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.     

Correlation of CYP2D6 genotype with perhexiline phenotypic metabolizer status

Perhexiline is metabolized by CYP2D6 and has concentration-related hepatoxicity and peripheral neuropathy. The risk of toxicity is reduced using therapeutic drug monitoring. CYP2D6 genotyping before therapy may allow earlier appropriate dosing. This study aimed to determine whether assessment of CYP2D6 genotype in patients on perhexiline could predict accurately metabolizer status as determined by the perhexiline metabolic ratio (MR). Blood samples from patients stabilized on perhexiline were analysed for CYP2D6 genotype and for concentrations of perhexiline and its hydroxy metabolite. The MR was determined. Of 74 patients, five were poor metabolizers (PM) defined by a MR<0.4, and the remainder were extensive metabolizers (EM). The genotypes were: *1/*1 (n=21), *1/*4 (n=18), *1/*2 (n=12), *1/*3 (n=2), *1/*5 (n=1), *1/*9 (n=2), *1/*10 (n=2), *2/*4 (n=4), *2/*2 (n=3), *4/*41 (n=3), *2/*41 (n=1), *41/*41 (n=1), *4/*9 (n=1), *4/*5 (n=1), *5/*6 (n=1) and *4/*6 (n=1). Allele frequencies were consistent with those reported in population studies. The 3 PMs with the lowest MR were predicted by genotype (*4/*5, *5/*6, *4/*6). The other 2 PMs had intermediate metabolizer genotypes and were on CYP2D6 inhibiting drugs. Amongst the EMs, the highest MR was associated with *1 and *2 allele combinations and the MR was progressively lower with the presence of alleles with intermediate function (*9, *10, *41) followed by alleles with no functional product (*3, *4, *5, *6). Thus, a gene-dose effect was observed. Genotype predicted PM phenotype and also intermediate metabolizers. Determination of CYP2D6 genotype before therapy with perhexiline may help predict perhexiline dose requirements and reduce the risk of perhexiline concentration-related toxicity.     

Effect of CYP2D6 genotypes on the metabolism of haloperidol in a Japanese psychiatric population.

We investigated the effect of CYP2D6 genotypes on plasma levels of haloperidol (HAL) and reduced haloperidol (RHAL) in 88 Japanese schizophrenic inpatients being treated with HAL. Some subjects carrying CYP2D6*5 allele (CYP2D6*1/CYP2D6*5, CYP2D6*5/CYP2D6*10) showed extremely high concentrations of both HAL and RHAL, and the groups with CYP2D6*5 allele seemed to have higher plasma concentrations of HAL (1.14+/-0.69 ng/ml/mg) and RHAL (1.10+/-1.05 ng/ml/mg) than the other groups. Among those without CYP2D6*5 allele, there were no significant differences in plasma concentrations of HAL and RHAL between those without CYP2D6*10 allele (HAL=0.68+/-0.31 ng/ml/mg, RHAL=0.28+/-0.37 ng/ml/mg), those with one CYP2D6*10 (HAL=0.70+/-0.23 ng/ml/mg, RHAL=0.31+/-0.16 ng/ml/mg) and those with two CYP2D6*10 alleles (HAL=0.69+/-0.14 ng/ml/mg, RHAL=0.40+/-0.09 ng/ml/mg), although there was a tendency of higher plasma concentration of RHAL in those with two CYP2D6*10 alleles. At a lower daily dosage of HAL (<10 mg/day), the subjects with two or one CYP2D6*10 allele(s) showed significantly higher plasma concentrations of RHAL (0.43+/-0.23 ng/ml/mg, 0.34+/-0.16 ng/ml/mg) than those without CYP2D6*10 allele (0.18+/-0.16 ng/ml/mg). The results of this study indicate that CYP2D6*10 allele plays significant but modest role in HAL metabolism in Japanese; nevertheless, we should not lump CYP2D6*10 allele with CYP2D6*5 allele because these two mutated alleles seem to have different impacts in the metabolism of HAL.     

Effect of the CYP2D6 genotype on metoprolol metabolism persists during long-term treatment

The beta1 selective beta-blocker metoprolol is metabolized predominantly but not exclusively by CYP2D6. Due to the polymorphism of the CYP2D6 gene, CYP2D6 activity varies markedly between individuals. Consequently, after short-term administration metoprolol plasma concentrations were found to be several fold higher in poor metabolizers than in extensive metabolizers. However, it is currently not known, whether the impact of the CYP2D6 polymorphism persists during long-term therapy, since alternate mechanisms of elimination or metabolism could be effective in this setting. The study comprised 91 Caucasian patients on long-term treatment with metoprolol (median duration of treatment 12.6 months; median daily drug dose: 47.5 mg/day). Metoprolol and alpha-OH-metoprolol plasma concentrations were assessed by HPLC. Genotyping detected the null alleles (*0): *3, *4, *5, *6, *7, *8, *12, *14, *15, the alleles *9, *10 and *41 associated with reduced enzymatic activity as well as the fully functional alleles *1 and *2. Genotype and allele frequencies were in accordance with published frequencies for the German population. The plasma metabolic ratio of metoprolol/alpha-OH-metoprolol was markedly affected by the genotype (P < 0.0001). In accordance, median adjusted metoprolol plasma concentrations were 6.2- and 3.9-fold higher in patients with *0/*0 genotypes (n = 8) and intermediate genotypes (n = 10), respectively, as compared to those with two fully functional alleles (n = 31; P < 0.01). In summary, the pronounced effect of the CYP2D6 genotype persists during long-term therapy, affecting both metabolic ratio and metoprolol plasma concentration.     

Pharmacokinetics of the antianginal agent perhexiline: relationship between metabolic ratio and steady-state dose

AIMS: 1) To develop an estimate of oral clearance (CL(Px)/F) for the antianginal agent perhexiline based on the ratio of cis-OH-perhexiline metabolite/parent perhexiline plasma concentrations at steady-state (C(OHPx,ss)/C(Px,ss)). 2) To determine whether the ratio measured in the first fortnight of treatment (C(i)(OHPx)/C(i)(Px)) may be used to guide patient dosing with perhexiline, a drug with a narrow therapeutic index, long half-life and saturable metabolism via CYP2D6. METHODS: Two retrospective studies were conducted reviewing patient records and data obtained from routine monitoring of plasma perhexiline and cis-OH-perhexiline concentrations. RESULTS: Study 1 (n=70). At steady-state, the frequency distributions of CL(Px)/F and C(OHPx,ss)/C(Px,ss) were consistent with CYP2D6 metabolism. Putative poor metabolizers (approximately 8%) were identified by CL(Px)/F< or =50 ml min(-1) or C(OHPx,ss)/C(Px,ss)< or =0.3. A group of patients with CL(Px)/F> or =950 ml min(-1) may have been ultra-rapid metabolizers. In this group, the high CL(Px)/F values suggest extensive first-pass metabolism and poor bioavailability. In patients with therapeutic plasma perhexiline concentrations (0.15-0.60 mg l(-1)), the variability in dose appeared directly proportional to CL(Px)/F (r2=0.741, P<0.0001). Study 2 (n=23). Using C(i)(OHPx)/C(i)(Px) patients were tentatively identified as poor, extensive and ultra-rapid metabolizers, with CL(Px)/F of 23-72, 134-868 and 947-1462 ml min(-1), respectively, requiring doses of 10-25, 100-250 and 300-500 mg day(-1), respectively. CONCLUSIONS: The cis-OH-perhexiline/perhexiline concentration ratio may be useful for optimizing individual patient treatment with the antianginal agent perhexiline.     

Analysis of the CYP2D6 gene mutations and their consequences for enzyme function in a West African population

The data on differences in the metabolic handling of the CYP2D6 probe drugs sparteine and debrisoquine, and the relationship between phenotype and genotype and gene frequencies for several mutant CYP2D6 alleles in African populations are limited and sometimes controversial. Therefore, in a West African population (Ghana), we investigated (i) the phenotype for sparteine debrisoquine by phenotyping 201 individuals with both drugs and (iii) the genotype for CYP2D6 (n = 326) and debrisoquine (n = 201) oxidation, (ii) the coregulatory control of sparteine and alleles *3 and *4 in 133 individuals and for the alleles *1, *2, *3, *4, *5, *6, *7, *8, *9, *10, *14, *16, *17, *2b, *2xN, *2bxN in 193 individuals. Of the 326 individuals phenotyped with sparteine, eight had a metabolic ratio (MR)sp > 20 corresponding to a poor metabolizer frequency of 2.5% [95% (confidence interval) CI = 1.06-4.77]. The prevalence of the poor metabolizer phenotype for debrisoquine oxidation was 3% (95% CI = 1.1-6.39) with six of the 201 individuals having a MR greater than 12.6. The distribution of the MR of sparteine was trimodal whereas MR of debrisoquine was unimodally distributed with a pronounced kurtosis. In individuals phenotyped with both drugs, there was a significant correlation between the MRs (r(s) = 0.63, P < 0.001). The CYP2D6 alleles *1, *2 and *17 were the most common functional alleles occurring with frequencies of 43.7, 10.6 and 27.7%, respectively. The three other observed functional alleles *2xN, *10 and *20 had much lower frequencies (1.6%, 3.1% and 0.3%, respectively). Of the eight non-functional alleles, only *4 (6.3%) and *5 (6.0%) could be found. The allele *5 occurred with the same frequency as in Caucasian populations (4.1%) but the *4 allele had a much lower frequency (Caucasians 19.5%). One individual with *1/*1 was a poor metabolizer for sparteine and debrisoquine indicating the existence of as yet unknown non-functional alleles in this West African population. Although the prevalence of poor metabolizers and the number of heterozygotes for non-functional alleles was much lower in Ghanaians, the median MRsp of 0.7 was significantly higher in this population compared with a median MRsp of 0.4 in Caucasians, indicating a lower metabolic clearance for CYP2D6 substrates in the West Africans. The lower metabolic activity in Ghanaians could not be explained solely by the high frequency of the *17 allele, which is associated with an impairment of CYP2D6 enzyme function. In addition, a higher median MRsp of 0.5 corresponding to metabolic clearance of 346 ml/min was observed among extensive metabolizers with the genotype *1/*1. Thus, compared with the median of MRsp = 0.28 (CLmet 573 ml/min) in Caucasians homozygous for *1, the metabolic clearance of sparteine was 40% lower on average in respective Ghanaians.     

Impact of CYP2D6 intermediate metabolizer alleles on single-dose desipramine pharmacokinetics

This study utilized cytochrome P450 2D6 (CYP2D6) genotypes to explain variability of desipramine pharmacokinetics in a cohort of non-poor metabolizer individuals. In an interaction study utilizing desipramine as a probe, genotyping for the CYP2D6*3, *4, *5 and *6 alleles was used to screen out CYP2D6 poor metabolizers. Individuals were categorized according to these and additional alleles (CYP2D6*2, *9, *10, *17, *41 and x2). Genotypes of individuals heterozygous for two or three of *2, *17 and *41 alleles were confirmed by molecular haplotyping. Pharmacokinetic parameters of desipramine were analysed according to CYP2D6 category. Molecular haplotyping was necessary to definitively categorize four of 16 individuals. A subject who had unusually high plasma elimination half-time, exposure and metabolic ratios carried an intermediate metabolizer (IM) *9 allele in combination with a non-functional allele. This combination has a population frequency of less than 1 : 200. Individuals with *1/*1, *1/*2 and *2/*2 genotypes had lower than average plasma elimination half-time, exposure and metabolic ratios. For desipramine, additional genotyping of CYP2D6 IM alleles helped define subgroups of the CYP2D6-positive cohort. This suggests that genotyping for IM alleles will aid in interpretation of clinical trials involving CYP2D6 substrates. Due to the diversity of IM alleles, molecular haplotyping may be necessary to fully characterize CYP2D6 genotype-phenotype relationships.     

Polymorphisms in CYP2D6 duplication-negative individuals with the ultrarapid metabolizer phenotype: a role for the CYP2D6*35 allele in ultrarapid metabolism?

Ultrarapid drug metabolism mediated by CYP2D6 is associated with inheritance of alleles with duplicated or amplified functional CYP2D6 genes. However, genotyping for duplicated CYP2D6 alleles only explains a fraction (10-30%) of the ultrarapid metabolizer phenotypes observed in Caucasian populations. Using a sample of CYP2D6 duplication-negative ultrarapid metabolizer subjects and selected control subjects with extensive metabolism, we examined parts of the CYP2D7 pseudogene, and the promoter region and 5'-coding sequence of CYP2D6 for polymorphisms possibly associated with the ultrarapid metabolizer phenotype. In an initial screening of 17 subjects (13 ultrarapid metabolizers and four extensive metabolizers), we identified three DNA variants in the 5'-end of the CYP2D7 pseudogene and 29 variants in the 5'-end of the CYP2D6 gene. Five variants were then selected for examination in a larger sample of subjects having the ultrarapid metabolizer (n = 27) or extensive metabolizer phenotype (n = 77). Subsequent statistical analyses of allele, genotype and estimated haplotype distributions showed that the 31A allele of the 31G > A (Val(II)Met) polymorphism was significantly more frequent in ultrarapid metabolizer subjects than in extensive metabolizer subjects (P = 0.04). Also, estimation of haplotype frequencies suggested that one of the haplotypes with the 31A variant was significantly more frequent among the ultrarapid metabolizers compared with the extensive metabolizers (P = 0.03). The average metabolic ratio was significantly lower in subjects possessing the 31A allele compared with subjects homozygous for the 31G allele (P = 0.02). We also observed a nonsignificant over-representation of the G-allele of a - 1584 C > G promoter polymorphism in the ultrarapid metabolizer group. Since our results are based on a relatively low number of subjects, further studies on larger samples and functional analyses of the polymorphisms detected are necessary to determine the role of the 31G > A and - 1584C > 6 variants in CYP2D6 duplication-negative ultrarapid metabolizer subjects.     

Depot haloperidol treatment in outpatients with schizophrenia on monotherapy: impact of CYP2D6 polymorphism on pharmacokinetics and treatment outcome

Haloperidol and several other antipsychotic drugs are at least partially metabolized by the polymorphic cytochrome P450 2D6 (CYP2D6). The interindividual variation in metabolic capacity of CYP2D6 might be of importance when dosing. In this study, 26 outpatients with schizophrenia and depot haloperidol as monotherapy were genotyped. The authors found 1 patient with no functional alleles, 8 with one functional allele, 16 with two functional alleles, and 1 with three functional alleles. The daily dose of haloperidol ranged from 0.45 to 14.29 mg. Steady state plasma concentrations were measured at peak (range, 1.6-67 nmol/L) and at trough (range, 1.0-49 nmol/L). The Positive and Negative Syndrome scale for Schizophrenia and the Extrapyramidal Symptom Rating Scale were used to evaluate the clinical effect. The authors found a clear correlation between haloperidol plasma concentration and number of active CYP2D6 alleles. No correlation was found between plasma concentration of haloperidol or number of CYP2D6 alleles and treatment outcome or side effects. A model to predict plasma concentration from dose and number of active CYP2D6 alleles was formed from the obtained data by means of multiple linear regression.     

CYP2C19 genotype predicts steady state escitalopram concentration in GENDEP

In vitro work shows CYP2C19 and CYP2D6 contribute to the metabolism of escitalopram to its primary metabolite, N-desmethylescitalopram. We report the effect of CYP2C19 and CYP2D6 genotypes on steady state morning concentrations of escitalopram and N-desmethylescitalopram and the ratio of this metabolite to the parent drug in 196 adult patients with depression in GENDEP, a clinical pharmacogenomic trial. Subjects who had one CYP2D6 allele associated with intermediate metabolizer phenotype and one associated with poor metabolizer (i.e. IM/PM genotypic category) had a higher mean logarithm escitalopram concentration than CYP2D6 extensive metabolizers (EMs) (p = 0.004). Older age was also associated with higher concentrations of escitalopram. Covarying for CYP2D6 and age, we found those homozygous for the CYP2C19*17 allele associated with ultrarapid metabolizer (UM) phenotype had a significantly lower mean escitalopram concentration (2-fold, p = 0.0001) and a higher mean metabolic ratio (p = 0.0003) than EMs, while those homozygous for alleles conferring the PM phenotype had a higher mean escitalopram concentration than EMs (1.55-fold, p = 0.008). There was a significant overall association between CYP2C19 genotypic category and escitalopram concentration (p = 0.0003; p = 0.0012 Bonferroni corrected). In conclusion, we have demonstrated an association between CYP2C19 genotype, including the CYP2C19*17 allele, and steady state escitalopram concentration.     

Elucidation of the genetic basis of the common 'intermediate metabolizer' phenotype for drug oxidation by CYP2D6

A subgroup of 10-15% of Caucasians are termed phenotypical 'intermediate metabolizers' of drug substrates of CYP2D6 because they have severely impaired yet residual in-vivo function of this cytochrome P450. Genotyping based on the currently known CYP2D6 alleles does not predict this phenotype satisfactorily. A systematic sequencing strategy through 1.6 kb of the CYP2D6 5'-flanking sequence revealed six mutations of which three were exclusively associated with the functional CYP2D6*2 allele (-1496 C to G; -652 C to T; and -590 G to A), two were associated with the nonfunctional *4 and with the functional *10-alleles (-1338 C to T and -912 G to A) and one (-1147 A to G) was seen in all *2, *4 and *10-alleles investigated. The -1496 C to G mutation was found to be polymorphic within CYP2D6*2 alleles. In a family study, the wild-type CYP2D6 *2[-1496 C] and the novel variant [-1496 G] allele co-segregated with lower and higher CYP2D6 in-vivo function, respectively, as shown by phenotyping using sparteine as probe drug. In a representative population sample selected for genotypes comprising one CYP2D6*2 and one non-functional allele, the median urinary metabolic ratio (MRs) for sparteine oxidation was 4.4-fold reduced in individuals with the variant allele (*2[-1496 G], MRs = 0.53, n = 27) compared with individuals lacking the mutation (*2[-1496 C], MRs = 2.33, n = 12; P < 0.0001). The mutation -1496 C to G has an estimated frequency of approximately 20% in the general population and allows establishment of a genotype for the identification of over 60% of intermediate metabolizers in Caucasian populations.     

Allele and genotype frequencies of polymorphic cytochromes P4502D6, 2C19 and 2E1 in aborigines from western Australia

The polymorphisms of the important xenobiotic metabolizing enzymes CYP2D6, CYP2C19 and CYP2E1 have been studied extensively in a large number of populations and show significant heterogeneity in the frequency of different alleles/genotypes and in the prevalence of the extensive and poor metabolizer phenotypes. Understanding of inter-ethnic differences in genotypes is important in prediction of either beneficial or adverse effects from therapeutic agents and other xenobiotics. Since no data were available for Australian Aborigines, we investigated the frequencies of alleles and genotypes for CYP2D6, CYP2C19 and CYP2E1 in a population living in the far north of Western Australia. Because of its geographical isolation, this population can serve as a model to study the impact of evolutionary forces on the distribution of different alleles for xenobiotic metabolizing enzymes. Twelve CYP2D6 alleles were analysed. The wild-type allele *1 was the most frequent (85.81%) and the non-functional alleles (*4, * 5, * 16) had an overall frequency of less than 10%. Only one subject (0.4%) was a poor metabolizer for CYP2D6 because of the genotype *5/*5. For CYP2C19, the frequencies of the *1 (wild-type) and the non-functional (*2 and *3) alleles were 50.2%, 35.5% and 14.3%, respectively. The combined CYP2C19 genotypes (*2/*2, *2/*3 or *3/*3) correspond to a predicted frequency of 25.6% for the CYP2C19 poor metabolizer phenotype. For CYP2EI, only one subject had the rare c2 allele giving an overall allele frequency of 0.2%. For CYP2D6 and CYP2C19, allele frequencies and predicted phenotypes differed significantly from those for Caucasians but were similar to those for Orientals indicating a close relationship to East Asian populations. Differences between Aborigines and Orientals in allele frequencies for CYP2D6* 10 and CYP2E1 c2 may have arisen through natural selection, or genetic drift, respectively.     

CYP2D6^＊10B基因型对中国人普罗帕酮对映体药动学的影响

AIM: To study the relationship between genotype of CYP2D6*10B and pharmacokinetics of propafenone enantiomers. METHODS: Genotype of 17 healthy Chinese HAN subjects was determined by an allele specific amplification method. The blood samples (0-15 h) of the subjects were taken after oral administration of a single dose (400 mg) of propafenone hydrochloride. Concentrations of propafenone enantiomers in plasma were measured by a reverse-phase HPLC with precolumn derivatization. RESULTS: Seventeen subjects characterized for CYP2D6*10B genotype included (*1/*1) (n=4), (*1/*10) (n=5) and (*10/*10) (n=8). The metabolic ratios (lg MR) of the three genotypes were -2.68+/-0.23, -2.2+/-0.7, and -1.1+/-0.5, respectively. The AUC of the three groups were (1534+/-334), (1891+/-793), (3171+/-1075) microg.h.L(-1) for S-enantiomer and (1136+/-345), (1467+/-817), (2277+/-745) microg.h.L(-1) for R-enantiomer, respectively. The AUC of propafenone enantiomers in *10/*10 is about 1.5-2 times of that of *1/*10 group or *1/*1 group, and the CL of both enantiomers in *10/*10 is only half of that of *1/*10 group or *1/*1 group (P<0.05). CONCLUSION: CYP2D6*10B alleles induce the declined activity of CYP2D6 and impair the metabolism of propafenone.     

Effects of the CYP 2D6 genotype and cigarette smoking on the steady-state plasma concentrations of fluvoxamine and its major metabolite fluvoxamino acid in Japanese depressed patients

The effects of the cytochrome P450 (CYP) 2D6 genotype and cigarette smoking on the steady-state plasma concentrations (C(ss)) of fluvoxamine (FLV) and its demethylated metabolite fluvoxamino acid (FLA) were studied in 49 Japanese depressed patients receiving FLV 200 mg/d. The C(ss) of FLV and FLA were measured by HPLC, and the wild-type allele (*1) and two mutated alleles causing absent (*5) or decreased (*10) CYP 2D6 activity were identified by PCR methods. The patients were divided into three genotype groups by the number of mutated alleles: 12 cases with no (*1/*1), 27 cases with one (*1/*5 and *1/*10), and 10 cases with two (*5/*10 and *10/*10) mutated alleles. The means +/- SD of the C(ss) of FLV and FLA and the FLA/FLV ratio of all patients were 169.1 +/- 147.5 ng/mL, 83.9 +/- 52.7 ng/mL, and 0.71 +/- 0.50, respectively. The C(ss) of FLV and FLA were not significantly different among the three genotype groups. However, the FLA/FLV ratio was significantly lower in the patients with one (P < 0.05) and two (P < 0.01) mutated alleles than in those with no mutated allele. There was no significant difference between nonsmokers (n = 34) and smokers (n = 15) in these values. In the stepwise multiple regression, the C(ss) of FLA (P < 0.05) and FLA/FLV ratio (P < 0.001) showed significant negative correlations with the number of mutated alleles, and the FLA/FLV ratio was significantly (P < 0.05) lower in women than in men. The present study suggests that the CYP 2D6 genotype and cigarette smoking have no major impact on the C(ss) of FLV and FLA, though CYP 2D6 is involved in the demethylation of FLV.     

等位基因特异扩增法研究中国人CYP2D6中速代谢的相关基因

目的　建立CYP2D6*10B的等位基因特异扩增法(ASA-PCR),以探讨中国人CYP2D6中速代谢的基因分型。方法　采用两步扩增法得到CYP2D6*10B等位基因特异片段,分析健康中国汉族人CYP2D6*10B等位基因,并探讨基因分型结果与右美沙芬表型分型结果的相关性。结果　35名表型为极快代谢受试者(VEMs)中,CYP2D6*10B以杂合子(wt/m)为主占57%;29名中速代谢受试者(IMs)以突变型纯合子(m/m)为主占69%;慢代谢受试者(PM)基因型为m/m。CYP2D6*10Bm/m组的MR明显大于wt/m组和野生型组(wt/wt)。结论　ASA-PCR法有快速、准确的优点,可用于CYP2D6中速代谢的检测与研究。     

Effect of cyp2d6*10 allele on the pharmacokinetics of loratadine in chinese subjects.

Loratadine is known to be a substrate for both CYP3A4 and CYP2D6 based on a previous in vitro study. In view of the large interindividual variability in loratadine pharmacokinetics and the greater genetically determined variability of CYP2D6 activity than of CYP3A4 in vivo, we hypothesized that CYP2D6 polymorphisms may contribute to the pharmacokinetic variability of loratadine. The purpose of this study was to evaluate the effect of CYP2D6 genotype (specifically the CYP2D6*10 allele) on the pharmacokinetics of loratadine in Chinese subjects. Three groups of healthy male Chinese subjects were enrolled: group I, homozygous CYP2D6*1 (*1/*1, n=4); group II, heterozygous CYP2D6*10 (*1/*10 or *2/*10, n=6); and group III, homozygous CYP2D6*10 (*10/*10, n=7) carriers. Each subject received a single oral dose of 20 mg of loratadine under fasting conditions. Multiple blood samples were collected over 48 h, and the plasma concentrations of loratadine and its metabolite desloratadine were determined by high-performance liquid chromatography. In comparing homozygous CYP2D6*10 (group III) to heterozygous CYP2D6*10 (group II) to homozygous CYP2D6*1 (group I) subjects, loratadine oral clearance values were 7.17+/- 2.54 versus 11.06+/-1.70 versus 14.59+/-2.43 l/h/kg, respectively [one-way analysis of variance (ANOVA), p<0.01], and the corresponding metabolic ratios [area under the plasma concentration-time curve (AUC)(desloratadine)/AUC(loratadine)] were 1.55+/-0.73 versus 2.47+/- 0.46 versus 3.32+/- 0.49, respectively (one-way ANOVA, p<0.05), indicating a gene-dose effect. The results demonstrated that CYP2D6 polymorphism prevalent in the Chinese population significantly affected loratadine pharmacokinetics.     

Steady-state plasma levels of nortriptyline and its hydroxylated metabolites in Japanese patients: impact of CYP2D6 genotype on the hydroxylation of nortriptyline

The authors investigated the impact of the CYP2D6 genotype on steady-state concentrations of nortriptyline (NT) and its metabolites, trans-10-hydroxynortriptyline (EHNT) and cis-10-hydroxynortriptyline in a Japanese population of psychiatric patients. Forty-one patients (20 men and 21 women) were orally administered nortriptyline hydrochloride. The allele frequencies of the CYP2D6*5 and CYP2D6*10 were 4.9% and 34.1%, respectively. Significant differences in NT concentrations corrected for dose and weight were observed between the subjects with no mutated alleles and those with one mutated allele (mean +/- SD for no mutated alleles vs. one mutated allele: 70.3 +/- 25.4 vs. 98.4 +/- 36.6 ng/mL x mg(-1) x kg(-1); t = 2.54, dcf = 33, p < 0.05) and between the subjects with no mutated alleles and two mutated alleles (no mutated alleles vs. two mutated alleles: 70.3 +/- 25.4 vs. 147 +/- 31.1 ng/mL x mg(-1) x kg(-1); t = 5.87, df = 19, p < 0.0001). Also, a significant difference in the NT/EHNT ratio, which is representative of the hydroxylation ratio of NT, was observed between the subjects with no mutated alleles and those with two mutated alleles (no mutated alleles vs. two mutated alleles: 0.82 +/- 0.30 vs. 2.71 +/- 0.84; t = 7.86, df = 19, p < 0.0001). Multiple regression analysis showed that the number of mutated alleles of CYP2D6, which was the only significant factor, accounted for 41% and 48% of the variability in log(NT corrected for dose and weight) and log(NT/EHNT), respectively.     

CYP2D6 genotype and phenotype in Amerindians of Tepehuano origin and Mestizos of Durango, Mexico

Although the drug-metabolizing enzyme CYP2D6 has been studied extensively in subjects of differing ethnicities, limited CYP2D6 pharmacogenetic data are available for the Amerindian population and Mestizos of Mexico. Dextromethorphan hydroxylation phenotype was studied in Tepehuano Amerindian (n = 58) and Mestizo (n = 88) subjects, and 195 individuals (85 Tepehuano Amerindians and 110 Mestizos) were genotyped by polymerase chain reaction-restriction fragment length polymorphism methods to identify the frequencies of the CYP2D6*3, *4, *6, and *10 alleles. Tepehuano Amerindian subjects lacked the poor metabolizer (PM) phenotype, whereas in Mestizos the PM phenotype frequency was 6.8%. The CYP2D6*3, *6, and *10 alleles were not found in Tepehuano Amerindians. The CYP2D6*4 allele had a low frequency (0.006) in this Amerindian group. In the Mestizo group, the CYP2D6*3, *4, and *10 alleles had frequencies of 0.009, 0.131, and 0.023, respectively. The CYP2D6*6 allele was not found in Mestizos. The genotype-phenotype association was strongly statistically significant (r(2) = .45; P = .005) in Mestizos. The Tepehuano population was found to have a low phenotypic and genotypic CYP2D6 diversity and differed from other Amerindian groups. On the other hand, the frequencies of the CYP2D6 variant alleles in Mestizos were similar to those reported for whites.     

Phenotypes and genotypes for CYP2D6 and CYP2C19 in a black Tanzanian population

AIMS: CYP2D6 and CYP2C19 are polymorphically expressed enzymes that show marked interindividual and interethnic variation. The aim of this study was to determine the frequency of the defective alleles in CYP2D6 and CYP2C19 in Africans and to test whether the genotype for CYP2C19 is better correlated with the proguanil/cylcoguanil ratio than the mephenytoin S/R ratio. METHODS: Two hundred and sixteen black Tanzanians were phenotyped for CYP2D6 with the use of sparteine, and for CYP2C19 with the use of mephenytoin and proguanil. Of these 196 subjects were also genotyped for CYP2D6 (including the CYP2D6*1, CYP2D6*3 and CYP2D6*4 alleles) and 195 were genotyped for CYP2C19 (including the CYP2C19*1, CYP2C19*2 and the CYP2C19*3 alleles). Furthermore 100 subjects were examined for the allele duplication in CYP2D6, leading to ultrarapid metabolism, with long PCR. RESULTS: The sparteine metabolic ratio (MR) was statistically significantly higher in the Tanzanian group of homozygous, extensive metabolizers compared to a historical control group of white Danish extensive metabolizers. Only one poor metabolizer for CYP2D6 (MR=124 and genotype CYP2D6*1/CYP2D6*4 ) was found. The gene frequencies were 0.96 for the CYP2D6*1 allele and 0.04 for the CYP2D6*4 allele. No CYP2D6*3 alleles were found. Nine subjects had an allele duplication in CYP2D6 (9%). For CYP2C19 there were seven subjects (3. 6%) who were phenotyped as poor metabolizers, but only three subjects (1.5%) had a genotype (CYP2C19*2/CYP2C19*2 ) indicative of poor metabolism. The gene frequencies were 0.90 for the CYP2C19*1 allele and 0.10 for the CYP2C19*2 allele. No CYP2C19*3 alleles were found. The mephenytoin S/R ratios were not bimodally distributed. CONCLUSIONS: Both the genotyping and phenotyping results show that there is a substantial difference between an African black population and a Caucasian population in the capacity to metabolize drugs via CYP2D6 and CYP2C19.     

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.