What does it mean to be a poor metabolizer?

It is a predicted phenotype indicating that, based on the variant alleles a person carries, the relevant enzyme is expected to have little or no activity, so substrate drugs are metabolized very slowly; it is a classification of metabolic capacity, not a treatment instruction.

Can a person's genotype-predicted phenotype differ from how they actually metabolize a drug?

Yes. The phenotype is a prediction from genotype, and factors such as interacting drugs that inhibit or induce the enzyme, liver or kidney disease, and age can shift the observed metabolic activity away from the genotype-based prediction.

Metabolizer Phenotypes

A metabolizer phenotype is a summary label - poor, intermediate, normal, or ultrarapid - that describes how fast a person is predicted to metabolize the substrates of a particular enzyme, based on the combination of variant alleles they carry. These categories translate a complex genotype into an interpretable statement about metabolic capacity. This topic addresses how genotype is mapped to a metabolizer phenotype and how those phenotype terms have been standardized.

Definition

A metabolizer phenotype is a categorical description of an individual's predicted enzyme activity for a given pharmacogene - conventionally poor, intermediate, normal, or ultrarapid metabolizer - inferred from the function of the two inherited alleles (the diplotype).

Scope

The topic covers the standard metabolizer-phenotype categories, the principle of inferring phenotype from a diplotype (the pair of inherited alleles), the activity-score approach used for genes such as CYP2D6, and the effort to harmonize terminology so that laboratories report results consistently. It is reference material on classification and does not provide dosing instructions.

Core questions

How is an individual's genotype translated into a predicted metabolizer phenotype?
What do the categories poor, intermediate, normal, and ultrarapid metabolizer mean?
How does the activity-score method assign a phenotype from a diplotype?
Why is standardized phenotype terminology important across laboratories?

Key concepts

Diplotype (pair of inherited alleles)
Allele function assignment (no, decreased, normal, increased)
Activity score
Poor, intermediate, normal, and ultrarapid metabolizer categories
Genotype-to-phenotype translation
Standardized result terminology

Mechanisms

Each variant allele of a metabolizing gene is assigned a function - no function, decreased function, normal function, or increased function. Combining the two alleles a person carries (their diplotype) yields a predicted overall activity, which for some genes is captured numerically as an activity score and then mapped to a phenotype category. A person carrying two non-functional alleles is predicted to be a poor metabolizer, one or two normal alleles give a normal metabolizer, gene duplications can produce an ultrarapid metabolizer, and intermediate combinations fall between. Because this is a prediction from genotype, the observed phenotype can still be modified by drug interactions, organ function, and other factors.

Clinical relevance

Metabolizer phenotypes are the practical output of pharmacogenetic testing for metabolizing genes and are the form in which genotype information is communicated for interpretation. As reference content, this topic explains what the categories mean and how they are derived; it does not state what dose any phenotype should receive, which is determined by validated clinical guidelines applied by qualified clinicians to the whole clinical situation.

Epidemiology

The proportion of people in each metabolizer category varies widely by ancestral population, because the underlying allele frequencies differ. For some enzymes a notable fraction of certain populations are poor or ultrarapid metabolizers, which is why population data accompany phenotype interpretation.

History

The concept of distinct metabolizer groups arose from early twin and family studies showing bimodal drug-metabolism distributions. As genotyping replaced direct metabolic testing, the field needed consistent ways to translate genotypes into phenotype labels, leading to the activity-score system for CYP2D6 and to a 2017 Clinical Pharmacogenetics Implementation Consortium consensus standardizing the terms used in test reports.

Debates

How should the intermediate metabolizer category be defined?: Different schemes have drawn the activity-score boundaries between poor, intermediate, and normal metabolizers differently, and harmonization efforts have sought consensus so that the same genotype yields the same phenotype label across laboratories.

Key figures

Kelly Caudle
Teri Klein
Mary Relling
Andrea Gaedigk

Seminal works

caudle-2017
weinshilboum-2003

Frequently asked questions

What does it mean to be a poor metabolizer?: It is a predicted phenotype indicating that, based on the variant alleles a person carries, the relevant enzyme is expected to have little or no activity, so substrate drugs are metabolized very slowly; it is a classification of metabolic capacity, not a treatment instruction.
Can a person's genotype-predicted phenotype differ from how they actually metabolize a drug?: Yes. The phenotype is a prediction from genotype, and factors such as interacting drugs that inhibit or induce the enzyme, liver or kidney disease, and age can shift the observed metabolic activity away from the genotype-based prediction.