The highly polymorphic nature of the CYP2D6 gene is a great source of variability in CYP2D6 drug substrates responses. The current phenotypic classification of CYP2D6 includes poor, intermediate, extensive or ultra-rapid metabolizers of CYP2D6 drug substrates.
- Poor metabolizers have a low or no enzyme activity and are predicted by the presence of 2 non-functional alleles (activity score 0).
- Intermediate metabolizers have reduced enzymatic activity and are predicted by the presence of two reduced activity alleles, or one reduced function allele et one non-functional allele, or one functional allele and one non-functional allele (activity score 0.5-1). Based on the specific context, the phenotype may overlap with a poor metabolizer or an extensive metabolizer.
- Extensive metabolizers have normal enzymatic activity and are predicted by the presence of 2 functional alleles, or one functional allele and one reduced function allele (activity score 1-2).
- Ultra-rapid metabolizers have increased enzymatic activity, and are predicted by the presence of more than 2 functional alleles (activity score >2).