CYP2D6 Metabolizer Status Determination

Genetic Testing and Metabolizer Phenotypes

Genetic testing is the most reliable way to determine if an individual is a poor metabolizer of the CYP2D6 enzyme, as it can identify the presence of two inactive enzyme gene alleles that characterize poor metabolizer status, according to the American College of Medical Genetics 1, 2
The CYP2D6 enzyme has four main metabolizer phenotypes: Poor Metabolizer (PM), Intermediate Metabolizer (IM), Extensive Metabolizer (EM), and Ultra-rapid Metabolizer (UM), with each phenotype having distinct genetic characteristics 1
FDA-approved tests, such as the Roche AmpliChip® CYP450 Test, can identify CYP2D6 genotypes and predict metabolizer status, as recommended by the FDA 2, 3
Laboratory-developed tests can also identify specific allelic variants in the CYP2D6 gene that determine enzyme activity, according to the Mayo Clinic 4, 5

Poor metabolizers may experience higher than expected drug concentrations and increased risk of adverse reactions with standard doses of CYP2D6-metabolized medications, such as SSRIs like fluoxetine and paroxetine, as reported by the Mayo Clinic 4, 5
Relying solely on clinical response can be misleading, as drug interactions, diet, and other factors can affect metabolism independently of genetic status, according to the American College of Medical Genetics 3, 6
Phenocopying, where some medications can inhibit CYP2D6 and make an extensive metabolizer temporarily function as a poor metabolizer, should be considered when evaluating metabolizer status, as noted by the Mayo Clinic 4, 5

Genetic testing for CYP2D6 may be valuable in specific clinical scenarios, such as before starting medications with narrow therapeutic windows that are primarily metabolized by CYP2D6, as recommended by the Mayo Clinic 4
Testing may also be considered when taking multiple medications that are substrates or inhibitors of CYP2D6, according to the Mayo Clinic 5
The American Heart Association recommends that genetic testing for CYP2D6 not be used as a routine screening test for all patients, but rather in specific clinical scenarios to optimize medication therapy and reduce adverse effects, as published in Circulation 7