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Biomedical subjects

G R Wilkinson

Publications and source records attributed to G R Wilkinson.

At least 91 records · Page 5Linked to original sources
PubMed1989

Propranolol's metabolism is determined by both mephenytoin and debrisoquin hydroxylase activities.

The relative contributions of the debrisoquin and mephenytoin isozymes of hepatic cytochrome P-450 to the stereoselective metabolism of propranolol have been studied in a panel of volunteers of known oxidative phenotypes. Six subjects were extensive metabolizers of both debrisoquin and mephenytoin (EM). Four subjects were poor metabolizers of debrisoquin but rapid for mephenytoin (PMD). Five subjects were poor metabolizers of mephenytoin but rapid for debrisoquin (PMM), and one individual had a deficiency for both test compounds (PMD/M). Partial metabolic clearances of each propranolol enantiomer to 4-hydroxypropranolol (4-OH-P), the sulfate, and glucuronide conjugates of 4-OH-P, naphthoxylactic acid (NLA) and propranolol glucuronide, were estimated after a single oral dose of racemic propranolol (80 mg). The partial metabolic clearance of both enantiomers to total 4-OH-P in the PMD group was 75% less than in the EM and PMM groups, indicating the contribution of the debrisoquin isozyme to this route of metabolism. The R/S ratios for the clearance to 4-OH-P were similar between EM and PMD (2.5 +/- 0.5 vs 2.5 +/- 0.4, respectively), implying that the different enzymes involved in ring hydroxylation (i.e., the debrisoquin isozyme and other hydroxylases) have similar stereoselective preferences. The partial metabolic clearance to NLA was 55% less in the PMM group than in the EM and PMD groups, indicating that S-mephenytoin 4-hydroxylase contributes to the metabolic conversion of propranolol to NLA. The R/S ratios for the clearance to NLA were close to unity in all groups. The partial metabolic clearance to propranolol glucuronide also did not exhibit stereoselectivity and was similar in all groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Aryl Hydrocarbon Hydroxylases↗
PubMed1989

Influence of debrisoquin phenotype on the inducibility of propranolol metabolism.

The effects of rifampin (600 mg) once daily for 22 days on the total and fractional metabolic clearances of propranolol were determined in a group of six genetically extensive (EM) and six poor metabolizers (PM) of debrisoquin. The impaired ability of PMs to metabolize propranolol to the ring-oxidized metabolite 4-hydroxypropranolol was confirmed. The total oral clearance of propranolol increased about fourfold in both phenotypes from 219.2 +/- 52.8 to 976.7 L/hr in the EMs and from 75.0 +/- 12.6 to 289.8 +/- 78.2 L/hr in the PMs. The extent of induction of glucuronidation was similar in the two groups. 4-Hydroxylation was induced in both phenotypes but the increase was fifteenfold greater in EMs than in PMs. This would imply that the cytochrome P-450 determined by the debrisoquin allele or some coinherited 4-hydroxylase(s) was induced to a greater extent in EMs than PMs.

Adult↗
PubMed1989

Effects of ketoconazole on the polymorphic 4-hydroxylations of S-mephenytoin and debrisoquine.

Studies were undertaken in 12 normal, male subjects to determine whether a metabolic interaction occurs between ketoconazole and mephenytoin. A single dose (400 mg) of ketoconazole produced a reduction in the 0-8 h urinary R/S ratio of mephenytoin following oral administration (100 mg) of racemic drug and after 28 daily doses the median value was further reduced to 42.9% of its baseline value. Within 7 days following discontinuation of ketoconazole the enantiomeric ratio had returned to its pre-study value. These findings are consistent with ketoconazole being a potent in vivo inhibitor of mephenytoin's 4-hydroxylation and confirm the ability of such an interaction to be predicted by in vitro studies with human liver microsomes. By contrast, ketoconazole had a much smaller effect on the 0-8 h urinary metabolic ratio of debrisoquine, indicating that ketoconazole has a selective inhibitory effect on different forms of cytochrome P-450.

Adult↗
PubMed1989

Plasma binding and transport of diazepam across the blood-brain barrier. No evidence for in vivo enhanced dissociation.

The tissue uptake of extensively plasma-bound compounds is reportedly inconsistent with the conventional free-drug hypothesis limiting transport to unbound moiety in rapid intracapillary equilibrium with bound complex. Instead, protein-mediated/cell surface enhancement of dissociation has been postulated to occur in the microvasculature. This possibility was investigated by studying the passive transport of diazepam across the blood-brain barrier. Microdialysis probes placed within the vena cava and brain cortex were used to directly compare steady-state, interstitial unbound diazepam levels in both Wistar and genetically analbuminemic rats. The absence of albumin in the latter increased the unbound fraction of diazepam by almost fivefold; however, in both groups, the ratio of unbound concentrations in brain and blood at equilibrium was equal to unity. If enhanced dissociation occurred in the microvasculature, then the unbound brain level should have been greater than that in the systemic circulation. It is probable that earlier findings suggestive of protein-mediated transport reflect a nonequilibrium phenomenon. Comparison of the extent of diazepam's in vivo binding in blood by microdialysis to that estimated in vitro using conventional equilibrium dialysis with microcells showed good agreement, thus validating a widely accepted assumption of equivalency of these two values.

Animals↗
PubMed1988

The assessment of bioavailability in the presence of nonlinear elimination.

The simultaneous administration of an oral dose and intravenous tracer dose, as a method to determine bioavailability, was examined by means of computer simulation for drugs exhibiting Michaelis-Menten type elimination. A physiological pharmacokinetic model parameterized for man and including first-order absorption and elimination solely from the liver was employed. This tracer method provided good estimates of the true availability, with an error of 6% or less, over a wide range of dosing and dispositional conditions. Poorer estimates were noted when large doses of drugs with very short half-lives were considered. This poor performance was improved by administering the intravenous tracer at some time after the oral dose but an a priori basis for establishing this time was not apparent. The tracer method, therefore, appears to be a robust means of assessing, in man, oral bioavailability in the presence of Michaelis-Menten type elimination for drugs characterized by the general properties of the physiological model employed and with half-lives in excess of approximately 40 min. These findings together with the statistical power and simplicity of performance of the tracer method indicate that it is a valid technique for the assessment of bioavailability under a wide range of kinetic conditions.

Biological Availability↗
PubMed1988

Brain uptake of benzodiazepines: effects of lipophilicity and plasma protein binding.

The rapid intracarotid injection technique was used to determine the unidirectional brain uptake of a number of benzodiazepines in the rat. The drugs varied considerably in their lipophilicity and, within the series oxazepam, lorazepam, chlordiazepoxide, desmethyldiazepam and diazepam, brain extraction of unbound moiety was enhanced as the octanol-water (pH = 7.4) partition coefficient increased. However, with flunitrazepam and midazolam, two fluorine-containing benzodiazepines, extraction was more and less extensive, respectively, than predicted from their lipophilicities. Importantly, the uptake findings were consistent with the characteristic onsets of central effects of the drugs established clinically in humans. The effects of reversible protein binding on uptake also were investigated by the addition of albumin (0-8 g.dl-1) to the injectate. This affected markedly the unbound fraction, determined in vitro by equilibrium dialysis, and also the brain uptake of all drugs. As the unbound fraction was reduced, the unidirectional brain extraction ratio decreased in a curvilinear fashion toward zero. However, attempts to describe the data were unsuccessful using a conventional model based on transcapillary uptake of only unbound drug whose binding kinetics with albumin were assumed to be the same as those indicated by equilibrium dialysis. The observed brain extraction was greater than predicted, and the discrepancy became more apparent as binding and albumin concentration increased. The data for all of the benzodiazepines could be fitted, however, if the equilibrium association constant was assumed to be smaller in vivo than in vitro, so that the effective unbound fraction in the brain capillaries was substantially higher (5- to 25-fold, dependent on the particular drug) than that estimated in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Oxidative metabolism of hexobarbital in human liver: relationship to polymorphic S-mephenytoin 4-hydroxylation.

The major route of hexobarbital metabolism in humans involves hydroxylation at the 3'-position followed by oxidation to a 3'-keto metabolite. Studies were performed to characterize the form of cytochrome P-450 responsible for the initial oxidation. In vitro studies indicated that P-450MP purified from human livers, involved in the 4-hydroxylation of S-mephenytoin, efficiently catalyzed the 3'-hydroxylation of hexobarbital; moreover, polyclonal antibodies raised to this enzyme extensively inhibited such activity in human liver microsomes. S-Mephenytoin 4- and hexobarbital 3'-hydroxylase activities in microsomes from different livers were significantly correlated, and both activities were essentially absent in fetal liver preparations. Hexobarbital was also found to inhibit S-mephenytoin 4-hydroxylation and vice versa, with Ki values similar to the Km values for the measured pathways. These data suggested that in vivo metabolism of hexobarbital would be determined by the same genetic factor(s) responsible for polymorphic 4-hydroxylation of S-mephenytoin. This prediction was confirmed by the finding that, after oral administration of a single dose of hexobarbital (300 mg), the 24-hr urinary excretion of 3'-hydroxy- and 3'-ketohexobarbital in a "poor-metabolizer" was only one third of that in a subject of the "extensive-metabolizer" phenotype. Also, the plasma level of metabolites at 6 hr after administration was reduced, and that of unchanged drug was increased in the poor metabolizer. Finally, a markedly enhanced sedative effect was associated with the impaired metabolism. Accordingly, several lines of in vitro evidence indicate that hexobarbital 3'-hydroxylation is catalyzed by P-450MP.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytochrome P-450 Enzyme System↗
PubMed1987

Genetic predisposition to bladder cancer: ability to hydroxylate debrisoquine and mephenytoin as risk factors.

The hypothesis that the frequency distribution of indices of oxidative drug-metabolizing activity is different between patients with bladder cancer (n = 98) and age, sex-matched control subjects (n = 110) has been investigated. Urinary recovery ratios of debrisoquine and R/S ratios of mephenytoin have been measured in an 8-h urine sample after simultaneous administration of debrisoquine (10 mg) and racemic mephenytoin (100 mg). In addition, alcohol consumption, smoking habit, and acetylation phenotype (using 100 mg dapsone as a substrate) have been measured. Patients with bladder cancer were classified on histological criteria as having aggressive (Stage III) (34%) or nonaggressive (Stages I and II) (66%) disease. The median of the frequency distribution of the debrisoquine urinary recovery ratio in patients with aggressive bladder cancer was greater than in control subjects, and only four patients had recovery ratios lower than the mean of the control group. Using logistic regression analysis, efficient debrisoquine metabolism and a synergistic interaction between smoking and ethanol consumption were significant, independent risk factors, while S-mephenytoin hydroxylation and acetylation phenotype were not significant risk factors. In contrast, patients with non-aggressive bladder cancer had a significant, but weaker, association with rapid hydroxylation of S-mephenytoin, which was independent of a significant synergistic interaction between smoking and alcohol consumption. Acetylation phenotype and debrisoquine urinary recovery ratio were not associated with increased risk of nonaggressive cancer. These results are consistent with the concept that oxidative isozymes might be responsible for conversion of environmental agents to proximate bladder carcinogens in nonindustrial-related bladder cancer. They also suggest that different etiological factors are involved in the pathogenesis of aggressive and nonaggressive bladder cancer.

Acetylation↗
PubMed1987

S-mephenytoin 4-hydroxylase is inherited as an autosomal-recessive trait in Japanese families.

The 4-hydroxylation of S-mephenytoin exhibits polymorphism in both whites and Japanese such that the populations can be divided into extensive and poor metabolizers. To determine whether genetic constitution is a primary determinant in the expression of such metabolism, four extended Japanese families containing 13 sets of parent/offspring relationships were phenotyped for their mephenytoin 4-hydroxylation activity using the 8-hour urinary ratio of unchanged R- and S-mephenytoin as the trait measurement. The incidence of the poor metabolizer phenotype in these families was 2.2 times greater than that in an unrelated Japanese population. In three families in which both parents were poor metabolizers of mephenytoin, all six children also exhibited the poor metabolizer trait. The phenotype distribution for each family studied was consistent with the hypothesis that mephenytoin 4-hydroxylation activity is under diallelic, monogenic control, with the poor metabolizer phenotype being the autosomal recessive homozygous genotype and the extensive metabolizer phenotype including both the autosomal dominant and heterozygous genotypes.

Alleles↗
PubMed1987

Acute effects of halothane anesthesia on arterial and venous concentrations of propranolol in the dog.

The present study determined the effect of halothane on arterial and venous propranolol concentrations during a continuous intravenous infusion of propranolol. Arterial and venous concentrations of propranolol remained constant during the experiment in the group of dogs anesthetized only with pentobarbital. However, there was a rapid increase in arterial propranolol concentration from 88 +/- 10 ng/ml (mean +/- SEM) prior to halothane to 116 +/- 12 ng/ml (P less than 0.05) and 130 +/- 7 ng/ml (P less than 0.05) 60 and 120 min, respectively, after starting halothane anesthesia (2.0 MAC 1.74%). The increase in venous propranolol concentration lagged substantially behind that of the arterial concentration, so that, 60 min after starting halothane, the arterial to venous (A/V) concentration ratio increased from 1.13 +/- 0.05 to a maximum of 1.48 +/- 0.08. In contrast to the changes following halothane, no significant change in the A/V ratio occurred following fentanyl. Both halothane and fentanyl administration produced a small but significant increase in the free fraction of propranolol (P less than 0.05). The results from this study emphasize the importance of the choice of sampling sites in pharmacokinetic experiments, as well as excluding subtle pharmacokinetic changes during anesthesia before ascribing changes in drug effect to changes in sensitivity to the drug.

Anesthesia, Inhalation↗
PubMed1987

Polymorphism of propafenone metabolism and disposition in man: clinical and pharmacokinetic consequences.

The relationship between debrisoquine metabolic phenotype and the pharmacokinetics and pharmacodynamics of propafenone was studied in 28 patients with chronic ventricular arrhythmias (22 extensive metabolizers [EMs] and six poor metabolizers [PMs] of debrisoquine). EMs were characterized by a shorter propafenone elimination half-life (5.5 +/- 2.1 vs 17.2 +/- 8.0, p less than .001), lower average plasma concentration (Cp) (1.1 +/- 0.6 vs 2.5 +/- 0.5 ng/ml/mg daily dosage, p less than .001), and higher oral clearance (1115 +/- 1238 vs 264 +/- 48 ml/min, p less than .001). The active metabolite 5-hydroxypropafenone, assayed in 12 patients, was identified in nine of 10 EMs but in neither of the PMs. A lower incidence of central nervous system side effects was noted in EMs (14% vs 67%, p less than .01). The magnitude of QRS widening at any given propafenone Cp was greater in EMs than PMs. There was no significant difference between EMs and PMs in effective propafenone dose or frequency of antiarrhythmic response. Inhibition of debrisoquine 4-hydroxylation by propafenone was demonstrated both in vivo and in a human liver microsomal system in vitro. We conclude that propafenone is metabolized via the same cytochrome P-450 responsible for debrisoquine's 4-hydroxylation, and that its pharmacokinetics and concentration-response relationships and the incidence of central nervous system side effects are different in patients of different debrisoquine metabolic phenotype.

Adult↗
PubMed1987

Characterization and inhibition of mephenytoin 4-hydroxylase activity in human liver microsomes.

The in vivo metabolism in humans of the anticonvulsant mephenytoin exhibits stereoselectivity as well as genetic polymorphism of the 4-hydroxylation pathway. The characteristics of the involved cytochrome P-450 isozyme are, however, not known completely. Accordingly, the ability of human liver microsomes to metabolize mephenytoin and its enantiomers was investigated in vitro, and the ability of related anticonvulsants and other compounds to inhibit 4-hydroxylation was studied. Marked stereoselectivity was observed in the conversion of S-mephenytoin to its 4-hydroxy metabolite, but N-demethylation was essentially similar for both enantiomers. The intrinsic clearance (Vmax/Km) for 4-hydroxymephenytoin formation showed an almost 10-fold range in five livers and was 150- to 1000-fold greater than that for N-demethylation. Competitive inhibition of 4-hydroxylation was observed with ethotoin, mephobarbital, methsuximide and phensuximide, but not other commonly used anticonvulsants such as ethosuximide, phenobarbital, phenytoin and primidone. However, synthetic N-alkyl analogs of the latter compounds were found to be inhibitory. An aryl residue alpha to the carbonyl carbon of an N-alkyl lactam in a 5- or 6-membered ring, therefore, appears to be a minimal requirement for strong interaction with the 4-hydroxylase. Warfarin, but not diazepam, ketoconazole or iodochlorohydroxyquin, were also competitive inhibitors, but at much higher concentrations than the anticonvulsants. Competitive inhibition at concentrations similar to the Km of 4-hydroxymephenytoin formation (30-350 microM) may indicate that the isozyme is involved in the metabolism of the substrates under consideration and, therefore, their in vivo metabolism may be regulated to some extent by the same genetic factor(s) that determine mephenytoin's biotransformation.

Anticonvulsants↗
PubMed1987

Hepatic metabolism of tolbutamide: characterization of the form of cytochrome P-450 involved in methyl hydroxylation and relationship to in vivo disposition.

In vitro investigations suggest the same human liver cytochrome P-450 that catalyzes S-mephenytoin 4-hydroxylation, P-450MP, is responsible for methyl hydroxylation of the oral hypoglycemic agent tolbutamide. Tolbutamide hydroxylase activity copurified with P-450MP; electrophoretically homogenous P-450MP catalyzed both tolbutamide and S-mephenytoin hydroxylation. Each substrate competitively inhibited hydroxylation of the other, and anti-P-450MP inhibited tolbutamide hydroxylation in human liver microsomes. Significant correlation between tolbutamide and S-mephenytoin hydroxylase activities was seen in a set of human liver samples. These findings suggested that subjects with a genetically determined impairment in ability to hydroxylate mephenytoin might also have deficient tolbutamide metabolism. However, plasma tolbutamide concentration-time profiles and urinary excretion of metabolites formed via the hydroxylation pathway were similar in four phenotypically poor and six extensive metabolizers of mephenytoin. We suggest that alteration of a substrate binding site of P-450MP may reduce its ability to hydroxylate S-mephenytoin but not tolbutamide.

Adult↗
PubMed1986

Disposition kinetics of encainide and metabolites.

Interpretation of plasma concentration data during encainide therapy is predicated on an understanding of the role of active metabolites during treatment. In over 90% of patients, encainide is rapidly biotransformed to O-desmethyl encainide (ODE) and 3-methoxy-O-desmethyl encainide (3-MODE), which persist in plasma hours after encainide itself is undetectable. This metabolism occurs in the liver, and encainide clearance is sufficiently high that a significant first-pass effect is seen during oral therapy (bioavailability 30 +/- 7%). In these extensive metabolizers, ODE and 3-MODE appear to mediate the arrhythmia suppression and electrocardiographic changes seen during encainide therapy. In less than 10% of patients, a genetic defect prevents expression of the enzyme responsible for the rapid biotransformation of encainide. In this poor metabolizer subset, the systemic clearance of encainide is 10-fold lower than in extensive metabolizers (0.18 +/- .002 vs 1.9 +/- 0.2 liters/min), the first-pass effect is virtually absent (bioavailability 83% to 88%), plasma concentrations are higher and an antiarrhythmic effect may be seen at usual encainide doses. Minimally effective plasma concentrations appear to be 35 ng/ml (ODE), 100 ng/ml (3-MODE) and 300 ng/ml (encainide), making ODE one of the most potent sodium channel blockers yet used in man. The elimination half-life of encainide is 2.3 +/- 0.3 hours in extensive metabolizer patients. Despite this rapid elimination, encainide can be administered every 8 to 12 hours in both extensive and poor metabolizer subsets; this is because of slowly eliminated metabolites in extensive metabolizers and slower elimination of encainide itself (11.3 +/- 0.3 hours) in poor metabolizers.(ABSTRACT TRUNCATED AT 250 WORDS)

Anilides↗
PubMed1986

Altered distribution of debrisoquine oxidation phenotypes in patients with systemic lupus erythematosus.

Oxidative metabolism in patients with systemic lupus erythematosus (SLE) was studied using the antihypertensive drug, debrisoquine. The metabolism of this drug to its principal metabolite, 4-hydroxydebrisoquine, is catalyzed by a discrete isozyme of cytochrome P-450. The extent of this reaction exhibits genetic polymorphism, with 2 phenotypes, "poor metabolizers" and "extensive metabolizers," discernible in the normal population. We observed the poor metabolizer debrisoquine phenotype in 9 of 42 patients with idiopathic SLE (21%), in contrast with 12 of 147 healthy volunteers (8%), which is a significant difference in frequency (P less than 0.04). These data provide further evidence for altered oxidative metabolism in SLE and support the concept that genetic differences in oxidative metabolism of endogenous compounds, such as sex steroid hormones, or of xenobiotics might influence susceptibility to SLE.

Adult↗
PubMed1986

Polymorphic metabolism of mephenytoin in man: pharmacokinetic interaction with a co-regulated substrate, mephobarbital.

The simultaneous dosing of two drugs with co-regulated genetic polymorphisms determined by a single cytochrome P-450 isozyme could result in competitive inhibition of metabolism. We investigated this hypothesis in vivo by studying the interaction of mephobarbital and mephenytoin in eight normal subjects with wide variability in S-mephenytoin 4-hydroxylation. Each received oral racemic mephenytoin (100 mg) alone and, on a separate occasion, 1 hour after oral racemic mephobarbital (200 mg). After mephenytoin dosing alone, the 8-hour urinary enantiomeric (R/S) ratio indicated one poor (PM), one intermediate (IM), and six extensive (EM) metabolizers. Total intrinsic clearance of S-mephenytoin varied more than 100-fold, whereas the range for R-mephenytoin was only twofold. The urinary R/S ratio correlated (r = 0.92) with the enantiomeric ratio of the plasma AUCs over the same period, indicating no stereoselectivity in renal clearance. When mephenytoin was taken in the presence of mephobarbital, peak levels and AUC of S-mephenytoin increased while those of the R-enantiomer remained unchanged. Accordingly, the R/S ratios in both plasma and urine were reduced, with the change rank order-related to the control value of the total intrinsic clearance of S-mephenytoin (i.e., greatest in the most extensive EM). Thus the urinary R/S ratio can be used as a measure of the enantiomeric ratio of the plasma concentrations over the same time period of collection. Moreover, this ratio may be used to detect drug interactions that involve the cytochrome P-450 isozyme(s) responsible for the polymorphic 4-hydroxylation of mephenytoin.(ABSTRACT TRUNCATED AT 250 WORDS)

Administration, Oral↗