Cytochrome P450 System and Drug Interactions

The cytochrome P450 (CYP) enzymes are a superfamily of heme-containing monooxygenases that catalyse most oxidative drug metabolism in the human liver and intestine. A small number of isoforms — notably CYP3A4, CYP2D6, CYP2C9, CYP2C19, and CYP1A2 — handle the bulk of clinically used drugs. Because many drugs share these enzymes and can inhibit or induce them, the CYP system is the central mechanistic basis for metabolic drug-drug interactions.

Definition

The cytochrome P450 system is a superfamily of heme-thiolate monooxygenase enzymes that oxidise drugs and other xenobiotics using molecular oxygen and NADPH; shared use, inhibition, and induction of its isoforms underlie most metabolic drug-drug interactions.

Scope

The topic covers the structure and catalytic cycle of cytochrome P450 enzymes, the major human drug-metabolising isoforms and their nomenclature, and the mechanisms of CYP inhibition and induction that produce drug-drug interactions. It treats the CYP system as a chemical and pharmacological topic; it describes how interactions arise and is not clinical dosing guidance.

Core questions

• How does a cytochrome P450 enzyme oxidise a drug at the molecular level?
• Which human CYP isoforms metabolise most clinically used drugs?
• How are CYP enzymes named and organised into families and subfamilies?
• How do enzyme inhibition and induction cause drug-drug interactions?
• Why do CYP-mediated interactions differ so much between individuals?

Key concepts

• Heme-thiolate monooxygenase
• CYP catalytic cycle
• Major isoforms (CYP3A4, 2D6, 2C9, 2C19, 1A2)
• CYP nomenclature (family/subfamily)
• Enzyme inhibition (reversible and mechanism-based)
• Enzyme induction
• Drug-drug interactions
• Substrates, inhibitors, and inducers

Mechanisms

Each cytochrome P450 enzyme contains a heme iron coordinated by a cysteine thiolate; in its catalytic cycle the enzyme binds substrate, accepts electrons from NADPH via cytochrome P450 reductase, activates molecular oxygen, and inserts one oxygen atom into the substrate while reducing the other to water. The human genome encodes many CYP genes organised by sequence identity into families (sharing >40%) and subfamilies (>55%), but only a handful — chiefly CYP3A4, CYP2D6, CYP2C9, CYP2C19, and CYP1A2 — account for most drug oxidation. Interactions arise when one drug inhibits a CYP isoform (competitively, or irreversibly through mechanism-based inactivation) and thereby raises the concentration of a co-administered substrate, or induces the isoform (typically by activating nuclear receptors that increase enzyme expression) and thereby lowers substrate concentrations. The magnitude of these effects varies with the isoform, the drugs involved, and individual enzyme activity.

Clinical relevance

The cytochrome P450 system explains why combining certain drugs can substantially raise or lower their blood levels, which is why CYP substrate, inhibitor, and inducer profiles are evaluated during drug development and interaction prediction. Knowledge of the major isoforms also frames how genetic differences and other factors alter metabolism. This entry describes those mechanisms as reference knowledge and does not provide individualised dosing or interaction-management instructions.

Evidence & guidelines

Characterisation of CYP enzymes draws on enzymology, recombinant-enzyme and microsome studies, and human pharmacokinetic interaction studies, with gene and isoform nomenclature maintained by a standing committee. Regulatory drug-interaction guidance (for example from the US FDA and EMA) specifies how CYP-based interaction potential should be studied during development, but this topic entry is an educational overview rather than a protocol.

History

Cytochrome P450 was named in the early 1960s for a microsomal pigment whose carbon-monoxide complex absorbs light at 450 nm, and its role as the oxygen-activating catalyst of drug oxidation was established over the following decade. The cloning and sequencing of CYP genes from the 1980s onward led to a systematic family-and-subfamily nomenclature, and recognition of the small set of dominant human drug-metabolising isoforms made the system the organising framework for predicting metabolic drug-drug interactions.

Key figures

• F. Peter Guengerich
• David R. Nelson
• Daniel W. Nebert
• Jiunn H. Lin

Seminal works

• guengerich-2001
• nelson-2004

Frequently asked questions

Why does CYP3A4 matter so much for drug interactions?

CYP3A4 is the most abundant hepatic and intestinal CYP isoform and metabolises a large fraction of marketed drugs, so inhibitors or inducers of CYP3A4 can affect the levels of many co-administered medicines.

What is the difference between enzyme inhibition and induction?

Inhibition reduces a CYP enzyme's activity and tends to raise substrate drug levels, often within hours, whereas induction increases enzyme amount and activity over days and tends to lower substrate levels.