Polymorphism of dextromethorphan metabolism: relationships between phenotype, genotype and response to the administration of encainide in humans.

The polymorphism of dextromethorphan and encainide metabolism is genetically determined and is related to the activity of hepatic CYP2D6. In order to examine the relations between CYP2D6 phenotype, genotype and the electrocardiographic response to the oral administration of encainide, 110 healthy subjects were studied. Metabolic ratios were calculated in urine after oral administration of 40 mg of dextromethorphan and in plasma obtained 2.5 h after oral administration of 50 mg of encainide. Encainide-induced electrocardiographic changes were measured 2.5 h after oral administration of the drug. Genotype was determined in 52 subjects. Results showed that phenotype, either extensive or poor metabolizer, for CYP2D6-dependent metabolism could be identified from the dextromethorphan metabolic ratio calculated in urine, from the encainide metabolic ratio calculated in plasma and from the genotype. However, despite the fact that the changes in atrioventricular (PR) and intraventricular (QRS) conduction times produced by encainide were different in extensive and poor metabolizer subjects and correlated with CYP2D6 activity, the electrocardiographic response was never 100% specific and sensitive for the identification of either phenotype. Moreover, genotypic identification of heterozygous and homozygous extensive metabolizer subjects did not predict CYP2D6 activity, as determined by dextromethorphan and encainide metabolic ratios, or encainide response, as determined by intraventricular and atrioventricular changes. Thus, CYP2D6 activity does not fully predict the electrocardiographic effects of encainide, and genotype, as determined in our study, cannot replace the determination of metabolic ratio in predicting CYP2D6 activity and encainide response in extensive metabolizer subjects.