Low frequency of slow debrisoquine hydroxylation in a native Chinese population

Actual and potential drug interactions associated with methadone

OBJECTIVE

To identify and characterize methadone-related drug interactions, as well as factors accounting for the variability in manifesting these interactions clinically.

DESIGN

Systematic review of the primary literature.

METHODS

Over 200 articles, reports of clinical trials, and case reports were reviewed. Studies and case reports were included if they revealed either quantitative or qualitative methods to identify, evaluate severity of, or compare methadone-related drug interactions.

RESULTS OF DATA SYNTHESIS

The evidence base associated with methadone drug interactions is underdeveloped in general, as the majority of references found were case reports or case series. Most of the studies and reports focused on inpatients receiving methadone maintenance treatment (MMT) that were between 20 and 60 years of age, taking 200 mg/day of methadone or less. Evidence supporting the involvement of lesser known cytochrome P450 enzymes such as 2B6 is emerging, which may partially explain the inconsistencies previously found in studies looking specifically at 3A4 in vitro and in vivo. Genetic variability may play a role in the pharmacokinetics and pharmacodynamics of many medications, including methadone.

CONCLUSIONS

Drug interactions associated with methadone and their clinical significance are still poorly understood in general. Many tertiary drug information references and review articles report interactions associated with methadone in a general sense, much of which is theoretical and not verified by case reports, much less well-designed clinical trials. The majority of drug interaction reports that do exist were performed in the MMT population, which may differ significantly from chronic pain or cancer pain populations.

Differences in drug pharmacokinetics between East Asians and Caucasians and the role of genetic polymorphisms

Interethnic variability in pharmacokinetics can cause unexpected outcomes such as therapeutic failure, adverse effects, and toxicity in subjects of different ethnic origin undergoing medical treatment. It is important to realize that both genetic and environmental factors can lead to these differences among ethnic groups. The International Conference on Harmonization (ICH) published a guidance to facilitate the registration of drugs among ICH regions (European Union, Japan, the United States) by recommending a framework for evaluating the impact of ethnic factors on a drug's effect, as well as its efficacy and safety at a particular dosage and dosage regimen. This review focuses on the pharmacokinetic differences between East Asians and Caucasians. Differences in metabolism between East Asians and Caucasians are common, especially in the activity of several phase I enzymes such as CYP2D6 and the CYP2C subfamily. Before drug therapy, identification of either the genotype and/or the phenotype for these enzymes may be of therapeutic value, particularly for drugs with a narrow therapeutic index. Furthermore, these differences are relevant for international drug approval when regulatory agencies must decide if they accept results from clinical trials performed in other parts of the world.

Geographical/interracial differences in polymorphic drug oxidation. Current state of knowledge of cytochromes P450 (CYP) 2D6 and 2C19

The isoenzymes which catalyse the polymorphic hydroxylations of debrisoquine/sparteine and S-mephenytoin are cytochromes P450 2D6 and P450 2C19 (CYP2D6 and CYP2C19), respectively. CYP2D6 is involved in the stereospecific metabolism of several important groups of drugs, for example antiarrhythmics, antidepressants and neuroleptics. About 7% of Caucasians but only 1% of Orientals are poor metabolisers (PMs) of debrisoquine. The most common mutated allele CYP2D6B in Caucasian PMs is almost absent from their Oriental counterparts. On the other hand, the mean activity of CYP2D6 in Oriental extensive metabolisers (EMs) is lower than that in Caucasian EMs. This is due to the frequent distribution of a partially deficient CYP2D6 allele causing a Pro34-->Ser amino acid exchange in as many as 50% of Oriental alleles. This is the molecular genetic basis for slower metabolism of antidepressants and neuroleptics observed in Oriental compared with Caucasian people, and consequently for the lower dosages of these drugs used. While CYP2D6 catalyses the metabolism of lipophilic bases only, CYP2C19 is involved in the metabolism of acids (e.g. S-mephenytoin), bases (e.g. imipramine and omeprazole) and neutral drugs (e.g. diazepam). About 3% of Caucasians and 12 to 22% of Orientals are PMs of S-mephenytoin. Polymerase chain reaction-based genotyping techniques recently became available for the two CYP2C19 mutated alleles m1 and m2, which cause no enzyme to be expressed. M1 accounts for about 80% of the mutations responsible for the PM phenotypes in Caucasians, Oriental and Black people. Diazepam is partially demethylated by CYP2C19, and the high frequency of mutated alleles in Orientals is probably the reason why such populations have a slower metabolism and are treated with lower doses of diazepam than Caucasians. Omeprazole is to a major extent hydroxylated by CYP2C19, and there is an approximately 10-fold difference in oral clearance between EMs and PMs of S-mephenytoin. The separation of Caucasians from Orientals is fairly recent in the evolutionary process (40,000 to 60,000 years ago); the separation of Black from Caucasian/Oriental people occurred much earlier, about 150,000 years ago. As pronounced differences have been found between Caucasians and Orientals in the CYP2D6 and CYP2C19 enzymes, it might be expected that Black people will show even greater differences in this respect. Some studies have been performed with Black participants, but the picture is not clear. The mean CYP2D6 activity in Black EMs seems to be lower than that in Caucasian EMs and similar to that of Oriental EMs.(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency of human CYP2D6 mutant alleles in a normal Chinese population

Substantial differences in the pharmacogenetics of debrisoquine hydroxylation exist between Caucasians and Chinese populations. Among Chinese, not only is the frequency of the poor metabolizer phenotype low, the contribution of the 29B mutation in the CYP2D6 gene is insignificant. By contrast the frequency of D6-J is very common (approximately 0.6) and is clearly related to the activity of debrisoquine hydroxylase. Its presence however, does not predict the poor metabolizer phenotype. The D6-C mutation was also not detectable in our population.

Distinct phenotypes and genotypes of debrisoquine hydroxylation among Europeans and Chinese

Images

Genetic analysis of the interethnic difference between Chinese and Caucasians in the polymorphic metabolism of debrisoquine and codeine

The Far Eastern and Caucasian populations are strikingly different with respect to the debrisoquine/sparteine hydroxylation polymorphism. The number of poor metabolizers, as defined for Caucasians, is very low among Chinese and Japanese. We investigated the molecular basis for this difference by analysis of the CYP2D6 gene in 115 Chinese subjects, combined with phenotypic classification of codeine and debrisoquine metabolism. A correlation between the rates of metabolism of these two drugs and genotype, as analyzed by RFLP using XbaI, was observed among the Chinese. A high frequency (37%) of alleles indicative of gene insertions (reflected by XbaI 44kb fragments) was recorded in the Chinese, but was not associated with the poor metabolizer phenotype, as it is in Caucasians. PCR amplification of part of the CYP2D6 gene with mutation specific primers for CYP2D6A (29A) and CYP2D6B (29B) allelic variants revealed that the XbaI 44kb fragment in Chinese apparently contains a functional CYP2D6 gene, in contrast to the situation among Caucasians. The results provide a molecular explanation of the interethnic difference in the metabolism of drugs affected by the debrisoquine hydroxylation polymorphism.

A comparison of the pharmacokinetics of codeine and its metabolites in healthy Chinese and Caucasian extensive hydroxylators of debrisoquine

1. The kinetics of codeine and metabolites were studied in eight unrelated healthy Chinese subjects following a single oral dose of 50 mg codeine phosphate. The data were compared with those from eight Caucasian subjects who were matched with the Chinese group according to their metabolic ratio (MR) of debrisoquine. 2. Mean values of Cmax (445 nmol l-1) and AUC (1660 nmol l-1 h) of codeine in the Chinese were significantly higher than those in the Caucasians (292 nmol l-1 and 1010 nmol l-1 h). Thus plasma clearance was significantly lower (P less than 0.02) and the plasma half-life was longer (P less than 0.05) in the Chinese. 3. Partial clearance by glucuronidation was significantly lower (0.79 +/- 0.14 s.d. vs 1.42 +/- 0.48 s.d. 1 h-1 kg-1) in Chinese than in Caucasians. 4. The total urinary recovery of drug-related material in 48 h urine was similar in Chinese (82.2%) and Caucasians (84.4%). The recovery of unchanged codeine was significantly higher in Chinese (5.7%) than in Caucasians (3.3%). 5. The AUC ratios of codeine relative to its 6-glucuronide, morphine and norcodeine were 1:9, 35:1 and 4:1, respectively in Chinese. The corresponding ratios in Caucasians were 1:15, 50:1 and 6:1. 6. There was no significant difference between Chinese and Caucasians in the renal clearances of codeine and its primary metabolites. 7. Large interethnic differences in the kinetics of codeine have been shown. The Chinese are less able to metabolise codeine mainly because of a lower efficiency in glucuronidation.

Unimodal distribution of the metabolic ratio for debrisoquine in Cuna Amerindians of Panama

1. The metabolic oxidation of debrisoquine (DB) was studied in 89 non-related Cuna Amerindian subjects. 2. Means and standard deviations for urinary recoveries of the intact drug and its 4-hydroxy metabolite (4-HD) were: %DB: 6.8 +/- 4.5; %4-HD: 16.0 +/- 9.1; %sum: 22.8 +/- 12.0. The log10 metabolic ratios for DB (LMRDB) were distributed within a single mode of insignificant skewness (-0.01, P greater than 0.10), which was unimodal (log kernel density and maximum likelihood methods) and normal (chi 2 = 22.5; d.f. = 15; P greater than 0.09; power of the test greater than 80%). 3. Therefore, no poor metabolizers (95% C.I.: 0.1%, 5.2%) were detected in the population sample studied.

Evidence for the polymorphic oxidation of debrisoquine in the Thai population

Debrisoquine polymorphism has been studied extensively in Caucasian populations. The prevalence of the poor metaboliser phenotype is 3-10% in European and American Caucasian populations but appears to be very low in Asian populations. This study was carried out to determine the metabolic oxidation status in 173 Thai subjects. Phenotyping was performed using the metabolic ratio (MR) calculated as the 0-8 h urinary output of debrisoquine/0-8 h urinary output of 4-hydroxydebrisoquine after oral administration of 10 mg debrisoquine hemisulphate. Two subjects (1.2%) were phenotyped as poor metabolisers; they had MR values of 13.17 and 92.04. The incidence of the poor metaboliser phenotype of debrisoquine oxidation of 1.2% seems to be lower in the Thai population compared with that in various Caucasian populations.

Interindividual and interethnic differences in the demethylation and glucuronidation of codeine

1. The 8 h urinary excretion of codeine and seven of its metabolites was compared in 149 healthy Swedish Caucasians and 133 healthy Chinese following a single oral dose of 25 mg codeine phosphate. 2. The total 8 h urinary recovery of drug-related material was 74 +/- 24% in the Caucasians and 60 +/- 14% in the Chinese (P less than 0.001). The excretion of unchanged codeine was significantly higher in the Chinese (7.2%) compared with the Caucasians (4.3%, P less than 0.001). 3. The Caucasians excreted significantly greater proportions of codeine-6-glucuronide (C6G) (62%) than the Chinese (44%) (P less than 0.001). The frequency distribution of the log metabolic ratio (MR) for glucuronidation (codeine/C6G) was shifted towards higher values in the Chinese population. Males in both groups and Chinese smokers had significantly lower glucuronidation MRs than females and non-smokers in the respective populations (P less than 0.001). 4. The frequency distribution of the MR for O-demethylation (codeine/morphine (M) + M-3 and M-6-glucuronide (M3G and M6G) + normorphine (NM) was highly skewed in the Caucasians, suggestive of a bimodal distribution. There was a 160-fold interindividual variation in this MR. A unimodal distribution of the log O-demethylation MR was observed in Chinese. The Caucasians excreted less M and more M6G than did the Chinese (P less than 0.001). 5. Significantly more norcodeine (NC) and less NC-glucuronide (NCG) were excreted in the Chinese compared with the Caucasians (P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical significance of the sparteine/debrisoquine oxidation polymorphism

The sparteine/debrisoquine oxidation polymorphism results from differences in the activity of one isozyme of cytochrome P450, the P450db1 (P450 IID1). The oxidation of more than 20 clinically useful drugs has now been shown to be under similar genetic control to that of sparteine/debrisoquine. The clinical significance of this polymorphism may be defined by the value of phenotyping patients before treatment. The clinical significance of such polymorphic elimination of a particular drug can be analyzed in three steps: first, does the kinetics of active principle of a drug depend significantly on P450db1?; second, is the resulting pharmacokinetic variability of any clinical importance?; and third, can the variation in response be assessed by direct clinical or paraclinical measurements? It is concluded from such an analysis that, in general, the sparteine/debrisoquine oxidation polymorphism is of significance in patient management only for those drugs for which plasma concentration measurements are considered useful and for which the elimination of the drug and/or its active metabolite is mainly determined by P450db1. At present, this applies to tricyclic antidepressants and to certain neuroleptics (e.g. perphenazine and thioridazine) and antiarrhythmics (e.g. propafenone and flecainide). Phenotyping should be introduced in to clinical routine under strictly controlled conditions to afford a better understanding of its potentials and limitations. The increasing knowledge of specific substrates and inhibitors of P450db1 allows precise predictions of drug-drug interactions. At present, the strong inhibitory effect of neuroleptics on the metabolism of tricyclic antidepressants represents the best clinically documented and most relevant example of such an interaction.