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Biomedical subjects

S Cholerton

Publications and source records attributed to S Cholerton.

At least 19 recordsLinked to original sources
PubMed1999

The effect of fluconazole on cyclophosphamide metabolism in children.

Fluconazole is increasingly used in children receiving chemotherapy. Many of these patients are being treated with cyclophosphamide, which must undergo hepatic metabolism to produce active alkylating species. As a consequence of the cytochrome P-450 inhibitory properties of fluconazole, a potential interaction exists between these two agents that could influence the therapeutic effect of cyclophosphamide. To investigate this interaction, a retrospective case series of patients was chosen from a population of children with a previously established profile of cyclophosphamide metabolism. Twenty-two children who were not receiving other therapy known to influence drug metabolism were selected and analyzed in terms of fluconazole treatment; of these, nine were receiving fluconazole and thirteen were identified as controls. Study design was not randomized. The plasma clearance of cyclophosphamide was lower in patients receiving fluconazole [mean(SD) 2.4(0.71) versus 4.2(1.2) l/h/m2, p =.001]. In vitro studies were performed to characterize the interaction between fluconazole and cyclophosphamide in six human liver microsomes. The concentration of fluconazole required to reduce the production of 4-hydroxycyclophosphamide to 50% of control values (IC50) varied between 9 and 80 microM (median 38 microM). Further studies of the effect of fluconazole on 4-hydroxycyclophosphamide production in vivo are warranted to determine whether this interaction reduces the therapeutic effect of cyclophosphamide in clinical practice.

Adolescent↗
PubMed1998

Variability of coumarin 7- and 3-hydroxylation in a Jordanian population is suggestive of a functional polymorphism in cytochrome P450 CYP2A6.

OBJECTIVE: To determine the variability of coumarin 7- and 3-hydroxylation in a human population and to evaluate the evidence for the existence of genetic polymorphism in these pathways. 7-Hydroxylation of coumarin is considered to be a detoxication pathway, whilst 3-hydroxylation, which predominates in rats, leads to hepatotoxicity in the rat. Coumarin metabolic phenotypes could aid in refining the risk evaluation for humans of dietary and environmental exposure to coumarin and for the chronic use of coumarin in high doses as a drug to treat lymphoedema and certain cancers. METHODS: Healthy male and female Jordanian volunteers (n = 103) were administered 2 mg coumarin by mouth and collected their 0-8-h urines. These, together with pre-dose blank urines, were analysed by selected-ion monitoring gas chromatography mass spectrometry for their content of the coumarin metabolites 7-hydroxycoumarin (70HC) and 2-hydroxyphenylacetic acid (2OHPAA), the latter arising from the 3-hydroxylation pathway. RESULTS: After coumarin administration, excretion of both 70HC and 2OHPAA was highly variable. A coumarin metabolic ratio (2OHPAA/7OHC) was suggestive of polymorphism. At least one subject had a metabolic response similar to an individual known to be both phenotypically and genotypically (CYP2A6 gene) 7-hydroxylation-deficient. CONCLUSION: In the light of the finding of high variability and possible polymorphism in both the 7- and 3-hydroxylation of coumarin in a human population. we recommend a reappraisal of the risk evaluation of human exposure to coumarin, particularly in pharmaceutical doses.

Adult↗
PubMed1998

Lack of association between the dopamine D2 receptor gene allele DRD2*A1 and cigarette smoking in a United Kingdom population.

The dopamine D2 receptor gene contains a TaqI repeat fragment length polymorphism creating two alleles DRD2*A1 and DRD2*A2. It has been previously suggested that the lesser allele, DRD2*A1, is more prevalent in individuals who are susceptible to impulsive/addictive/compulsive behaviour, for example, alcoholics, polysubstance abusers and tobacco smokers. We genotyped a series of 104 smokers and 117 non smokers and compared the allele frequencies between the groups. A subset (n = 87) of the smoking population also completed the Classification of Smoking by Motives questionnaire and were given scores for five criteria that drive smoking: automatic, dependence, sedative, stimulant and indulgence. Another subset (n = 52) completed the Fagerstrom Tolerance Questionnaire and were given scores for nicotine dependence. We did not find any increase in allele A1 frequency when comparing smokers to non smokers. Furthermore, neither measure of dependence was affected by possession of the A1 allele; the only difference between DRD2*A1 bearing and DRD2*A2 homozygous individuals in terms of smoking motives was found in the scores for indulgence; the former having a moderately reduced score (by 17%, p < 0.05). We conclude that, in the individuals studied, the dopamine D2 receptor TaqI locus does not affect the drive to smoke. This may be caused by the locus being unrelated to impulsive/addictive/compulsive behaviour, the polymorphism being in linkage disequilibrium with another distinct locus or, alternatively, smoking may represent a behaviour that is not directly comparable to impulsive/addictive/compulsive behaviours previously associated with the DRD2*A1 allele.

Alleles↗
PubMed1997

A single amino acid substitution (Leu160His) in cytochrome P450 CYP2A6 causes switching from 7-hydroxylation to 3-hydroxylation of coumarin.

Human populations are thought to metabolize coumarin almost exclusively by 7-hydroxylation. We have identified an individual who is homozygous for a single amino acid substitution (Leu160His) in the cytochrome P450 CYP2A6 arising from the variant CYP2A6*2 allele. On administration of coumarin (2 mg orally) no detectable 7-hydroxycoumarin was excreted in the 0-8-hr urine, rather, approximately 50% of the dose was eliminated as 2-hydroxyphenylacetic acid, the end-product of coumarin 3-hydroxylation. His immediate family members, who were heterozygous for the CYP2A6*2 allele, excreted little 2-hydroxyphenylacetic acid and mainly 7-hydroxycoumarin, when similarly tested. These findings raise a question regarding human risk evaluations for environmental coumarin exposures, since 7-hydroxylation is regarded as a detoxication pathway, but 3-hydroxylation as the process required to lead to macromolecular covalent binding of coumarin. Persons homozygous for the CYP2A6*2 allele may constitute 1-25% of various populations.

Adolescent↗
PubMed1996

Automated gas chromatographic assay for amlodipine in plasma and gingival crevicular fluid.

This paper describes an automated capillary gas chromatographic method for the determination of amlodipine in plasma, and in sub-microlitre volumes of gingival crevicular fluid (GCF), in order to assess if amlodipine is present in GCF under conditions of gingival overgrowth, as has been shown for nifedipine, another dihydropyridine drug. Liquid-liquid extraction followed by derivatisation was employed to isolate amlodipine and render it suitable for gas chromatography. Amlodipine was analysed in plasma and GCF of four patients undergoing amlodipine therapy for cardiovascular disorders, three of whom had significant gingival overgrowth. Amlodipine was detected in the plasma of all patients and in massive concentrations in the GCF of those patients with overgrowth, 23- to 290-fold greater than in their plasma. Like nifedipine, amlodipine sequestration into GCF appears to be linked with gingival overgrowth.

Amlodipine↗
PubMed1996

Trimethylamine N-oxidation in Turkish women with bacterial vaginosis.

Bacterial vaginosis (BV) is the most common cause of vaginal discharge in women of child-bearing age. A common symptom of this condition is a fishy-smelling vaginal discharge. Trimethylamine (TMA) is the substance which is primarily responsible for this distinctive odour. The ability to metabolize TMA is polymorphically distributed such that the majority of individuals metabolize a large part of the body burden of TMA to the odourless trimethylamine N-oxide (TMAO) which is excreted in urine with a small amount of TMA. However, in certain individuals, N-oxidation of TMA is impaired which results in the excretion of large amounts of TMA in the urine, breath, sweat and vaginal secretions. In the present study the metabolism of TMA to TMAO was determined in women with clinically diagnosed BV, women with fishy-smelling vaginal discharge but with no other evidence of BV and control women with no evidence of fishy-smelling vaginal discharge. An index of TMA N-oxidation was established for all subjects after analysis of a 24 h urine sample for free TMA and total TMA after reduction of TMAO. We show that, irrespective of diagnosis of BV or not, women with fishy-smelling vaginal discharge excrete significantly more free TMA and have a similarly significantly reduced capacity to N-oxidize TMA when compared to healthy control women. Thus, the results of this study suggest that a woman's ability to metabolize TMA to TMAO is an important factor which predisposes to a fishy-smelling vaginal discharge.

Adolescent↗
PubMed1995

The N-oxidation of trimethylamine in a Jordanian population.

The ability to oxidise trimethylamine (TMA) to trimethylamine N-oxide (TMAO) is distributed polymorphically within a British white population with the majority of individuals excreting greater than 90% of total urinary TMA as TMAO. The opposite extreme is characterised by a rare inborn error of TMA N-oxidation known as the fish-odour syndrome. However there is a lack of information regarding inter-individual variability in the N-oxidation of TMA in other ethnic groups. In this study the urinary excretion of TMA and TMAO was determined over a period of 24 h in 82 Jordanian subjects. A frequency distribution histogram of % of total urinary TMA excreted as TMAO revealed that the majority of subjects excreted greater than 80% of the total urinary TMA as TMAO, however eight subjects (9.7%) excreted less than 80% of the total TMA as TMAO. In a previous study of 169 white British subjects only one (0.6%) excreted less than 80% of the total TMA as TMAO. The results suggest that the prevalence of compromised ability to N-oxidise TMA may be higher in a Jordanian population than in a British population.

Adult↗
PubMed1995

A genetic polymorphism in coumarin 7-hydroxylation: sequence of the human CYP2A genes and identification of variant CYP2A6 alleles.

A group of human cytochrome P450 genes encompassing the CYP2A, CYP2B, and CYP2F subfamilies were cloned and assembled into a 350-kb contig localized on the long arm of chromosome 19. Three complete CYP2A genes--CYP2A6, CYP2A7, and CYP2A13--plus two pseudogenes truncated after exon 5, were identified and sequenced. A variant CYP2A6 allele that differed from the corresponding CYP2A6 and CYP2A7 cDNAs previously sequenced was found and was designated CYP2A6v2. Sequence differences in the CYP2A6v2 gene are restricted to regions encompassing exons 3, 6, and 8, which bear sequence relatedness with the corresponding exons of the CYP2A7 gene, located downstream and centromeric of CYP2A6v2, suggesting recent gene-conversion events. The sequencing of all the CYP2A genes allowed the design of a PCR diagnostic test for the normal CYP2A6 allele, the CYP2A6v2 allele, and a variant--designated CYP2A6v1--that encodes an enzyme with a single inactivating amino acid change. These variant alleles were found in individuals who were deficient in their ability to metabolize the CYP2A6 probe drug coumarin. The allelic frequencies of CYP2A6v1 and CYP2A6v2 differed significantly between Caucasian, Asian, and African-American populations. These studies establish the existence of a new cytochrome P450 genetic polymorphism.

Alleles↗
PubMed1994

Identification of the major human hepatic cytochrome P450 involved in activation and N-dechloroethylation of ifosfamide.

Two NADPH-dependent metabolic routes for the anticancer drug ifosfamide, 4-hydroxylation (activation) and N-dechloroethylation (a detoxication pathway), were studied in human liver microsomes to identify the cytochrome P450 enzymes involved. Naringenin, a grapefruit aglycone and an inhibitor of cytochrome P450 3A4 (CYP3A4)-catalysed reactions, was found to inhibit ifosfamide activation and N-dechloroethylation by human liver microsomes. IC50 for both reactions was of the order of 70 microM. The CYP3A4-specific inhibitor triacetyloleandomycin inhibited ifosfamide N-dechloroethylation by human liver microsomes with an IC50 of approximately 10 microM. Furthermore, anti-human CYP3A4 antiserum inhibited by about 80% N-dechloroethylation of ifosfamide by human liver microsomes. The relative levels of cytochromes P450 1A, 2C, 2E and 3A4 in 12 human livers were determined by western blotting analysis. A strong correlation (P < 0.001) was observed between CYP3A4 expression and both activation and N-dechloroethylation of ifosfamide. A role for human CYP3A4 in both pathways of ifosfamide metabolism was thus demonstrated. This was substantiated by the observation that the nifedipine oxidase activities of the 12 samples of human liver microsomes correlated with ifosfamide activation (P < 0.009) and N-dechloroethylation (P < 0.001). These findings have important clinical implications. The involvement of the same key cytochrome P450 enzyme in both reactions prohibits selective inhibition of the N-dechloroethylation pathway, as might be desirable to reduce toxic side effects. They also demonstrate the need to consider interaction with co-administered drugs that are CYP3A4 substrates.

Antibodies↗
PubMed1994

Genotyping for polymorphisms in xenobiotic metabolism as a predictor of disease susceptibility.

Polymorphisms in many xenobiotic metabolizing enzymes occur leading to variation in the level of enzyme expression in vivo. Enzymes showing such polymorphisms include the cytochrome P450 enzymes CYP1A1, CYP1A2, CYP2A6, CYP2D6, and CYP2E1 and the phase two metabolism enzymes glutathione S-transferase MI (GSTMI) and arylamine N-acetyltransferase 2 (NAT2). In the past, these polymorphisms have been studied by phenotyping using in vivo administration of probe drugs. However, the mutations which give rise to several of these polymorphisms have now been identified and genotyping assays for polymorphisms in CYP1A1, CYP2A6, CYP2D6, CYP2E1, GSTMI, and NAT2 have been developed. Specific phenotypes for several of the polymorphic enzymes have been associated with increased susceptibility to malignancy, particularly lung and bladder cancer, and Parkinson's disease. These associations are likely to be due to altered activation or detoxication of chemicals initiating these diseases, including components of tobacco smoke and neurotoxins. The substrate specificity and tissue distribution of polymorphic enzymes implicated in disease causation discussed with particular reference to previously described disease-phenotype associations.

Animals↗
PubMed1992

Mutant debrisoquine hydroxylation genes in Parkinson's disease.

The frequency of fifteen genotypes of CYP2D6 (debrisoquine 4-hydroxylase) in 53 patients with Parkinson's disease was determined by the polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) analyses and compared with the findings in 72 healthy controls. The commonest mutant allele, CYP2D6B, was twice as frequent among patients as in controls, with an approximate relative risk ratio of 2.70 (95% confidence interval 1.14-6.41; p = 0.0063) for subjects homozygous or heterozygous for this allele.

Aged↗
PubMed1992

Comparison of a novel thin-layer chromatographic-fluorescence detection method with a spectrofluorometric method for the determination of 7-hydroxycoumarin in human urine.

A novel method for the determination of 7-hydroxycoumarin in human urine which combines thin-layer chromatography (TLC) with fluorescence detection (FD) has been devised. The limit of detection (1 ng/ml) enables determination of 7-hydroxycoumarin after both administration of coumarin and environmental exposure to this fragrance material. When compared to a spectrofluorometric method of analysis, the TLC-FD method proved to be more selective for the analysis of 7-hydroxycoumarin in human urine.

Administration, Oral↗
PubMed1992

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

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

Cytochrome P-450 Enzyme System↗
PubMed1992

The pharmacogenetics of chemical carcinogenesis.

The human body is endowed with a large number of xenobiotic chemical metabolizing enzymes, a significant proportion of which are polymorphic and thus render one individual at greater or lesser risk than another of chemically-induced disease. All examples of genetic polymorphism of chemical metabolizing enzymes have been reviewed in relation to their potential to activate and detoxicate procarcinogens and promutagens. Many examples are cited whereby phenotype can act as a carcinogenic risk factor. With the availability of a large amount of DNA sequence data for chemical metabolizing enzymes there has emerged a number of polymerase chain reaction (PCR) strategies aimed at discerning one metabolic phenotype or another. This is seen as a very positive and democratic scientific development, widening the franchise for studies of disease risk. Nevertheless, it is argued that, at these early stages with many laboratory-based scientists scarcely familiar with epidemiological study design, a cautious approach should obtain when interpreting single studies.

Animals↗