Using population pharmacokinetic analyses of drugs metabolized by CYP2D6 to study the genotype-phenotype translation

Cytochrome P450 2D6 (CYP2D6) is an important drug-metabolizing enzyme exhibiting extensive interindividual variability predominantly caused by genetic polymorphism. Predicting CYP2D6 function based on genotype may be used to personalize pharmacotherapy, but the process of translating CYP2D6 genotype into predicted phenotype is complex and has suffered from a lack of consensus. The Clinical Pharmacogenetics Implementation Consortium and Dutch Pharmacogenetics Working Group have proposed a standardized translation scheme based on the activity score system aiming to facilitate more consistent CYP2D6 genotype-phenotype translation. However, this system remains suboptimal particularly with regards to decreased function alleles and substrate-specific behaviour. This review summarizes the process and challenges for functional assignment of CYP2D6 alleles. We discuss population pharmacokinetics (popPK) as a tool for estimating CYP2D6 function and present findings from three popPK meta-analyses quantifying the impact of individual CYP2D6 alleles in the metabolism of vortioxetine, tedatioxetine and brexpiprazole. Findings from these analyses indicate that the activity values currently assigned to decreased function alleles CYP2D6*9, *17 and *41 overestimate their function. Moreover, the CYP2D6*2 allele exhibited reduced activity in the metabolism of brexpiprazole, indicating substrate-specific behaviour. Considering the totality of the evidence, the activity score system may be further refined to better reflect the enzyme function associated with these alleles.