Clinical Information

The cytochrome P450 (CYP) family of enzymes is responsible for primary metabolism of many drugs. CYP450s are oxidative/dealkylating enzymes localized in the microsomes of many tissues including the intestines and liver. One of the CYP enzymes, CYP2D6, is wholly or partially responsible for the metabolism of many commonly prescribed drugs such as some analgesics, anticonvulsants, antidepressants, antipsychotics, antiemetics, antihypertensives, antiestrogens, antineoplastics, antipsychotics, antiretrovirals, antitussives, beta-blockers, cardioactive drugs, H-2 blockers, stimulants, sympathomimetic and other drug classes.

The CYP2D6 gene is highly variable with over 100 named alleles. The gene may be deleted, duplicated, and multiplied, and can have multiple sequence variations. In addition, some individuals have genes that are hybrids of CYP2D6 and the CYP2D7 pseudogene. Some individuals have altered CYP2D6 variants that result in synthesis of enzyme devoid of catalytic activity, or an enzyme with diminished catalytic activity. These individuals may process CYP2D6-metabolized medications more slowly depending upon the gene variant found on each chromosome. CYP2D6 duplications and multiplications involving active alleles may result in ultrarapid metabolism of CYP2D6-metabolized drugs. CYP2D6 genotype results are used to predict ultrarapid, ultrarapid to extensive (normal), extensive (normal), extensive (normal) to intermediate, intermediate, intermediate to poor, and poor metabolizer phenotypes for a sample.(see Table 1)

Table 1. Enzyme Activity of Individual Star Alleles

CYP2D6 phenotype is predicted based upon the number of functional, partially functional, and nonfunctional alleles present in a sample.(see Table 2)

Table 2. Phenotype Assignment of CYP2D6

* Phenotyping was derived from the Human Cytochrome P450 (CYP) Allele Nomenclature Committee website and the PharmGKB website for the related Clinical Pharmacogenetics Implementation Consortium guidelines.

**Ultra-Rapid Metabolizer, UM; Extensive Metabolizer, EM; Intermediate Metabolizer, IM; Poor Metabolizer, PM

There are instances where a phenotype prediction is not categorical and, in these instances, a range of possible phenotypes will be given. It should be noted that other laboratories may use different phenotype prediction methods as there is no consensus on this at this time. However, the method used here represents the findings of the majority of literature available at this time. Individuals without a detectable gene alteration will have the predicted phenotype of an extensive drug metabolizer and are designated as CYP2D6 *1/*1.

Dosing drugs that are metabolized by CYP2D6 may require adjustment based on the individual patient's genotype. Patients who are poor metabolizers may require lower than usual doses to achieve optimal response in the case of drugs that are inactivated by the CYP2D6 enzyme, and higher than usual doses in the case of drugs that are activated by CYP2D6 enzyme. Alternatively, patients who are ultrarapid metabolizers may benefit from increased doses in the case of drugs that are inactivated by CYP2D6 enzyme, and lower doses in the case of drugs that are activated by CYP2D6. In the absence of clear guidance from FDA on dosing for various metabolizer phenotypes, patients with either ultrarapid or poor metabolism may benefit from therapeutic drug monitoring or switching to another comparable drug that is not primarily metabolized by CYP2D6.