Phenotypic debrisoquine 4-hydroxylase activity among extensive metabolizers is unrelated to genotype as determined by the Xba-I restriction fragment length polymorphism

Determination of dextromethorphan metabolic phenotype by salivary analysis with a reference to genotype in Chinese patients receiving renal hemodialysis

BACKGROUND

The polymorphic metabolism of debrisoquin and sparteine by cytochrome P450IID6 (CYP2D6) is genetically determined. Determination of the CYP2D6 metabolic phenotype with conventional urine analytic methods is not feasible in anuric patients with renal failure. The possibility of using salivary analysis, with dextromethorphan as a probe drug, to determine the CYP2D6 metabolic phenotype in patients with renal failure was evaluated.

METHODS AND RESULTS

One hundred four Chinese patients with renal failure were recruited. All 104 patients were receiving hemodialysis. Saliva was collected before and at 3 hours after each patient took a capsule of dextromethorphan hydrobromide (30 mg). Four patients were excluded because of insufficient samples of saliva. The distribution of logarithms of the metabolic ratios (log[MR]) in the 100 patients appeared to be normal. Administration of quinidine sulfate (200 mg twice daily) to nine of the patients significantly and markedly increased the dextromethorphan metabolic ratios. The metabolic ratios of nine patients pretreated with quinidine were higher than any of the 100 patients with renal failure who did not receive quinidine pretreatment. A metabolic ratio of 33 separated these two groups. Genomic deoxyribonucleic acid was extracted from whole blood in a subset of patients. Polymerase chain reaction (PCR)-based methods were used to detect the CYP2D6 and B mutant genes. Mutant B alleles (which are common in white poor metabolizers) of CYP2D6 genes were not detected in any of the 47 subjects tested. A PCR-based test of cytosine (C188) to thymine (T188) polymorphism at 188 base pairs in exon 1 of CYP2D6 genes was performed in 61 patients. Subjects who were homozygous for C188 had significantly (p = 0.0067) lower log[MR] values than those who were homozygous for T188.

CONCLUSIONS

Determination of dextromethorphan metabolic ratios in saliva is feasible in patients with renal failure requiring hemodialysis. All subjects in this study appeared to be "extensive metabolizer" phenotype for CYP2D6, and no poor metabolizer was identified. From the results with quinidine pretreatment, a metabolic ratio of 33 is suggested to be a tentative antimode for identification of poor metabolizers in patients with renal failure.

Molecular heterogeneity of the XbaI defined 44kb allele of the CYP2D locus within the Caucasian population

1. Cytochrome P450 debrisoquine (CYP2D6) activity is polymorphic and under genetic control. Most Caucasians are extensive metabolizers, but 5%-10% are poor metabolizers. 2. Restriction fragment length polymorphism analysis of the CYP2D6 locus identifies a 29kb XbaI fragment, either normal (D6-wt) or mutated, and three mutated XbaI alleles (44kb, 11.5kb and 16 + 9kb). The 44kb allele was initially considered as a poor metabolizer allele owing to a D6-B mutation, but cases of 44kb allele not carrying the D6-B, and therefore potentially functional, have been found. The degree of molecular heterogeneity of this allele was investigated by phenotype and genotype analysis of families. 3. Thirty-one French Caucasian families, representing 117 individuals, possessing at least one 44kb allele in each family were selected. Phenotypes were determined using dextromethorphan, and the XbaI, NcoI and BamH1 RFLPs of 42 independent chromosomes were analyzed. 4. 80% of the XbaI 44kb alleles carried the CYP2D6-B mutation and had an additional NcoI fragment (12.5kb or 4.8kb). The remaining 20% did not carry the CYP2D6-B or A mutations and had no extra NcoI fragment. 5. Information on three families demonstrated that 44kb alleles not carrying the CYP2D6-B mutation were associated with the extensive metabolizer phenotype. 6. We conclude that a substantial percentage of XbaI 44kb alleles is associated with a functional CYP2D gene, and therefore, that the XbaI 44kb allele is not consistently a poor metaboliser allele.