Eugen Steiner

Polymorphic debrisoquin hydroxylation in 757 Swedish subjects

The metabolic ratios (MRs) between debrisoquin and 4‐hydroxydebrisoquin in urine after a single oral dose of 10 mg debrisoquin were bimodally distributed in 757 healthy, white Swedish volunteers. Forty‐one subjects (5.4%) had an MR >12.6 and were classified as slow debrisoquin hydroxylators. The MR was reproducible in urine stored at + 8° C for 1 week and at − 20° C over a period of 5 yers. Collection intervals of 6 or 12 hours gave the same MR. Intraindividual repeatability of the debrisoquin phenotyping test was established in 37 subjects examined twice at least 2 weeks apart. The calculated frequency of the single allele that is believed to control deficient debrisoquin hydroxylation is similar among white Swedish people, as among other white groups examined so far; however, it is significantly different from the frequency in certain Oriental groups. Detailed comparisons of the prevalence of slow debrisoquin hydroxylation in different ethnic groups are not possible due to shortcomings in current epidemiologie techniques used (small materials, the location of the antimode distinguishing rapid and slow hydroxylators unknown, and family studies missing).

A family study of genetic and environmental factors determining polymorphic hydroxylation of debrisoquin

Debrisoquin hydroxylation capacity determined as the ratio of debrisoquin to 4‐OH‐debrisoquin (DMR) in urine after a single oral dose (10 mg) was studied in 52 nuclear families comprising 226 subjects. The relative importance of genetic and environmental factors for DMR was studied by path analysis. There was a significant negative correlation between DMR and coffee intake but no significant correlations between DMR and sex, age, alcohol intake, or smoking habits. Path analysis showed that genetic heritability was 0.79 while cultural heritability was only 0.06. Complex segregation analysis gave evidence for a major locus with incomplete dominance (d = 0.28) between a recessive and an additive gene. The frequency of the major gene was 0.31, allowing an estimate of the frequency of slow hydroxylators in the Swedish population of 9.4%. There was also evidence for a multifactorial component accounting for 14% of the total variation. It was not possible to distinguish between the different genotypes within the rapid hydroxylator phenotype. Our data agree with previous studies in British and German populations showing that two alleles at a major autosomal locus can explain most of the observed variation in DMR. The frequency of slow hydroxylators in Sweden is very similar to that reported in other European studies. The debrisoquin metabolic phenotype seems to be extensively controlled by a monogenic system and not significantly influenced by environmental factors or age.

Phenotypic consistency in hydroxylation of desmethylimipramine and debrisoquine in healthy subjects and in human liver microsomes

The 2‐hydroxylation of desmethylimipramine (DMI) and the 4‐hydroxylation of debrisoquine (D) were studied in healthy subjects and in human liver microsomes. A single oral dose of DMI (25 mg) was given to 18 healthy subjects previously phenotyped with D (13 rapid and five slow hydroxylators). Urine was collected for 24 hr and DMI and total 2‐hydroxydesmethylimipramine (2‐OH‐DMI) levels were determined by HPLC. The urinary ratio DMI/2‐OH‐DMI correlated strongly (r = 0.92) with the urinary ratio of D to 4‐hydroxydebrisoquine (D/4‐OH‐D). The two hydroxydations were also studied in human liver microsomes from 10 different subjects. Formation rates of the hydroxylated metabolites correlated strongly (r = 0.869). Moreover, D competitively inhibited the 2‐hydroxylation of DMI. These findings suggest that both are hydroxylated by the same cytochrome P‐450 isozyme.

Inhibition of desipramine 2‐hydroxylation by quinidine and quinine

Urinary excretion of desipramine (DMI) and 2‐hydroxydesipramine (2‐OH‐DMI) after single oral doses of 25 mg DMI was investigated in seven rapid and three slow debrisoquin hydroxylators, before and after pretreatment with either quinidine or its diastereoisomer quinine. After treatment with 800 mg quinidine daily for 2 days, excretion of 2‐OH‐DMI decreased by 96% in rapid hydroxylators and 68% in slow hydroxylators. After treatment with 750 mg quinine/day for 2 days, excretion of 2‐OH‐DMI in rapid hydroxylators was 54% lower than during the control experiment, whereas in slow hydroxylators no significant changes in the excretion pattern were observed. Unchanged DMI constituted only a minor fraction of recovered drug and no significant changes in its recovery were observed in either phenotypic group after pretreatment with quinidine or quinine. Thus both quinidine and quinine decreased the excretion of 2‐OH‐DMI. At similar doses the effect of quinidine was much stronger than that of quinine, virtually transforming rapid hydroxylators into slow hydroxylators. The mechanism probably involves a stereoselective inhibition of DMI 2‐hydroxylation.

Differences in the inhibitory effect of cimetidine on desipramine metabolism between rapid and slow debrisoquin hydroxylators

The disposition of a 25 mg single oral dose of desipramine was investigated in five rapid and four slow hydroxylators of debrisoquin before and during oral administration of 1200 mg cimetidine daily. AUC and elimination half‐life of desipramine increased during cimetidine administration in rapid but not in slow hydroxylators. This was the result of a decrease in overall clearance. The urinary recovery of 2‐hydroxydesipramine was significantly decreased in rapid hydroxylators during cimetidine administration. We conclude that cimetidine inhibits the metabolism of desipramine in rapid but not in slow hydroxylators.

Hydroxylation of desmethylimipramine: Dependence on the debrisoquin hydroxylation phenotype

The 2‐hydroxylation of desmethylimipramine (DMI) was studied in 14 healthy subjects previously phenotyped with respect to debrisoquin hydroxylation. After a single oral dose (25 mg), slow hydroxylators of debrisoquin had significantly lower total and metabolic clearances and longer plasma half‐lives of DMI and excreted less 2‐hydroxydesmethylimipramine than did rapid hydroxylators. These findings strengthen the hypothesis that the hydroxylations of debrisoquin and DMI may be under common enzymatic control.

Time course of enzyme induction in humans: Effect of pentobarbital on nortriptyline metabolism

To study the effect of induction we gave six male volunteers 10 mg nortriptyline three times a day for 4 weeks and 0.2 gm pentobarbital on days 8 to 21. Plasma and urinary levels of nortriptyline and metabolites were measured. The rate and extent of induction of the enzyme(s) were estimated by a model with use of nortriptyline concentrations. There was a marked decrease of nortriptyline levels after 2 days of pentobarbital treatment. Total clearance of nortriptyline increased more than twofold (range, 1.6‐fold to 4.1‐fold). Apparent metabolic clearance by 10‐hydroxylation increased markedly. The decrease in nortriptyline levels was more rapid than the increase after pentobarbital cessation, fitting with the theory of the model. The induction of nortriptyline metabolism is probably mainly the result of an increase in a non‐CYP 2D6 P450 isozyme, possibly CYP 3A4 or a CYP 2C form. More knowledge of induction characteristics of drugs should lead to better predictions of decreased effects and appearance of adverse effects. The kinetic model used for analysis of our data could then be useful.

Relationship between uptake and elimination of toluene and debrisoquin hydroxylation polymorphism

The toxicokinetics of toluene were studied in six healthy subjects. Three of the subjects were phenotyped as rapid hydroxylators of debrisoquin and three subjects were phenotyped as slow hydroxylators of debrisoquin. The subjects were exposed in an exposure chamber to toluene vapor (3.25 mmol/m3) for 4 hours. Solvent concentrations in blood and the metabolites, hippuric acid and o‐cresol, in urine were measured during the exposure period and 3 hours after exposure. The capacity to metabolize debrisoquin was determined in three volunteers who had earlier experimentally been exposed to toluene. The uptake of toluene was about 3 mmol, or 50% of the inhaled dose in both rapid and slow hydroxylators. There were no significant differences between the two phenotypic groups with regard to concentration of toluene in blood, apparent blood clearance of toluene, or excretion of hippuric acid and o‐cresol.

Ultrafiltration compared with equilibrium dialysis in the determination of unbound phenytoin in plasma

Protein binding of phenytoin (PHT) was studied in 36 patients with normal kidney function and six uremic patients. We compared a newly introduced ultrafiltration (UF) technique (EMIT Free Level System I) for measuring unbound PHT in plasma with an equilibrium dialysis (ED) method. The precision of the UF method was satisfactory (the CV within samples was 4.2%, and between days, 3.4%). PHT concentrations were measured with both homogeneous enzyme immunoassay and high performance liquid chromatography using plasma samples from epileptic patients. The values obtained agreed well. Unbound concentrations of PHT determined by the UF technique and ED were identical in both normal and uremic plasma obtained from patients under treatment. Unbound concentrations of PHT correlated significantly to total concentrations in both groups of patients. The investigated UF technique thus appears to be accurate for measuring unbound concentrations of PHT in plasma. Controlled clinical studies are required to show that this is a cost-effective clinical service.

Induction of theophylline metabolism by pentobarbital.

Theophylline concentrations in plasma and urine were determined during maintenance treatment in nine healthy volunteers during one dosage interval before and after 10 days of simultaneous treatment with pentobarbital (100 mg each night). During the pentobarbital period, total plasma clearance of theophylline increased by 40% (range -4-79%), whereas renal clearance remained unchanged. It is concluded that therapeutic doses of pentobarbital induce the metabolism of theophylline with marked interindividual variation.