The contribution of genetically determined oxidation status to inter-individual variation in phenacetin disposition

Pharmacogenetics of Agents Acting on the Central Nervous System

This article will review the various agents affecting the central nervous system (CNS) such as the analgesics, antidepressants, anticonvulsants, antipsychotics, and benzodiazepines. Most of the research in pharmacogenetics with the CNS agents have been conducted in the antidepressants. The cytochrome 450 IID6 isozyme system has been shown to influence the disposition of the antidepressants and antipsychotics. Amitriptyline metabolism to nortriptyline and nortriptyline conversion to its 10-OH metabolite were shown to be influenced by the IID6 isozyme. Interestingly, imipramine metabolism to desipramine is only partially related to the IID6 isozyme. Biotransformation of imipramine to its 2-OH metabolite was shown to be affected by the IID6 isozyme, but its metabolism to the 10-OH remains to be investigated. Of the antipsychotic drugs, haloperidol and thioridazine are two agents most studied. Haloperidol is converted to a reduced metabolite via a ketone reductase enzyme. The reduced metabolite is oxidized back to Haloperidol. This oxidation pathway was reported to be affected by the IID6 isozyme. Thioridazine metabolism to mesoridazine and conversion of codeine to morphine appear to be also influenced by CP-450 IID6. Other 450 isozymes are reported to be involved with other CNS agents.

Genetics of caffeine consumption and responses to caffeine

RATIONALE

Caffeine is widely consumed in foods and beverages and is also used for a variety of medical purposes. Despite its widespread use, relatively little is understood regarding how genetics affects consumption, acute response, or the long-term effects of caffeine.

OBJECTIVE

This paper reviews the literature on the genetics of caffeine from the following: (1) twin studies comparing heritability of consumption and of caffeine-related traits, including withdrawal symptoms, caffeine-induced insomnia, and anxiety, (2) association studies linking genetic polymorphisms of metabolic enzymes and target receptors to variations in caffeine response, and (3) case-control and prospective studies examining relationship between polymorphisms associated with variations in caffeine response to risks of Parkinson's and cardiovascular diseases in habitual caffeine consumers.

RESULTS

Twin studies find the heritability of caffeine-related traits to range between 0.36 and 0.58. Analysis of polysubstance use shows that predisposition to caffeine use is highly specific to caffeine itself and shares little common disposition to use of other substances. Genome association studies link variations in adenosine and dopamine receptors to caffeine-induced anxiety and sleep disturbances. Polymorphism in the metabolic enzyme cytochrome P-450 is associated with risk of myocardial infarction in caffeine users.

CONCLUSION

Modeling based on twin studies reveals that genetics plays a role in individual variability in caffeine consumption and in the direct effects of caffeine. Both pharmacodynamic and pharmacokinetic polymorphisms have been linked to variation in response to caffeine. These studies may help guide future research in the role of genetics in modulating the acute and chronic effects of caffeine.

Structure, function, regulation and polymorphism and the clinical significance of human cytochrome P450 1A2

Human CYP1A2 is one of the major CYPs in human liver and metabolizes a number of clinical drugs (e.g., clozapine, tacrine, tizanidine, and theophylline; n > 110), a number of procarcinogens (e.g., benzo[a]pyrene and aromatic amines), and several important endogenous compounds (e.g., steroids). CYP1A2 is subject to reversible and/or irreversible inhibition by a number of drugs, natural substances, and other compounds. The CYP1A gene cluster has been mapped on to chromosome 15q24.1, with close link between CYP1A1 and 1A2 sharing a common 5'-flanking region. The human CYP1A2 gene spans almost 7.8 kb comprising seven exons and six introns and codes a 515-residue protein with a molecular mass of 58,294 Da. The recently resolved CYP1A2 structure has a relatively compact, planar active site cavity that is highly adapted for the size and shape of its substrates. The architecture of the active site of 1A2 is characterized by multiple residues on helices F and I that constitutes two parallel substrate binding platforms on either side of the cavity. A large interindividual variability in the expression and activity of CYP1A2 has been observed, which is largely caused by genetic, epigenetic and environmental factors (e.g., smoking). CYP1A2 is primarily regulated by the aromatic hydrocarbon receptor (AhR) and CYP1A2 is induced through AhR-mediated transactivation following ligand binding and nuclear translocation. Induction or inhibition of CYP1A2 may provide partial explanation for some clinical drug interactions. To date, more than 15 variant alleles and a series of subvariants of the CYP1A2 gene have been identified and some of them have been associated with altered drug clearance and response and disease susceptibility. Further studies are warranted to explore the clinical and toxicological significance of altered CYP1A2 expression and activity caused by genetic, epigenetic, and environmental factors.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Variation in CYP1A2 activity and its clinical implications: influence of environmental factors and genetic polymorphisms

CYP1A2 is involved in the metabolism of several widely used drugs and endogenous compounds, and in the activation of procarcinogens. Both genetic and environmental factors influence the activity of this enzyme. The current knowledge regarding factors influencing the activity of CYP1A2 is summarized in this review. Substrates, inhibitors and inducers of CYP1A2 activity, as well as phenotyping probes, are discussed. The functional significance and clinical importance of CYP1A2 gene polymorphisms are reviewed and interethnic differences in the distribution of CYP1A2 variant alleles and haplotypes are summarized. Finally, future perspectives for the possible clinical applications of CYP1A2 genotyping are discussed.

Identification of novel polymorphisms in the 5' flanking region of CYP1A2, characterization of interethnic variability, and investigation of their functional significance

CYP1A2 activity has been demonstrated to be bimodally or trimodally distributed in several populations, consistent with a codominant or recessive functional genetic polymorphism. However, studies aimed at identifying polymorphisms in CYPIA2 have not yet adequately accounted for this distribution pattern. To search for functional polymorphisms, we performed genome-walking, polymerase chain reaction (PCR) sequencing, and cloning, and identified three novel polymorphisms in the 5' flanking region of CYP1A2: a T-3591G substitution, a G-3595T substitution, and a T-3605 insertion. The frequency of the T-3591G substitution was determined by a PCR-restriction fragment length polymorphism assay, and found to be significantly higher (P < 0.0001) in Taiwanese (allele frequency 0.128, n = 125) compared to Caucasians (0.017, n = 87) or African Americans (0.024, n = 104). The functional consequence of the T-3591G and the G-3595T substitutions was determined by site-directed mutagenesis followed by transient transfection experiments. The T-3591G mutation was shown to be nonfunctional, while although the G-3595T mutation appeared to result in an increase in promoter activity, this was only to a small degree and therefore unlikely to be important in vivo. In addition, we report 532 bases of 5' flanking sequence further upstream than that reported to date, and four sequence discrepancies compared to the original published sequence (G-3649C, deltaT-3650, deltaA-4072, and C-4093 ins).

Pharmacokinetic drug interactions of new antidepressants: a review of the effects on the metabolism of other drugs

Seven of the newest antidepressants are the serotonin-selective reuptake inhibitors (fluoxetine, sertraline, paroxetine, and fluvoxamine [currently approved in the United States only for obsessive-compulsive disorder]), a serotonin-norepinephrine reuptake inhibitor (venlafaxine), a postsynaptic serotonin antagonist-presynaptic serotonin reuptake inhibitor (nefazodone), and a presynaptic-postsynaptic noradrenergic-serotonergic receptor antagonist (mirtazapine). Many of these drugs are potent inhibitors of the cytochrome P-450 enzymes (CYPs) of the liver. The isoforms of the CYPs most relevant to the use of antidepressants are CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4. CYP inhibition may affect the metabolism of numerous drugs in several classes that are substrates for these isoenzymes, with potentially serious consequences. To minimize the potential for an adverse event, the practitioner must remember the drug-drug interactions, and possible consequences when one of these antidepressants is being prescribed. A "primer" on drug metabolism is included herein, which serves as a basis for understanding these interactions., Each of the isoenzymes of the CYPs is discussed in relationship to the drugs they metabolize, and appropriate cautions are recommended for concurrent administration of these new antidepressants and other drugs most frequently prescribed to elderly patients.

Polymorphisms of CYP1A1 and GSTM1 influence the in vivo function of CYP1A2

Differences in human cancer susceptibility have been attributed to polymorphisms of carcinogen metabolizing enzymes. Our efforts have focused on the systems responsible for metabolism of aromatic and heterocyclic amines found in cigarette smoke and in cooked foods. Cytochrome P4501A2 (CYP1A2), which catalyzes aromatic and heterocyclic amine N-oxidation, has been implicated as a risk factor in both urinary bladder and colorectal cancer. In the present study we used the results of caffeine phenotyping experiments to measure the effects of cigarette smoke and compounds present in meat cooked at high temperature on CYP1A2 activity. Subjects in the smoking cessation study had mean CYP1A2 activity of 17.8 (expressed as the urinary molar ratio of [17X + 17U]/137X) while smoking: however, this activity decreased to 10.9 three weeks after cessation of smoking. Subjects in the cooked meat feeding study had mean CYP1A2 activity of 9.01 after 1 week of consuming meat cooked at low temperature, but this value increased to 12.7 after 1 week of consuming meat cooked at high temperature. Because no association has been identified between differences in CYP1A2 activity and variations in the CYP1A2 structural gene, we sought to determine whether the activities of other carcinogen metabolizing enzymes are involved in the regulation of CYP1A2 activity. CYP1A2 activity was higher in individuals who express the GSTM1 null allele compared to those expressing the GSTM1*A,B allele, 10.2 vs. 8.5 for unexposed conditions and 15.0 vs. 12.3 for exposed conditions. CYP1A1 genotyping demonstrated that individuals possessing the Ile/Ile CYP1A1 genotype had greater mean CYP1A2 activity than those who had the heterozygous Ile/Val allelic variant of the CYP1A1 gene. However, upon exposure to cigarette smoke or high-temperature cooked meat, individuals possessing the heterozygous form of the CYP1A1 gene had significantly increased CYP1A2 activity (18.1) compared to those with the more common Ile/Ile CYP1A1 genotype (13.3). These results indicate that CYP1A2, CYP1A1, and GSTM1 gene-gene interactions could be important confounders in the interpretation of molecular epidemiology studies.

Determination of clozapine in serum by radioreceptor assay versus high-performance liquid chromatography: possible detection of hydroxy-metabolites

Clozapine is an antipsychotic drug with few extra-pyramidal motor side-effects, used to treat schizophrenia which is resistant to classical neuroleptic therapy. This report shows that norclozapine but not clozapine-N-oxide has the same D2 receptor affinity as clozapine. Assay results suggest a bimodal distribution which may be explained by CYP1A2 polymorphism. Extensive metabolizers could produce other active metabolites, probably other hydroxy-clozapine derivatives.

Drug metabolizing enzymes related to laboratory medicine: cytochromes P-450 and UDP-glucuronosyltransferases

Many studies on drug metabolism have been carried out during the last decades using protein purification, molecular cloning techniques and analysis of polymorphisms at phenotype and genotype levels. These researchers led to a better understanding of the role of drug metabolizing enzymes in the biotransformation of drugs, pollutants or foreign compounds and of their use in laboratory medicine. The metabolic processes commonly involved in the biotransformation of xenobiotics have been classified into functionalization reaction (phase I reactions), which implicate lipophilic compounds. These molecules are modified via monooxygenation, dealkylation, reduction, aromatization, hydrolysis and can be substrates for the phase II reactions, often called conjugation reactions as they conjugate a functional group with a polar, endogenous compound. This review, devoted to cytochromes P-450 (CYP) and UDP-glucuronosyltransferases (UGT), describes essentially the genetic polymorphisms found in humans, their clinical consequences and the methods to assess the phenotypes or genotypes, with a view to studying the interindividual differences in drug monooxygenation and drug glucuronidation. Variations in drug glucuronidation reported here focused essentially on variations due to physiological factors, induction, drug interactions and genetic factors in disorders such as Gilbert's Syndrome and Crigler-Najjar type I and II diseases.

Metabolic polymorphisms

Polymorphisms have been detected in a variety of xenobiotic-metabolizing enzymes at both the phenotypic and genotypic level. In the case of four enzymes, the cytochrome P450 CYP2D6, glutathione S-transferase mu, N-acetyltransferase 2 and serum cholinesterase, the majority of mutations which give rise to a defective phenotype have now been identified. Another group of enzymes show definite polymorphism at the phenotypic level but the exact genetic mechanisms responsible are not yet clear. These enzymes include the cytochromes P450 CYP1A1, CYP1A2 and a CYP2C form which metabolizes mephenytoin, a flavin-linked monooxygenase (fish-odour syndrome), paraoxonase, UDP-glucuronosyltransferase (Gilbert's syndrome) and thiopurine S-methyltransferase. In the case of a further group of enzymes, there is some evidence for polymorphism at either the phenotypic or genotypic level but this has not been unambiguously demonstrated. Examples of this class include the cytochrome P450 enzymes CYP2A6, CYP2E1, CYP2C9 and CYP3A4, xanthine oxidase, an S-oxidase which metabolizes carbocysteine, epoxide hydrolase, two forms of sulphotransferase and several methyltransferases. The nature of all these polymorphisms and possible polymorphisms is discussed in detail, with particular reference to the effects of this variation on drug metabolism and susceptibility to chemically-induced diseases.

The role of individual human cytochromes P450 in drug metabolism and clinical response

Recent advances in the study of human cytochromes P450 by protein purification, molecular cloning techniques and analysis of polymorphisms has led to increased understanding of the role of the various forms in the metabolism of clinically important drugs. In particular, the substrate specificity of one form, CYP2D6, is well established. CYP2D6 shows polymorphism, with 5-10% of Caucasians (poor metabolizers) not expressing this enzyme. The molecular basis of this deficiency is now well understood and methods for the detection of poor metabolizers are discussed, as well as the effect of the polymorphism on drug metabolism. Substrate specificities and possible polymorphisms in other cytochromes P450 are also discussed.

P450 enzymes. Inhibition mechanisms, genetic regulation and effects of liver disease

Multiple hepatic P450 enzymes play an important role in the oxidative biotransformation of a vast number of structurally diverse drugs. As such, these enzymes are a major determinant of the pharmacokinetic behaviour of most therapeutic agents. There are several factors that influence P450 activity, either directly or at the level of enzyme regulation. Drug elimination is decreased and the incidence of drug interactions is increased when there is competition between 2 or more drugs for oxidation by the same P450 enzyme. The available knowledge concerning the relationship between the presence of certain functional groups within the drug structure and inhibition of P450 activity is increasing. In many instances, it is possible to associate inhibition with certain drug classes, e.g. antimycotic imidazoles and macrolide antibiotics. Disease states, especially those with hepatic involvement, and the genetic makeup of the individual are conditions in which some P450s may be downregulated (that is, the enzyme concentrations in liver are decreased), with associated slower rates of drug elimination. In these individuals, dosages of drugs that are substrates for downregulated P450s should be decreased. Exposure to environmental pollutants as well as a large number of lipophilic drugs can result in induction (upregulation) of P450 enzyme activity. This raises the issue of previous approaches to the study of P450 induction in vivo. The use of human hepatocyte preparations in culture is a promising new direction that could assist the determination of modifications to drug therapy necessitated by exposure to inducing agents. Until such information is obtained, however, the use of drugs known to increase the microsomal expression of particular P450s, and increase associated drug oxidation capacity in humans, should be used with caution.

Metabolism of paracetamol and phenacetin in relation to debrisoquine oxidation phenotype

The metabolism of paracetamol and phenacetin has been studied in subjects previously phenotyped as either extensive or poor metabolisers of debrisoquine (EM and PM, respectively), in order to examine the relationship between phenacetin and paracetamol activation and debrisoquine oxidation status. In separate experiments, paracetamol and phenacetin were administered orally to groups of 5 EM and 5 PM subjects, and the excretion of metabolites measured for 24 h. There were no differences between EM and PM subjects in the excretion of metabolites. After phenacetin, 0.82 of the dose was recovered in urine, mostly as paracetamol glucuronide (51%) and sulphate (30%), with smaller amounts of free paracetamol (4%) and the mercapturate (5%) and cysteine conjugates (5%), 2-hydroxyphenetidine (5%) and N-hydroxyphenacetin (0.5%). Following paracetamol, 0.87 of the dose was recovered, with similar proportions of paracetamol-derived metabolites. It is concluded that the debrisoquine oxidation phenotype is unrelated to either the metabolic activation of phenacetin and paracetamol, or to their overall metabolic clearance.

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.

Human cytochrome P-450PA (P-450IA2), the phenacetin O-deethylase, is primarily responsible for the hepatic 3-demethylation of caffeine and N-oxidation of carcinogenic arylamines

Aromatic amines are well known as occupational carcinogens and are found in cooked foods, tobacco smoke, synthetic fuels, and agricultural chemicals. For the primary arylamines, metabolic N-oxidation by hepatic cytochromes P-450 is generally regarded as an initial activation step leading to carcinogenesis. The metabolic activation of 4-aminobiphenyl, 2-naphthylamine, and several heterocyclic amines has been shown recently to be catalyzed by rat cytochrome P-450ISF-G and by its human ortholog, cytochrome P-450PA. We now report that human hepatic microsomal caffeine 3-demethylation, the initial major step in caffeine biotransformation in humans, is selectively catalyzed by cytochrome P-450PA. Caffeine 3-demethylation was highly correlated with 4-aminobiphenyl N-oxidation (r = 0.99; P less than 0.0005) in hepatic microsomal preparations obtained from 22 human organ donors, and both activities were similarly decreased by the selective inhibitor, 7,8-benzoflavone. The rates of microsomal caffeine 3-demethylation, 4-aminobiphenyl N-oxidation, and phenacetin O-deethylation were also significantly correlated with each other and with the levels of immunoreactive human cytochrome P-450PA. Moreover, a rabbit polyclonal antibody raised to human cytochrome P-450PA was shown to inhibit strongly all three of these activities and to inhibit the N-oxidation of the carcinogen 2-naphthylamine and the heterocyclic amines, 2-amino-6-methyldipyrido-[1,2-a:3',2'-d]imidazole and 2-amino-3-methylimidazo[4,5-f]-quinoline. Human liver cytochrome P-450PA was also shown to catalyze caffeine 3-demethylation, 4-aminobiphenyl N-oxidation, and phenacetin O-deethylation. Thus, estimation of caffeine 3-demethylation activity in humans may be useful in the characterization of arylamine N-oxidation phenotypes and in the assessment of whether or not the hepatic levels of cytochrome P-450PA, as affected by environmental or genetic factors, contribute to interindividual differences in susceptibility to arylamine-induced cancers.

Poor sulphoxidation ability in patients with food sensitivity

Patients with well defined reactions to foods were examined for their ability to carry out both sulphur and carbon oxidation reactions by using carbocisteine and debrisoquine as probe compounds. The proportion of poor sulphoxidisers (58 of 74) was significantly greater than that of a previously determined normal control population (67 of 200; p less than 0.005). The proportion of poor carbon oxidisers was not significantly different from the controls. Metabolic defects may play a part in the pathogenesis of adverse reactions to foods.

Differential inhibition of human liver phenacetin O-deethylation by histamine and four histamine H2-receptor antagonists

1. The effects of histamine and four histamine H-2 receptor antagonists on phenacetin O-deethylation by microsomal preparations of four human livers was quantified by a radiometric-thin layer chromatographic method. 2. Histamine and three of these drugs, namely cimetidine, ranitidine and famotidine, were weak inhibitors of this cytochrome P-450-catalysed O-deethylation, but mifentidine was a potent competitive inhibitor with a Ki in the range 40-70 microM. 3. Cimetidine, histamine and mifentidine are all 4(5)-substituted imidazole derivatives, and the contrast between the very weak inhibitory effects of cimetidine and histamine, and the more potent effect of mifentidine, suggests that the imidazole moiety may play little role in the inhibition of phenacetin O-deethylase by mifentidine. 4. The demonstration that cimetidine, ranitidine and histamine were all poor inhibitors of phenacetin oxidation further suggests the possible lack of identity between the human liver cytochrome P-450 isoenzymes responsible for catalyzing the oxidation of metoprolol and phenacetin. This follows from recognizing that metoprolol oxidation is known, from both in vivo and in vitro studies, to be strongly inhibited by both of these H-2 receptor antagonists and from in vitro studies also to be inhibited by histamine.

Phenacetin O-deethylation by human liver microsomes: kinetics and propranolol inhibition

1. Phenacetin O-deethylation catalysed by human liver microcomes has been examined over a substrate concentration range of 2.5 to 700 microM using preparations of eight human liver samples. Michaelis-Menten kinetics described phenacetin oxidation satisfactorily in five of these samples; apparent Km values were in the range of 17.7 to 38.4 microM. 2. In the three other livers a single rectangular hyperbolic relationship did not describe the substrate saturation data adequately; analyses in these three cases requiring two classes of catalytic site. The apparent Km value for the higher affinity class of site in these three samples was within the range quoted above, but limitations imposed by assay sensitivity and phenacetin solubility obviated accurate characterization of the lower affinity class. 3. Estimates of Vmax for the high affinity class of site in the eight livers varied eleven-fold and there was no correlation between either Km or Vmax and microsomal cytochrome P-450 specific content, NADPH cytochrome c (P-450) reductase specific activity or ethylmorphine demethylase activity. 4. Propranolol was a potent competitive inhibitor of phenacetin deethylation with apparent Ki values of 2 to 7 microM describing its effect on the higher affinity class of activity. Propranolol was also an inhibitor of the lower affinity phenacetin deethylase identified in three of the livers, however the mechanism of inhibition could not be characterized. 5. These data suggest the possibility that propranolol oxidation may be mediated in part, by one or more human liver cytochrome P-450 species catalyzing phenacetin oxidation.

Genetic factors in toxicology: implications for toxicological screening

Methods of assessing the toxicity of xenobiotics have improved substantially during the last decade. However, as compounds become generally safer, the problem of individual variation in response assumes increasing relative importance. Environmental factors such as age, health and nutritional status, and interactions with other xenobiotics account for some of this variation, but genetic differences between individuals and races have important implications. In a few cases, Mendelian loci which control drug susceptibility (e.g., to isoniazid) have been described. However, in most cases the exact mode of inheritance has not yet been determined due to the problems of carrying out genetic studies in man. It is well established that many loci that are polymorphic in man are also so in laboratory animals, so much of this genetic variation should be picked up in preclinical screening, and could be used to more accurately predict potential variation in toxicity in man. Unfortunately, most toxicologists use only a single stock of laboratory animals, which does not show whether the response to a given xenobiotic is under genetic control. The design of animal tests would be improved by using more than one strain of genetically defined animals, and by paying more attention to genetic variation in responses to xenobiotics, both in animals and man.

Pharmacogenetics of dextromethorphan O-demethylation in man

The metabolism of dextromethorphan has been investigated from the aspect of genetically determined intersubject differences of oxidative drug metabolism in man. For this purpose, the urinary elimination of dextromethorphan and dextrorphan, which is the major O-demethylated metabolite in urine, has been studied in selected drug hydroxylation phenotypes. Dextromethorphan O-demethylation co-segregates with polymorphic debrisoquine hydroxylation, whereas no such co-segregation exists with the independently controlled mephenytoin polymorphism in man. The urinary dextromethorphan over dextrorphan metabolic ratio was validated for linearity of O-demethylation vs dose administered, and for varying urine collection intervals at different urinary pH values. A 94% repeatability of the dextromethorphan metabolic ratio could be established in extensive and poor metabolizer phenotypes. In a preliminary study, different rates of N-, O- and N,O- demethylation of dextromethorphan to yield D-methoxymorphinane, dextrorphan and D-hydroxymorphinane, respectively, were found in extensive- (Sprague-Dawley) and poor-metabolizer (female dark Agouti) rat strains. The observed interphenotype differences in man and the interstrain variations in an experimental animal model indicate that dextromethorphan O-demethylation is catalysed by the debrisoquine-type cytochrome P-450 isozyme. Therefore, the common genetic control of debrisoquine and dextromethorphan metabolism indicates that dextromethorphan might be used as a safe and innocuous substitute for debrisoquine in future routine phenotyping in the field of human pharmacogenetics of oxidative drug metabolism.

The relationship between the acetylator and the sparteine hydroxylation polymorphisms

Thirty-eight healthy white British Caucasian subjects were hydroxylator phenotyped with sparteine and acetylator phenotyped with sulphadimidine. The results showed that there was no significant difference in the mean sparteine metabolic ratio between eight rapid acetylator extensive hydroxylators and 27 slow acetylator extensive hydroxylators.

Polymorphism in drug metabolism--implications for drug toxicity

Genetic polymorphism of the drug metabolism pathways is commonly encountered both for man and laboratory animal species. It is a major source of variable metabolism and linked events such as response to drugs and toxic substances. In man the cytochrome P-450 isozyme system exhibits considerable polymorphism. Several independent genetic polymorphisms regulating metabolic oxidation at C-, N-, and S-centres have been recently characterised. This phenomenon appears to be a powerful factor in determining biochemical individuality with respect to the oxidative metabolism of drugs and responsiveness to therapeutic agents. Of considerable importance is the recognition of the existence of phenotypes within the individual polymorphism, characterised by an impaired ability to effect metabolic oxidation. Evidence suggests that this factor can determine an increased susceptibility to experience exaggerated pharmacological effects and adverse reactions to several drugs. Laboratory animal species also exhibit polymorphism with respect to several drug metabolic pathways but compared with man, this has been less extensively researched. The study of intra-species differences in metabolism of drugs and toxic substances can be of value: when it occurs it may signal its possible occurrence in man and animal strain models of the human metabolic polymorphisms facilitate the laboratory study of inherited susceptibility to toxicants.

Formation of reactive metabolites of phenacetin in humans and rats

The metabolism of phenacetin to reactive intermediates in humans was estimated from the excretion of thio adducts in urine. N-Hydroxyphenacetin, a precursor of reactive metabolites, was also quantified. Following an oral dose of phenacetin (10 mg/kg) to humans, these metabolites in 24 h urine were: paracetamol-3-cysteine, 4.4% dose; paracetamol-3-mercapturate, 3.9%; 3-thiomethylparacetamol, 0.4%; N-hydroxyphenacetin, 0.5%. Rats showed a considerable increase in N-hydroxyphenacetin excretion after chronic dosing with phenacetin at high dosage (500 mg/kg) for one month. chronic dosing with a low dose (50 mg/kg) did not increase N-hydroxyphenacetin excretion, but a marked increase occurred on concomitant administration of aspirin and caffeine.

The genetic control of phenformin 4-hydroxylation

Previously published results of phenformin 4-hydroxylation in 195 unrelated white British volunteers and 87 family members of 27 randomly selected probands have been subjected to genetic analysis. The results clearly show that about 9% of this population has a genetically determined defect in carrying out this oxidation reaction. The character for the defect is inherited in a Mendelian autosomal recessive fashion. The polymorphism shows a substantial degree of dominance.

Purification and characterization of the rat liver microsomal cytochrome P-450 involved in the 4-hydroxylation of debrisoquine, a prototype for genetic variation in oxidative drug metabolism

Genetic polymorphism in oxidative drug metabolism is perhaps best exemplified in the case of debrisoquine 4-hydroxylase activity, where the incidence of deficient metabolism ranges from 1% to 30% in various populations and this defect is also linked to an impaired ability to metabolize a number of other drugs effectively. Sprague-Dawley (SD) rats possess this activity, but females of the DA strain do not, although total cytochrome P-450 (P-450) levels are similar. We have purified, by using debrisoquine 4-hydroxylase activity as an assay, a minor P-450 to electrophoretic homogeneity from male SD rats and designate this as P-450UT-H. P-450UT-H differs from eight other purified rat liver P-450s as judged by peptide mapping and immunochemical analysis and thus appears to be isozymic with these other P-450s. P-450UT-H exhibited considerably more debrisoquine 4-hydroxylase activity than any of the other purified P-450s and, on a total P-450 basis, more than total microsomal P-450. Antibodies raised against P-450UT-H specifically recognized P-450UT-H and inhibited more than 90% of the debrisoquine hydroxylase activity present in SD rat liver microsomes. The level of P-450UT-H in SD rat liver microsomes accounted for less than 10% of the total P-450, as judged by immunochemical quantitation. These assays also indicated that the level of P-450UT-H in female DA rat liver microsomes is only about 5% of that in male or female SD rat liver microsomes, consonant with the view that deficiency of this form of P-450 is responsible for the defective debrisoquine 4-hydroxylase activity in the former animals.(ABSTRACT TRUNCATED AT 250 WORDS)