PubMed Health⌕ Search

Biomedical subjects

D L Rees

Publications and source records attributed to D L Rees.

9 recordsLinked to original sources
PubMed1995

Human hepatic cytochrome P450 2C9 catalyzes the rate-limiting pathway of torsemide metabolism.

Tolyl methylhydroxylation is the rate-limiting step in the elimination of torsemide, a newly developed diuretic, in humans in vivo. Kinetic and inhibitor studies with human liver microsomes and complementary DNA-expressed enzyme were performed to identify the cytochrome P450 (CYP) isoform responsible for torsemide tolyl methylhydroxylation to predict factors that might alter clearance in patients receiving torsemide. As in vivo, tolyl methylhydroxylation was the major biotransformation pathway in human liver microsomes. Microsomal tolyl methyl-hydroxylation kinetics followed Michaelis-Menten kinetics, with the mean apparent Km for the reaction being 11.2 +/- 1.3 microM. The microsomal reaction was almost completely abolished by the specific CYP2C9 inhibitor sulfaphenazole and was inhibited competitively by the alternative CYP2C9 substrate tolbutamide. Torsemide tolyl methylhydroxylase activity in microsomes from 16 human livers correlated significantly (rs = .81-.88) with tolbutamide and phenytoin hydroxylation, both CYP2C9-mediated reactions. Complementary DNA-expressed CYP2C9 catalyzed torsemide tolyl methylhydroxylation with an apparent Km (23 microM) similar to that observed for human liver microsomes and the IC50 values for sulfaphenazole inhibition of the reaction were essentially identical for the two enzyme sources. Taken together, these data demonstrate that human hepatic torsemide tolyl methylhydroxylation is catalyzed predominantly, if not solely, by CYP2C9. The implications of this finding for the regulation of torsemide metabolism in vivo are discussed.

Aryl Hydrocarbon Hydroxylases↗
PubMed1993

Site-directed mutation studies of human liver cytochrome P-450 isoenzymes in the CYP2C subfamily.

Evidence from human studies in vivo and in vitro strongly suggests that the methylhydroxylation of tolbutamide and the 4-hydroxylation of phenytoin, the major pathways in the elimination of these two drugs, are catalysed by the same cytochrome P-450 isoenzyme(s). In the present study we used site-directed mutagenesis and cDNA expression in COS cells to characterize in detail the kinetics of tolbutamide and phenytoin hydroxylations by seven CYP2C proteins (2C8, 2C9 and variants, and 2C10) in order to define the effects of small changes in amino acid sequences and the likely proteins responsible in the metabolism of these two drugs in man. Tolbutamide was hydroxylated to varying extents by all expressed cytochrome P-450 isoenzymes, although activity was much lower for the expressed 2C8 protein. While the apparent Km values for the 2C9/10 isoenzymes (71.6-131.7 microM) were comparable with the range of apparent Km values previously observed in human liver microsomes, the apparent Km for 2C8 (650.5 microM) was appreciably higher. The 2C8 enzyme also showed quite different sulphaphenazole inhibition characteristics. The 4-hydroxylation of phenytoin was also more efficiently catalysed by the 2C9/10 enzymes. These enzymes showed similarities in kinetics of phenytoin hydroxylation and sulphaphenazole inhibition compared with human liver phenytoin hydroxylase. Also of interest was the observation that, among the 2C9 variants, small differences in amino acid composition could appreciably affect both tolbutamide and phenytoin hydroxylations. The amino acid substitution Cys-144-->Arg increased both the rates of tolbutamide and phenytoin hydroxylations, while the Leu-359-->Ile change had a greater effect on phenytoin hydroxylation. We conclude that: (1) although 2C8 and 2C9/10 proteins metabolize tolbutamide. only 2C9/10 proteins play a major role in human liver; (2) 2C9/10 proteins also appear to be chiefly responsible for phenytoin hydroxylation; and (3) subtle differences in the amino acid composition of these 2C9/10 proteins can affect the functional specificities towards both tolbutamide and phenytoin.

Amino Acid Sequence↗
PubMed1993

Tolbutamide hydroxylation in humans: lack of bimodality in 106 healthy subjects.

The tolbutamide hydroxylation capacity was studied in 106 healthy unrelated volunteers from the Australian population. Following a 500 mg oral dose of tolbutamide, the ratio of metabolites (hydroxytolbutamide plus carboxytolbutamide) to unchanged tolbutamide excreted in urine from 6 to 12 h post-dose (urinary metabolic ratio, MR) was determined. Metabolic ratio values did not appear bimodally distributed, even following various transformations of the data (i.e. Log10, inverse, Log10 inverse). A poor metabolizer (PM) subject from a previous clinical study, however, could be distinguished (MR value 159) from the above subjects (MR value range 324-3033), particularly from the histogram plot of inverse tolbutamide metabolic ratio. The poor metabolizer's parents had metabolic ratio values (526 and 478) that were at the lower end of the range of metabolic ratios obtained from the population study, and may indicate that they both have a heterozygous genotype and that a recessive form of inheritance is most likely. As the hydroxylations of tolbutamide and phenytoin are closely linked, the incidence of slow tolbutamide metabolizers is likely to be similar to that for phenytoin (about 1:500) and this is consistent with the failure to detect a single poor tolbutamide metabolizer in our random sample of 106 individuals.

Adolescent↗
PubMed1980

Eskornade in the treatment of diurnal incontinence in children.

This study examined 75 children with daytime wetting and particularly those with some form of bladder neck incompetence. There was a good response in 75% of cases to Eskornade (containing phenylpropanolamine, an alpha adrenergic agonist), but a lesser response in children with bladder instability. Large doses of this preparation have sometimes been necessary to produce urinary continence and side effects, although common, were usually minor and transient.

Adolescent↗
PubMed1979

Training in urology.

Explore the source record for details and available documents.

Education, Medical, Graduate↗
PubMed1978

Bladder instability in women with recurrent cystitis.

Factors in the aetiology of bladder instability in women with recurrent cystitis are analysed. A multifactorial approach to patient management is advocated and supported by a treatment failure rate of only 3.6% with these patients over a mean follow-up of nearly 2 years.

Adult↗
PubMed1977

Psychiatric aspects of recurrent cystitis in women.

Using both psychometric questionnaires and psychiatric interview it has been shown that women with recurrent cystitis have significantly more psychiatric symptoms (particularly those of anxiety) than the population as a whole. By comparison with the expected prevalence, the study group shows a 3-fold increase in psychiatric symptoms antedating micturition symptoms, and a 10-fold increase in psychiatric symptoms overall. Significant differences in psychometric profiles have been demonstrated between different clinical subgroups of patients (e.g. bladder instability, outlet obstruction and dyspareunia). A multifactorial approach (including an awareness of psychiatric factors) to patients complaining of recurrent cystitis can produce a treatment failure rate as low as 4.4%.

Adolescent↗
PubMed1975

Urodynamic findings in adult females with frequency and dysuria.

Urodynamic findings, and in particular measures of outflow obstruction in a consecutive series of 156 women presenting with recurrent increased frequency and dysuria, have been analysed. Poor correlation between these various measures has been demonstrated; at most only 8% of these patients have demonstrable outflow obstruction. More than a quarter of the patients had bladder instability.

Adolescent↗