Drug-Metabolizing Enzyme Variants

Most drugs are chemically transformed before they leave the body, largely by a set of metabolizing enzymes dominated by the cytochrome P450 (CYP) family. Inherited variation in the genes encoding these enzymes alters how quickly a drug is broken down or activated, and is one of the best-characterized sources of variable drug response. This topic covers the principal drug-metabolizing enzyme variants and the metabolizer phenotypes they define.

Definition

Drug-metabolizing enzyme variants are inherited differences in the genes encoding the enzymes that biotransform drugs — most prominently the cytochrome P450 enzymes — that increase, decrease, or abolish enzyme activity and thereby change a drug's metabolism and resulting exposure.

Scope

The entry covers the major phase I (oxidative, chiefly cytochrome P450) and phase II (conjugating) drug-metabolizing enzymes, the functional consequences of common variant alleles, and the poor, intermediate, normal, and ultrarapid metabolizer phenotypes derived from genotype. It is a reference overview and does not give drug-specific dosing instructions.

Core questions

• Which enzymes carry out most clinically relevant drug metabolism?
• How do variant alleles change enzyme activity?
• How is genotype translated into a metabolizer phenotype?
• Why does the same metabolizer status have opposite consequences for a prodrug versus an active drug?

Key concepts

• Cytochrome P450 (CYP) enzyme family
• Phase I and phase II metabolism
• Metabolizer phenotypes: poor, intermediate, normal, ultrarapid
• Star-allele nomenclature and activity scores
• Prodrug activation versus drug inactivation
• Gene duplication and deletion (copy-number variation)
• Phenoconversion by drug interactions

Mechanisms

Drug metabolism is conventionally divided into phase I reactions (often oxidation by cytochrome P450 enzymes such as CYP2D6, CYP2C9, CYP2C19, and CYP3A4/5) and phase II conjugation reactions (such as glucuronidation by UGT enzymes or thiopurine methylation by TPMT). Variant alleles can reduce or eliminate enzyme function, while gene duplication can increase it; these are summarized as activity scores and translated into metabolizer phenotypes. For a drug that is inactivated by an enzyme, reduced activity raises exposure and the risk of concentration-related toxicity, whereas for a prodrug that the enzyme activates, reduced activity lowers the formation of the active compound and may reduce efficacy — so the clinical direction depends on whether the enzyme inactivates or activates the drug (Ingelman-Sundberg, 2004; Evans & McLeod, 2003). Enzyme activity inferred from genotype can also be modified by co-administered inhibitors or inducers, a phenomenon termed phenoconversion.

Clinical relevance

Knowledge of enzyme-variant phenotypes helps explain why some patients experience reduced efficacy or concentration-dependent adverse effects at standard doses, and informs the rationale for genotype-aware appraisal. This entry is a reference description of mechanisms; it is not a basis for individual dosing decisions, which rely on validated guidelines and clinical assessment.

Epidemiology

Variant alleles of the major drug-metabolizing enzymes are common, and large sequencing studies show that their frequencies — and the resulting proportions of poor, intermediate, and ultrarapid metabolizers — differ substantially across worldwide populations, with many functionally important rare variants concentrated in particular ancestries (Zhou, Ingelman-Sundberg & Lauschke, 2017).

Evidence & guidelines

The Clinical Pharmacogenetics Implementation Consortium (CPIC) and related bodies publish peer-reviewed guidelines that translate metabolizer phenotypes for enzymes such as CYP2D6 and CYP2C19 into prescribing considerations for specific drug classes, providing graded, regularly updated reference standards (Hicks et al., 2015).

History

Inherited differences in drug metabolism were recognized from mid-twentieth-century observations of variable isoniazid acetylation and debrisoquine hydroxylation, the latter leading to the characterization of CYP2D6 polymorphism. Molecular cloning of the cytochrome P450 genes and, later, population-scale sequencing extended this into a detailed catalogue of enzyme-variant alleles and their global distribution.

Debates

How should rare and novel enzyme variants be classified?

Population sequencing reveals many rare variants of uncertain function that fall outside established star-allele systems, and methods for assigning their functional impact and incorporating them into phenotype prediction remain an active area of work.

Key figures

• Magnus Ingelman-Sundberg
• William Evans
• Mary Relling
• Volker Lauschke

Seminal works

• ingelman-sundberg-2004
• evans-mcleod-2003
• zhou-2017

Frequently asked questions

What does it mean to be a 'poor metabolizer'?

It describes a phenotype, inferred from genotype, in which the activity of a particular metabolizing enzyme is markedly reduced or absent, so drugs handled by that enzyme are cleared more slowly or, for prodrugs, activated less.

Why is CYP2D6 so frequently discussed in pharmacogenomics?

CYP2D6 metabolizes a large share of commonly used drugs, is highly polymorphic with alleles ranging from no activity to gene duplication, and therefore produces a wide spectrum of metabolizer phenotypes.