Polymorphism of debrisoquine hydroxylation among Finns and Lapps

Characterization of CYP2D6 genotypes and metabolic profiles in the Portuguese population: pharmacogenetic implications

Aim:CYP2D6 codes for a protein that is vastly involved in the metabolism of various substances. This gene is highly polymorphic, which influences the enzymatic activity and contributes to the huge variability in the enzyme hydroxylation capacity. Different metabolic profiles determine the processing of xenobiotics and endobiotics, thereby influencing disease risk, therapeutic efficacy and side effects, or toxicity of xenobiotics. The aim of this work was to characterize CYP2D6 polymorphisms and predict metabolic profiles in the Portuguese population. Subjects & methods: The study comprised 300 Portuguese unrelated healthy adult volunteers. Genetic analysis included allelic discrimination and copy number determination with TaqMan® probes by real-time PCR and allele duplications of CYP2D6*1, CYP2D6*2, CYP2D6*4 and CYP2D6*10 were confirmed by long PCR and PCR-RFLP. Results: The percentages of poor and ultrarapid metabolizers found in this Portuguese population were 6.3 and 4.7%, respectively. Discussion & conclusion: Compared with other studies, such as in Spaniards, the allelic frequencies observed were similar, with some exceptions, such as for CYP2D6*10, which is higher in the Portuguese population, and for CYP2D6*6 and duplication of *1 and *2, appearing in lower frequencies in the present study. These results allow the determination of the frequency of the most relevant CYP2D6 polymorphisms and the prediction of the enzyme metabolic activity, being of much importance for the CYP2D6 pharmacogenetics approach in the Portuguese population. The data presented here are significant for the study of genetic variability influencing CYP2D6 activity, to improve the effectiveness and safety of xenobiotics exposure, also can be used as a tool in clinical practice for the development of individualized pharmacotherapy.

Pharmacogenetics of debrisoquine and its use as a marker for CYP2D6 hydroxylation capacity

Debrisoquine hydroxylation polymorphism is by far the most thoroughly studied genetic polymorphism of the CYP2D6 drug-metabolizing enzyme. Debrisoquine hydroxylation phenotype has been the most used test in humans to evaluate CYP2D6 activity. Two debrisoquine hydroxylation phenotypes have been described: poor and extensive metabolizers. A group with a very low debrisoquine metabolic ratio within the extensive metabolizers, named ultrarapid metabolizers, has also been distinguished. This CYP2D6 variability can be for a large part alternatively determined by genotyping, which appears to be of clinical importance given CYP2D6 involvement in the metabolism of a large number of commonly prescribed drugs. CYP2D6 pharmacogenetics may then become a useful tool to predict drug-related side effects, interactions or therapeutic failures. However, a number of reasons appear to have made research into this field lag behind. The present review focuses on the relevance of genetics and environmental factors for determining debrisoquine hydroxylation phenotype, as well as the relevance of CYP2D6 genetic polymorphism in psychiatric patients treated with antipsychotic drugs.

Bioinformatics Research on Inter-racial Difference in Drug Metabolism I. Analysis on Frequencies of Mutant Alleles and Poor Metabolizers on CYP2D6 and CYP2C19

The enzyme activities of CYP2D6 and CYP2C19 show a genetic polymorphism, and the frequency of poor metabolizers (PMs) on these enzymes depends on races. In the present study, the frequencies of mutant alleles and PMs in each race were analyzed based on information from published studies, considering the genetic polymorphisms of CYP2D6 and CYP2C19 as the causal factors of racial and inter-individual differences in pharmacokinetics. As a result, it was shown that there were racial differences in the frequencies of each mutant allele and PMs. The frequencies of PMs on CYP2D6 are 1.9% of Asians and 7.7% of Caucasians, and those of PMs on CYP2C19 are 15.8% of Asians and 2.2% of Caucasians. Based on the results, it was suggested that there would be racial differences in the frequencies of PM subjects whose blood concentrations might be higher for drugs metabolized by these enzymes. Additionally, it was suggested that enzyme activities would vary according to the number of functional alleles even in subjects judged to be extensive metabolizers (EMs). In the bridging study, genetic information regarding CYP2D6 and CYP2C19 of the subjects will help extrapolate foreign clinical data to a domestic population.

CYP2D6-related oxidation polymorphism in Italy

The distribution of the oxidation polymorphism related to cytochrome CYP2D6 (debrisoquine type) was determined in 246 healthy Italian volunteers. Phenotyping was based on HPLC determination of the dextrometorphan/dextrorphan concentration ratio (metabolic ratio) in urine samples collected over an 8 h interval following a single oral 30 mg dose of dextromethorphan hydrobromide. Urinary excretion of dextromethorphan showed a wide interindividual variability, ranging from < or = 0.04 to 3.9% and from 0.5 to 79.6% of the dose, respectively. Metabolic ratios ranged from < or = 0.001 to 6.6. Eleven of the 246 subjects showed a metabolic ratio greater than 0.30, indicating that 4.5% of the population could be ascribed to the poor metabolizer status. The frequency of the poor metabolizer phenotype in this population is within the range described for other Caucasian ethnic groups.

Polymorphism in CYP1A1 and CYP2D6 genes: possible association with susceptibility to lung cancer

Polymorphism of CYP2D6 gene encoding for debrisoquine hydroxylase was determined genotypically for 94 controls and 77 lung cancer patients using a polymerase chain reaction-based application. Both of the point mutations that give rise to deficient alleles of the CYP2D6 gene are detectable by this method. Out of the 94 healthy controls, 3 individuals (3.2%) had poor metabolizer (PM) genotypes, whereas no PM genotypes were detected in the lung cancer patient group. We observed no difference in the allelic frequencies for either homozygous extensive metabolizers (EMs) or heterozygous EMs between the lung cancer patients and the healthy controls. However, the absence of the poor metabolizer genotype (0/77) in the lung cancer patients is compatible with the hypothesis that there is an increased risk of lung cancer for individuals who are extensive metabolizers of debrisoquine. Another member of the cytochrome P450 gene superfamily that has attracted interest for its potential role in human pulmonary carcinogenesis is the CYP1A1 gene. In CYP1A1 gene studies, a polymorphic site assessable to MspI gives rise to two different hybridizable fragments in a Southern blot analysis (alleles C1 and C2, respectively). The C2C2 genotype has previously been associated with an increased risk of lung cancer. So far 74 lung cancer patients, 30 patients with lung diseases other than cancer, and 118 healthy controls have been studied for CYP1A1 gene polymorphism. No association between the MspI restriction fragment length polymorphism in the CYP1A1 gene and lung cancer susceptibility has been found.

Polymorphism of debrisoquine and mephenytoin hydroxylation among Estonians

Debrisoquine and S-mephenytoin hydroxylation polymorphisms were studied in 156 unrelated native Estonians. The hydroxylation phenotypes were assessed by coadministration of mephenytoin with debrisoquine or dextromethorphan. The frequency of the poor metaboliser phenotype of debrisoquine/dextromethorphan was 4.5% (95% confidence interval 1.2-7.8%), and that of mephenytoin was 3.9% (95% confidence interval 0.9-6.9%) among Estonians, which is very similar to what has been reported in other Caucasian populations.

Debrisoquine oxidation polymorphism in a Tasmanian population

The debrisoquine hydroxylation phenotype was studied in 152 unselected healthy Tasmanian subjects, who were mostly Caucasians of British ancestry. Following a 10 mg oral dose of debrisoquine (D), the ratio of D/4-hydroxydebrisoquine excreted in 8-h urine (metabolic ratio, MR) was determined. MR values were bimodally distributed. Thirteen subjects (8.6%) had MR values from 13.8 to 93.3 and were considered to be poor metabolisers of D, while the others were extensive metabolisers with MR values of 0.04 to 5.4. The D hydroxylation phenotype was not associated with sex. These findings confirm the constancy of D polymorphism in a Caucasian population even after migration to another country.

The effects of diltiazem on hepatic drug metabolizing enzymes in man using antipyrine, trimethadione and debrisoquine as model substrates

Six healthy male subjects were given single oral doses of antipyrine (7 mg kg-1), trimethadione (4 mg kg-1) and debrisoquine (10 mg) before and during diltiazem treatment (30 mg three times daily orally for 8 days). Antipyrine clearance decreased from 33.7 +/- 9.1 to 22.5 +/- 4.9 ml min-1 (P less than 0.05, mean +/- s.e. mean) after diltiazem treatment without any significant change in apparent volume of distribution (0.59 +/- 0.06 to 0.60 +/- 0.04 1 kg-1), resulting in an increase in antipyrine elimination half-life from 13.4 +/- 4.8 to 19.7 +/- 3.2 h (P less than 0.05). The formation clearance of antipyrine to 4-hydroxyantipyrine was decreased significantly from 10.8 +/- 2.7 to 6.6 +/- 2.7 ml min-1 (P less than 0.05), while that to 3-hydroxymethylantipyrine and norantipyrine was not altered by diltiazem. The metabolic ratio of debrisoquine (urinary excretion of debrisoquine/4-hydroxydebrisoquine) was increased significantly from 0.70 +/- 0.05 to 1.95 +/- 0.20 (P less than 0.05), while that of trimethadione (serum concentration of dimethadione/trimethadione) was not changed significantly (0.48 +/- 0.08 vs 0.41 +/- 0.06) after diltiazem treatment. Diltiazem selectively inhibits cytochrome P-450 isoenzymes.

Hydroxylation polymorphisms of debrisoquine and mephenytoin in European populations

European data on the polymorphic metabolism of debrisoquine, sparteine, dextromethorphan and mephenytoin have been collected. No significant difference in phenotype frequencies was found between the separate series for debrisoquine, sparteine and dextromethorphan, which supports the claim that these probe drugs reflect the same enzyme polymorphism. The mean frequency of the phenotype slow debrisoquine metaboliser was 7.65% based on 5005 determinations. The overall mean reflecting all three drugs and 8764 determinations was 7.40%. This is consistent with a gene frequency of 0.27 (95% confidence interval 0.26-0.28). The overall mean of the phenotype slow metaboliser of mephenytoin was 3.52% corresponding to a gene frequency of 0.19 (confidence interval 0.17-0.20). The incidence of slow metabolism of debrisoquine and possibly also of S-mephenytoin was homogeneous in the samples from European populations. This is of considerable interest as interethnic differences are now being found both in the phenotypic characters as well as the genotypes of polymorphic drug oxidation.