydroxymetoprolol were quantified by high-performance liquid chromatography. RESULTS: Mean (+/-SD) AUCs of S-metoprolol were 190 +/- 99 ng/ml.h in UMs, 366 +/- 158 in EMs, and 1,804 +/- 300 in PMs. For R-metoprolol, the AUCs were 127 +/- 72 ng/ml.h in UMs, 261 +/- 126 in EMs, and 1,746 +/- 319 in PMs. The concentrations of R-metoprolol and S-metoprolol, respectively, needed to obtain a half-maximum reduction in heart rate were estimated as 20 and 21 ng/ml in PMs, 11 and 17 ng/ml in EMs, and 7 and 11 ng/ml in UMs. CONCLUSION: A slight enantiopreference towards metabolism of R-metoprolol by CYP2D6 was observed in EMs and even more in the UM group, but the effect was far from being enantioselective.     

Relationships between body composition parameters and fluorouracil pharmacokinetics

AIMS: To verify whether fluorouracil (FU) clearance (CL) and volume of distribution (V(ss)) are better correlated with specific body compartments, such as body cell mass (BCM), total body water (TBW) or fat free mass (FFM), rather than with body surface area (BSA) or total body weight (BW). METHODS: Thirty-four patients (13 females and 21 males) affected by colorectal cancer and receiving FU as adjuvant therapy entered the study. CL and Vss were determined after a 2 min i.v. injection of FU (425 mg m(-2)) and leucovorin (20 mg m(-2)). Body composition, in terms of BCM, TBW and FFM, was evaluated non-invasively by bioelectrical impedance analysis (BIA). RESULTS: Significant but poor correlations were found between CL or V(ss) and most anthropometric parameters, including BIA-derived measures (r2 range=0.10-0.21). However, when multiple regression analysis was performed with sex, TBW and FFM as independent variables, the correlations improved greatly. The best correlation was obtained between CL and sex (r2=0.44) and between V(ss) and sex (r2=0.36). FFM-normalized CL was significantly higher in women than in men (0.030+/-0.008 vs 0.022+/-0.005 l min(-1) kg)(-1); 95% CI of difference 0.012, 0.003; P=0.003), suggesting that FU metabolism is more rapid in females. Surprisingly, V(ss) was highly correlated with CL (r2=0.67; CL=0.52+V(ss) x 0.040). This finding may either be explained by extensive drug metabolism in extra-hepatic organs or by variable inactivation on first-pass through the lung. Both these hypotheses need experimental validation. CONCLUSIONS: The pharmacokinetics of FU are better predicted by FFM and TBW than by standard anthropometric parameters and predictions are sex-dependent. The use of BIA may lead to improved dosing with FU.     

Pharmacokinetics of citalopram in relation to genetic polymorphism of CYP2C19.

The study was designed to define the contribution of cytochrome p450 2C19 (CYP2C19) and cytochrome p450 3A4 (CYP3A4) to citalopram N-demethylation and to evaluate the relationship between the disposition of citalopram and CYP2C19 genotype. A single oral 40-mg dose of citalopram was administered to eight extensive metabolizers and five poor metabolizers recruited from 77 healthy Chinese volunteers whose genotypes and phenotypes were predetermined. The plasma concentrations of citalopram and desmethylcitalopram were determined by high-performance liquid chromatography. It was found that the genotype of CYP2C19 had a significant effect on the N-demethylation of citalopram. Poor metabolizers with m1 mutation had higher area under the plasma concentration versus time curve (AUC0--> infinity ) values than did extensive metabolizers. Terminal elimination half-life (t1/2) values of citalopram in poor metabolizers were significantly higher than the values in extensive metabolizers who were either homozygous or heterozygous with CYP2C19*1. The oral clearance (CLoral) of citalopram in poor metabolizers was significantly lower than that of extensive metabolizers. The AUC0--> infinity and maximum plasma concentration (Cmax) of desmethylcitalopram in poor metabolizers were significantly lower than the values of extensive metabolizers. The results show that CYP3A4 is not the major enzyme in the N-demethylation of citalopram among extensive metabolizers. The polymorphism of CYP2C19 plays an important role in the N- demethylation of citalopram in vivo. The extensive metabolizers and poor metabolizers of CYP2C19 had significant difference in disposition of citalopram in vivo.     

CYP2C19多态性对奥美拉唑药代动力学的影响

目的:研究中国健康受试者细胞色素P4502C19(CYP2C19)多态性对奥美拉唑体内药代动力学的影响。方法:筛选12名健康男性和12名健康女性受试者,采用随机分组、双交叉的试验方案,每组分别服用一种奥美拉唑7d,洗脱期7d,第2周期交换用药。用LC-MS/MS方法测定每周期第1天和第7天多个时间点血药浓度,计算两种奥美拉唑的药代动力学参数。检测受试者基因位点CYP2C19*2(681G>A)和CYP2C19*3(636G>A),按照基因型分成快代谢型、中等代谢型和慢代谢型。结果:慢代谢型、中等代谢型和快代谢型在药代动力学参数t1/2、MRT0-t、CL、Vd、AUC0-t、AUC0-∞中存在显著性差异(P<0.05),连续给药后基因多态性对药物代谢的影响相对减小。结论:CYP2C19多态性与奥美拉唑的代谢密切相关,临床上应关注基因多态性对奥美拉唑代谢的影响。     

Studies on drug interactions between esomeprazole, amoxicillin and clarithromycin in healthy subjects

OBJECTIVE: A combination of esomeprazole, amoxicillin and clarithromycin may be used for Helicobacter pylori eradication. We explored the potential for interactions between these drugs. METHODS: In 2 randomized, 4-way crossover studies, healthy CYP2C19 extensive metabolizers (EMs) received esomeprazole 40 mg once daily (n = 20) or 20 mg twice daily (b.i.d.) (n = 20), clarithromycin 500 mg b.i.d., amoxicillin 1 g b.i.d. or the combination of the 3 drugs for 7 days. In a third randomized, 2-way, crossover study, 6 healthy CYP2C 19 poor metabolizers (PMs) received esomeprazole 40 mg once daily with and without clarithromycin 500 mg b.i.d. for 1 week. RESULTS: Triple therapy with esomeprazole 40 mg increased the area under the plasma concentration-time curve during the dosing interval (AUCtau) from 13.31 micromol x h/l (11.12-15.93) for esomeprazole alone to 22.69 micromol x h/l (18.94-27.17) for triple treatment. Respective AUCtau values with esomeprazole 20 mg b.i.d. were 4.97 micromol.h/l (3.97-6.21) and 11.29 micromol x h/l (9.03-14.12). Clarithromycin and amoxicillin plasma levels were largely unchanged by combination therapy. In PMs, the esomeprazole AUC also approximately doubled when administered in combination with clarithromycin. All treatments were well tolerated. CONCLUSION: Clarithromycin decreases the metabolism rate of esomeprazole, leading to approximately doubled AUC values, both in EMs and PMs.     

CYP3A4 mediates dextropropoxyphene N-demethylation to nordextropropoxyphene: human in vitro and in vivo studies and lack of CYP2D6 involvement

The individual cytochrome P450 isoforms in dextropropoxyphene N-demethylation to nordextropropoxyphene were determined and the pharmacokinetics of dextropropoxyphene and nordextropropoxyphene in cytochrome P4502D6 (CYP2D6) extensive (EM) and poor (PM) subjects were characterized. Microsomes from six CYP2D6 extensive metabolizers and one CYP2D6 poor metabolizer were used with isoform specific chemical and antibody inhibitors and expressed recombinant CYP enzymes. Groups of three CYP2D6 EM and PM subjects received a single 65-mg oral dose of dextropropoxyphene, and blood and urine were collected for 168 and 96 h, respectively. Nordextropropoxyphene formation in vitro was not different between the CYP2D6 extensive metabolizers (Km = 179 +/- 74 microM, Cl(int) = 0.41 +/- 0.26 ml mg(-1)h(-1)) and the PM subject (K = 225 microM, Cl(int) = 0.19 ml mg(-1) h(-1)) and was catalysed predominantly by CYP3A4. There was no apparent difference in the pharmacokinetics of dextropropoxyphene and nordextropropoxyphene in CYP2D6 EM and PM subjects. CYP3A4 is the major CYP enzyme catalysing the major metabolic pathway of dextropropoxyphene metabolism. Hence variability in the pharmacodynamic effects of dextropropoxyphene are likely due to intersubject variability in hepatic CYP3A4 expression and/or drug-drug interactions. Reported CYP2D6 phenocopying is not due to dextropropoxyphene being a CYP2D6 substrate.     

Can absolute oral bioavailability in humans be predicted from animals? A comparison of allometry and different indirect methods

The objective of this study was to predict absolute bioavailability in humans from animal data using interspecies scaling as well as indirect approaches. Five different methods were used to predict absolute bioavailability in humans: (i) absolute bioavailability vs body weight (allometric approach); (ii) F = CL(IV)/CL(oral); (iii) F = 1-[CL(IV)/Q]; (iv) F = 1-[CL(oral)/Q]; and (v) F = Q/[Q + CL(oral)]. Methods II-V are indirect approaches, where predicted i.v. or oral clearance and hepatic blood flow (Q) (1500 ml/min) were used to predict absolute bioavailability in humans. Fifteen drugs were tested and the results of this study indicate that all five approaches predict absolute bioavailability with different degrees of accuracy, and are therefore unreliable for the accurate prediction of absolute bioavailability in humans from animal data. In conclusion, although the above-mentioned approaches do not accurately predict absolute bioavailability, a rough estimate of absolute bioavailability is possible using these approaches.     

Proguanil disposition and toxicity in malaria patients from Vanuatu with high frequencies of CYP2C19 mutations.

The increasing resistance of falciparum malaria to common antimalarial drugs has renewed interest in the compound proguanil normally metabolized to cycloguanil, a strong dihydrofolate reductase inhibitor, via the cytochrome P450 isozyme CYP2C19. The relationship between CYP2C19 genotypes and proguanil metabolism was therefore studied in 100 uncomplicated malaria patients on Malakula island in Vanuatu, where a CYP2C19-related poor metabolizer genotype status was known to be frequent. The patients (median age, 7 years) with Plasmodium falciparum or P. vivax infections, received proguanil treatment for 3 days in daily doses corresponding to adult doses of 300-500 mg. Capillary blood samples were collected on filter paper for determining both human CYP2C19 mutations by polymerase chain reaction and mutation-specific restriction enzyme digestion and blood concentrations of proguanil and its metabolites by high-performance liquid chromatography. The frequencies of the defective alleles, CYP2C19*2 and CYP2C19*3, were 0.57 and 0.25, respectively. The patients were genotyped as 68 CYP2C19-related poor metabolizers and 32 extensive metabolizers. Proguanil concentrations were higher and cycloguanil and 4-chlorophenylbiguanide concentrations were lower in poor compared to extensive metabolizers. Among the extensive metabolizers, 27 were heterozygous and five were homozygous for unmutated alleles. The tendency of an intermediate degree of proguanil metabolism in heterozygous extensive metabolizers as compared to homozygous extensive metabolizers and poor metabolizers suggests the trend towards the existence of a gene dose effect. Mild adverse events (mainly gastro-intestinal symptoms) were often reported and positively correlated with proguanil concentrations. The incidence was, however, similar in poor and extensive metabolizers. In conclusion, our data demonstrate an association between CYP2C19 mutations and poor metabolism of proguanil.     

Inhibitory effect of troleandomycin on the metabolism of omeprazole is CYP2C19 genotype-dependent

1. Eighteen healthy CYP2C19 genotyped male subjects were administered a 20-mg oral dose of omeprazole (OP) alone or received troleandomycin (TAO) 500 mg daily for 2 days before the dose of OP was administered. Blood samples were obtained and OP 5-hydroxyomeprazole (5-OH-OP) and OP sulfone in plasma were determined by reversed-phase HPLC. 2. The mean C(max), AUC and CL for OP in poor metabolizers (PMs) were greater with TAO than without TAO. The C(max) and AUC of 5-OH-OP in PMs were significantly (p < 0.05) less with TAO than without TAO. The differences in 5-OH-OP between heterozygous extensive metabolizers (EMs) with TAO versus without TAO were similar to those observed in PMs, except for the AUC. However, in homozygous EMs, there were no statistical differences for the effect of TAO. 3. The effect of TAO on the metabolism of OP and its two principal metabolites differs in different genotype groups of CYP2C19. CYP3A4 not only plays a dominant role in the formation of OP sulfone, but also it contributes to the 5-hydroxylation of OP. Both CYP2C19 and CYP3A contribute to the further elimination of 5-OH-OP and OP sulfone.     

Pharmacokinetics of efavirenz when co-administered with rifampin in TB/HIV co-infected patients: pharmacogenetic effect of CYP2B6 variation

The goal of this study was to determine the effect of CYP2B6 genetic variation on the steady-state pharmacokinetics of efavirenz (600 mg/d) in TB/HIV co-infected patients receiving concomitant rifampin, a potent CYP inducer. In the 26 patients studied, CYP2B6 c.516GG, GT, and TT genotype frequencies were 0.27, 0.50, and 0.23, respectively. Mean plasma efavirenz area under the curve was significantly higher in patients with CYP2B6 c.516TT than in those with GT (107 vs 27.6 microg x h/mL, P< .0001) or GG genotype (107 vs 23.0 microg x h/mL, P< .0001). Apparent oral clearance (CL/F) was significantly lower in patients with CYP2B6 c.516TT than in those with GT genotype (2.1 vs 8.4 mL/min/kg, P<0.0001) and GG genotype (2.1 vs 9.9 mL/min/kg, P< .0001). No differences in efavirenz exposure or CL/F existed between patients with CYP2B6 c.516GT and GG genotypes. Our results indicate that CYP2B6 c.516TT genotype can be used to identify efavirenz poor metabolizers in patients co-treated with rifampin.     

CYP2C19 polymorphism and proton pump inhibitors

Proton pump inhibitors such as omeprazole (esomeprazole), lansoprazole, pantoprazole and rabeprazole are eliminated by the hepatic route and the polymorphic CYP2C19 is mainly involved in their metabolism. In different populations three phenotypes have been identified: extensive metabolizers, poor metabolizers and individuals carrying one wild type and one mutant allele (het extensive metabolizers). Systemic exposure to the proton pump inhibitors as expressed by the AUC (area under the plasma level time profiles) is 5-12-times higher in poor metabolizers than in extensive metabolizers. As the pharmacodynamic response (elevation of intragastric pH) to the proton pump inhibitors is related directly to their AUC, a much higher pH can be monitored over 24 hr in poor metabolizers than in extensive metabolizers. Furthermore, clinical efficacy of all proton pump inhibitors depend on maintaining intragastric pH above certain threshold levels and significantly higher eradication rates of Helicobacter pylori have been observed in patients of the poor metabolizers and het extensive metabolizers phenotype if compared to extensive metabolizers. Likewise, limited data suggest that proton pump inhibitors-induced healing rates in gastro-oesophageal reflux disease are apparently higher in poor metabolizers/het extensive metabolizers than in extensive metabolizers of CYP2C19. Therefore initial genotyping for this enzyme and higher dosage in extensive metabolizers is likely to improve the clinical efficacy of proton pump inhibitors.     

Effect of the CYP2C19 oxidation polymorphism on fluoxetine metabolism in Chinese healthy subjects

AIMS: The study was designed to investigate whether genetically determined CYP2C19 activity affects the metabolism of fluoxetine in healthy subjects. METHODS: A single oral dose of fluoxetine (40 mg) was administrated successively to 14 healthy young men with high (extensive metabolizers, n=8) and low (poor metabolizers, n = 6) CYP2C19 activity. Blood samples were collected for 5-7 half-lives and fluoxetine, and norfluoxetine were determined by reversed-phase high performance liquid chromatography. RESULTS: Poor metabolizers (PMs) showed a mean 46% increase in fluoxetine peak plasma concentrations (Cmax, P < 0.001), 128% increase in area under the concentration vs time curve (AUC(0, infinity), P < 0.001), 113% increase in terminal elimination half-life (t(1/2)) (P < 0.001), and 55% decrease in CLo (P < 0.001) compared with extensive metabolizers (EMs). Mean +/- (s.d) norfluoxetine AUC(0, 192 h) was significantly lower in PMs than that in EMs (1343 +/- 277 vs 2935 +/- 311, P < 0.001). Mean fluoxetine Cmax and AUC(0, infinity) in wild-type homozygotes (CYP2C19*1/CYP2C19*1) were significantly lower than that in PMs (22.4 +/- 3.9 vs 36.7 +/- 8.9, P < 0.001; 732 +/- 42 vs 2152 +/- 492, P < 0.001, respectively). Mean oral clearance in individuals with the wild type homozygous genotype was significantly higher than that in heterozygotes and that in PMs (54.7 +/- 3.4 vs 36.0 +/- 8.7, P < 0.01; 54.7 +/- 3.4 vs 20.6 +/- 6.2, P < 0.001, respectively). Mean norfluoxetine AUC(0, 192 h) in PMs was significantly lower than that in wild type homozygotes (1343 +/- 277 vs 3163 +/- 121, P < 0.05) and that in heterozygotes (1343 +/- 277 vs 2706 +/- 273, P < 0.001), respectively. CONCLUSIONS: The results indicated that CYP2C19 appears to play a major role in the metabolism of fluoxetine, and in particular its N-demethylation among Chinese healthy subjects.