A study of the debrisoquine hydroxylation polymorphism in a Nigerian population

African Genetic Diversity: Implications for Cytochrome P450-mediated Drug Metabolism and Drug Development

Genetic diversity is greater in Africa than in other continental populations. Genetic variability in genes encoding drug metabolizing enzymes may contribute to the high numbers of adverse drug reactions reported in Africa. We reviewed publications (1995-April 2016) reporting frequencies of known cytochrome P450 (CYP) variants in African populations. Using principal components analysis (PCA) we identified CYP alleles of potential clinical relevance with a marked difference in distribution in Africa, compared with Asian and Caucasian populations. These were CYP2B6*6, CYP2C8*2, CYP2D6*3, CYP2D6*17, CYP2D6*29, CYP3A5*6, and CYP3A5*7. We show clearly that there is greater diversity in CYP distribution in Africa than in other continental populations and identify a need for optimization of drug therapy and drug development there. Further pharmacogenetic studies are required to confirm the CYP distributions we identified using PCA, to discover uniquely African alleles and to identify populations at a potentially increased risk of drug-induced adverse events or drug inefficacy.

Cytochrome P450 pharmacogenetics in African populations

The Cytochrome P450 (CYP450) family of enzymes is involved in the oxidative metabolism of many therapeutic drugs, carcinogens and various endogenous substrates. These enzymes are highly polymorphic at an inter-individual and inter-ethnic level. Polymorphisms or genetic variations account for up to 30% of inter-individual differences seen in a variety of drug responses. The frequencies of the different metabolizer categories (slow, intermediate, extensive and ultra-rapid), the distribution of genetic variants, genotype-phenotype correlations and the clinical importance of the CYP450 enzymes have been extensively documented in Caucasian and Oriental populations. Limited data exists for African populations, despite the fact that this knowledge is critically important for these populations who experience a heavy burden of communicable and non-communicable diseases. In addition, the costs incurred through adverse drug reactions and non-responsiveness to therapy could be reduced through the wide-scale application of pharmacogenetics. This review provides an overview and investigation of CYP450 genotypic and phenotypic reports published from 1980 to present in African populations. Our findings confirm the high degree of variability that is expected when comparing individuals of African origin to other ethnic groups and also highlight the distribution of clinically relevant CYP450 alleles amongst the various African populations. The notable discordance in genotypic and phenotypic data amongst African populations exemplifies the need for in-depth and well-orchestrated molecular and pharmacological investigations of these populations in the future, for which whole genome sequencing and association studies will be critical.

Opioid metabolism

Clinicians understand that individual patients differ in their response to specific opioid analgesics and that patients may require trials of several opioids before finding an agent that provides effective analgesia with acceptable tolerability. Reasons for this variability include factors that are not clearly understood, such as allelic variants that dictate the complement of opioid receptors and subtle differences in the receptor-binding profiles of opioids. However, altered opioid metabolism may also influence response in terms of efficacy and tolerability, and several factors contributing to this metabolic variability have been identified. For example, the risk of drug interactions with an opioid is determined largely by which enzyme systems metabolize the opioid. The rate and pathways of opioid metabolism may also be influenced by genetic factors, race, and medical conditions (most notably liver or kidney disease). This review describes the basics of opioid metabolism as well as the factors influencing it and provides recommendations for addressing metabolic issues that may compromise effective pain management. Articles cited in this review were identified via a search of MEDLINE, EMBASE, and PubMed. Articles selected for inclusion discussed general physiologic aspects of opioid metabolism, metabolic characteristics of specific opioids, patient-specific factors influencing drug metabolism, drug interactions, and adverse events.

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.

Interethnic differences in genetic polymorphisms of CYP2D6 in the U.S. population: clinical implications

DNA polymorphisms have been identified in the genes encoding a number of the cytochrome P450 (CYP) enzymes, leading to wide interindividual variation in drug clearance. CYP2D6 metabolizes a significant number of clinically used medications, and genetic variants of the CYP2D6 isozyme that result in varying levels of metabolic activity are of clinical importance in some settings. The exact nature of the clinical effect caused by polymorphisms of the gene depends on the drug in question and the specific variant alleles expressed, as individual variants result in differing phenotypes with a range of levels of enzymatic activity. Compromised drug efficacy due to CYP2D6 variation has been documented with a variety of agents, and this review considers a number of examples, including the 5-HT(3)-receptor antagonists, which are used in oncology supportive care for the prophylaxis of nausea and vomiting. CYP2D6 is involved in the metabolism of all of the most commonly available agents, except granisetron, and their efficacy and side effects may therefore be affected by the CYP2D6 polymorphism. Significant interethnic differences in CYP2D6 allele frequencies have been demonstrated from studies across many countries. However, incidences of polymorphisms in the U.S. population have been challenging to characterize because of the country's wide ethnic diversity. The CYP2D6 polymorphism may become more important as robust clinical tests become widely available and as the use of multiple medications and the attendant risk for drug-drug interactions increases.

3,4-Dehydrodebrisoquine, a novel debrisoquine metabolite formed from 4-hydroxydebrisoquine that affects the CYP2D6 metabolic ratio

Considerable unexplained intersubject variability in the debrisoquine metabolic ratio (urinary debrisoquine/4-hydroxydebrisoquine) exists within individual CYP2D6 genotypes. We speculated that debrisoquine was converted to as yet undisclosed metabolites. Thirteen healthy young volunteers, nine CYP2D6*1 homozygotes [extensive metabolizers (EMs)] and four CYP2D6*4 homozygotes [poor metabolizers (PMs)] took 12.8 mg of debrisoquine hemisulfate by mouth and collected 0- to 8- and 8- to 24-h urines, which were analyzed by gas chromatography-mass spectrometry (GCMS) before and after treatment with beta-glucuronidase. Authentic 3,4-dehydrodebrisoquine was synthesized and characterized by GCMS, liquid chromatography-tandem mass spectrometry, and (1)H NMR. 3,4-Dehydrodebrisoquine is a novel metabolite of debrisoquine excreted variably in 0- to 24-h urine, both in EMs (3.1-27.6% of dose) and PMs (0-2.1% of dose). This metabolite is produced from 4-hydroxydebrisoquine in vitro by human and rat liver microsomes. A previously unstudied CYP2D6*1 homozygote was administered 10.2 mg of 4-hydroxydebrisoquine orally and also excreted 3,4-dehydrodebrisoquine. EMs excreted 6-hydroxydebrisoquine (0-4.8%) and 8-hydroxydebrisoquine (0-1.3%), but these phenolic metabolites were not detected in PM urine. Debrisoquine and 4-hydroxydebrisoquine glucuronides were excreted in a highly genotype-dependent manner. A microsomal activity that probably does not involve cytochrome P450 participates in the further metabolism of 4-hydroxydebrisoquine, which we speculate may also lead to the formation of 1- and 3-hydroxydebrisoquine and their ring-opened products. In conclusion, this study suggests that the traditional metabolic ratio is not a true measure of the debrisoquine 4-hydroxylation capacity of an individual and thus may, in part, explain the wide intragenotype variation in metabolic ratio.

Inter-individual variation in the metabolic activation of the antimalarial biguanides

The aryl-biguanides proguanil and chlorproguanil were developed as part of a collaborative programme between ICI and the Liverpool School of Tropical Medicine during the 1940s. The compounds were characterized by their absence of host toxicity. However, the rapid development of parasite resistance to the actions of these drugs and the development of the 4-aminoquinoline, chloroquine, severely limited their use. The subsequent widespread development of parasite resistance to chloroquine, together with the observations that the magnitude of dihydrofolate reductase inhibitor resistance (the site of action of the biguanides) developed to pyrimethamine is not directly correlated with biguanide resistance(1,2). has resulted in renewed interest in these drugs. In particular, proguanil is now the drug of choice for malaria prophylaxis, in combination with chloroquine; used in combination with a suitable sulphonamide, it may be of value in malaria therapy.

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.

4-Hydroxylation of debrisoquine by human CYP1A1 and its inhibition by quinidine and quinine

A panel of 15 recombinant cytochromes P450 expressed in human B-lymphoblastoid cells was used to study debrisoquine 4-hydroxylation. Both CYP2D6 and CYP1A1 carried out the reaction. The apparent K(m) (micromolar) and V(max) (picomoles per minute per picomole of P450) for CYP2D6 were 12.1 and 18.2 and for CYP1A1 were 23.1 and 15.2, respectively. CYP1A1 debrisoquine 4-hydroxylase was inhibited by the CYP1A1 inhibitor alpha-naphthoflavone and the CYP1A1 substrate 7-ethoxyresorufin. Additionally and surprisingly, this reaction was also inhibited by quinidine and quinine, with respective IC(50) values of 1.38 +/- 0.10 and 3.31 +/- 0.14 microM, compared with those for CYP2D6 debrisoquine 4-hydroxylase of 0.018 +/- 0.05 and 3.75 +/- 2.07 microM, respectively. Anti-CYP1A1 monoclonal antibody (mAb) 1-7-1 abolished CYP1A1 debrisoquine hydroxylase and anti-CYP2D6 mAb 50-1-3 eradicated CYP2D6 debrisoquine 4-hydroxylase. Three further CYP2D6-specific reactions were tested: dextromethorphan O-demethylation, bufuralol 1'-hydroxylation, and sparteine dehydrogenation. The CYP2D6 specificity, judged by the CYP2D6/CYP1A1 activity ratios was 18.5, 7.0, 6.0, and 1.6 for dextromethorphan, bufuralol, sparteine, and debrisoquine, respectively. Thus, debrisoquine is not a specific CYP2D6 substrate and quinidine is not a specific CYP2D6 inhibitor. These findings have significant implications for the conduct of in vitro drug metabolism inhibition studies and underscore the fallacy of "specific chemical inhibitors" of a supergene family of enzymes that have overlapping substrate specificities. The use of highly specific mAbs in such studies is mandated. It is unclear as yet whether these findings have implications for the relationship between CYP2D6 genotype and in vivo debrisoquine 4-hydroxylase activity.

Pharmacogenetics and psychopharmacotherapy

Response to drugs can vary between individuals and between different ethnic populations. The biological (age, gender, disease and genetics), cultural and environmental factors which contribute to these variations are considered in this review. The most important aspect is the genetic variability between individuals in their ability to metabolize drugs due to expression of 'polymorphic' enzymes. Polymorphism enables division of individuals within a given population into at least two groups, poor metabolisers (PMs) and extensive metabolisers (EMs) of certain drugs. The two most extensively studied genetic polymorphisms are those involving cytochrome P450 2D6 (CYP2D6) and CYP2C19. CYP2D6 metabolizes a number of antidepressants, antipsychotics, beta-adrenoceptor blockers, and antiarrhythmic drugs. About 7% of Caucasians and 1% of Asians are PMs of CYP2D6 substrates. CYP2C19 enzyme participates in the metabolism of omeprazole, propranolol and psychotropic drugs such as hexobarbital, diazepam, citalopram, imipramine, clomipramine and amitriptyline. The incidence of PMs of CYP2C19 substrates is much higher in Asians (15-30%) than in Caucasians (3-6%). Variations in metabolism of psychotropic drugs result in variations in their pharmacokinetic parameters. This may lead to clinically significant intra- and inter-ethnic differences in pharmacological responses. Such variations are discussed in this review. Differential receptor-mediated response may play a role in ethnic differences in responses to antipsychotics and tricyclic antidepressants, but such pharmacodynamic factors remain to be systematically investigated. The results of studies of ethnic differences in response to psychopharmacotherapy appear to be discrepant, most probably due to limitations of study design, small sample size, inadequately defined study sample, and lack of control of confounding factors. The clinical value of understanding pharmacogenetics is in its use to optimize therapeutic efficacy, to prevent toxicity of those drugs whose metabolism is catalysed by polymorphic isoenzymes, and to contribute to the rational design of new drugs. Finally, applications and impact of pharmacogenetics in the field of psychopharmacotherapy are discussed.

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.

Genetic polymorphism of drug metabolising enzymes in African populations: implications for the use of neuroleptics and antidepressants

Metabolism of most drugs influences their pharmacological and toxicological effects. Drugs particularly affected are those with a narrow therapeutic window and that are subjected to considerable first-pass metabolism. Much of the interindividual and interethnic differences in effects of drugs is now attributable to genetic differences in their metabolism. Genetic polymorphisms have been described for many drug-metabolising enzymes in Caucasian and Oriental populations, the most well-characterised being those for cytochrome P450 2D6, cytochrome P450 2C19, glutathione S-transferases, and N-acetyl transferase 2. African populations have been studied to a lesser extent, but it is apparent that populations within Africa are heterogeneous with respect to these polymorphisms. In addition, although some allelic variants are common to all populations throughout the world (e.g., CYP2D6*5), some allelic variants are specific for an African population (e.g., CYP2D6*17). The polymorphisms give rise to enzymes with changed or no activity towards drug substrates. Two of the most important enzymes for metabolism of neuroleptics and other psychoactive drugs are CYP2D6 and CYP2C19. This article compares the current information on polymorphisms of these two enzymes in African and other populations and discusses the implications of these polymorphisms for neuropharmacotherapy.

Ethnicity and antipsychotic response

OBJECTIVE

To review the data generated by studies examining interethnic/racial differences in response to antipsychotics.

DATA SOURCES

A MEDLINE search (1966-1996) identified all articles examining differences in antipsychotic response among Caucasians, Asians, Hispanics, and African-Americans, as well as articles evaluating postulated mechanisms for these differences.

STUDY SELECTION

All abstracts, studies, and review articles were evaluated.

DATA SYNTHESIS

Ethnic/racial differences in response to antipsychotic medications have been reported and may be due to genetics, kinetic variations, dietary or environmental factors, or variations in the prescribing practices of clinicians. Studies suggest that Asians may respond to lower doses of antipsychotics due to pharmacokinetic and pharmacodynamic differences. Research relevant to African-Americans is limited, but some studies suggest that differences in this group may be due to clinician biases and prescribing practices, rather than to pharmacokinetic or pharmacodynamic variability.

CONCLUSIONS

Future research directed at validating the hypotheses that different ethnic/racial groups show variations in response to antipsychotics should focus on homogeneous ethnic groups, use recent advances in pharmacogenetic testing, and control for such variables as observer bias, gender, disease chronicity, dietary and environmental factors, and exposure to enzyme-inducing and -inhibiting agents. Clinicians should be aware that potential interethnic/racial differences in pharmacodynamics and pharmacokinetics may exist that can alter response to antipsychotics.

Clinical pharmacokinetics and metabolism of chloroquine. Focus on recent advancements

This paper presents the current state of knowledge on chloroquine disposition, with special emphasis on stereoselectivity and microsomal metabolism. In addition, the impact of the patient's physiopathological status and ethnic origin on chloroquine pharmacokinetics is discussed. In humans, chloroquine concentrations decline multiexponentially. The drug is extensively distributed, with a volume of distribution of 200 to 800 L/kg when calculated from plasma concentrations and 200 L/kg when estimated from whole blood data (concentrations being 5 to 10 times higher). Chloroquine is 60% bound to plasma proteins and equally cleared by the kidney and liver. Following administration chloroquine is rapidly dealkylated via cytochrome P450 enzymes (CYP) into the pharmacologically active desethylchloroquine and bisdesethylchloroquine. Desethylchloroquine and bisdesethylchloroquine concentrations reach 40 and 10% of chloroquine concentrations, respectively; both chloroquine and desethylchloroquine concentrations decline slowly, with elimination half-lives of 20 to 60 days. Both parent drug and metabolite can be detected in urine months after a single dose. In vitro and in vivo, chloroquine and desethylchloroquine competitively inhibit CYP2D1/6-mediated reactions. Limited in vitro studies and preliminary data from clinical experiments and observations point to CYP3A and CYP2D6 as the 2 major isoforms affected by or involved in chloroquine metabolism. In vitro efficacy studies did not detect any difference in potency between chloroquine enantiomers but, in vivo in rats, S(+)-chloroquine had a lower dose that elicited 50% of the maximal effect (ED950) than that of R(-)-chloroquine. Stereoselectivity in chloroquine body disposition could be responsible for this discrepancy. Chloroquine binding to plasma proteins is stereoselective, favouring S(+)-chloroquine (67% vs 35% for the R-enantiomer). Hence, unbound plasma concentrations are higher for R(-)-chloroquine. Following separate administration of the individual enantiomers, R(-)-chloroquine reached higher and more sustained blood concentrations. The shorter half-life of S(+)-chloroquine appears secondary to its faster clearance. Blood concentrations of the S(+)-forms of desethylchloroquine always exceeded those of the R(-)-forms, pointing to a preferential metabolism of S(+)-chloroquine.

Geographical/interracial differences in polymorphic drug oxidation. Current state of knowledge of cytochromes P450 (CYP) 2D6 and 2C19

The isoenzymes which catalyse the polymorphic hydroxylations of debrisoquine/sparteine and S-mephenytoin are cytochromes P450 2D6 and P450 2C19 (CYP2D6 and CYP2C19), respectively. CYP2D6 is involved in the stereospecific metabolism of several important groups of drugs, for example antiarrhythmics, antidepressants and neuroleptics. About 7% of Caucasians but only 1% of Orientals are poor metabolisers (PMs) of debrisoquine. The most common mutated allele CYP2D6B in Caucasian PMs is almost absent from their Oriental counterparts. On the other hand, the mean activity of CYP2D6 in Oriental extensive metabolisers (EMs) is lower than that in Caucasian EMs. This is due to the frequent distribution of a partially deficient CYP2D6 allele causing a Pro34-->Ser amino acid exchange in as many as 50% of Oriental alleles. This is the molecular genetic basis for slower metabolism of antidepressants and neuroleptics observed in Oriental compared with Caucasian people, and consequently for the lower dosages of these drugs used. While CYP2D6 catalyses the metabolism of lipophilic bases only, CYP2C19 is involved in the metabolism of acids (e.g. S-mephenytoin), bases (e.g. imipramine and omeprazole) and neutral drugs (e.g. diazepam). About 3% of Caucasians and 12 to 22% of Orientals are PMs of S-mephenytoin. Polymerase chain reaction-based genotyping techniques recently became available for the two CYP2C19 mutated alleles m1 and m2, which cause no enzyme to be expressed. M1 accounts for about 80% of the mutations responsible for the PM phenotypes in Caucasians, Oriental and Black people. Diazepam is partially demethylated by CYP2C19, and the high frequency of mutated alleles in Orientals is probably the reason why such populations have a slower metabolism and are treated with lower doses of diazepam than Caucasians. Omeprazole is to a major extent hydroxylated by CYP2C19, and there is an approximately 10-fold difference in oral clearance between EMs and PMs of S-mephenytoin. The separation of Caucasians from Orientals is fairly recent in the evolutionary process (40,000 to 60,000 years ago); the separation of Black from Caucasian/Oriental people occurred much earlier, about 150,000 years ago. As pronounced differences have been found between Caucasians and Orientals in the CYP2D6 and CYP2C19 enzymes, it might be expected that Black people will show even greater differences in this respect. Some studies have been performed with Black participants, but the picture is not clear. The mean CYP2D6 activity in Black EMs seems to be lower than that in Caucasian EMs and similar to that of Oriental EMs.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison of the prevalence of the poor metabolizer phenotype for CYP2D6 between 203 Hmong subjects and 280 white subjects residing in Minnesota

Genetic polymorphism of the P450IID6 (CYP2D6) enzyme system can be an important component of the variability in response to drug therapy. Interpopulation differences in the prevalence of deficiencies of drug-metabolizing enzymes may be clinically important in the selection and dosage of drug therapies for patients. Since 1980, the State of Minnesota has had more than a 1000% increase in population of Hmong refugees from Laos. The Hmong are frequently treated in our institution's international clinic with virtually no systematically acquired knowledge about the ability of this relatively ethnically pure population to metabolize commonly used Western medications. To further our knowledge of drug metabolism in this population, we identified the prevalence of the poor metabolizer phenotype for CYP2D6 in a sample population of Hmong subjects and compared this prevalence to that in a sample population of white subjects. Urine collected after ingestion of dextromethorphan in 237 healthy Hmong and 280 healthy white volunteers was analyzed by HPLC. Based on probit plots of the metabolic ratios (dextro-methorphan/dextrorphan), 8.9% of Hmong subjects and 6.1% of white subjects were assigned the poor metabolizer phenotype (difference not significant). Weak associations were found between body surface area and metabolic ratio for both Hmong and white men and between smoking status and metabolic ratio for white subjects only. We conclude that the prevalence of poor metabolizers for the CYP2D6 enzyme system is similar between Hmong subjects and white subjects residing in Minnesota and that an antimode of 0.3 for metabolic ratio appears to be reasonable for the populations studied.

Genetic differences in drug disposition

Genetic polymorphisms of drug metabolizing enzymes are well recognized. This review presents molecular mechanisms, ontogeny and clinical implications of genetically determined intersubject variation in some of these enzymes. Included are the polymorphic enzymes N-acetyl transferase, cytochromes P4502D6 and 2C, which have been well described in humans. Information regarding other Phase I and Phase II polymorphic pathways, such as glutathione and methyl conjugation and alcohol and acetaldehyde oxidation continues to increase and are also discussed. Genetic factors effecting enzyme activity are frequently important determinants of the disposition of drugs and their efficacy and toxicity. In addition, associations between genetic differences in these enzymes and susceptibility to carcinogens and teratogens have been reported. Ultimately, the application of knowledge regarding these genetic factors of enzyme activity may guide medical therapy and minimize xenobiotic-induced disease.

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.

Dextromethorphan O-demethylation polymorphism in Jordanians

The O-demethylation of dextromethorphan (DMT) to dextrorphan (DRP) was studied in 241 unrelated, healthy Jordanian volunteers (171 males, 70 females). Urine was collected for 8 h following a single oral dose of DMT bromhydrate 30 mg. A thin-layer chromatographic (TLC) technique was used to identify the metaboliser phenotype. The frequency of the poor metaboliser phenotype was found to be 2.9% (approximate 95% confidence interval 0.8-5.0%). Applying the Hardy-Weinberg Law, the frequency of the recessive autosomal gene controlling poor metabolism was 0.17 (95% confidence interval 0.108-0.232).

Oxidative polymorphism of dextromethorphan in a Burundi population

The wide availability, metabolism by the same cytochrome P450 as debrisoquine and, above all, the inocuity of dextromethorphan (DMP) favour the frequent choice of this drug as the test substance in determining oxidation phenotypes. 100 healthy Burundian volunteers (94 m and 6 f) in this study ingested 50 mg DMP bromhydrate, i.e. 38.5 mg of DMP base. Urine was collected for 8 h following the dose and TLC was used to analyse it. The method was particularly useful in view of its low cost, speed and the ease of applying it to a large study group. 5% of the Burundian subjects were poor metabolizers.

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.

Genetic polymorphism of sparteine/debrisoquine oxidation: a reappraisal

Polymorphic oxidation of the sparteine/debrisoquine-type has been shown to account for much of the interindividual variation in the metabolism, pharmacokinetics and pharmacodynamics of an increasing number of drugs, including some antiarrhythmic, antidepressant and beta-adrenoceptor antagonist agents. Impaired hydroxylation of these drugs results from the absence of the enzyme cytochrome P450IID6 in the livers of poor metabolisers, who constitute 6% to 10% of Caucasian populations. The clinical importance of the phenomenon has to be explored further and for most sparteine/debrisoquine-related substrates there is a need for controlled prospective studies to define the consequences to the patient of impaired or enhanced drug oxidation.

The genetic polymorphism of debrisoquine/sparteine metabolism--clinical aspects

It has been established that the metabolism of more than twenty drugs, including antiarrhythmics, beta-adrenoceptor antagonists, antidepressants, opiates and neuroleptics is catalyzed by cytochrome P-450dbl. The activity of this P-450 isozyme is under genetic rather than environmental control. This article discusses the therapeutic implications for each of the classes of drugs affected by this genetic polymorphism in drug metabolism. Not only are the problems associated with poor metabolizers who are unable to metabolize the compounds discussed, but it is also emphasized that it is difficult to attain therapeutic plasma concentrations for some drugs in high activity extensive metabolizers.

Acetylator status of kwashiorkor children in Ibadan (south-west Nigeria)

Acetylator status was determined in 25 kwashiorkor children, aged between 8 months and 3 years and in 25 age-matched control group of healthy children after a single oral dose of sulphamethazine (40 mg/kg body weight) and by measuring the acetylated sulphamethazine in blood samples, collected 6 h after the administration of sulphamethazine. The percentage of slow acetylators among kwashiorkor children was 40% while among the control group of children it was 48%. The difference between the two groups was not significant. Therefore, it is probable that the slow acetylator status of the Nigerian African children may not be a contributing factor for the development of kwashiorkor, a syndrome of protein-energy malnutrition. Furthermore, the polymorphic activity of N-acetyl transferase enzyme may not be impaired in kwashiorkor.

Non-correlation between debrisoquine and metoprolol polymorphisms in the Venda

1. The metabolic 4-hydroxylation of debrisoquine has been studied in a group of 98 black African villagers in Vendaland. 2. The metabolic alpha-hydroxylation of metoprolol has been studied in 94 of the same black African villagers. 3. A 4% prevalence of poor oxidative metabolism of debrisoquine and a 7.4% incidence of poor oxidation of metoprolol were found. The 4% result for debrisoquine differs considerably from the 19% found in San Bushmen, 30% in Hong Kong Chinese, 9% in Britains and 0% in Nigerians and Japanese, whilst the 7.4% result for metoprolol compares with 8.4% in Britains but differs from 0% in Nigerians and 4.1% in San Bushmen. 4. None of the poor oxidative metabolizers of debrisoquine were also poor oxidative metabolizers of metoprolol. This is contrary to results in British and Nigerian subjects where defective oxidation of metoprolol co-segrates with that of debrisoquine. 5. No similarities were found between the Venda metabolic ratio (MR) distributions and either extensive or poor MR distributions in Britains or Nigerians.

Metoprolol alpha-hydroxylation polymorphism in the San Bushmen of southern Africa

1. The metabolic oxidation of metoprolol has been studied in a group of 98 San Bushmen. 2. The amounts of metoprolol and alpha-hydroxy metoprolol excreted in 0-8 h urine collection, after dosing with 100 mg metoprolol, were measured and the metabolic ratio (% dose excreted as metoprolol/% dose excreted as alpha-hydroxy metoprolol) calculated. 3. Frequency distribution and probit plots of the metabolic rate data showed a bimodal distribution with 4.1% of the population exhibiting slow metabolism with an MR greater than 10. 4. These results are much less than found in Caucasians (8.4%) but very different from the unimodal distribution found for Nigerians. 5. A previous study in the same group of Bushmen had revealed that 18 of 96 subjects were poor or non-metabolizers of debrisoquine to 4-hydroxy debrisoquine, but only one of the poor metoprolol metabolizers was a poor metabolizer of debrisoquine. 6. On the basis of these results, the claim of debrisoquine type of polymorphism for beta-adrenoceptor antagonists found in Caucasians cannot be extrapolated to the San Bushmen, and one must query the use of debrisoquine as measure of oxidative status in any group other than Caucasians.

Further analysis of sparteine oxidation in a Japanese population and comparison with data observed in different ethnic populations

1. Data on the oxidation polymorphism of sparteine (SP) studied in 84 unrelated Japanese subjects of whom two (2.4%) were classified as poor metabolizers (PMs) were re-evaluated. The data were obtained from 6-hour urinary excretion ratios of SP to 2- and 5-dehydrosparteines (DHS), after an oral dose of 100 mg of SP sulphate. 2. Urinary excretion of both SP and DHS correlated with the SP/DHS ratio (rs = 0.862 and -0.756, respectively, P less than 0.001). In addition, urinary excretion of 2-DHS, 5-DHS or total DHS discriminated between PMs and extensive metabolizers (EMs). There was also a highly significant correlation (rs = 0.669, P less than 0.001) between the urinary excretion of 2- and 5-DHS. 3. These re-evaluated results on the oxidation polymorphism of SP indicate that 2- and 5-DHS formation from SP shares a common metabolic pathway (presumably via the same P-450 isozyme), and that the SP/DHS ratio, conventionally used as a discriminating index between PMs and EMs, quantitatively reflects the capacity of 2- and 5-DHS formation. 4. The benefit of using a shorter (6 h) collection period for assessing the individual oxidation phenotype of SP and inter-ethnic comparison of SP oxidation is also discussed.

Polymorphism of the 4-hydroxylation of debrisoquine in the San Bushmen of southern Africa

1. The metabolic oxidation of debrisoquine has been studied in a group of 96 San Bushmen. 2. The amounts of debrisoquine and 4-hydroxy-debrisoquine excreted in 0-8 h urine were measured and the metabolic ratio (% dose as debrisoquine/% dose as 4-hydroxy-debrisoquine) calculated. 3. On the basis of Caucasian criteria, that metabolic ratios greater than 12.6 represent poor metabolizers, 19% of the Bushmen were poor metabolizers in contrast to the 8-10% found in Caucasian studies. 4. Probit plots showed four modes may be present in the data, which may represent at least three isozymes of the relevant enzyme which may also differ from the Caucasian isozymes.

Animal extrapolation and the challenge of human heterogeneity

The capacity of animal models to predict the responses of humans to carcinogenic agents in light of the occurrence of human heterogeneity is assessed in this paper. It is widely accepted that human susceptibility to toxic substances, including carcinogens, is highly variable. At the same time, it is believed that the conventional rodent models, which are usually highly inbred and reared in standard ways, display a very homogeneous response to toxic agents, including carcinogens. The question then becomes, To which narrow band of the broad spectrum of human responses can specific animal models likely be extrapolated? First, the occurrence of human heterogeneity is examined with respect to a broad range of biological characteristics (e.g., aryl hydrocarbon hydroxylase activity, epoxide hydrase activity, glutathione S-transferase activity, beta-glucuronidase activity, debrisoquine hydroxylation, and DNA adduct formation), with particular emphasis on those which affect responses to carcinogens. Second, the occurrence of heterogeneity for selected animal models for these characteristics is assessed and the outcomes are related to the spectrum of human responses noted above.

The metabolism of debrisoquine in man: (1) regioselectivity of hydroxylation and (2) aberrant oxidative metabolism in two sibling patients with carbimazole-induced agranulocytosis

The regioselectivity of the metabolic hydroxylation of debrisoquine has been determined in 43 healthy British white volunteers and the priority was found to be in the order 4 greater than 7 greater than 6 greater than 5 greater than 8. The order of preference for hydroxylation position was independent of debrisoquine 4-hydroxylation phenotype. The extent of total aromatic hydroxylation varied widely between individuals and was largely independent of the extent of 4-hydroxylation, and thus of the influence of the DH/DL locus. Two sisters and their blood relations all excreted comparatively large amounts of the phenolic metabolites in their urine, indicating some genetic basis for the control of aromatic oxidation of debrisoquine in man. These same two sisters had previously developed agranulocytosis in association with carbimazole therapy.

Racial differences in drug responses--a comparative study of trimazosin and alpha 1-adrenoceptor responses in normotensive Caucasians and West Africans

The possible racial differences in alpha 1-adrenoceptor responsiveness and the blood pressure and heart rate responses following alpha 1-adrenoceptor antagonism with trimazosin have been investigated in matched groups of six Caucasians and six Nigerians. There were no significant differences between the racial groups in the blood pressure and heart rate responses to oral (200 mg) and intravenous (100 mg) trimazosin. alpha 1-adrenoceptor responsiveness was similar in both groups after placebo and following both active treatments. There were only minor pharmacokinetic differences with the Caucasians having a larger volume of distribution, and a longer terminal elimination half-life for the metabolite, 1-hydroxy-trimazosin. These results suggest a similarity in peripheral vascular alpha 1-adrenoceptor mechanisms and show no major significant racial differences in the pharmacokinetics and pharmacodynamics of trimazosin.

The polymorphic oxidation of beta-adrenoceptor antagonists. Clinical pharmacokinetic considerations

Wide variability in response to some drugs such as debrisoquine can be attributed largely to genetic polymorphism of their oxidative metabolism. Most beta-blockers undergo extensive oxidation. Anecdotal reports of high plasma concentrations of certain beta-blockers in poor metabolisers (PMs) of debrisoquine have claimed that the oxidation of these drugs is under polymorphic control. Subsequently, controlled studies have shown that debrisoquine oxidation phenotype is a major determinant of the metabolism, pharmacokinetics and some of the pharmacological actions of metoprolol, bufuralol, timolol and bopindolol. The poor metaboliser phenotype is associated with increased plasma drug concentrations, a prolongation of elimination half-life and more intense and sustained beta-blockade. Phenotypic differences have also been observed in the pharmacokinetics of the enantiomers of metoprolol and bufuralol. In vivo and in vitro studies have identified some of the metabolic pathways which are subject to the defect, viz. alpha-hydroxylation and O-demethylation of metoprolol and 1'- and possibly 4- and 6-hydroxylation of bufuralol. In contrast, the overall pharmacokinetics and pharmacodynamics of propranolol, which is also extensively oxidised, are not related to debrisoquine polymorphism, although 4'-hydroxypropranolol formation is lower in poor metabolisers. As anticipated, the disposition of atenolol which is eliminated predominantly unchanged by the kidney and in the faeces, is unrelated to debrisoquine phenotype. The clinical significance of impaired elimination of beta-blockers is not clear. If standard doses of beta-blockers are used in poor metabolisers, these subjects may be susceptible to concentration-related adverse reactions and they may also require less frequent dosing for control of angina pectoris.

Genetically determined variability in acetylation and oxidation. Therapeutic implications

The clinical significance of two separate genetic polymorphisms which alter drug metabolism, acetylation and oxidation is discussed, and methods of phenotyping for both acetylator and polymorphic oxidation status are reviewed. Particular reference is made to the dapsone method, which provides a simple means of distinguishing fast and slow - and possibly intermediate - acetylators, and to the sparteine method which allows a clear separation of oxidation phenotypes. Although acetylation polymorphism has been known for some time, definite indications for phenotyping are few. It is doubtful whether acetylator phenotype makes a significant difference to the outcome in most isoniazid treatment regimens, and peripheral neuropathy from isoniazid in slow acetylators is easily overcome by pyridoxine administration. However, in comparison with rapid acetylators, slow acetylators receiving isoniazid have an increased susceptibility to phenytoin toxicity, and perhaps also to carbamazepine toxicity. It is also possible that rapid acetylators receiving isoniazid attain higher serum fluoride concentrations from enflurane and similar anaesthetics than do similarly treated slow acetylators. Thus, when drug interactions of these types are suspected, phenotyping for acetylator status may be advisable. If routine monitoring of serum procainamide and N-acetylprocainamide concentrations is practised, phenotyping of subjects prior to therapy with these agents should not be necessary. Although acetylator phenotype influences serum concentrations of hydralazine, when this drug is given in combination with other drugs acetylator phenotype has not been shown to influence the therapeutic response. Slow acetylator phenotype along with female gender and the presence of HLA-DR antigens appear to be risk factors in the development of hydralazine-induced systemic lupus erythematosus (SLE). Determination of acetylator phenotype may therefore help determine susceptibility to this adverse reaction. In the case of sulphasalazine, adult slow acetylators require a lower daily dose of the drug than fast acetylators in order to maintain ulcerative colitis in remission without significant side effects. It is therefore advisable to determine acetylator phenotype prior to sulphasalazine therapy. Work on the association of acetylation polymorphism with various disease states is also reviewed. It is possible that a higher incidence of bladder cancer is associated with slow acetylation phenotype - especially in individuals exposed to high levels of arylamines. The question as to whether idiopathic SLE is more common in slow acetylators remains unresolved. There appears to be no difference between fa

Genetic polymorphism of mephenytoin p(4')-hydroxylation: difference between Orientals and Caucasians

The genetically controlled mephenytoin p(4')-hydroxylation capacity was determined in 118 Caucasians and 70 Orientals. After an oral dose of 50 or 100 mg of racemic mephenytoin, the amount of p(4')-hydroxymephenytoin in 24 h urine was measured by gas chromatography. Bimodal distribution was found with 9/70 (13%) Orientals and 5/118 (4%) Caucasians demonstrating deficient p(4')-hydroxylation. The statistically significant difference between Orientals and Caucasians (P less than 0.05) was accounted for by the high incidence of poor metabolizers among the Japanese subjects, 7/31 (23%). The frequency among Chinese subjects, 2/39 (5%), was similar to the frequency among Caucasians.

Statistical analysis of polymorphic drug metabolism data using the Rosin Rammler Sperling Weibull distribution

The Rosin Rammler Sperling Weibull distribution and its use in the analysis of complex data is explained with reference to metoprolol and acebutolol AUC values and isoniazid plasma concentrations. The technique is then applied to sparteine and debrisoquine data to resolve populations into distinct sub-groups. Goodness of fit is measured by applying the chi 2 test to the untransformed data. The method is simple to use and sub-groups can be identified rapidly. Each sub-group can be characterised by a simple exponential equation.

Spectral binding studies of the polymorphically metabolized drugs debrisoquine, sparteine and phenformin by cytochrome P-450 of normal and hydroxylation deficient rat strains

The mechanisms of polymorphic drug hydroxylation of debrisoquine, sparteine and related drugs in vivo have been investigated using Cyt P-450 preparations of inbred rat strains as an in vitro model of the poor and extensive metabolizer phenotypes found in various rat strains and in man. Optical difference spectroscopy with debrisoquine, sparteine, phenformin and three other drugs (selected test compounds with proven or suspected hydroxylation polymorphisms in man) exhibited Type 1 binding in normal Sprague-Dawley, Fischer and Lewis Cyt P-450, whereas no Type I drug binding was found in the hydroxylation deficient DA rat liver Cyt P-450. Cyt P-450 content and Type II drug binding of metiamide was the same in normal and hydroxylation deficient rat liver microsomes. The pronounced Type I drug binding in extensive hydroxylation Cyt P-450 and the defective Type I binding in DA Cyt P-450 in vitro, therefore, closely parallels the polymorphic hydroxylation pattern of these test drugs found in the four rat strains studied in vivo. Consequently, missing binding properties of Cyt P-450 or of its micro-environment might represent the enzymatic defect underlying the genetically determined hydroxylation deficiency of polymorphically metabolized drugs in the poor metabolizer phenotype in the DA rat and, by inference, in man.

Defective metabolism of metoprolol in poor hydroxylators of debrisoquine

Eight healthy volunteers received oral metoprolol 200 mg once daily for a week. The AUC, half-life and duration of beta-adrenoceptor blockade on day 7 was much greater in two subjects than in the remaining six. This suggested that the metabolism of metoprolol was impaired in two and the effect was therefore prolonged. Subsequent testing of oxidation phenotype with oral debrisoquine showed that the subjects with high metoprolol availability were also poor hydroxylators of debrisoquine. The urinary debrisoquine/4-hydroxydebrisoquine ratio was highly correlated with metoprolol AUC, half-life and beta-adrenoceptor blockade at 24 h. Thus patients with a genetic defect in drug oxidation, when treated with metoprolol, are likely to have high plasma concentrations and a prolonged effect.

The polymorphic 4-hydroxylation of debrisoquine in a Saudi arab population

1. Debrisoquine 4-hydroxylation was polymorphic in 102 Saudi arab volunteers, the population comprising one phenotypically poor metabolizer and 101 phenotypically extensive metabolizers of debrisoquine. 2. Mean urinary recoveries of drug and metabolite were low in Saudis (15 +/- 10% dose, mean +/- S.D.), which compared well with previously studied populations of Egyptians (16%) and Ghanaians (18%), but which were lower than those seen in a UK white population (41%). 3. Saudis, like Egyptians, were more extensive metabolizers of debrisoquine than UK whites, as judged by the metabolic ratio (% dose as debrisoquine/% dose as 4-hydroxy-debrisoquine eliminated in the urine). 4. Neither sex, urine collection period nor urinary recovery of drug and metabolite had any statistically significant effect upon the distribution of metabolic ratios in Saudis. 5. The frequency of the DL allele controlling the recessive poor metabolizer trait was 0.099 +/- 0.049 (+/- S.E.M.) in Saudis, which compared well to Egyptians (0.118 +/- 0.059) but was significantly lower than that for UK whites (0.298 +/- 0.030). 6. These findings raise questions regarding the efficacy and safety in Saudis of drugs undergoing oxidative metabolism which have been evaluated for usage in European white subjects.