Defective oxidation of drugs: pharmacokinetic and therapeutic implications

Estimation of Population Pharmacokinetic Parameters of Free-Phenytoin in Adult Epileptic Patients

BACKGROUND

Serum concentrations of free-phenytoin (F-PHT) obtained in adult epileptic patients receiving PHT in monotherapy were analyzed to estimate the Michaelis-Menten pharmacokinetic parameters.

METHODS

Steady-state F-PHT serum concentrations, PHT dosing history, and associated information were collected prospectively. The maximum metabolic rate (Vm) and Michaelis-Menten constant (Km) of F-PHT and their interindividual variability data were estimated using nonlinear mixed effects modeling (NONMEM).

RESULTS

Twenty-nine patients with two or more available steady-state F-PHT serum concentrations (total of 63 dose/serum concentration pairs) met the inclusion criteria. Patients were taking PHT (100-500 mg/day) in monotherapy. The population estimates of F-PHT for Vm and Km were 9.1 mg/kg/day and 7.3 mg/L, respectively. The model was prospectively evaluated in a small group (seven) of additional patients.

CONCLUSIONS

The recommended daily dose in this population to achieve a F-PHT concentration of 1.5 mg/L is 6.1 mg/kg.

Polymorphic cytochrome P450 2D6: humanized mouse model and endogenous substrates

Cytochrome P450 2D6 (CYP2D6) is the first well-characterized polymorphic phase I drug-metabolizing enzyme, and more than 80 allelic variants have been identified for the CYP2D6 gene, located on human chromosome 22q13.1. Human debrisoquine and sparteine metabolism is subdivided into two principal phenotypes--extensive metabolizer and poor metabolizer--that arise from variant CYP2D6 genotypes. It has been estimated that CYP2D6 is involved in the metabolism and disposition of more than 20% of prescribed drugs, and most of them act in the central nervous system or on the heart. These drug substrates are characterized as organic bases containing one nitrogen atom with a distance about 5, 7, or 10 A from the oxidation site. Aspartic acid 301 and glutamic acid 216 were determined as the key acidic residues for substrate-enzyme binding through electrostatic interactions. CYP2D6 transgenic mice, generated using a lambda phage clone containing the complete wild-type CYP2D6 gene, exhibits enhanced metabolism and disposition of debrisoquine. This transgenic mouse line and its wild-type control are models for human extensive metabolizers and poor metabolizers, respectively, and would have broad application in the study of CYP2D6 polymorphism in drug discovery and development, and in clinical practice toward individualized drug therapy. Endogenous 5-methoxyindole- thylamines derived from 5-hydroxytryptamine were identified as high-affinity substrates of CYP2D6 that catalyzes their O-demethylations with high enzymatic capacity and specificity. Thus, polymorphic CYP2D6 may play an important role in the interconversions of these psychoactive tryptamines, including a crucial step in a serotonin-melatonin cycle.

Effect of terbinafine on the pharmacokinetics and pharmacodynamics of desipramine in healthy volunteers identified as cytochrome P450 2D6 (CYP2D6) extensive metabolizers

Terbinafine-CYP2D6 inhibition was evaluated by assessing 48-hour concentration-time profiles of the tricyclic antidepressant desipramine in 12 healthy volunteers identified as extensive cytochrome P450 2D6 (CYP2D6) metabolizers by genotyping and phenotyping. Pharmacokinetics was evaluated at baseline (50 mg oral desipramine given alone), steady state (after 250 mg oral terbinafine for 21 days), and 2 and 4 weeks after terbinafine discontinuation. Pharmacodynamics was evaluated before and 2 hours after each desipramine administration, using Mini-Mental Status Examination (MMSE) and EGG. Terbinafine administration inhibited CYP2D6 metabolism, as indicated by the significant increase in desipramine C(max) (19 ng/ml vs. 36 ng/ml) and AUC0-infinity (482 ng.h/ml vs. 2383 ng.h/ml) and decrease in AUC0-24 and C(max) of the CYP2D6-mediated metabolite, 2-hydroxydesipramine. In addition, the C(max) and AGUC0-infinity of desipramine and metabolite were still elevated 4 weeks after terbinafine discontinuation. Caution should be exercised when coprescribing terbinafine and drugs metabolized by CYP2D6, particularly those with a narrow therapeutic index.

Pharmacokinetic aspects of chloroquine-induced pruritus: influence of dose and evidence for varied extent of metabolism of the drug

The significance of a pharmacokinetics basis in chloroquine (CQ)-induced pruritus was investigated by determining the disposition of the drug in two groups of volunteers; pruritus positive and pruritus negative. Single oral dose of 600 mg CQ was administered to each of 36 volunteers, 18 for each of the two groups. After a washout period of 9 months, 150 mg single oral dose of the drug was given to 12 of the same volunteers, six each from the two groups. Blood and urine samples were collected at predetermined times following administration of each dose. Concentrations of CQ and its major metabolite, desethylchloroquine (CQM), were measured in plasma and urine using an established HPLC method. Results showed that the ratio, AUC (CQ)/AUC (CQM), as well as AUC(0-48 h) and 24-h urinary CQ excretion were all significantly higher (P<0.05) in pruritus-positive compared to pruritus-negative volunteers, following administration of the 600-mg CQ dose. Also, urinary drug-metabolite ratios monitored over 0-48 h postdose were markedly higher in the pruritus positive group. However, after administration of the 150-mg dose, 24-h urinary CQ collection and urinary drug-metabolite ratios were highly comparable between the two groups (P>0.1). This study indicates that there might be a decreased metabolism of CQ in subjects susceptible to CQ-induced pruritus following ingestion of a therapeutic dose. It also suggests that the extent of metabolism of CQ in this group may be influenced by the dose of the drug. Comparatively higher CQ levels in pruritus susceptible subjects may possibly be responsible for the pruritus experienced by such individuals when given therapeutic regimen.

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.

Cytochrome P450 enzymes and drug metabolism--basic concepts and methods of assessment

1. The cytochrome P450 enzyme family is one of the major drug metabolizing systems in man. 2. Factors such as age, gender, race, environment, and drug treatment may have considerable influence on the activity of these enzymes. 3. There are now well-established in vitro techniques for assessing the role of specific cytochrome P450 enzymes in the metabolism of drugs, as well as the inhibitory or inducing effects of drugs on enzyme activity. In vitro data have been utilized to predict clinical outcomes (i.e., pharmacokinetic interactions), with close correlations between in vitro and in vivo data. 4. This information can be of considerable practical assistance to clinicians, to help with rational prescribing or to prevent or minimize the potential for drug interactions.

Characterization of stereoselectivity and genetic polymorphism of the debrisoquine hydroxylation in man via analysis of urinary debrisoquine and 4-hydroxydebrisoquine by capillary electrophoresis

Using capillary zone electrophoresis with a phosphate buffer at pH 2.5 containing 50 mM heptakis-(2,3,6-tri-O-methyl)-beta-CD as chiral selector, the separation of the enantiomers of the main metabolite of debrisoquine (DEB), 4-hydroxydebrisoquine (4-OHDEB), is reported. For extraction of underivatized urinary DEB, S-4-OHDEB and R-4-OHDEB, a procedure using disposable cartridges containing a polystyrene-based polymer was developed. A few nL of the extracts were analyzed in a 60 cm fused-silica capillary of 50 microns ID and solute detection was effected at 195 nm. For all three compounds, a mean (n = 5) recovery of about 73% and a detection limit of about 150 ng/mL were noted. Data obtained with urines that were received for routine phenotyping with DEB and mephenytoin confirmed the almost exclusive formation of S-4-OHDEB. Under the described conditions, no R-4-OHDEB could be detected. With these data and those obtained employing no chiral selector in the buffer, differentiation between extensive metabolizer phenotypes (EM) and poor metabolizer phenotypes (PM) for DEB was unambiguously possible by the presence of a significant peak and no (or minor) peak for 4-OHDEB, respectively. Data obtained for ten EM subjects and five PM subjects were found to agree with those generated by the routine assay based on gas chromatography. The capillary electrophoretic assays described are simple, reproducible (relative standard deviation of peak area ratios < 3%), require no sample derivatization, consume no halogenated organic solvents, and operate with inexpensive separation columns as well as small amounts of chemicals.

Drug-mediated inactivation of cytochrome P450

1. Multiple forms of cytochrome P450 (CYP) catalyse the oxidation of chemicals of endogenous and exogenous origin, including drugs, carcinogens, steroids and eicosanoids. However, this unusual low substrate specificity also makes CYP susceptible to inhibition by a wide range of drugs, leading to pharmacokinetic interactions of potential clinical significance. 2. Some drugs are converted by CYP to reactive metabolites that bind covalently to sites within the active centre of the same CYP. Such mechanism-based inhibition leads to CYP inactivation or complexation. These processes give rise to long-term effects on drug pharmacokinetics, as the inactivated or complexed CYP must be replaced by newly synthesized CYP protein. 3. Drugs that inactivate CYP generally possess recognizable functional groups that are oxidized to reactive products. Thus, drugs with side chains containing unsaturated carbon-carbon bonds and furan ring systems are associated with CYP inactivation. Nitrogen-containing systems may also inactivate CYP. 4. Metabolites formed from drugs containing alkylamino and methylenedioxy functionalities can trap CYP as inert complexes without eliciting inactivation. However, the functional effects of inactivation and complexation on drug pharmacokinetics are indistinguishable. Drugs that elicit CYP complexation include the first generation macrolide antibiotics, but newer analogues appear much safer. Some antidepressants, antiepileptics and tuberculostatic agents have been associated with CYP complexation.

Single and multiple dose pharmacokinetics of nefazodone in subjects classified as extensive and poor metabolizers of dextromethorphan

1. The single and multiple dose pharmacokinetics of nefazodone (NEF) and its active metabolites hydroxynefazodone (HO-NEF) and m-chlorophenyl-piperazine (mCPP) were evaluated in subjects classified as extensive metabolizers (EM) or poor metabolizers (PM) of dextromethorphan. 2. In a parallel design study, 10 subjects from each phenotype received either 50 mg or 200 mg oral doses of NEF as single doses on Day 1 and multiple (twice daily) doses on Days 12-22. 3. Serial plasma and urine samples were collected at specified time intervals after dosing on Days 1, 16, 18, 20 and 22. Plasma samples were analyzed for NEF, HO-NEF and mCPP. Urine samples were analyzed for mCPP and its metabolite p-hydroxy-mCPP (p-HO-mCPP) before and after hydrolyzing the samples with beta-glucuronidase. 4. For the 200 mg dose group, the single dose plasma results showed no significant differences in pharmacokinetic parameters for NEF and HO-NEF in EM compared with PM subjects. However, for mCPP, Cmax was 89 ng ml-1 in the PM subjects compared with 44 ng ml-1 in the EM subjects, AUC was higher in the PM than EM subjects (1642 ng ml-1 h and 412 ng ml-1 h, respectively), and mCPP elimination half-life increased from 6.1 h in the EM subjects to 16.4 h in the PM subjects. Upon multiple dosing, plasma levels for NEF and all metabolites reached steady state within 3 days of dosing in both groups of subjects. Steady state pharmacokinetic parameters for NEF and HO-NEF in EM and PM subjects were not significantly different. The steady state Cmax and AUC values for mCPP in the PM subjects were 182 ng ml-1 and 1706 ng ml-1 h, respectively, compared with 49.6 ng ml-1 and 182 ng ml-1 h in the EM subjects. 5. The cumulative urinary excretion of mCPP and p-HO-mCPP was different for EM and PM subjects. Excretion of total mCPP and total p-HO-mCPP was approximately four-fold lower and five-fold higher, respectively, in the EM subjects than PM subjects. 6. These results indicate that the conversion of mCPP to p-HO-mCPP is attributable to metabolism by cytochrome P450 2D6. The differences in mCPP pharmacokinetic parameters in PM subjects did not affect the time required for NEF and its metabolites to attain steady state or the number of adverse experiences in either group of subjects. Based on the results of this study, NEF may be dosed to EM and PM patients without regard to their cytochrome P450 2D6 phenotype.

Investigation of xenobiotic metabolism by CYP2D6 and CYP2C19: importance of enantioselective analytical methods

Investigations into the genetic polymorphism of drug metabolism have involved specific models to screen poor and extensive metabolisers of xenobiotics. Debrisoquine, sparteine, S-mephenytoin and dextromethorphan are particularly well known. They have been extensively described in the literature and are used to phenotype human subjects before performing investigations with new drugs which are believed to be under the control of a genetic polymorphism. Dextromethorphan, debrisoquine and sparteine are good substrates for CYP2D6, whereas the S-enantiomer of mephenytoin is a good substrate for CYP2C19, both being two isozymes of cytochrome P-450. In many drugs, the hepatic microsomal oxidative metabolism involving stereogenic centres congregates either with CYP2D6 or with CYP2C19 or, in certain cases, with both of them. The availability of both CYP2D6 from poor and extensive metabolisers and an enantioselective assay would allow genetic polymorphism in drug biotransformation to be investigated in vitro ex vivo at an early stage of drug development before the IND (investigational new drug). Single-dose investigations in vivo can also be performed when only minimal pre-clinical toxicological data are available and produce more reliable results than in vitro studies. This paper focuses on the problem of genetic polymorphism in drug development and specifically discusses some relevant knowledge gained in the last two decades on enantioselective bioassays. Specific examples are given.

Characterization of the genetic polymorphism of dihydrocodeine O-demethylation in man via analysis of urinary dihydrocodeine and dihydromorphine by micellar electrokinetic capillary chromatography

The genetic polymorphism of dihydrocodeine O-demethylation in man via analysis of urinary dihydrocodeine (DHC) and dihydromorphine (DHM) by micellar electrokinetic capillary chromatography is described. Ten healthy subjects which are known to be extensive metabolizers for debrisoquine ingested 60 mg of DHC and collected their 0-12 h urines. In these samples, about 1% of the administered DHC equivalents are shown to be excreted as DHM. Premedication of 50 mg quinidine sulfate to the same subjects is demonstrated to significantly reduce (3-4 fold) the amount of O-demethylation of DHC, a metabolic step which is thereby demonstrated to co-segregate with the hydroxylation of debrisoquine. Thus, in analogy to codeine and other substrates, extensive and poor metabolizer phenotypes for DHC can be distinguished. Using the urinary DHC/DHM metabolic ratio to characterize the extent of O-demethylation, the metabolic ratio ranges of extensive and poor metabolizers in a frequency histogram are shown to partially overlap. Thus, classification of borderline values is not unequivocal and DHC should therefore not be employed for routine pharmacogenetic screening purposes. Nevertheless, the method is valuable for metabolic research and preliminary data demonstrate that the same assay could also be used to explore the metabolism of codeine.

Antidepressant drug interactions and the cytochrome P450 system: a critical appraisal

Evaluation of the inhibitory effects of antidepressant drugs on hepatic cytochrome P450 isoenzymes can allow clinicians to predict drug interactions occurring via this mechanism a priori. The magnitude and clinical relevance of these drug interactions are dependent on many factors including concentration of the isoenzyme inhibitor, contribution of active metabolites with isoenzyme inhibitory activity, and toxicity of the concomitant medication. This review summarizes information to date regarding the inhibitory potency of fluoxetine, fluvoxamine, paroxetine, sertraline, nefazodone, and venlafaxine on five isoenzyme systems: CYP2D6, CYP3A3/4, CYP1A2, CYP2C9, and CYP2C19. Both in vitro and in vivo isoenzyme inhibition data are presented and correlated to drug interactions of clinical relevance.

Characterization of dextromethorphan N-demethylation by human liver microsomes. Contribution of the cytochrome P450 3A (CYP3A) subfamily

In an effort to identify the human cytochromes P450 involved in the N-demethylation of dextromethorphan, the kinetics of 3-methoxymorphinan formation were studied in microsomal enzyme systems. Under initial rate conditions, 3-methoxymorphinan formation demonstrated single enzyme Michaelis-Menten kinetics using microsomes obtained from three human livers (Km: 0.52-0.71 mM; Vmax: 375-812 pmol/mg protein/min). B-lymphoblastoid cells expressing CYP3A4 incubated with 0.4 mM dextromethorphan catalyzed the formation of 3-methoxymorphinan at a rate of 22 pmol product/mg protein/min. Midazolam, a prototypic substrate for CYP3A4 and CYP3A5, competitively inhibited dextromethorphan N-demethylation by two human liver microsomal samples with Ki values of 46 +/- 10 and 63 +/- 8 microM. At a dextromethorphan concentration of 0.4 mM, gestodene (100 microM) inhibited 3-methoxymorphinan formation by approximately 50%. Immunoinhibition of dextromethorphan N-demethylation using rabbit anti-CYP3A4 antibodies resulted in a 60% decrease in 3-methoxymorphinan formation at a dextromethorphan concentration of 0.4 mM. Additional inhibition studies using furafylline, coumarin, sulfaphenazole, mephenytoin, quinidine, and diethyldithiocarbamic acid, which are selective inhibitors of CYP1A2, CYP2A6, CYP2C8/9, CYP2Cmp, CYP2D6, and CYP2E1, respectively, demonstrated no substantial inhibition of dextromethorphan N-demethylation. Correlation analysis was performed using the rate of 3-methoxymorphinan formation at a concentration of 1 mM dextromethorphan and immunoquantified levels of CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, and CYP3A5 and their associated characteristic catalytic activities. A significant correlation was observed between dextromethorphan N-demethylase activity and midazolam 1'- and 4-hydroxylase activity (r2 = 0.77 and 0.69 respectively, N = 19, P < 0.01); the exclusion of those samples containing both CYP3A4 and CYP3A5 increased the correlation significantly (r2 = 0.87 and 0.91 respectively, N = 12, P < 0.01). In the absence of CYP3A5, a significant correlation was observed between 3-methoxymorphinan formation and the sample's erythromycin N-demethylase activity (r2 = 0.94, N = 12, P < 0.01), testosterone 6 beta-hydroxylase activity (r2 = 0.96, N = 7, P < 0.01) and relative immunoquantified levels of CYP3A4 (r2 = 0.96, N = 12, P < 0.01). Inclusion of those samples expressing CYP3A5 in addition to CYP3A4 reduced the magnitude of the observed correlation. No significant correlation between 3-methoxymorphinan formation and the sample's relative immunoquantified levels of or form-selective activity associated with CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19 (or CYP2Cmp), CYP2D6, and CYP2E1 was observed. In conclusion, dextromethorphan N-demethylation appears to be catalyzed primarily by CYP3A4 and to a lesser extent by CYP3A5 in vitro in humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Screening for hydroxylation and acetylation polymorphisms in man via simultaneous analysis of urinary metabolites of mephenytoin, dextromethorphan and caffeine by capillary electrophoretic procedures

Phenotypes for hydroxylation can be predicted by using mephenytoin and dextromethorphan as substrates, whereas phenotypes for acetylation can be determined with caffeine as probe drug. After single-dose administration of one of these drugs, of two of them simultaneously, or of the three drugs together, the major urinary metabolites (4-hydroxymephenytoin; dextrorphan, 3-methoxymorphinan, 3-hydroxymorphinan; 5-acetylamino-6-amino-3-methyluracil as decomposition product of 5-acetylamino-6-formylamino-3-methyluracil, 1-methylxanthine, respectively) of these substrates were analyzed by capillary electrophoretic techniques. No sample pretreatment other than enzymatic hydrolysis of the conjugated compounds was applied. Assays based on micellar electrokinetic capillary chromatography are shown to allow simultaneous and unambiguous phenotyping with mephenytoin and dextromethorphan or mephenytoin and caffeine. Simultaneous screening for all three polymorphisms with a single injection of a hydrolyzed urine is shown to be possible via use of multiwavelength absorption detection only. Phenotypes determined by electrokinetic capillary techniques are shown to agree with those obtained by analysis with customary assays based on high-performance liquid chromatography.

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).

Individual variation in first-pass metabolism

Individual variation in pharmacokinetics has long been recognised. This variability is extremely pronounced in drugs that undergo extensive first-pass metabolism. Drug concentrations obtained from individuals given the same dose could range several-fold, even in young healthy volunteers. In addition to the liver, which is the major organ for drug and xenobiotic metabolism, the gut and the lung can contribute significantly to variability in first-pass metabolism. Unfortunately, the contributions of the latter 2 organs are difficult to quantify because conventional in vivo methods for quantifying first-pass metabolism are not sufficiently specific. Drugs that are mainly eliminated by phase II metabolism (e.g. estrogens and progestogens, morphine, etc.) undergo significant first-pass gut metabolism. This is because the gut is rich in conjugating enzymes. The role of the lung in first-pass metabolism is not clear, although it is quite avid in binding basic drugs such as lidocaine (lignocaine), propranolol, etc. Factors such as age, gender, disease states, enzyme induction and inhibition, genetic polymorphism and food effects have been implicated in causing variability in pharmacokinetics of drugs that undergo extensive first-pass metabolism. Of various factors considered, age and gender make the least evident contributions, whereas genetic polymorphism, enzymatic changes due to induction or inhibition, and the effects of food are major contributors to the variability in first-pass metabolism. These factors can easily cause several-fold variations. Polymorphic disposition of imipramine and propafenone, an increase in verapamil first-pass metabolism by rifampicin (rifampin), and the effects of food on propranolol, metoprolol and propafenone, are typical examples. Unfortunately, the contributions of these factors towards variability are unpredictable and tend to be drug-dependent. A change in steady-state clearance of a drug can sometimes be exacerbated when first-pass metabolism and systemic clearance of a drug are simultaneously altered. Therefore, an understanding of the source of variability is the key to the optimisation of therapy.

Pharmacokinetics of a new thienodiazepine platelet activating factor receptor antagonist (E6123) in laboratory animals. Is there a metabolic polymorphism in the rhesus monkey?

1. The pharmacokinetics of E6123, a platelet activating factor receptor antagonist, were studied after i.v. and oral administration to rat, guinea-pig, dog and rhesus monkey. Plasma concentrations of E6123 were determined by h.p.l.c. with UV detection. 2. After i.v. dosing (1 mg/kg), the plasma concentration-time curves fitted a two-compartment model. The half-lives for the terminal phases (t1/2) in rat, dog, and guinea-pig showed very little inter-individual variation, but t1/2 in the monkey (n = 4) varied more than four-fold. The distribution parameters were very similar in rat, dog and monkey (Vc and Vss approx. 1.2 and 1.5 l/kg, respectively) but slightly higher values were found in the guinea-pig, which also showed the lowest plasma protein binding. 3. After oral dosing (1 mg/kg), the maximum plasma concentrations were obtained within 0.3-3.0 h in all species. The half-life for each individual animal was almost the same as that after i.v. dosing. The mean bioavailabilities of E6123 in rat, guinea-pig and dog were about 65, 95 and 81%, respectively, but the values for monkey were again highly variable (range 32-99%). 4. The high variability in the monkey was confirmed by i.v. administration to a further 10 animals. The mean half-lives for the terminal phase in extensive metabolizers (EMs) (n = 4) and poor metabolizers (PMs) (n = 10) were approx. 1 and 4 h, respectively. 5. The rank order for total body clearance of E6123 was: rat > monkey (EMs) > dog > guinea-pig > monkey (PMs).

Understanding the role of oxyradicals in general and in toxic hepatic damage can help safer drug design

The existence and importance of free radicals are well established both theoretically and in in vivo experiments. Methods and techniques are now available to demonstrate the formation of free radical metabolites of natural substances and xenobiotics. Theories of the biological effects of free radicals could be unified and used for a better understanding of the complex processes of the organism. Pharmacology can gain profit from this multidisciplinary approach, which helps to design safer drugs. The theories of free radicals and the results of experiments with antioxidants could help to decrease the oxidative stress caused by pharmaceutical chemicals.

Determination of debrisoquine and metabolites in human urine by gas chromatography-mass spectrometry

A gas chromatographic-mass spectrometric analysis has been developed for the determination of debrisoquine and its metabolites in the urine of healthy individuals (controls) and patients with chronic renal failure. The sensitive and specific assay comprises selected-ion monitoring of the drug and the metabolites 4-hydroxydebrisoquine and 8-hydroxydebrisoquine using guanoxan as the internal standard. The limit of detection is ca. 0.2 microgram/ml. The clinical study shows that the healthy individuals and patients with chronic renal failure can be divided in two groups of extensive metabolizers and poor metabolizers, respectively. The extensive metabolizers excreted large amounts of 4-hydroxydebrisoquine and minor amounts of 8-hydroxydebrisoquine. The poor metabolizers excreted small amounts of 4-hydroxy metabolite, and no 8-hydroxydebrisoquine was detected in the urine.

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.

Debrisoquine oxidation polymorphism in a Tasmanian population

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

Impairment of debrisoquine 4-hydroxylase and related monooxygenase activities in the rat following treatment with propranolol

The effect of repetitive oral administration of propranolol on hepatic microsomal drug metabolizing enzyme activities in the rat was investigated. Propranolol ring (4-, 5- and 7-)hydroxylase activities were markedly decreased, but, interestingly, N-desisopropylase activity was increased after propranolol administration. A marked decrease in enzyme activity after propranolol pretreatment was also observed with debrisoquine 4-hydroxylation. In addition, a similar decrease was observed with imipramine 2-hydroxylation which co-segregates with debrisoquine/sparteine type polymorphic drug oxidation, but not with imipramine N-demethylation. These results suggest the selective impairment of debrisoquine 4-hydroxylase by propranolol pretreatment.

Metabolic phenotypes and colorectal neoplasia

It now appears likely that the development of colonic adenomas and carcinomas involves a series of steps in which environmental or endogenous carcinogens induce or promote neoplasia through the accumulation of multiple, specific genetic mutations. Genetic predisposition to this process may take the form of inherited defects in control of cellular proliferation as in familial polyposis coli, or genetically determined polymorphism which affects enzyme activities relevant to the production or detoxication of carcinogens. Genetic effects may also influence levels of hormones and/or their target cell receptors which regulate the metabolic and proliferative activity of colonocytes. This review highlights data suggesting a role for polymorphism associated with xenobiotic acetylation, hydroxylation, and conjugation with glutathione in the metabolism of potential carcinogens, as well as for dehydroepiandrosterone in the metabolic control of cell proliferation. The study of genetically determined polymorphism in colorectal cancer may provide new insights into the epidemiology of cancer and result in new methods for the detection of higher risk groups.

Factors affecting the enterohepatic circulation of oral contraceptive steroids

Oral contraceptive steroids may undergo enterohepatic circulation, but it is relevant for only estrogens, because these compounds can be directly conjugated in the liver. Animal studies show convincing evidence of the importance of the enterohepatic circulation, but studies in humans are much less convincing. The importance of the route and the rate of metabolism of ethinyl estradiol are reviewed. Some antibiotics have been reported anecdotally to reduce the efficacy of oral contraceptive steroids, but controlled studies have not confirmed this observation. Although gut flora are altered by oral antibiotics, the blood levels of ethinyl estradiol are not reduced, and one antibiotic at least (cotrimoxazole) enhances the activity of ethinyl estradiol.

Interspecies comparisons in toxicology: the utility and futility of plasma concentrations of the test substance

A classical dilemma in toxicology is how the dose administered relates to the dose delivered to the target site. Plasma concentrations of the test substance may be misleading since the concentration of any given substance in the plasma may not be representative of its concentration in tissues. Furthermore, a given tissue concentration of a xenobiotic can evoke responses which are highly species-dependent. While evaluating toxicity data within one species, plasma concentrations reflect the effects of route of administration, bioavailability, dose level, multiple dosing, age, gender, etc. However, when toxicity data is compared across species, the relevance of plasma concentrations depends on the nature of the toxicity. Reversible, pharmacodynamic effects often correlate with plasma concentrations, although there may be marked interspecies differences in dose-response relationships. Irreversible effects, if pharmacodynamic in origin, often correlate better with the intensity/duration of the pharmacodynamic response, rather than with plasma concentration. On the other hand, irreversible effects, if chemically mediated, may not correlate at all with plasma concentration, the lesions being caused by reactive metabolites of fleeting existence, which rarely survive long enough to leave their site of synthesis. They cannot be measured in the plasma nor predicted from plasma concentrations of the parent xenobiotic. The limitations of plasma concentrations in interpreting the toxicology of substances which are tissue-sequestered, which are subject to pharmacogenetic factors, or which show plasma concentrations that are not proportional to dose are also discussed. Mention is made of possible alternatives to plasma concentrations in assessing exposure in toxicology studies.

Variation of benzbromarone elimination in man--a population study

The plasma benzbromarone concentration-time profile in a healthy subject who retained the compound much longer than other individuals is described. The data suggested that determination of the 24 h plasma concentration of the parent drug after a single oral dose of 100 mg benzbromarone would be an appropriate procedure to determine the elimination phenotype. Based on this procedure, 148 of 153 healthy individuals (97%) in a population study were found to eliminate benzbromarone rapidly. In one subject the 24 h benzbromarone plasma concentration was very similar to that observed in the individual who had been more fully characterized. Four participants gave intermediate results. The data are compatible with a bimodal or trimodal distribution of different benzbromarone elimination phenotypes.

Drug interactions in hypertensive patients. Pharmacokinetic, pharmacodynamic and genetic considerations

Antihypertensive treatment has proven benefits, and the number of patients being treated with these drugs is significant. Hypertensive patients may have other medical illnesses for which they receive medications, and interactions between antihypertensive agents and other drugs is likely. Some of these interactions may lead to undesirable effects or even loss of blood pressure control. However, drug interactions can also be beneficial when 2 antihypertensive drugs with different pharmacological actions are prescribed in combination and with a clear therapeutic objective in mind. Clinicians should be aware of the mechanisms and the consequences of the different types of interaction in hypertensive patients, so that a desired pharmacological response can be achieved with the fewest side effects in the patients.

Inhibitory studies of mexiletine and dextromethorphan oxidation in human liver microsomes

The cytochrome P-450dbl isozyme (P-450bdl) is responsible for the genetic sparteine-debrisoquine type polymorphism of drug oxidation in humans. To investigate the relationship between mexiletine oxidation and the activity of this isozyme, cross-inhibition studies were performed in human liver microsomes with mexiletine and dextromethorphan, a prototype substrate for P-450dbl. The formation of hydroxymethylmexiletine and p-hydroxymexiletine, two major mexiletine metabolites, was competitively inhibited by dextromethorphan. Mexiletine competitively inhibited the high affinity component of dextromethorphan O-demethylation. In addition, there was a good agreement between the apparent Km values for the formation of both mexiletine metabolites and the high affinity component of dextromethorphan O-demethylation and their respective apparent Ki values. Several drugs were tested for their ability to inhibit mexiletine oxidation. Quinidine, quinine, propafenone, oxprenolol, propranolol, ajmaline, desipramine, imipramine, chlorpromazine and amitryptiline were competitive inhibitors for the formation of hydroxymethylmexiletine and p-hydroxymexiletine as for prototype reactions of the sparteine-debrisoquine type polymorphism. Amobarbital, valproic acid, ethosuximide, caffeine, theophylline, disopyramide and phenytoin, known to be non-inhibitors of P-450dbl activity, were found not to inhibit the formation of these mexiletine metabolites. Moreover, the formation of both metabolites was strongly inhibited by an antiserum containing anti-liver/kidney microsomes antibodies type I (anti-LKMI) directed against P-450dbl. These data suggest that the formation of two major metabolites of mexiletine is predominantly catalysed by the genetically variable human liver P-450dbl.

Evidence for the polymorphic oxidation of debrisoquine in the Thai population

Debrisoquine polymorphism has been studied extensively in Caucasian populations. The prevalence of the poor metaboliser phenotype is 3-10% in European and American Caucasian populations but appears to be very low in Asian populations. This study was carried out to determine the metabolic oxidation status in 173 Thai subjects. Phenotyping was performed using the metabolic ratio (MR) calculated as the 0-8 h urinary output of debrisoquine/0-8 h urinary output of 4-hydroxydebrisoquine after oral administration of 10 mg debrisoquine hemisulphate. Two subjects (1.2%) were phenotyped as poor metabolisers; they had MR values of 13.17 and 92.04. The incidence of the poor metaboliser phenotype of debrisoquine oxidation of 1.2% seems to be lower in the Thai population compared with that in various Caucasian populations.

Polymorphic sparteine metabolism and amyotrophic lateral sclerosis

The activity of detoxication with the cytochrome P450 family of enzymes was studied in patients with amyotrophic lateral sclerosis (ALS). The metabolic ratio (MR) of sparteine sulphate was measured, and the population of phenotypes for sparteine nitro (N)-oxidation was determined in 30 patients with ALS and 41 controls for the assay of the sparteine N-oxidation by the cytochrome P450 enzymes. A lower MR and higher frequency of efficient metabolizers were observed in the ALS group, suggesting that there is efficient sparteine N-oxidation by cytochrome P450 pathways in ALS. This tendency was more marked in 15 patients with ALS aged under 60 years.

The isolation, identification and structure of a new hydroxylated metabolite of benzbromarone in man

1. The metabolic fate of the uricosuric drug, benzbromarone, in man was reinvestigated. Plasma and urine samples obtained from healthy subjects after administration of a single oral dose of 100 mg were analysed by h.p.l.c. and g.l.c.-mass spectrometry; bromobenzarone and benzarone, previously assumed to be the debrominated metabolites of benzbromarone, were not detectable. 2. Instead, two metabolites (M1 and M2) were present in plasma samples, which had plasma elimination rates lower than those of the parent drug. 3. One of the metabolites (M1) was identified as 1'-hydroxy-benzbromarone using g.l.c.-mass spectrometric analysis of trimethylsilylated and methylated extracts. Chromatographic and spectroscopic data for this metabolite were identical to those of the synthetic compound.

Antiarrhythmic efficacy, clinical electrophysiology, and pharmacokinetics of 3-methoxy-O-desmethyl encainide (MODE) in patients with inducible ventricular tachycardia or fibrillation

In most patients, the clinical effects of therapy with encainide are mediated by the generation of the active metabolites O-desmethyl encainide and 3-methoxy-O-desmethyl encainide (MODE). Data from in vitro and animal studies have indicated that MODE has electrophysiologic and pharmacokinetic features that make its further evaluation desirable; in earlier studies, we found that MODE suppressed chronic high-frequency nonsustained ventricular arrhythmias at plasma concentrations of 50-160 ng/ml. We now report the clinical electrophysiology, antiarrhythmic activity, and pharmacokinetics of MODE in 17 patients with inducible ventricular tachyarrhythmias (VTs) in whom programmed electrical stimulation was performed before drug administration and after one or two sequences of loading and maintenance infusions of MODE. Because the relation between plasma concentration and effect had been incompletely defined, a dose-titration approach was adopted: available pharmacokinetic data were used to construct loading and maintenance infusion regimens that were predicted to attain low plasma concentrations in initial patients while higher infusion rates were evaluated in subsequent patients. MODE prevented VT induction in three of 17 patients and VT cycle length was increased by greater than or equal to 100 msec in a further seven of 17; most responses to MODE occurred at plasma concentrations greater than 556 ng/ml (greater than 1 SD above mean plasma MODE during encainide therapy). Response to MODE did not predict subsequent response to oral therapy with encainide. MODE increased intracardiac conduction times, QT intervals during atrial and ventricular pacing, and right ventricular effective refractory periods (RVERP); changes in RVERP were most prominent at rapid pacing rates, while changes in intracardiac conduction were rate-independent at cycle lengths between 400 and 600 msec. Plasma MODE concentrations measured during electrophysiology study correlated well with those predicted by the pharmacokinetic simulations (r = 0.91, p less than 0.001). Serial plasma sampling after programmed electrical stimulation indicated a minimum MODE elimination half-life of 8.2 +/- 5.4 hours. Side effects were confined to three instances of asymptomatic conduction system depression in subjects with latent conduction system disturbances. We conclude that MODE slows intracardiac conduction, delays repolarization, and can suppress or substantially modify inducible VT. Moreover, it was only with the adoption of the dose-titration strategy that we were able to safely demonstrate that plasma MODE concentrations higher than those routinely observed during encainide therapy were required to substantially alter cardiac electrophysiology.

Genetically determined N-acetylation and oxidation capacities in Japanese patients with non-occupational urinary bladder cancer

Genetically determined polymorphisms of N-acetylation and oxidative capacity have been studied using dapsone and metoprolol in 51 Japanese patients with spontaneous bladder cancer and 203 healthy control subjects. The results for N-acetylation pharmacogenetics were against the initial expectation that there would be a preponderance of slow acetylators in the cancer group, as 3 such patients (5.9%) were found as compared to 13 (6.4%) in the healthy group. There was no poor metabolizer (PM) of metoprolol in the cancer group, whereas in the healthy group one (0.5%) was a PM. There were no significant differences between the groups in the frequency of slow acetylator and poor oxidiser phenotypes, or in the frequency distribution profiles of acetylation (monoacetyldapsone/dapsone) and oxidative metabolic ratio (log metoprolol/alpha-hydroxymetoprolol). The results indicate that neither N-acetylation nor the debrisoquine/sparteine-type oxidative phenotype and/or capacity represent a genetic predisposition to spontaneous bladder carcinogenesis in Japanese patients. In the normal Japanese population there is a great predominance of rapid acetylators and extensive oxidisers.

Relevance of genetic polymorphism in drug metabolism in the development of new drugs

Drugs whose principal metabolic pathways are under polymorphic genetic regulation may show considerable interindividual pharmacokinetic variability. This could lead to clinically significant differences in the pharmacological responses of some patients and so might lead the pharmaceutical industry to stop development of the drug. This can be prevented and there are several measures that can be taken to avoid such premature termination of development. They include studies in vitro with human liver samples, and clinical pharmacological experiments designed specifically to examine possible genetic polymorphism in the disposition of the drug.

The effects of encainide versus diltiazem on the oxidative metabolic pathways of antipyrine

The effects of diltiazem and encainide on the pharmacokinetics and metabolism of antipyrine were compared in nine healthy male volunteers. Diltiazem 90 mg every 8 hours for 5 days decreased the oral clearance of antipyrine from 2.34 to 1.86 L/hour (p less than 0.05) and increased half-life from 12.7 to 15.9 hours (p less than 0.05). Diltiazem reduced the formation rate constants for 3-hydroxymethylantipyrine by 27% (p less than 0.05) and 4-hydroxyantipyrine by 37% (p less than 0.05). There was also a 21% reduction in the formation rate constant for norantipyrine (0.05 less than p less than 0.10). Encainide 25 mg every 8 hours for 5 days had no apparent effect on the oral clearance or half-life of antipyrine, or on the formation rate constants for metabolites of antipyrine. In contrast to a previously published report in rats, encainide, unlike diltiazem, does not inhibit the oxidative metabolism of antipyrine in humans.

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.

Effect of quinidine on the dextromethorphan O-demethylase activity of microsomal fractions from human liver

1. The kinetics of dextromethorphan O-demethylation were measured in microsomes prepared from five human livers, both in the absence and in the presence of quinidine. 2. For each liver and over the concentration range of dextromethorphan examined (4.2-3400 microM), this reaction involved an enzymatic component of high affinity, with an apparent Michaelis-Menten constant (Km) of 4.6 +/- 1.8 microM (mean +/- s.d.) and a maximum velocity (Vmax) of 4.2 +/- 3.5 nmol mg-1 h-1 (mean +/- s.d.). 3. Quinidine was a potent and competitive inhibitor of the activity of this component (mean Ki +/- s.d. of 0.025 +/- 0.008 microM) as it is for other oxidation reactions which have already been found to co-segregate with the debrisoquine-type polymorphism. 4. With microsomes from four of the five livers studied, there was evidence of a second enzymatic component of activity characterized by a similar Vmax and about 20-fold higher Km compared with the high affinity component. The activity of this low affinity component was unaffected by quinidine in the concentrations studied.