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

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 2C8*2 allele in Botswana: Human genetic diversity and public health implications

Human cytochrome P450 2C8 is a highly polymorphic gene and shows variation according to ethnicity. The CYP2C8*2 is a slow drug metabolism allele and shows 10-24% frequency in Black populations. The objective of this study was to assess the prevalence of CYP2C8*2 allele in Botswana among the San (or Bushmen) and the Bantu ethnic groups. For that purpose we recruited 544 children of the two ethnicities in three districts of Botswana from primary schools, collected blood samples, extracted DNA and genotyped them through PCR-based restriction fragment length polymorphism analysis. The results demonstrated that in the San the prevalence of the CYP2C8*2 allele is significantly higher than among the Bantu-related ethnic groups (17.5% and 8.5% for San and Bantu, respectively; P=0.00002). These findings support the evidence of a different genetic background of the San with respect to Bantu-related populations, and highlight a possible higher risk of longer drug clearance or poor level of activation of pro-drugs among the San group.

Pharmacogenetic comparison of CYP2D6 predictive and measured phenotypes in a South African cohort

The relationship between genetic variation in CYP2D6 and variable drug response represents a potentially powerful pharmacogenetic tool. However, little is known regarding this relationship in the genetically diverse South African population. The aim was therefore to evaluate the relationship between predicted and measured CYP2D6 phenotype. An XL-PCR+Sequencing approach was used to determine CYP2D6 genotype in 100 healthy volunteers and phenotype was predicted using activity scores. With dextromethorphan as the probe drug, metabolic ratios served as a surrogate measure of in vivo CYP2D6 activity. Three-hour plasma metabolic ratios of dextrorphan/dextromethorphan were measured simultaneously using semi-automated online solid phase extraction coupled with tandem mass spectrometry. Partial adaptation of the activity score system demonstrated a strong association between genotype and phenotype, as illustrated by a kappa value of 0.792, inter-rater discrepancy of 0.051 and sensitivity of 72.7%. Predicted phenotype frequencies using the modified activity score were 1.3% for poor metabolisers (PM), 7.6% for intermediate metabolisers (IM) and 87.3% for extensive metabolisers (EM). Measured phenotype frequencies were 1.3% for PM, 13.9% for IM and 84.8% for EM. Comprehensive CYP2D6 genotyping reliably predicts CYP2D6 activity in this South African cohort and can be utilised as a valuable pharmacogenetic tool.

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.

Genetic polymorphism in cytochrome P450 2D6 (CYP2D6): Population distribution of CYP2D6 activity

Cytochrome P-450 2D6 (CYP2D6) is involved in the metabolism of many therapeutic drugs even though the enzyme represents a small proportion of the total CYP content of human liver. In vivo phenotyping with probe drug substrates such as debrisoquine and dextromethorphan showed a clear separation between poor metabolizers (PM) and extensive metabolizers (EM). This polymorphism may affect susceptibility to environmental disease, as suggested by molecular epidemiologic studies that found an association between CYP2D6 metabolizer phenotype and cancer risk; however, this association is not consistent. There are only a few examples of CYP2D6 involvement in toxicant mechanism of action, but this has not been extensively studied. Gene probe studies documented a number of genetic polymorphisms that underlie CYP2D6 metabolizer phenotypes. The EM group carries the wild-type (*1) or active (*2) variant alleles, while the PM group carries the *3, *4, *5, or *6 alleles, all of which code for a protein that has lower or null CYP2D6 activity. The current analysis characterizes (a) influence of genotype on phenotype based upon in vivo metabolism studies of probe drugs and (b) frequency of the major genotypes in different population groups is also characterized. These data were then incorporated into Monte Carlo modeling to simulate population distributions of CYP2D6 activity. This analysis reproduced the bimodal distributions commonly seen in phenotyping studies of Caucasians and found extensive population variability in enzyme activity, as indicated by the 9- to 56-fold difference between the PM modal median and the total population median CYP2D6 activity. This substantial degree of interindividual variability in CYP function indicates that assessments involving CYP2D6 substrates need to consider the full distribution of enzyme activity in refining estimates of internal dose in health assessments of xenobiotics.

Revealing the paradox of drug reward in human evolution

Neurobiological models of drug abuse propose that drug use is initiated and maintained by rewarding feedback mechanisms. However, the most commonly used drugs are plant neurotoxins that evolved to punish, not reward, consumption by animal herbivores. Reward models therefore implicitly assume an evolutionary mismatch between recent drug-profligate environments and a relatively drug-free past in which a reward centre, incidentally vulnerable to neurotoxins, could evolve. By contrast, emerging insights from plant evolutionary ecology and the genetics of hepatic enzymes, particularly cytochrome P450, indicate that animal and hominid taxa have been exposed to plant toxins throughout their evolution. Specifically, evidence of conserved function, stabilizing selection, and population-specific selection of human cytochrome P450 genes indicate recent evolutionary exposure to plant toxins, including those that affect animal nervous systems. Thus, the human propensity to seek out and consume plant neurotoxins is a paradox with far-reaching implications for current drug-reward theory. We sketch some potential resolutions of the paradox, including the possibility that humans may have evolved to counter-exploit plant neurotoxins. Resolving the paradox of drug reward will require a synthesis of ecological and neurobiological perspectives of drug seeking and use.

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.

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.

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.

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.

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.

Absence of polymorphism of sparteine oxidation in the South African Venda

1 This study has found no occurrence of poor metabolism of sparteine within a South African Venda population of 97 subjects. 2 On the basis of MR (metabolic ratio) the mean and distribution of the results are very similar to those found in Ghanaians. 3 The distribution is also similar to that for fast metabolizers in Caucasians. 4 It is concluded that different P450 cytochromes are responsible for immediate oxidation of debrisoquine and sparteine, but that both may be activated by the same P450 reductase.

Unimodal distribution of the metabolic ratio for debrisoquine in Cuna Amerindians of Panama

1. The metabolic oxidation of debrisoquine (DB) was studied in 89 non-related Cuna Amerindian subjects. 2. Means and standard deviations for urinary recoveries of the intact drug and its 4-hydroxy metabolite (4-HD) were: %DB: 6.8 +/- 4.5; %4-HD: 16.0 +/- 9.1; %sum: 22.8 +/- 12.0. The log10 metabolic ratios for DB (LMRDB) were distributed within a single mode of insignificant skewness (-0.01, P greater than 0.10), which was unimodal (log kernel density and maximum likelihood methods) and normal (chi 2 = 22.5; d.f. = 15; P greater than 0.09; power of the test greater than 80%). 3. Therefore, no poor metabolizers (95% C.I.: 0.1%, 5.2%) were detected in the population sample studied.

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.

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.