Poor Metabolizers Of Debrisoquine Research Articles

Polymorphic ability to metabolize propranolol alters 4-hydroxypropranolol levels but not beta blockade.

The ability to hydroxylate debrisoquine is known to be polymorphically distributed, with about 8% to 9% of the North American Caucasian population being poor metabolizers. We have shown that the ability to 4-hydroxylate propranolol is also polymorphically determined and that it cosegregates with ability to metabolize debrisoquine, such that poor debrisoquine metabolizers produce much less 4-hydroxypropranolol (4-OH propranolol) than do extensive metabolizers. There was no significant difference, however, between plasma propranolol concentrations after either single or multiple doses in the two groups. Despite the substantial difference in production of the pharmacologically active 4-OH metabolite, no difference was seen in the extent of beta-blockade induced in the extensive and poor metabolizers, which implies that 4-OH propranolol does not contribute substantially to beta-blockade.

The relationship between debrisoquine oxidation phenotype and the pharmacokinetics and pharmacodynamics of propranolol.

The pharmacokinetics and pharmacodynamics of propranolol (80 mg by mouth) were studied in seven extensive and four poor metabolisers of debrisoquine. Evidence for impairment of the 4'-hydroxylation of propranolol was found in poor metabolisers. However, no significant difference was detected in the oral clearance of unchanged drug between the two groups of debrisoquine oxidation phenotypes. Poor metabolisers of debrisoquine did not experience more intense or more prolonged beta-adrenoceptor blockade than extensive metabolisers of debrisoquine.

Bufuralol metabolism in human liver: a sensitive probe for the debrisoquine-type polymorphism of drug oxidation.

The genetically controlled polymorphism causing decreased metabolism of debrisoquine is closely related to that of the metabolism of bufuralol and numerous other drugs and has important clinical consequences. A sensitive in vitro assay was developed which quantifies the production of 1'-hydroxy-bufuralol (carbinol) from bufuralol in human liver microsomes. Initial formation rates of carbinol suggested Michaelis-Menten kinetics with an apparent KM of 61 and 171 mumol l-1 and Vmax of 3.2 and 5.8 nmol mg-1 microsomal protein h-1 in two human liver samples. The Vmax in microsomes of thirty-two liver samples was 4.2 +/- 1.0 (SD) nmol carbinol mg-1 protein h-1. Metabolism of debrisoquine in vivo, as expressed by the 'metabolic ratio' of debrisoquine over 4-OH debrisoquine correlated (r = -0.65, P less than 0.01; n = 18) with carbinol production rate in microsomes in vitro. Microsomes of one individual identified as poor metabolizer of debrisoquine in vivo showed reduction of carbinol formation to 1.97 nmol mg-1 h-1. Mixing his microsomes with those of an extensive metabolizer resulted in additive formation of carbinol excluding mediation of the defect by a soluble inhibitor. These data support the concept of a primary defect in microsomal oxidation of bufuralol. The described assay offers a sensitive tool to investigate the molecular mechanism of the 'debrisoquine polymorphism'.

Urinary bile acid and bile alcohol excretion does not reflect the genetic polymorphism of debrisoquine hydroxylation.

Excretion of the major urinary bile alcohol 27-nor-5 beta-cholestane-3 alpha, 7 alpha, 12 alpha, 24,25- pentol , and of cholic, chenodeoxycholic, deoxycholic and lithocholic acid was measured in 24 h urine collections of 10 extensive and seven poor metabolizers of debrisoquine. There was no significant difference of the excretion of these cholesterol metabolites between the two groups, indicating that cholesterol hydroxylation to bile alcohols and bile acids is probably not controlled by the same genes responsible for the 'debrisoquine-type' hydroxylation polymorphism.

Pharmacogenetics of mephenytoin: a new drug hydroxylation polymorphism in man.

Inherited deficiency in mephenytoin hydroxylation was observed in a family study. It is important that the propositus was of the extensive metabolizer phenotype for the genetically controlled hydroxylation of debrisoquine. Thus, a genetic polymorphism of drug hydroxylation was suspected for mephenytoin. A population study of mephenytoin hydroxylation, combined with identification of extensive and poor debrisoquine hydroxylation phenotypes, was carried out in 221 unrelated normal volunteers. Twelve of them (5%) exhibited defective aromatic hydroxylation of mephenytoin, and 23 (10%) could be identified as poor metabolizers of debrisoquine. Amongst these 35 subjects with a drug hydroxylation deficiency, 3 (or 0.5%; 1 female, 2 males) displayed both defects simultaneously. A panel study of 10 extensive and 10 poor metabolizers of mephenytoin showed that the ability to perform aromatic hydroxylation of the demethylated mephenytoin metabolite nirvanol (5-phenyl-5-ethylhydantoin) was co-inherited with the mephenytoin hydroxylation polymorphism. Family studies suggested that poor metabolizer phenotypes of nirvanol and mephenytoin were most likely to have the homozygous genotype for an autosomal recessive allele of deficient aromatic drug hydroxylation. Intra-subject comparison of the debrisoquine and mephenytoin hydroxylation phenotypes in these subjects indicated that deficiency in the two drug hydroxylations occurred independently. Consequently, the co-inheritance of extensive and poor hydroxylation of mephenytoin and nirvanol, respectively, represents a new drug hydroxylation polymorphism in man.

The fate of orally administered [4-14C]phenytoin in two healthy male volunteers.

A recovery study was conducted to determine whether phenytoin (DPH), like the barbiturates, is metabolized via the recently discovered N-glucosidation pathway. Virtually 100% of the ingested 14C-labelled doses in two subjects could be accounted for in the excreta within 5 days, with 35% in feces and 65% in urine. Radioactivity in the urine was entirely due to free and conjugated 5-(4-hydroxy-phenyl)-5-phenylhydantoin (p-HPPH) and the dihydrodiol, and that in the feces mostly due to the unmetabolized drug. There was no indication of phenytoin N-glucoside being excreted in either the urine or feces of either subject, although one of the subjects was known to possess a particularly strong N-glucosidation capacity for barbiturates. The other subject was a poor metabolizer of debrisoquine and sparteine. Nevertheless, the DPH disappearance from serum and the DPH metabolite excretion in urine were virtually alike in these two subjects, indicating that the debrisoquine 4-hydroxylating and DPH hydroxylating capacities may be separable entities.

Metabolic oxidation of methaqualone in extensive and poor metabolisers of debrisoquine.

The metabolism of methaqualone to the glucuronides of 5 C-monohydroxy metabolites and to the N-oxide has been studied in 2 groups of healthy young adults phenotyped as extensive and poor metabolisers of debrisoquine. No significant interphenotype differences were observed with respect to the excretion of any of the 6 metabolites. It is probable that the genetic regulation of the pathways leading to these metabolites is at a locus other than that which is responsible for the regulation of the oxidation of debrisoquine, guanoxan, phenacetin, phenytoin and sparteine.

Influence of the genetically controlled deficiency in debrisoquine hydroxylation on antipyrine metabolite formation.

The influence of the genetically controlled deficiency in debrisoquine hydroxylation on antipyrine metabolite formation was studied by giving 500 mg antipyrine to 14 extensive and 10 poor metabolizers of debrisoquine. The pharmacokinetics of antipyrine were determined on the basis of the saliva concentration time curve and the cumulative urinary excretion of 4-hydroxyantipyrine, norantipyrine, 3-hydroxymethyl-antipyrine, and 3-carboxyantipyrine was measured for 32 h following drug administration. Antipyrine elimination half-life, volume of distribution, and total clearance were almost equal for the two groups. Significant differences in the excretion of antipyrine metabolites were not observed, except for 3-hydroxymethyl-antipyrine which was excreted in poor metabolizers about 30% less than in extensive metabolizers (p less than 0.01). However, this difference only reached borderline significance (p less than 0.1) when clearance values for production of this metabolite were calculated. It is concluded that different species of the drug-oxidizing enzymes (cytochrome P-450 system) are involved in the metabolism of debrisoquine and antipyrine. Possibly the enzyme responsible for hydroxylating debrisoquine is partly involved in the formation of 3-hydroxymethyl-antipyrine.