When discussing PGx, we classify a person according to their phenotype (their metabolic capacity for a given enzyme).
A poor metabolizer (PM) is a person who lacks a fully functional enzyme or has reduced expression of the enzyme and, therefore, exhibits decreased metabolism of the drug(s) that are substrates for that CYP enzyme. This person would require lower doses of the drug. A PM who receives a standard dose is more likely to experience unwanted side effects or toxicity. A PM can also experience diminished effects with pro-drugs. Pro-drugs need to be metabolized into active compounds before they are effective.
An intermediate metabolizer (IM) has one wild-type (normal) copy of the gene and one absent or dysfunctional copy. The IM group is very heterogeneous.
A person with normal enzyme activity is called a 'wild type' or an extensive metabolizer (EM). This person should respond to standard drug dosages. Most people are EMs, and this is the population in which most dosing regimens have been developed in clinical trials.
An ultrarapid metabolizer (UM) will require a higher dose than usual since they have a polymorphism (mutation) that codes for a more efficient enzyme or more expression of an enzyme. They will eliminate the drug more quickly. A UM may resist standard treatments, and adjusting the dosage before therapy is achieved may take some time.
