Dextromethorphan O-demethylation polymorphism in Jordanians

Dextromethorphan metabolic phenotyping in an Iranian population

OBJECTIVE

CYP2D6 polymorphism of drug metabolism represents an important source of interindividual and interethnic variation in drug response. Since this polymorphism has not been studied in an Iranian population, the present study was undertaken.

METHODS

Two hundred healthy unrelated Iranian subjects participated in this study. Phenotyping was based on high-performance liquid chromatography determination of the dextromethorphan/total dextrorphan molar ratios as metabolic ratios (MRs) in plasma samples collected at 3 h after oral administration of 30 mg dextromethorphan hydrobromide. Since the dextromethorphan detection limit of 5 ng/ml achieved in the simultaneous assay for dextromethorphan and its metabolites was not adequate to identify intermediate metabolizers (IMs), 80 of 200 samples selected randomly were also assayed using a modified, more sensitive procedure with a dextromethorphan detection limit of 1 ng/ml.

RESULTS

Poor and extensive metabolizers (EMs) could be identified distinctly. A 520-fold interindividual variation in dextromethorphan MRs was observed in this study. In contrast to undetectable dextrorphan and hydroxymorphinan concentrations, clearly determinable dextromethorphan concentrations higher than 10 ng/ml were observed in plasma samples of poor metabolizers (PMs). Considering the antimode of 0.3, five (2.5%, 95% confidence interval of 0.34-4.66) volunteers were identified as PMs. Using the more sensitive method, dextromethorphan was quantified in 4 (one PM) of 80 samples. Excluding the PM, a Shapiro-Wilk test indicated a non-normal distribution of MRs (P < 0.01) in the latter population.

CONCLUSIONS

From this study it can be concluded that the frequency of PMs in an Iranian population is 2.5% (95% confidence interval of 0.34-4.66). IMs could be identified using dextromethorphan plasma assays with detection limits of at least 1 ng/ml. However, the phenotype-genotype relationships in this respect remain to be established.

Bioinformatics Research on Inter-racial Difference in Drug Metabolism I. Analysis on Frequencies of Mutant Alleles and Poor Metabolizers on CYP2D6 and CYP2C19

The enzyme activities of CYP2D6 and CYP2C19 show a genetic polymorphism, and the frequency of poor metabolizers (PMs) on these enzymes depends on races. In the present study, the frequencies of mutant alleles and PMs in each race were analyzed based on information from published studies, considering the genetic polymorphisms of CYP2D6 and CYP2C19 as the causal factors of racial and inter-individual differences in pharmacokinetics. As a result, it was shown that there were racial differences in the frequencies of each mutant allele and PMs. The frequencies of PMs on CYP2D6 are 1.9% of Asians and 7.7% of Caucasians, and those of PMs on CYP2C19 are 15.8% of Asians and 2.2% of Caucasians. Based on the results, it was suggested that there would be racial differences in the frequencies of PM subjects whose blood concentrations might be higher for drugs metabolized by these enzymes. Additionally, it was suggested that enzyme activities would vary according to the number of functional alleles even in subjects judged to be extensive metabolizers (EMs). In the bridging study, genetic information regarding CYP2D6 and CYP2C19 of the subjects will help extrapolate foreign clinical data to a domestic population.

Dextromethorphan phenotyping and haloperidol disposition in schizophrenic patients

This study examined the relationship between the metabolic ratios of dextromethorphan/dextrorphan, haloperidol disposition, and the incidence of extrapyramidal side effects in schizophrenic patients. Eighteen schizophrenic patients were phenotyped with a test dose of dextromethorphan prior to the initiation of haloperidol treatment. The metabolic ratio of dextromethorphan/dextrorphan was determined in each patient. Patients were treated with oral haloperidol 10 mg/day for 2 weeks. Blood samples for haloperidol and reduced haloperidol were obtained at week 2 of haloperidol treatment. Haloperidol and reduced haloperidol plasma concentrations were assayed by HPLC with electrochemical detection. Significant correlations of dextromethorphan/dextrorphan metabolic ratios vs. plasma haloperidol concentrations, reduced haloperidol concentrations, and reduced haloperidol/haloperidol ratios were found (r = 0.726, P = 0.0007; r = 0.782, P = 0.0001; and r = 0.619, P = 0.006, respectively). Ten patients who experienced extrapyramidal side effects had higher reduced haloperidol concentrations and reduced haloperidol/haloperidol ratios than the other patients (2.49 +/- 1.42 [S.D.] ng/ml vs. 1.10 +/- 0.46 ng/ml, P = 0.014 and 0.287 +/- 0.102 vs. 0.192 +/- 0.065, P = 0.030). The former also had a trend to have higher haloperidol concentrations and dextromethorphan/dextrorphan ratios than the latter (8.04 +/- 2.91 ng/ml vs. 5.83 +/- 1.79 ng/ml, P = 0.066 and 0.023 +/- 0.017 vs. 0.011 +/- 0.010, P = 0.077). Phenotyping patients has the potential to assist clinicians in predicting plasma drug concentrations during the subsequent neuroleptic drug treatment. Further research with phenotyping and psychotropic drug metabolism in psychiatric patients is needed.

Dextromethorphan metabolism in Jordanians: dissociation of dextromethorphan O-demethylation from debrisoquine 4-hydroxylation

The concentrations of dextromethorphan (DM) and its metabolites dextrorphan (DRP), 3-methoxymorphinan (MM) and 3-hydroxymorphinan (HM) were measured in 8 h urine samples from 266 unrelated healthy Jordanian subjects following oral administration of 30 mg dextromethorphan hydrobromide and using a rapid, sensitive and precise HPLC method with fluorometric detection. The frequency of the 'poor' metabolizer status of DM-O-demethylation as judged by log DM/DRP was found to be 6.8% with a 95% confidence interval of 3.8-9.8%. There was a strong correlation between log DM/DRP and log total non-O-demethylated compounds (NODM)/total O-demethylated metabolites (ODM) metabolic ratios (r = 0.96, P < 0.01). However, one subject with log DM/DRP of 0.05 that classifies him as a poor metabolizer was found to have a log NODM/ODM of -0.73 which is in the range of extensive metabolizer status suggesting the presence of another cytochrome P450 isoenzyme involved in dextromethorphan O-demethylation. Dextromethorphan N-demethylation to 3-methoxymorphinan was detected in 55.3% of individuals. Furthermore, a dissociation between dextromethorphan O-demethylation and debrisoquine (D) 4-hydroxylation has been observed. Among the 116 subjects phenotyped with both dextromethorphan and debrisoquine, 7 were poor metabolizers of both, three were poor metabolizers of debrisoquine and extensive metabolizers of dextromethorphan whilst 4 were poor metabolizers of dextromethorphan and extensive metabolizers of debrisoquine, one of whom was reclassified as an extensive metabolizer of dextromethorphan using log NODM/ODM to characterize dextromethorphan metabolizer status.