A Küpfer

Polymorphic dextromethorphan metabolism: co-segregation of oxidative O-demethylation with debrisoquin hydroxylation

Dextromethorphan hydrobromide, 25 mg po, was given to 268 unrelated Swiss subjects to study urinary drug and metabolite profiles. Rates of O‐demethylation yielding the main metabolite dextrorphan were expressed by the urinary dextromethorphan/dextrorphan metabolic ratio. We found a bimodal distribution of this parameter in our population study, which indicates that there are two phenotypes for dextromethorphan O‐demethylation. The antimode at a metabolic ratio of 0.3 separated the poor metabolizer (PM; n = 23; prevalence of 9%) from extensive metabolizer (EM) phenotypes. Urinary output of dextrorphan was <6% of the dose in all PMs and was 50% in the 245 EMs. Pedigree analysis of 14 family studies revealed an autosomal‐recessive transmission of deficient dextromethorphan O‐demethylation. In these families, 37 heterozygous genotypes could be identified; however, through use of the urinary drug and metabolite analysis it was not possible to identify the heterozygous genotypes within the EM phenotype group. Co‐segregation of dextromethorphan O‐demethylation with debrisoquin 4‐hydroxylation was also studied. Complete concordance of the two phenotypic assignments was obtained, with a Spearman rank correlation coefficient of rs = 0.78 (n = 62; P < 0.0001) for dextromethorphan and debrisoquin metabolic ratios. Presumably the two drug oxidation polymorphisms are under the same genetic control. Thus the innocuousness and ubiquitous availability of dextromethorphan render it attractive for worldwide pharmacogenetic investigations in man.

Aminopyrine demethylation measured by breath analysis in cirrhosis.

The method of measuring the rate of aminopyrine demethylation by breath analysis was assessed in 23 normal subjects and 20 patients with cirrhosis. Carbon 14 aminopyrine specifically labeled at the two N‐methyl groups was administered by mouth in a dose of 9 mg/kg, including a total radioactivity of 2 μCi. The decay of the specific activity of 14C02 in breath (kb) was found to correlate (r = 0.91) with the disappearance of aminopyrine from plasma (Kp). In normal volunteers, kb was 22.4%/hr; in patients with alcoholic and nonalcoholic cirrhosis it was depressed to 8.4%/hr (p < 0.001). The degree of functional impairment found with the breath test was similar to the sulfobromophthalein (BSP) disappearance curve and the galactose elimination capacity. Although many questions relating to the aminopyrine breath test remain open, our data confirm and extend previous studies of 14CO2 breath analysis after 14C‐aminopyrine administration. It is concluded that it represents a simple and noninvasive procedure which quantitatively reflects the microsomal function of the cirrhotic liver.

Stereoselective mephobarbital hydroxylation cosegregates with mephenytoin hydroxylation

The 8‐hour urinary recovery of 4‐hydroxy‐mephobarbital has been measured after oral administration of racemic mephobarbital (90 mg) in 17 extensive (EM) and six poor (PM) metabolizer phenotypes of mephenytoin. The recovery of this metabolite was measurable in every EM and ranged from 2.5% to 48% (10.9% ± 1.9% of dose), but was not detected in any PM (<1% of dose). In EMs, the 8‐hour urine recovery of 4‐OH‐mephobarbital after mephobarbital was approximately half that of 4‐OH‐mephenytoin over the same time after mephenytoin administration. One EM received similar doses of R‐ and S‐mephobarbital on separate occasions. Urinary recovery of 4‐OH‐mephobarbital was 33% and <1%, respectively. These results suggest that mephobarbital is stereoselectively hydroxylated by the same drug metabolizing enzyme that is responsible for the stereoselective aromatic hydroxylation of mephenytoin.

Mephenytoin hydroxylation deficiency: Kinetics after repeated doses

Deficient aromatic hydroxylation of S‐mephenytoin was observed in an index subject during a kinetic study of stereoselective metabolism of mephenytoin. A genetic basis for this defect was suggested by decreased urinary recovery of 3‐methyl‐5‐(4‐hydroxyphenyl)‐5‐ethylhydantoin (4‐OH‐M) in the 24 hr after oral racemic mephenytoin in two brothers of the propositus. The parents and a third brother had urinary recoveries of 4‐OH‐M of the same order as in a group of 20 normal subjects. The kinetic implications of this defect were studied in the index subject and compared with four normal subjects after a single oral dose of differentially radiolabeled pseudoracemic mephenytoin (5 µCi of 14C‐S‐mephenytoin, 45 µCi of H3‐R‐mephenytoin, and 11.5 µmol/kg of both S‐ and R‐mephenytoin) followed by single oral doses of 1.4 mmol of unlabeled racemic mephenytoin daily the next 4 days. In normal subjects, there was substrate stereoselective metabolism with the S‐enantiomer rapidly excreted as 4‐OH‐M and the R‐enantiomer slowly excreted as 5‐phenyl‐5‐ethylhydantoin (PEH). Stereoselective metabolism persisted during repeated dosing. In the hydroxylation‐deficient subject, there was no evidence of stereoselective metabolism, recovery of 4‐OH‐M was low, and both enantiomers were slowly excreted, predominantly as PEH. Plasma PEH concentrations and urinary PEH excretion rates were approximately twice that in normal subjects. Thus a genetic deficiency in ability to hydroxylate S‐mephenytoin results in the S‐enantiomer metabolization by the alternate route of demethylation to PEH that cumulates, thereby, in comparison to the normal, effectively doubling the dose of total hydantoin.

Stereoselectivity of the Arene Epoxide Pathway of Mephenytoin Hydroxylation in Man

Summary: Stereoselective metabolism of mephenytoin has been investigated in four normal subjects by comparing urinary recoveries of hydroxylated metabolites after administration of racemic RS‐mephenytoin (1.4 mmol/day) and R‐mephenytoin (0.7 mmol/day) on separate occasions. Gas chromatography‐mass spectrometry was employed to measure the urinary recovery of 3‐methyl‐5‐(4‐hydroxyphenyl)‐5‐ethylhydantoin (4‐OH‐M) and mephenytoin catechol, methylcatechol, and dihydrodiol metabolites. Following a single oral dose of racemic mephenytoin, 4‐OH‐M, mephenytoin catechol, and methylcatechol metabolites were identified in urine mainly as conjugates, whereas the dihydrodiol metabolite was recovered mainly in its unconjugated form. Urinary elimination of each metabolite was similar on days 1 and 10 of chronic racemic mephenytoin administration. Following R‐mephenytoin administration, urinary recoveries of hydroxylated metabolites were five to 10 times smaller than after administration of the racemic drug. This implies substrate‐stereoselective hydroxylation of the S‐enantiomer of mephenytoin. In one subject with a genetic deficiency of aromatic mephenytoin hydroxylation deficiency, the excretion of each hydroxylated mephenytoin metabolite after RS‐mephenytoin administration was decreased to 5–15% of the values found in the four extensively hydroxylating study volunteers. The impaired formation of hydroxylated mephenytoin metabolites in genetic hydroxylation deficiency, in conjunction with stereoselective hydroxylation of S‐mephenytoin via an extensive NIH shift in normal man, is consistent with the hypothesis that the formation of the S‐mephenytoin arene oxide is under genetic control and represents the initial enzymatic reaction of stereoselective aromatic mephenytoin hydroxylation. The formation of this potentially reactive metabolite of S‐mephenytoin may have implications in mephenytoin‐induced toxicity.

Pharmacokinetics of R‐Enantiomeric Normephenytoin During Chronic Administration in Epileptic Patients

Summary: Stereoselective metabolism has been demonstrated for mephenytoin (MHT), R‐MHT being demethylated to the pharmacologically active metabolite 5‐phenyl‐5‐ethylhydantoin (PEH; nirvanol), and S‐MHT undergoing aromatic hydroxylation to 4‐OH‐MHT, with formation of an intermediate arene oxide metabolite. PEH is responsible for the therapeutic effect, whereas 4‐OH‐MHT is rapidly eliminated by the kidneys. The arene oxide metabolite may have implications in MHT toxicity. The metabolism of PEH is also stereospecific. In the present study, the R‐enantiomer of PEH (R‐PEH; R‐normephenytoin) was administered chronically during 8 weeks to four epileptic patients, as a single dose every 3 days. The half‐lives of R‐PEH ranged from 77.7 to 175.8 h, and correlated closely with the creatinine clearance. Mean urinary recovery of R‐PEH was 86.6% of the dose at steady state, with 4‐OH‐PEH accounting for only 5%. This indicates that, unlike Nirvanol (a racemic mixture of R‐ and S‐PEH), R‐PEH is only minimally metabolized, even after several weeks of treatment and despite potential enzymatic autoinduction and heteroinduction by other antiepileptic drugs. Complete blood counts and liver function tests revealed no alteration, and no other adverse effects were noted. If arene oxide intermediate metabolites are indeed involved in the toxicity of MHT and nirvanol, R‐PEH may represent a safer alternative.