J.R. Idle

Animal modelling of human polymorphic drug oxidation—the metabolism of debrisoquine and phenacetin in rat inbred strains

The metabolism of debrisoquine (5mg kg−1 orally) was investigated in females of 7 strains of rat. Two major metabolic pathways, those of 4‐ and 6‐ hydroxylation were found to be polymorphic. The DA strain eliminated in urine only 7–10% of the dose as 4‐hydroxy‐debrisoquine together with 31–55% debrisoquine while the corresponding values for the Lewis strain were 44–55% and 11–17% respectively. Accordingly, DA and Lewis rats were proposed as models for the human PM (poor metabolizer) and EM (extensive metabolizer) drug oxidation phenotypes. To further test this model, DA and Lewis rats were given phenacetin (200 mg kg−1 orally). This underwent O‐de‐ethylation to paracetamol (52–55%) and aromatic 2‐hydroxylation (7–8%) in Lewis rats. The corresponding findings in DA rats were 35–40% O‐de‐ethylation and 12–13% 2‐hydroxylation. It is suggested that, with respect to both debrisoquine and phenacetin, Lewis and DA inbred rat strains afford a model of oxidative drug metabolism for the human EM and PM phenotypes respectively.

A genetic polymorphism of the N-oxidation of trimethylamine in humans.

Trimethylamine (TMA) and its N‐oxide (TMAO) are normal components of human urine. They are present in the diet and also derived from the enterobacterial metabolism of precursors such as choline. Dietary TMA is almost entirely metabolized to and excreted as TMAO. However, the extent to which TMA undergoes N‐oxidation appears to be polymorphic in a British white population study (n = 169). Two propositi were identified with relative TMA N‐oxidation deficiency that was further confirmed by oral challenge with TMA (600 mg). The study of the families of the two propositi, as well as those of two identified subjects with trimethylaminuria, under both normal dietary conditions and after oral TMA challenge strongly indicates that the conditions of impaired N‐oxidation is inherited as a recessive trait. It is proposed that the N‐oxidation of TMA in humans is polymorphic and under single gene diallelic control in which individuals who are homozygous for the variant allele exhibit marked N‐oxidation deficiency and trimethylaminuria.

Debrisoquin hydroxylation polymorphism among Ghanaians and Caucasians.

The alicyclic and aromatic hydroxylation of debrisoquin was studied in Ghanaians. As in a previously studied Caucasian population, the alicyclic 4‐hydroxylation of debrisoquin in Ghanaians was polymorphic. Three phenotypes were observed: homozygous extensive metabolizers (58%), heterozygous extensive metabolizers (36%), and homozygous poor metabolizers (6%). In contrast, British Caucasians are primarily monomorphic extensive metabolizers (92%) and homozygous poor metabolizers comprise 8% of the population. Urinary recovery of the drug and its hydroxylated metabolites was significantly less in the Ghanaian subjects. In both Ghanaian and British populations, aromatic hydroxylation producing 5‐, 6‐, 7ȁ0, and 8‐hydroxydebrisoquin was shown to parallel the alicyclic 4‐hydroxylation of debrisoquin, and thus to be controlled by the same gene locus. Debrisoquin is advocated as a tool for uncovering polymorphism in drug oxidation and its interethnic variations.

The metabolism of 14C-labelled trimethylamine and its N-oxide in man

The metabolism and elimination of 14C-labelled trimethylamine and its N-oxide (100 mg orally) were studied in three male volunteers. For both compounds the urine was the major route of elimination, with 95% of the administered 14C being voided in the first 24 h. No radioactivity was found in expired air. The majority (greater than 95%) of the urinary 14C from both compounds was excreted as trimethylamine N-oxide.

Some observations on the oxidation phenotype status of Nigerian patients presenting with cancer

The hypothesis is being explored that there may be an association between genetically determined oxidation status and propensity to develop carcinoma in response to environmental chemical carcinogens. For this purpose, the genetic structure of a normal, healthy Nigerian population with respect to oxidation status, has been compared with that found for a group of 59 Nigerian patients presenting with carcinoma of the liver and gastrointestinal tract. Genetically determined oxidation status was assessed by measuring the extent of oxidation of a probe drug, debrisoquine, to its major metabolite, 4-hydroxydebrisoquine. The cancer group contained a disproportionately large number of individuals who were extensive oxidizers compared to the controls (2 P = 0.0045). The findings support the view that genetically determined oxidation status may be an important host factor in influencing responsiveness to chemical carcinogens that require oxidative metabolic activation.

Dissociation of co‐regulatory control of debrisoquin/phenformin and sparteine oxidation in Ghanaians

The ability to oxidize sparteine to form 2‐ and 5‐dehydrosparteine was studied in 154 healthy Ghanaians. Although the urinary metabolic sparteine/dehydrosparteines ratio varied widely (from 0.14 to 12.5), in contrast to observations in several Caucasian population groups the ratios were not bimodally distributed and no phenotypically poor oxidizers of sparteine were found. The ability of these same subjects to oxidize debrisoquin and phenformin was also studied in 141 and 143 subjects. Of the 141 subjects dosed with debrisoquin, 10 proved to be poor oxidizers, and of the 143 subjects dosed with phenformin, 11 were poor oxidizers. All the poor oxidizers of debrisoquin were also poor oxidizers of phenformin. The 10 confirmed poor metabolizers of debrisoquin, who had debrisoquin metabolic ratios ranging from 14.4 to 52.0, had sparteine metabolic ratios ranging only from 0.15 to 12.5. Whereas Caucasian poor metabolizers of sparteine excrete <2.0% of a dose as dehydrosparteines, the mean excretion of dehydrosparteines in our 10 subjects was 20.6% ± 13.2%. The overall rank correlation between the sparteine and debrisoquin metabolic ratios was low (rs = 0.47), while the coefficient of determination for linear regression (r2) was only 0.17. Our data show that the ability of Ghanaians to oxidize sparteine is largely independent of their capacity for debrisoquin oxidation and is indicative of a major interethnic difference in the genetic control of these reactions.

A study of the debrisoquine hydroxylation polymorphism in a Nigerian population.

1. The metabolic oxidation of debrisoquine has been studied in a group of 123 Nigerian volunteers. 2. All subjects excreted unchanged drug together with five oxidation products, namely, 4-, 5-, 6-, 7- and 8-hydroxy-debrisoquine. 3. The 4-hydroxylation reaction exhibits polymorphism; ten subjects were defective in their ability to effect this reaction. 4. The incidence (q) of the allele governing impaired 4-hydroxylation (DL) among Nigerians was calculated as being 0.28 (95% confidence limit of 0.20-0.37). 5. An association was demonstrated between the ability to effect 4-hydroxylation and 6- and 7-hydroxylation of debrisoquine, suggesting that the alleles controlling alicyclic oxidation also influence aromatic hydroxylation.

Genetic polymorphism of phenformin 4-hydroxylation

The ability to oxidize a single 50‐mg dose of phenformin to its 4‐hydroxy metabolite was determined in 195 individuals. Variations in the urinary ratio of phenformin/4‐hydroxyphenformin ranged from 1 to 184. Family studies were consistent with the hypothesis that this variability resulted from a single gene mode of inheritance in which impaired hydroxylation of phenformin appears as an autosomal recessive trait. Both genotype frequencies and the degree of dominance of the extensive metabolizer phenotype over the recessive showed a remarkable resemblance to those described for debrisoquine 4‐hydroxylation, which was confirmed by the high degree of correlation (rs = 0.785, P < 0.0001) between the phenformin ratio and the debrisoquine metabolic ratio. Such close agreement between the metabolism of these drugs may indicate that the same genetic control is in operation. Such genetic polymorphism of phenformin hydroxylation may have important implications for therapeutic response and for the possibility of toxic effects in a few individuals.

A population and familial study of the defective alicyclic hydroxylation of debrisoquine among Egyptians.

1. Debrisoquine hydroxylation exhibited profound variation in 72 Egyptian volunteers. 2. The frequency distribution histogram of the metabolic ratio (ratio unchanged drug: 4-hydroxy metabolite in 0-8 h urine) was polymodal. 3. From family data it was possible to define more clearly than before the heterozygous characteristics. 4. Egyptians appear in general to be more extensive oxidizers of debrisoquine than do English subjects. 5. Ramadan fasting was found to lower the absorption of debrisoquine.

Influence of oxidation polymorphism on phenformin kinetics and dynamics

Plasma and urinary kinetics and responses of blood lactate, pyruvate, and glucose after a single 50‐mg phenformin dose were investigated in eight subjects of known debrisoquin oxidation phenotype, four poor metabolizers (PM) and four extensive metabolizers (EM). Higher peak plasma concentrations of phenformin (152.2 ± 12.7 ng/ml; mean ± SE) and a greater plasma AUC (779 ± 99 ng · hr · ml−1) were reached in PM than in EM (99.8 ± 13.7 ng/ml and 549 ± 47 ng · hr · ml−1). Although the urinary excretion of unchanged phenformin was greater in PM between 2 and 24 hr after dosing than in EM, excretion of 4‐hydroxy‐phenformin could not be detected in most samples collected from PM but was present in every sample from EM. Blood lactate concentrations increased dramatically in PM but fell in EM after phenformin. There were no changes in either blood pyruvate or glucose levels. The results may help to explain lactic acidosis in patients given phenformin in the absence of other predisposing factors.