A family and population study of the genetic polymorphism of debrisoquine oxidation in a white British population

Normal metabolism of debrisoquine and theophylline in a slow tolbutamide metaboliser

The metabolism of debrisoquine and theophylline has been studied in a healthy male who was identified as a slow hydroxylator of tolbutamide. Tolbutamide clearance in this subject was three-fold lower than the lowest tolbutamide clearance observed in other healthy males and the drug's half-life was approximately three-fold longer. Despite this, his ability to 4-hydroxylate debrisoquine and both N-demethylate and 8-hydroxylate theophylline was normal. Along with previously published information the data from this subject suggest that tolbutamide hydroxylation, debrisoquine hydroxylation, theophylline N-demethylation, and theophylline 8-hydroxylation involve four distinct isozymes of cytochrome P-450. Furthermore, this report illustrates the difficulties of using the metabolic clearance of a model drug to predict the ability of an individual to clear a range of metabolised drugs.

Lack of congruence of S-carboxymethyl-L-cysteine sulphoxidation and debrisoquine 4-hydroxylation in a Caucasian population

One-hundred-and-twenty volunteers and three families were investigated for possible association between the sulphoxidation of S-carboxymethyl-L-cysteine and the debrisoquine hydroxylation polymorphism. The observed individual variations in these two metabolic reactions were shown not to be concordant (rs = 0.068) and any heritable factors controlling the major aspects of these phenomena do not co-segregate.

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

Oxidation phenotype and the metabolism and action of beta-blockers

Variability in response to some drugs such as debrisoquine can be attributed to genetic polymorphism of their oxidative metabolism. Most beta-adrenoceptor antagonists (beta-blockers) are extensively metabolised via oxidative routes. Anecdotal reports of high plasma concentrations of certain beta-blockers in poor metabolisers of debrisoquine (PM) have claimed that their oxidation is under polymorphic control. 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 and timolol. The PM phenotype is associated with an increased drug bioavailability, a prolongation of elimination half-life and more intense and sustained beta-blockade. Phenotypic differences were also noted in the pharmacokinetics of the enantiomers of metoprolol. In vivo and in vitro work has identified some of the metabolic pathways which are subject to the defect, namely, the alpha-hydroxylation and the O-dealkylation of metoprolol and the 1'-hydroxylation of bufuralol. In contrast, the pharmacokinetics and pharmacodynamics of propranolol which is also extensively oxidised, are not related to debrisoquine polymorphism, although 4'-hydroxypropranolol formation is lowered in PM subjects. The clinical significance of impaired elimination of beta-blockers is unclear. If standard doses of beta-blockers are used in PM subjects, they may be susceptible to concentration-related adverse reactions and they may also require lower and less frequent dosing for control of angina pectoris.

Timolol and atenolol: relationships between oxidation phenotype, pharmacokinetics and pharmacodynamics

The pharmacokinetics and pharmacodynamics of atenolol and timolol were studied in six extensive and four poor metabolisers of debrisoquine. There was a significant correlation between the debrisoquine to 4-hydroxydebrisoquine ratio and the area under the plasma concentration time curve (AUC) for timolol (rs = 0.75, P less than 0.02). The mean of the AUC values for timolol was significantly greater in the poor metabolisers than in the extensive metabolisers (P less than 0.05). There was a significant correlation between the debrisoquine to 4-hydroxydebrisoquine ratio and beta-adrenoceptor blockade 24 h after dosing with timolol (rs = 0.66, P less than 0.05). The mean degree of beta-adrenoceptor blockade was significantly greater in the poor metabolisers than in the extensive metabolisers 24 h after dosing with timolol (P less than 0.01). There was no relation between the debrisoquine to 4-hydroxydebrisoquine ratio and the pharmacokinetics or pharmacodynamics of atenolol.

Inter-ethnic difference in sparteine oxidation among Ghanaians and Germans

Sparteine oxidation, which exhibits genetic polymorphism in various Caucasian populations, was studied in 154 unrelated Ghanaian subjects. Mean total urinary recovery of sparteine and 2- and 5-dehydrosparteines (56.6 +/- 16.6% of the dose) was not different from Caucasians. In contrast to Caucasians, amongst whom 6.3 to 9% of the population are poor metabolizers (PM), no PM subject was observed in the Ghanaian population sample. The data appear to indicate allelic differences between Caucasians and Ghanaians in the gene encoding the synthesis of the P-450 isozyme involved in polymorphic sparteine oxidation.

Effect of oxidative polymorphism (debrisoquine/sparteine type) on hepatic first-pass metabolism of bufuralol

Bufuralol is a beta-adrenoceptor blocking drug whose oxidative metabolism is under the same genetic control as debrisoquine and sparteine. The pharmacokinetics of bufuralol were studied in 10 healthy subjects (7 extensive and 3 poor metabolizers of debrisoquine) after oral and intravenous administration. In extensive metabolizers the systemic availability of bufuralol was 43%. Poor metabolizers were characterized by a considerable increase in systemic availability due to a corresponding decrease in hepatic first-pass metabolism. After oral administration of bufuralol non-linear kinetics may occur.

Interindividual variation of beta-adrenoceptor blocking drugs, plasma concentration and effect: influence of genetic status on behaviour of atenolol, bopindolol and metoprolol

Ten healthy subjects whose genetic oxidative phenotype had been determined (6 extensive and 4 poor metabolizers of the debrisoquine-sparteine type of polymorphism) received single oral doses of 3 beta-blockers: atenolol, bopindolol and metoprolol. The plasma concentrations and the extent of the decrease in exercise-induced tachycardia were determined. The oxidative polymorphism was only significant for substances that had a high hepatic first pass metabolism, such as metoprolol. The metabolic pathway under genetic control was highly stereoselective. This observation must be taken into account when assessing the relation between the plasma concentration and effect of these drugs, which are often administered as racemic mixtures.

Genetic polymorphism in drug oxidation: in vitro studies of human debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase activities

A sensitive, specific assay utilizing fluorescence-HPLC has been developed for determining the 1'-hydroxylation of bufuralol by human liver. The 1'-hydroxylation of the isomers of bufuralol varied threefold, both the Vmax and the Km for the (+) isomer being greater than the corresponding values for the (-) isomer. Debrisoquine was a competitive inhibitor of the 1'-hydroxylation of both isomers and of the racemate of bufuralol. Both isomers and the racemate of bufuralol were competitive inhibitors of debrisoquine 4-hydroxylase activity. The competitive inhibition of debrisoquine and bufuralol of each other's metabolism, together with the similarity in the values for Km and Ki, support the conclusion that the same form of cytochrome P-450 catalyses these two reactions.

Characterization of a human liver cytochrome P-450 involved in the oxidation of debrisoquine and other drugs by using antibodies raised to the analogous rat enzyme

Debrisoquine 4-hydroxylase activity is a prototype for genetic polymorphism in oxidative drug metabolism in humans; approximately 10% of Caucasian populations exhibit the poor metabolizer phenotype, and the clearance of at least 14 other drugs has been shown to be deficient in patients exhibiting this phenotype. Antibodies prepared to a cytochrome P-450 shown to be responsible for debrisoquine 4-hydroxylation in rats were found to inhibit the oxidation of debrisoquine and sparteine, encainide, and propranolol, three other drugs suggested to be associated with this phenotype, in human liver microsomes. The antibodies did not inhibit the oxidation of seven other cytochrome P-450 substrates. The antibodies recognized a single polypeptide of Mr51,000 after combined sodium dodecyl sulfate/polyacrylamide electrophoresis and immunochemical staining of human liver microsomes. The intensity of this band was significantly correlated with debrisoquine 4-hydroxylase activity when liver microsomes from 44 organ donors were examined. Immunoprecipitation of in vitro translation products of total liver RNA revealed major electrophoretic bands corresponding to the cytochrome P-450 in rats and humans. The level of translatable mRNA coding for the debrisoquine-hydroxylating cytochrome P-450 was an order of magnitude less in human liver than in rat liver. The availability of these antibodies provides a biochemical basis for further basic and clinical studies on the role of a particular cytochrome P-450 polymorphism in humans.

Characterization of a common genetic defect of cytochrome P-450 function (debrisoquine-sparteine type polymorphism)--increased Michaelis is Constant (Km) and loss of stereoselectivity of bufuralol 1'-hydroxylation in poor metabolizers

In order to define the mechanism of the debrisoquine-sparteine type genetic polymorphism of drug oxidation we studied the kinetics of bufuralol 1'-hydroxylation in liver microsomes from extensive and poor metabolizers and in a purified reconstituted human cytochrome P-450 isozyme with high activity for bufuralol 1'-hydroxylation, P-450[buf]. In extensive metabolizer microsomes the enzymatic reaction displayed apparent Michaelis-Menten kinetics and the (+)-isomer was preferentially metabolized. By contrast, the enzymatic reaction in poor metabolizer microsomes was characterized by a 4- to 5-fold increase in Km and by a loss of stereoselectivity. In a non-membraneous reconstituted system containing NADPH cytochrome P-450 reductase, a NADPH regenerating system and phospholipids, P-450[buf] exhibited an almost complete substrate stereoselectivity for (+)-isomer 1'-hydroxylation. It is concluded that the purified cytochrome P-450[buf] is the target of the debrisoquine-sparteine type oxidation polymorphism and that poor metabolizers have a quantitative or qualitative deficiency of this isozyme.

Metabolic oxidation phenotypes as markers for susceptibility to lung cancer

That bronchial carcinoma is not an inevitable consequence of cigarette smoking has stimulated the search for host factors that might influence the susceptibility of the individual smoker. One plausible host factor would be a polymorphic gene controlling the metabolic oxidative activation of chemical carcinogens, giving rise to wide inter-subject variation in the generation of cancer-inducing and/or promoting species. Recently, three genetic polymorphisms of human metabolic oxidation have been demonstrated (as characterized by debrisoquine, mephenytoin and carbocysteine), with the metabolism of several substrates exhibiting the phenomenon. Debrisoquine 4-hydroxylation segregates into two human phenotypes, each comprising characteristic metabolic capability. We report here the frequency of debrisoquine 4-hydroxylation phenotypes in age-, sex- and smoking history-matched bronchial carcinoma and control patients. Cancer patients showed a preponderance of probable homozygous dominant extensive metabolizers (78.8%) with few recessive poor metabolizers (1.6%) compared with smoking controls (27.8% and 9.0% respectively). We conclude that the gene controlling debrisoquine 4-hydroxylation may be a host genetic determinant of susceptibility to lung cancer in smokers and that it represents a marker to assist in assessing individual risk.

Impaired oxidation of debrisoquine in patients with perhexiline liver injury

Perhexiline maleate is an antianginal agent which depends on hepatic oxidation for its elimination. Its use may be complicated by the development of peripheral neuropathy and liver damage. The majority of patients with perhexiline neuropathy have an impaired ability to effect metabolic drug oxidation which is genetically determined. Information has not been available on drug oxidation capacity in patients with perhexiline liver injury. Drug oxidation was measured using an oxidation phenotyping procedure in four patients with perhexiline liver injury and in 70 patients with chronic liver disease serving as a control group. All four patients with perhexiline liver damage showed a substantial metabolic defect; three of the four patients (75%) showed a genetically determined impairment of oxidation capacity. The incidence of severely impaired oxidation capacity in the perhexiline group was significantly greater than in the patients with chronic liver disease (6/70; 8.6%) and in the healthy population (9%) (F = 0.0048). A clear association exists between perhexiline liver injury and diminished drug metabolic activity, suggesting that the propensity to develop perhexiline liver injury is, at least in part, genetically determined.

Genetic aspects of the polymodally distributed sulphoxidation of S-carboxymethyl-L-cysteine in man

Interindividual variation in the sulphoxidation of S-carboxymethyl-L-cysteine (750 mg p.o.) was investigated in 200 healthy volunteers. Nearly a 100-fold difference was observed between individuals with respect to the amount of sulphoxide metabolites detected in their 0-8 h urine (0.6 to 59.1% recovery). Such a difference was shown to be reproducible over several months in 40 subjects who spanned the entire range of capacities. Cumulative plots and maximum likelihood analysis of the distribution indicated that a bimodal model was most probable. Analysis of pedigree data obtained from 12 families suggested a genetic effect with overlying environmental influences.

Oxidation phenotyping in alcoholics with liver disease of varying severity

The propensity to develop alcoholic cirrhosis is probably, at least in part, genetically determined. A striking similarity exists histologically between perhexiline and alcohol-related hepatitis and both are potentially precirrhotic lesions. Liver damage due to perhexiline is associated with impaired drug oxidation capacity which is genetically determined and tested by use of debrisoquine. Oxidation phenotyping might be used to predict susceptibility to perhexiline liver damage; it might also predict the potential to develop alcoholic cirrhosis. Oxidation phenotyping was therefore undertaken, using debrisoquine in 100 alcoholic patients, 30 of whom had only fatty liver despite prolonged alcohol abuse, while the remaining 70 had alcoholic hepatitis and/or cirrhosis. One hundred patients with nonalcoholic chronic liver disease served as controls. The number of patients with severely impaired drug oxidation capacity (poor metabolizer phenotype) was similar in the alcoholic group (8%) and the nonalcoholic control group (7%). In particular, the incidence of the poor metabolizer phenotype was similar in alcoholics with severe liver disease (10%) and in those with only fatty change (3%). There appears to be no association between the susceptibility to develop alcoholic cirrhosis and drug oxidizing capacity.

Debrisoquine-type polymorphism of drug oxidation: purification from human liver of a cytochrome P450 isozyme with high activity for bufuralol hydroxylation

Indirect evidence suggests that the genetically defective metabolism of drugs such as debrisoquine and bufuralol observed in up to 10% of the population (poor metabolizers) is caused by the absence or functional deficiency of a cytochrome P450 isozyme. Using bufuralol-1'-hydroxylation to carbinol to optimize the procedure, 3 cytochrome P450 isozymes (P450A, P450buf, P450C) were purified to apparent electrophoretic homogeneity from human liver microsomes. P450buf had a specific activity of 20.3 nmol carbinol X nmol P450-1 X 15 min-1 as compared to microsomes (10.0 nmol carbinol X nmol P450(-1) X 15 min-1) when (+)-bufuralol was used as substrate. The stereoselective metabolism of (-)- and (+)-bufuralol to carbinol by purified P450buf [(-)/(+) ratio: 0.13] was strikingly different from that in the microsomes of either an extensive [(-)/(+) ratio: 0.4] or poor metabolizer [(-)/(+) ratio: 0.83] of bufuralol. We propose that this isozyme is the major bufuralol and debrisoquine hydroxylating species and is the target of the genetic deficiency.

Desipramine pharmacokinetics in Chinese and Caucasian volunteers

In order to better define the role of pharmacokinetic variation in reported cross-ethnic differences in dosing patterns of some psychoactive drugs, single dose kinetics of the tricyclic antidepressant desipramine (DMI) were studied in 14 Chinese and 16 Caucasian healthy volunteers. DMI and 2-OH-DMI concentrations were assayed with h.p.l.c. in serial plasma and 24 h urine samples over 5 days following an oral 100 mg dose of DMI. Mean total clearance of DMI ( CLDMI ) from plasma was significantly (P less than 0.05) higher in the Caucasians (123 +/- 57 l/h) than in the Chinese (73.5 +/- 38.8 l/h). There was no significant difference in the apparent clearance of DMI by hydroxylation, fraction of dose metabolized to the hydroxy metabolite, DMI t1/2 or plasma protein binding between the two groups. Trimodal distribution of CLDMI was found, with 4/14 (29%) Chinese demonstrating slow CL (less than 33 l/h) and 4/16 (25%) Caucasians rapid CL (greater than 195 l/h). Correcting CL values for the greater mean weight of the Caucasians did not alter the pattern of distribution. CLDMI did not correlate with body weight. Although environmental factors cannot be ruled out, these results are consistent with genetically based differences in hepatic metabolism, probably affecting pathways in addition to hydroxylation, and suggest that 30% of Orientals would be at risk for toxicity from routine doses of tricyclics or similarly metabolized drugs.

Polymorphic hydroxylation of perhexiline maleate in man

Long term perhexiline maleate therapy causes peripheral neuropathy and hepatic damage in certain subjects. An association between these adverse reactions and a genetically determined relative inability to hydroxylate debrisoquine has been described. This association could indicate either that the effects of perhexiline impair debrisoquine oxidation thus producing a phenocopy, or that perhexiline is polymorphically hydroxylated and that the polymorphism is controlled by the same alleles as control the debrisoquine polymorphism. To test the second possibility, a study investigating the hydroxylation status of a population of healthy volunteer subjects has been performed using perhexiline maleate. Hydroxylation phenotyping was performed on 50 normal volunteers. A standard oral dose was given and plasma and urinary perhexiline, 4-monohydroxyperhexiline (MI metabolite), and 4'monohydroxyperhexiline (MIII metabolite) was measured. The 24-hour plasma perhexiline concentration, the 24-hour plasma MI metabolite concentration, and 12 to 24-hour urinary MI metabolite excretion were clearly bimodal, suggesting the existence of a polymorphism for perhexiline hydroxylation. Poor metabolisers represent 6% of the population studied. Known poor metabolisers of debrisoquine are also poor metabolisers of perhexiline, while known extensive metabolisers of debrisoquine are also extensive metabolisers of perhexiline, indicating that in white British subjects the hydroxylation polymorphism is under identical genetic control for both compounds. The poor metaboliser sub-group exhibited the highest plasma perhexiline levels. Perhexiline phenotyping separates the poor and extensive metaboliser phenotypes much more clearly than other tests and defines a sub-group at risk from perhexiline toxicity. Pretreatment phenotyping using this test, followed by exclusion of poor metabolisers from perhexiline therapy, should substantially reduce the incidence of major adverse effects.

Competitive inhibition of sparteine oxidation in human liver by beta-adrenoceptor antagonists and other cardiovascular drugs

The rate of oxidation of sparteine by the 9000 x g supernatant fraction of a human liver was measured in the presence of various drugs which exert cardiovascular effects. Hexamethonium, ouabain, caffeine and isoproterenol had no effect on this rate, while alprenolol, metoprolol, oxprenolol, propranolol, timolol, pindolol, lidocaine, mexiletine, 17-n-pentyl-sparteine, tolazoline, quinine, quinidine, cinchonine and cinchonidine inhibited the in vitro reaction competitively. Stereoselective inhibition was observed between quinine (Ki = 15 microM) and quinidine (Ki = 0.06 microM). Genetic evidence suggests that the primary metabolism of sparteine depends on a single species of cytochrome P450. In vitro competitive inhibition of sparteine oxidation by a drug indicates that this drug is capable of occupying the same enzymatic site as sparteine. This may mean that the competing drug is also metabolized at that site and thereby subject to the same genetic variation as sparteine's oxidation; absence of inhibition excludes this possibility.

Polymorphic metabolism of beta-adrenoceptor antagonists

Most beta-adrenoceptor blockers undergo extensive oxidative metabolism. The evidence for polymorphism of the debrisoquine type is reviewed. The AUC and half-life of metoprolol were considerably greater in poor metabolisers (PM) of debrisoquine than in extensive metabolisers (EM). Metoprolol alpha-hydroxylation is impaired and O-dealkylation must also be affected. Polymorphism in the former route has been demonstrated in a population of 143 patients to be directly related to debrisoquine phenotype. Bufuralol AUC and half-life are much higher in PM than EM subjects. Hydroxylation at the 1 and 4 positions are affected. Genetic polymorphism for 1-hydroxylation has been shown in family and population studies. Propranolol 4-hydroxylation is defective in PM subjects of debrisoquine but propranolol AUC is not related to phenotype, presumably because other major pathways are unaffected. Oxidation phenotype correlates well with intensity and duration of beta-adrenoceptor blockade after metoprolol, PM subjects requiring only once-daily dosing. However, in EM subjects twice-daily dosing is required even if slow release preparations are used, since plasma metoprolol concentrations may remain negligible 24 h after dosing. The beta-adrenoceptor blocking effects of propranolol and bufuralol are unlikely to be influenced by oxidation status. Anecdotal reports of toxicity arising in PM subjects taking metoprolol or propranolol need to be substantiated. However, vomiting after the administration of bufuralol often occurs in poor metabolisers. Metabolic interactions with drugs sharing the same enzyme system are discussed. Debrisoquine and bufuralol competitively inhibit each other's metabolism in vitro. (ABSTRACT TRUNCATED AT 250 WORDS)

Human cytochromes P-450

Studies in vivo have provided evidence for a multiplicity of cytochromes P-450 in man, some of which are under independent monogenic control. Although the activity of cytochromes P-450 in man are generally lower than those of rat, this is by no means always the case. There are several important exceptions including the N-hydroxylation of 2-acetamidofluorene. Studies in vitro by a number of different techniques have confirmed the evidence from studies in vivo that there are multiple forms of human cytochrome P-450. In addition to differences in Vmax, the different forms of cytochrome P-450 may also exhibit marked differences in their apparent Km values. The implications that this may have for pharmacokinetics and toxicology are discussed. The polymorphism in the 4-hydroxylation of debrisoquine observed in vivo has been shown to be due to a defect in a specific form of cytochrome P-450 which appears to be under monogenic regulation. Cross-inhibition studies have enabled the specificity of this isozyme to be characterized. Such studies have also enabled the contribution of this isozyme of cytochrome P-450 to the oxidation of other substrates to be determined. Compounds investigated include bufuralol and phenytoin. Evidence from studies both in vivo and in vitro suggest that selective induction of different forms of cytochrome P-450 can occur in man. However, the number of different classes of inducer in man is not yet known. Human cytochromes P-450 have been purified to near homogeneity in several laboratories. Different forms of cytochrome P-450 purified from the same liver sample vary in molecular weight, chromatographic characteristics and substrate specificities.

Pharmacogenetics of mephenytoin: a new drug hydroxylation polymorphism in man

Inherited deficiency in mephenytoin hydroxylation was observed in a family study. It is important that the propositus was of the extensive metabolizer phenotype for the genetically controlled hydroxylation of debrisoquine. Thus, a genetic polymorphism of drug hydroxylation was suspected for mephenytoin. A population study of mephenytoin hydroxylation, combined with identification of extensive and poor debrisoquine hydroxylation phenotypes, was carried out in 221 unrelated normal volunteers. Twelve of them (5%) exhibited defective aromatic hydroxylation of mephenytoin, and 23 (10%) could be identified as poor metabolizers of debrisoquine. Amongst these 35 subjects with a drug hydroxylation deficiency, 3 (or 0.5%; 1 female, 2 males) displayed both defects simultaneously. A panel study of 10 extensive and 10 poor metabolizers of mephenytoin showed that the ability to perform aromatic hydroxylation of the demethylated mephenytoin metabolite nirvanol (5-phenyl-5-ethylhydantoin) was co-inherited with the mephenytoin hydroxylation polymorphism. Family studies suggested that poor metabolizer phenotypes of nirvanol and mephenytoin were most likely to have the homozygous genotype for an autosomal recessive allele of deficient aromatic drug hydroxylation. Intra-subject comparison of the debrisoquine and mephenytoin hydroxylation phenotypes in these subjects indicated that deficiency in the two drug hydroxylations occurred independently. Consequently, the co-inheritance of extensive and poor hydroxylation of mephenytoin and nirvanol, respectively, represents a new drug hydroxylation polymorphism in man.

Physiological factors affecting drug toxicity

Physiological factors that affect the fate of drugs in the body and thereby have effects on their pharmacology and toxicology involve the systems that control absorption, distribution, metabolism, and excretion. The main factors are disease, genetics, and age. Nutritional status, sex, hormonal status (e.g., the effects of pregnancy), and circadian rhythm have important influences. Maternal toxicity will affect the fetus. The absorption and excretion of drugs are frequently reduced by diseases. Excretion is reduced by impaired renal function, often found in the elderly, which may increase the toxic response. Distribution is affected by body weight and build, for example, the proportion of fat. The disposition of many drugs has been shown to be significantly influenced by circadian rhythms such that therapeutic doses may exhibit toxicity if administered at an inappropriate time of day. Metabolism is modified by environmental influences including previous food and drug experience, and various factors such as age, sex, and disease. Intersubject variations in drug disposition can be very great with possibly severe consequences for the individual; in this regard, knowledge of genetic polymorphism in drug metabolizing enzymes is rapidly increasing. The toxicology of a drug may be profoundly affected by a particular disease state, for example, the administration of a drug that might be a tumor promoter when a cancerous or precancerous condition exists. These effects are illustrated with examples from the literature and recent studies undertaken in the Bureau of Drug Research.

The genetic control of sparteine and debrisoquine metabolism in man with new methods of analysing bimodal distributions

Debrisoquine and sparteine tests were carried out in 215 random white British subjects. There is a high degree of correlation between the urinary 'metabolic ratios' of the two drugs. New mathematical techniques have been developed (1) to define phenotypes and (2) to identify the genotypes within the dominant phenotype. The members of 15 families were tested with both debrisoquine and sparteine. The results indicate that persons who are 'poor metabolisers' of sparteine are also 'poor metabolisers' of debrisoquine and are autosomal Mendelian recessives.

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

The oxidative O-de-ethylation and aromatic 2-hydroxylation of phenacetin have been investigated in panels of extensive (EM, n = 13) and poor (PM, n = 10) metabolizers of debrisoquine. The EM group excreted in the urine significantly more paracetamol (EM: 40.8 +/- 14.9% dose/0-8 h; PM: 29.2 +/- 8.7% dose/0-8 h, 2P less than 0.05) and significantly less 2-hydroxylated metabolites (EM: 4.7 +/- 2.3% dose/0-8 h; PM: 9.7 +/- 3.5% dose/0-8 h, 2P less than 0.005) than the PM group. Apparent first-order rate constants, calculated from pooled phenotype data, for overall elimination of phenacetin (k) and formation of paracetamol (kml) were higher in the EM group (EM: k = 0.191 +/- 0.151 h-1; kml = 0.091 +/- 0.025 h-1; PM: k = 0.098 +/- 0.035 h-1, 2P less than 0.05, kml = 0.052 +/- 0.019 h-1, 2P less than 0.05) than the PM group. The apparent first-order rate constant for 2-hydroxylation displayed no significant inter-phenotype differences. Correlation analysis demonstrated that genetically determined oxidation status accounted for approximately 50% of the inter-individual variability in phenacetin disposition encountered in this study.

Genetically determined oxidation capacity and the disposition of debrisoquine

1 The disposition in urine of debrisoquine and its hydroxylated metabolites has been studied in subjects of the 'extensive metabolizer' (EM; n = 5) and 'poor metabolizer' (PM; n = 5) phenotypes. The 4-hydroxylation of debrisoquine by PM subjects following a 10 mg oral dose was capacity-limited and displayed significant dose-dependency over a range of 1-20 mg. In contrast, the EM subjects' ability to perform this metabolic oxidation did not deviate from first-order kinetics over a dose range of 10-40 mg. 2 The disposition of debrisoquine in plasma following a 10 mg oral dose has been studied in EM (n = 4) and PM (n = 3) subjects. Whilst PM subjects displayed significantly higher plasma levels of debrisoquine at all time points following 1 h post-dosing, and higher values for areas under the plasma concentration-time curve (EM: 105.6 +/- 7.0 ng ml-1 h; PM: 371.4 +/- 22.4 ng ml-1 h, 2P less than 0.0001), neither debrisoquine plasma half-life (EM: 3.0 +/- 0.5 h; PM: 3.3 +/- 0.4 h) nor renal clearance of the drug (EM: 152.8 +/- 30.3 ml min-1; PM: 137 +/- 4.5 ml min-1) displayed significant inter-phenotype differences. 3 The results of these investigations show that the phenotyping of individuals for debrisoquine oxidation status by means of a 'metabolic ratio' derived from a single 0-8 h urine sample has a sound kinetic basis. The kinetic differences between the two phenotypes would strongly suggest that the metabolic defect manifested in PM subjects is one of pre-systemic elimination capacity.

Contribution of the genetic status of oxidative metabolism to variability in the plasma concentrations of beta-adrenoceptor blocking agents

The oxidative metabolism of bufuralol is under the same genetic control as that of debrisoquine and sparteine. 154 fasting volunteers received a 30 mg tablet of bufuralol and a blood sample was taken 3 h later. In poor metabolizers (8% of the sample) the plasma bufuralol concentrations were very high and the metabolite concentrations were low. The genetic oxidative status is a major source of interindividual variation in the plasma concentration of drugs that undergo oxidative metabolism.

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.

Comparison of two long-acting preparations of metoprolol with conventional metoprolol and atenolol in healthy men during chronic dosing

1 Eight healthy men received two long-acting formulations of metoprolol 200 mg (SA Astra, SR Geigy), conventional metoprolol 200 mg and atenolol 100 mg once daily for 1 week each in balanced, crossover fashion. There was a washout period of at least a week between each phase. 2 On the last day of each phase, post-exercise heart rate was recorded at intervals and compared to pretreatment values. Plasma metoprolol concentrations were measured. 3 The mean AUC was similar after each of the three formulations of metoprolol (relative bioavailability of SA and SR v conventional was 97%) but with SA and SR metoprolol the time to peak was significantly delayed by about 2 h. 4 In comparison to conventional metoprolol only metoprolol SA was associated with significantly higher plasma metoprolol concentrations at the end of a dosing interval (mean values: conventional, 25 ng/ml, SR 37 ng/ml, SA 51 ng/ml). 5 Mean (+/- s.d.) reduction in exercise tachycardia at the end of a dosing interval was significantly greater with atenolol (14.8 +/- 4.5%) and metoprolol SA (13.7 +/- 10.3%) than with metoprolol SR (10 +/- 8.4%) and conventional metoprolol (8.2 +/- 7.1%). 6 The variability in beta-adrenoceptor blockade at 24 h was much greater with each of the three metoprolol formulations than that with atenolol. This was explained by the variability in metoprolol metabolism. 7 Oxidation phenotype testing with debrisoquine showed there were six extensive metabolisers and two poor metabolisers. The AUC, half-life and response to metoprolol at 24 h were much greater in poor metabolisers. Response to atenolol was not influenced by phenotype.

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.

Ethnic differences in drug metabolism

Interethnic differences in drug-metabolising capacity may be substantial, and they are sufficiently frequent to warrant attention. Such differences may consist of different mean values of quantitative traits in separate populations, or of different frequency distributions as produced by the occurrence of genetic enzyme variants. The collection of population data requires the investigation of substantial numbers of subjects. This may be no problem if drug-metabolising enzymes occur in blood or are sufficiently stable in their tissues to allow investigation in vitro. However, if investigations require the use of probe drugs, new efforts are needed to adapt pharmacokinetic methods to make them suitable for population studies. This development of methods is further called for because genetic variants seem to be more easily detected through the assessment of particular metabolites than through the determination of pharmacokinetic parameters of the parent drug. Many studies with probe drugs comparing different populations have given results that are equivocal in terms of the nature-nurture interplay. However, a set of data with antipyrine has pointed to environmental factors as the principal determinant of differences in metabolising capacity, while data with debrisoquine have indicated monogenically controlled variation of one facet of the cytochrome P-450 system. In several instances, statistically significant differences between population means have been established by testing small numbers of subjects, numbers insufficient to establish distribution patterns that would allow the recognition of genetic polymorphism. The populations studied range from Greenlanders to South African Blacks, but most comparisons pertain to Caucasians and Orientals.

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.

Assay and characterisation of debrisoquine 4-hydroxylase activity of microsomal fractions of human liver

1 A method for the assay of debrisoquine 4-hydroxylase activity in vitro by microsomal fractions of human liver is described. The assay utilises gas chromatography-mass spectrometry with d9-4-hydroxydebrisoquine as internal standard. 2 The limit of detection of 4-hydroxydebrisoquine was 2 ng ml -1 and the coefficient of variation was 4.4%. 3 Debrisoquine 4-hydroxylase activity was linear with protein to concentrations above 2.1 mg ml -1 and with incubation times of at least 15 min. 4 Debrisoquine 4-hydroxylase is a microsomal enzyme with a requirement for NADPH. Activity was inhibited by carbon monoxide. It is concluded that the activity is catalysed by cytochrome P-450. 5 In three samples of human liver the mean value for Vmax of debrisoquine 4-hydroxylase activity was 69.9 +/- 14.3 pmol mg -1 min -1 and for Km it was 130 +/- 24 microM. 6 The only variable from smoking status, alcohol ingestion, sex of the patients, source of liver sample and presence of liver disease that had a significant effect on 4-hydroxylation of debrisoquine was the presence of liver disease. This was associated with a decrease in enzyme activity.

Polymorphic acetylation of sulphamethazine in rural bedouin and urban-dwellers in Saudi Arabia

1. The polymorphic acetylation of sulphamethazine (sulphadimidine, sulphamezathine) has been investigated in a population of 109 Saudi male arabs of rural bedouin origin and in 126 Saudi female arabs from urban cosmopolitan areas of Jeddah. 2. Rural males excreted 5-79% of the administered dose (1 X 5 g/m2 body surface area) in the 0-12 h urine and the urban females excreted 5-97%. 3. The frequency distribution of the ratio acetyl sulphamethazine/sulphamethazine was bimodal in rural, urban and the combined populations with a clear antimode at 70% acetylation of the recovered dose. 4. The incidence of slow acetylators was 67 X 9, 59 X 5 and 63 X 4% in the rural, urban and combined populations. The incidence of the As allele in Saudi arabs was thus 0 X 80+/-0 X 03 S.E.M., which is similar to that found in the neighbouring countries, of Egypt and Sudan. Since no significant difference in As frequency was apparent between the rural (pure) and urban (cosmopolitan) arabs, it is concluded that immigrants to Saudi Arabia from other muslim countries have not affected the gene frequencies with respect to acetylation. 5. Methodology of assessing acetylation phenotype is discussed. It would appear that urine analysis alone gives satisfactory discrimination between phenotypes.

Impaired oxidation of debrisoquine in patients with perhexiline neuropathy

The use of perhexiline maleate as an antianginal agent is occasionally associated with side effects, particularly neuropathy and liver damage. The reason why some individuals develop these toxic reactions is not clear, though some evidence suggests that they may result from impaired oxidative metabolism, due to genetic or hepatic factors, and consequential accumulation of the drug in toxic concentrations. Drug oxidation was measured with an oxidation phenotyping procedure in 34 patients treated with perhexiline, 20 of whom had developed neuropathy and 14 of whom had not. Most of the 20 patients with neuropathy, but not the unaffected patients, showed an impaired ability to effect metabolic drug oxidation. This impairment was independent of hepatic function, concurrent drug therapy, or tobacco or alcohol consumption. The fact that the ability to oxidise several drugs is genetically controlled points to a genetic susceptibility to developing neuropathy in response to perhexiline. Routine determination of the drug oxidation phenotype might lead to safer use of perhexiline by predicting patients who may be more at risk of developing a neuropathic reaction associated with its long-term use.