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G R Wilkinson

Publications and source records attributed to G R Wilkinson.

At least 73 records · Page 4Linked to original sources
PubMed1993

Absence of correlations among three putative in vivo probes of human cytochrome P4503A activity in young healthy men.

In vitro studies with human liver microsomes have shown that erythromycin N-demethylation, dapsone N-hydroxylation, and the 6 beta-hydroxylation of cortisol are all primarily mediated by P4503A4. Trait measurements to assess the in vivo level of activity of these separate oxidations have also been developed previously. This study investigated the relationships among the three phenotypic trait measurements in 30 young healthy white men. The frequency distributions of the trait values were all unimodal, with a twofold range for the erythromycin breath test and the urinary dapsone recovery ratio; the urinary 6 beta-hydroxycortisol/cortisol ratio was more variable, with a 17-fold range of values. No statistically significant correlations were observed among any of the trait measurements (dapsone recovery ratio versus erythromycin breath test: r = -0.07, p = 0.7; urinary 6 beta-hydroxycortisol/cortisol ratio versus erythromycin breath test: r = -0.12, p = 0.6; urinary 6 beta-hydroxycortisol/cortisol ratio versus dapsone recovery ratio: r = 0.13, p = 0.5. This lack of any relationship was unexpected and the reason(s) is unknown; however, it is possible that factors such as route of administration and extrahepatic metabolism in the intestinal epithelium and kidney are involved. Further studies are required to identify and validate the use of an appropriate in vivo probe of P4503A4 in humans.

Adult↗
PubMed1992

Activity of oxidative routes of metabolism of debrisoquin, mephenytoin, and dapsone is unrelated to the pathogenesis of vinyl chloride-induced disease.

The hypothesis, that cytochrome P4502D6, cytochrome P4502CMP, cytochrome P4503A4 or N-acetyl-transferase may form active intermediary metabolites that could be etiologically related to vinyl chloride-induced disease was investigated in 21 drug-free workers with previous development of vinyl chloride-induced disease and in 23 drug-free workers from the same plant who did not develop the syndrome. Each subject received simultaneous oral administration of debrisoquin (10 mg), mephenytoin (100 mg), and dapsone (100 mg). Measurement of the debrisoquin recovery ratio, the 8-hour recovery of 4-hydroxymephenytoin, and the dapsone recovery ratio in the subsequent 8-hour urine sample provided in vivo phenotypic indexes of cytochromes P4502D6, P4502CMP, and P4503A4 activity, respectively. An 8-hour blood sample was used to measure the acetylation ratio, a measure of N-acetyltransferase activity. The frequency distributions of each drug metabolizing activity were similar between groups. Within this small sample, there was no evidence to implicate cytochromes P4502D6, P4502CMP, and P4503A4 and N-acetyltransferase in the pathogenesis of vinyl chloride-induced disease.

Arylamine N-Acetyltransferase↗
PubMed1992

The disposition of dapsone in cirrhosis.

Acetylation and N-hydroxylation of dapsone were evaluated in drug-free, non-smoking, normal subjects and subjects with cirrhosis (n = 7 for each group) after oral administration of 100 mg dapsone. Acetylation was not correlated with oral dapsone clearance or reduced in cirrhosis (0.37 +/- 0.43 versus 0.52 +/- 0.32). Fractional metabolic clearance of dapsone to its hydroxylamine was associated with dapsone oral clearance (r = 0.96, p less than 0.001, n = 14). In patients with cirrhosis, liver disease was associated with a trend to reduction in oral clearance (22%) and metabolic clearance of dapsone (48%). Protein binding was minimally reduced by cirrhosis (73% +/- 1% versus 69% +/- 3% in patients with cirrhosis (p less than 0.02). The dapsone recovery ratio was validated as a phenotypic index of the metabolic clearance of dapsone (r = 0.74, p less than 0.05). In an extended comparison of 14 patients with cirrhosis to 70 control subjects, cirrhosis was associated with reductions of 28% in dapsone recovery ratio (p less than 0.001), and 37% in acetylation ratio (p less than 0.01). Neither dapsone recovery ratio nor acetylation ratio correlated with Pugh Score, conventional liver function tests, indocyanine green clearance, or phenotypic measures of S-mephenytoin hydroxylase or debrisoquin hydroxylase activity. We conclude that cirrhosis is associated with minor changes in dapsone disposition and that dosage modification is not required. In addition, there is evidence that cirrhosis has a selective influence on activity of individual isozymes of cytochrome P450.

Acetylation↗
PubMed1992

The ability to 4-hydroxylate debrisoquine is related to recurrence of bladder cancer.

Oxidative metabolism by cytochrome P450 enzymes is often involved in the activation of environmental procarcinogens. Debrisoquine, mephenytoin, and dapsone were used as in vivo probes for the activities of P4502D6, 2CMP, and 3A4, respectively, as well as dapsone for N-acetyltransferase, in order to assess the relationship between such activities and the relative risk of recurrence of bladder cancer. Urinary recovery ratios of debrisoquine and dapsone and the R/S ratio of mephenytoin were measured in an 0-8 h urine sample after simultaneous administration of debrisoquine (10 mg) and racemic mephenytoin (100 mg), and the administration of dapsone (100 mg) one week later, to patients undergoing local surgical resection of transitional cell bladder cancer of G-I, G-II, or G-III histopathology. In addition, plasma levels of dapsone and mono-acetyldapsone were determined in an 8 h plasma sample to determine the N-acetylation phenotype. Patients were followed for 3 years, to the time of tumour recurrence, or death. Three patients were lost to follow-up; of the remaining 95 patients, 55 had tumour recurrence. The debrisoquine recovery ratio was significantly greater in patients with recurrence than in individuals who remained disease-free. Among the 65 patients with non-aggressive (G-I and G-II) histopathology, two patients were lost to follow-up and 32 had tumour recurrence. In this subgroup, the debrisoquine recovery ratio was again found to be significantly greater in those individuals with tumour recurrence (p < 0.003).(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1992

Cholestasis in transplant patients--what is the role of cyclosporin?

The use of the drug cyclosporin is limited by toxicity. It would be advantageous to develop therapeutic monitoring of cyclosporin which would predict the development of clinical toxicity. In the present study, alternative methods of measuring cyclosporin levels were evaluated in a heterogenous population of transplant patients, comparing a fluorescent polarization immunoassay using a non-specific polyclonal antibody, which measures both cyclosporin and its main metabolites, and a specific high-performance liquid chromatography assay for unchanged cyclosporin in blood. Neither measured variable alone correlated with laboratory evidence of renal toxicity (serum creatinine) or liver toxicity (serum glutamate transaminase, lactic dehydrogenase, alkaline phosphatase, or serum bilirubin). The relationship between metabolites and parent cyclosporin was quantitated using the ratio of cyclosporin levels determined by fluorescent polarization immunoassay over levels determined by high-performance liquid chromatography. A cohort of patients with markedly elevated ratios of cyclosporin were identified. When patients' data were reviewed collectively and individually there was a correlation between an elevated ratio and the serum bilirubin (r = 0.41, P less than 0.001). This association could be either due to cyclosporin as a cause of cholestasis or to cholestasis from any cause resulting in metabolite accumulation. Further studies are needed to clarify the role of cyclosporin in hepatic dysfunction and develop early, specific markers for this cyclosporin-associated toxicity.

Adolescent↗
PubMed1992

Human liver microsomal N-hydroxylation of dapsone by cytochrome P-4503A4.

One of the major routes of elimination of dapsone (4,4'-diaminodiphenylsulfone) is by N-oxidation, to produce a hydroxylamine metabolite. The specific form of cytochrome P-450 (P-450) involved in this oxidation reaction was examined in human liver microsomal preparations previously characterized with respect to their content of several known P-450 enzymes. Among five preparations, the rank order of activity for dapsone hydroxylamine formation was most well correlated with the immunochemically determined level of P-4503A4 (r = 0.94, p less than 0.03). Moreover, inhibition of microsomal oxidation was observed with antibodies specific to P-4503A, with a maximum reduction of greater than 90%, but was not produced by antibodies specific to P-4501A2, P-4502CMP, or P-4502E1. Prior incubation of microsomes with gestodene (100 microM) or troleandomycin (20 microM), known selective mechanism-based inhibitors of P-4503A enzymes (in the presence of NADPH), led to 75% and 40% reductions in catalytic activity, respectively. In contrast, preincubation with increasing concentrations of alpha-naphthoflavone, a known activator of P-4503A4, increased dapsone N-hydroxylation in a concentration-dependent manner, with 5-fold activation being observed at 50 microM alpha-naphthoflavone. Finally, P-4503A4 isolated from human liver microsomes and cDNA-expressed P-4503A4 (in yeast) were both able to catalyze dapsone N-hydroxylation, with the latter preparation exhibiting a 3-fold activation in the presence of 100 microM alpha-naphthoflavone. Collectively, these findings demonstrate that N-oxidation of dapsone in human liver is predominantly mediated by P-4503A4, and they suggest that quantitative measurement of this metabolic pathway in vivo might serve as an index of the activity of this enzyme.

Antibodies↗
PubMed1991

Enantiospecific quantification of hexobarbital and its metabolites in biological fluids by gas chromatography/electron capture negative ion chemical ionization mass spectrometry.

A highly sensitive and specific assay based on gas chromatography/electron capture negative ion chemical ionization mass spectrometry has been developed for the analysis of the enantiomers of hexobarbital and its major metabolites in human urine and plasma. S-(+)-(5-2H3)hexobarbital and R-(-)-(5-2H3)hexobarbital were synthesized for clinical studies along with (+/-)-(1,5-2H6)hexobarbital and the deuterated major metabolites for use as internal and reference standards. Hexobarbital enantiomers and their metabolites were analyzed after pentafluorobenzyl and trimethylsilyl derivatization, following solid-phase extraction from plasma and urine. Intense negative ion spectra were observed for all of the derivatives. The base peak in the spectra corresponded to the M-pentafluorobenzyl anion [M-PFB]- except for 1,5-dimethylbarbituric acid, where M-. was the most abundant ion. The applicability of the method was demonstrated by following the plasma concentration-time profiles and urinary excretion in a male extensive metabolizer of mephenytoin who was given a pseudoracemic oral dose of hexobarbital containing equal 50 mg amounts of S-(+)-2(H0)hexobarbital and R-(-)-(2H3)hexobarbital. Marked stereoselective disposition was observed, with the R-(-)-enantiomer being more efficiently metabolized, primarily by alicyclic oxidation and ring cleavage.

Adult↗
PubMed1991

Differential effects of quinidine on the disposition of nifedipine, sparteine, and mephenytoin in humans.

The effects of quinidine on oxidative routes of drug metabolism mediated by different forms of cytochrome P450 were investigated in 10 healthy subjects. Each subject was studied on three different occasions and separately received oral administration of (1) a "cocktail" of nifedipine (5 mg), sparteine sulfate (90 mg), and mephenytoin (100 mg), (2) quinidine sulfate (200 mg), and (3) quinidine sulfate followed by the "cocktail" 1 hour later. Quinidine pretreatment significantly inhibited the aromatization of nifedipine to its major first-pass pyridine metabolite (M-0) and prolonged the elimination half-life of the calcium channel antagonist, both by about 40%. More marked inhibition of metabolism was observed with sparteine, and the formation of dehydrosparteine was abolished. A significant correlation was found between the 0-8-hour urinary ratio and the plasma concentration ratio of sparteine to dehydrosparteine obtained 4 hours after drug administration. No quinidine-induced changes were observed in the 4-hydroxylation of mephenytoin. The interaction between quinidine and nifedipine supports the involvement of a common P450 (P450IIIA4) in the metabolism of the two drugs.

Administration, Oral↗
PubMed1991

Phenotypic debrisoquine 4-hydroxylase activity among extensive metabolizers is unrelated to genotype as determined by the Xba-I restriction fragment length polymorphism.

1. The major pathway for 4-hydroxylation of debrisoquine in man is polymorphic and under genetic control. More than 90% of subjects (extensive metabolizers, EMs) have active debrisoquine 4-hydroxylase (cytochrome P450IID6) while in the remainder (poor metabolizers, PMs), cytochrome P450IID6 activity is greatly impaired. 2. Within the EM group, cytochrome P450IID6-mediated metabolism of a range of substrates varies widely. Some of this intra-phenotype non-uniformity may be explained by the presence of two subsets of subjects with different genotypes (heterozygotes and homozygotes). 3. Cytochrome P450IID6 substrates have not differentiated between these two genotypes. However, a restriction fragment length polymorphism (RFLP) which identifies mutant alleles of cytochrome P450IID6 locus has been described and can definitively assign genotype in some heterozygous EM subjects. 4. In this study, we used RFLP analysis and encainide as a model substrate to determine if non-uniformity in cytochrome P450IID6 activity among EMs is related to genotype. We tested the hypothesis that heterozygotes exhibit intermediate metabolic activity and that homozygous dominants exhibit the highest activity. We proposed encainide as a useful substrate for this purpose since cytochrome P450IID6 catalyzes not only its biotransformation to O-desmethyl encainide (ODE) but also the subsequent metabolism of ODE to 3-methoxy-O-desmethyl encainide (MODE). 5. A single 50 mg oral dose of encainide was administered to 139 normal volunteers and 14 PMs were identified. Urinary ratios among encainide, ODE and MODE in the remaining 125 EM subjects revealed a wide range of cytochrome P450IID6 activity. However, Southern blotting of genomic DNA digested with XbaI identified obligate heterozygotes in both extremes of all ratio distributions.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiography↗
PubMed1990

Differences in plasma binding of drugs between Caucasians and Chinese subjects.

Interracial differences in drug responsiveness can be accounted for, at least in part, by differences in drug disposition. To investigate whether reversible interactions with plasma constituents may be a contributing factor in such differences, the binding of a number of model drugs was studied in plasma obtained from healthy Caucasian and Chinese subjects (n = 8 in each group). The unbound fractions of drugs binding to sites I and II on albumin (warfarin, diazepam, and salicylic acid) were similar in the two groups, and there was no difference in the plasma albumin concentration. By contrast, the percentages of unbound diphenhydramine (26.40% +/- 6.46% versus 18.30% +/- 4.31, mean +/- SD) and propranolol (13.81% +/- 1.33% versus 11.68% +/- 2.37) were significantly (p less than 0.05) higher in Chinese subjects compared to Caucasians. A 30% difference was also observed in the nonlinear binding of disopyramide. These basic drugs interact with both alpha 1-acid glycoprotein and albumin, and the lower binding was associated with a lower plasma concentration of the acute-phase reactant in Chinese subjects. Kinetic analysis of the disopyramide binding isotherm was also suggestive of reduced binding capacity with no change in binding affinity. The reason for the racial difference in the alpha 1-acid glycoprotein level is unknown; however, for drugs extensively bound to this glycoprotein the resulting difference in unbound fraction would be expected to have predictable pharmacokinetic consequences that may result in differences in responsiveness. Determination of plasma binding, especially of drugs interacting with alpha 1-acid glycoprotein, should therefore be an essential component of comparative studies in subjects of different races.

Asian People↗
PubMed1990

Scleroderma is associated with differences in individual routes of drug metabolism: a study with dapsone, debrisoquin, and mephenytoin.

Exposure to certain environmental agents may induce a scleroderma-like syndrome in a small proportion of individuals. Differences in susceptibility could involve metabolic activation of a protoxin, with affected patients having a greater converting ability. This possibility was investigated in 84 patients with scleroderma and 108 control subjects with in vivo probes of specific pathways of metabolism. Scleroderma was associated with reduced hydroxylating activity for dapsone and S-mephenytoin, whereas the ability to hydroxylate debrisoquin and N-acetyl dapsone was similar in both groups. Logistic regression confirmed these associations based on the shift in frequency distribution. Individuals who were poor metabolizers for mephenytoin and only modest N-hydroxylators of dapsone had a tenfold increased risk of scleroderma (p = 0.008). Thus this combined metabolic impairment may be causally involved in the development of scleroderma or, alternatively, the disease may produce inhibition of selected metabolizing enzymes in a subset of patients.

Acetylation↗
PubMed1990

The contribution of N-hydroxylation and acetylation to dapsone pharmacokinetics in normal subjects.

The relative importance of N-hydroxylation and acetylation of dapsone to the oral clearance of dapsone (100 mg) was investigated in seven healthy volunteers. Plasma dapsone and monoacetyldapsone concentrations rose rapidly with subsequent similar monoexponential elimination. The oral clearance of dapsone was low (33 +/- 14 ml/min), with a threefold variability. Four subjects were identified as fast acetylators; however, differences in acetylation did not explain the variability in oral clearance. The cumulative urinary recoveries of dapsone and its hydroxylamine were approximately 20% of the dose. The formation clearance of hydroxylamine, which exhibited a tenfold intersubject variability, was closely associated with the oral clearance of dapsone (r = 0.96). Thus, the formation of the hydroxylamine is more important than acetylation in determining dapsone's intersubject variability in oral clearance. Variation in N-hydroxylation may have clinical consequences, because the hydroxylamine is considered to be important in dapsone-mediated toxicity.

Acetylation↗
PubMed1990

Induction of polymorphic 4'-hydroxylation of S-mephenytoin by rifampicin.

Studies were performed in 13 healthy subjects to determine whether treatment with rifampicin results in induction of the metabolism of mephenytoin. Daily dosing with 600 mg rifampicin for 22 days caused a three to eightfold increase in the 0-8 h urinary R/S ratio of mephenytoin following oral administration (100 mg) of racemic drug to extensive metabolizers of the anticonvulsant. This was accompanied by a 40 to 180% increase in the 0-8 h urinary excretion of the 4'-hydroxy metabolite. Four weeks after discontinuing rifampicin, both metabolic indices had returned to their baseline values. By contrast, rifampicin had no effect on either measures of metabolism in subjects of the poor metabolizer phenotype. Thus, it appears that the activity of the enzyme (P-450 MP) mediating the genetically determined 4'-hydroxylation of S-mephenytoin can be significantly modulated by enzyme inducing agents such as rifampicin and possibly environmental agents with a similar ability.

Adult↗
PubMed1989

Racial differences in drug response. Altered sensitivity to and clearance of propranolol in men of Chinese descent as compared with American whites.

To determine whether the pharmacokinetics and pharmacodynamics of beta-blockade differ among racial groups, we gave 10 men of Chinese descent and 10 American white men 10, 20, 40, and 80 mg of propranolol every eight hours; the dosages were given in random order, and each dose was given for one day. The degree of beta-blockade was measured as the reduction in the heart rate and blood pressure in the supine and upright positions and during treadmill exercise testing. The Chinese subjects had at least a twofold greater sensitivity to the beta-blocking effects of propranolol than the white subjects, as indicated by the mean (+/- SEM) plasma concentrations producing a 20 percent reduction in the heart rate in both the supine position (197 +/- 31 vs. 536 +/- 58 nmol per liter; P less than 0.05) and the upright position (131 +/- 27 vs. 343 +/- 39 nmol per liter; P less than 0.05) and after exercise testing (96 +/- 12 vs. 185 +/- 23 nmol per liter; P less than 0.05). In addition, the Chinese subjects had much greater sensitivity to the hypotensive effects of propranolol, as shown by the concentrations that reduced blood pressure by 10 percent in the supine position (73 +/- 5 vs. 748 +/- 7 nmol per liter; P less than 0.01) and in the upright position (89 +/- 5 vs. 401 +/- 6 nmol per liter; P less than 0.01). No difference in beta-receptor density or affinity of lymphocytes was found between the groups. The Chinese group had a 45 percent higher free fraction of propranolol in plasma, which may have contributed to the increased drug effect but cannot explain it entirely. This group metabolized propranolol more rapidly than the white group, which resulted in a 76 percent higher clearance of an oral dose (3740 +/- 737 vs. 2125 +/- 214 ml per minute; P less than 0.05) because of increased metabolism through multiple metabolic pathways. We conclude that Chinese men have greater sensitivity than white men to the effects of propranolol on heart rate and blood pressure. Decreased protein binding may be responsible in part, but most of the effect remains to be explained.

Adult↗