Enzyme Inhibition and Induction

Enzyme inhibition and induction are the two mechanisms by which one drug changes the metabolic clearance of another by acting on drug-metabolizing enzymes, chiefly the cytochrome P450 (CYP) family. Inhibition reduces an enzyme's activity and raises the exposure of its substrates; induction increases enzyme amount or activity and lowers substrate exposure. Together they account for a large share of clinically important pharmacokinetic drug interactions.

Definition

Enzyme inhibition is a decrease in the catalytic activity of a drug-metabolizing enzyme caused by another agent, raising substrate concentrations; enzyme induction is an increase in enzyme expression or activity, usually mediated by nuclear receptors, that accelerates substrate metabolism and lowers substrate concentrations.

Scope

This topic covers the molecular basis of competitive, mechanism-based (irreversible), and metabolite-based inhibition; the transcriptional induction of metabolizing enzymes through nuclear receptors such as PXR and CAR; the resulting direction and time course of changes in substrate exposure; and why these effects differ between enzymes. It is treated as a metabolic-interaction topic for reference, not as dosing guidance.

Core questions

• How do competitive and mechanism-based inhibition differ in onset and reversibility?
• How do nuclear receptors such as PXR and CAR drive enzyme induction?
• Why does inhibition act quickly while induction develops over days?
• How does the affected enzyme determine which substrates are involved?

Key concepts

• Cytochrome P450 (CYP) enzymes
• Reversible (competitive) inhibition
• Mechanism-based (irreversible) inhibition
• Enzyme induction
• Nuclear receptors PXR and CAR
• Substrate, inhibitor, and inducer roles
• UDP-glucuronosyltransferases (UGTs)
• Time course of inhibition versus induction

Mechanisms

Inhibition occurs when one drug occupies or inactivates an enzyme that metabolizes another. Reversible inhibition - most often competitive - depends on the inhibitor's concentration and reverses as it is cleared, so the interaction appears and resolves quickly. Mechanism-based (irreversible) inhibition forms a stable enzyme-inhibitor complex, so activity returns only as new enzyme is synthesized, giving a longer-lasting effect (Tanaka, 1998; Wilkinson, 2005). Induction works through gene regulation: xenobiotics activate nuclear receptors such as the pregnane X receptor (PXR) and constitutive androstane receptor (CAR), which upregulate transcription of CYP and conjugating enzymes; because it requires new protein synthesis, induction builds up and wanes over days to weeks (Tolson & Wang, 2010). Analogous inhibition and induction also affect phase II conjugating enzymes such as the UGTs (Miners et al., 2023).

Clinical relevance

Inhibition and induction explain why adding or stopping one medicine can change the exposure of another, a recurring theme in drug-interaction appraisal and pharmacovigilance. This entry describes the mechanisms and their typical direction and time course for reference and education; it is not a source of dosing or individualized treatment advice.

Evidence & guidelines

Understanding of these mechanisms rests on narrative and mechanistic reviews of cytochrome P450 and conjugating-enzyme biology and of nuclear-receptor-mediated induction. Regulatory agencies maintain dedicated guidance on in vitro and clinical evaluation of metabolism-based drug interactions, which informs how inhibitors and inducers are classified.

History

As individual cytochrome P450 isoforms were characterised from the 1980s onward, distinct substrates, inhibitors, and inducers were mapped to each enzyme, turning anecdotal interaction reports into a mechanistic framework. The later discovery that nuclear receptors PXR and CAR govern the induction of these enzymes provided the molecular explanation for the slower, transcription-dependent arm of metabolic interactions.

Key figures

• Grant R. Wilkinson
• Hongbing Wang
• Eiji Tanaka
• John O. Miners

Seminal works

• wilkinson-2005
• tolson-wang-2010
• tanaka-1998

Frequently asked questions

Why does enzyme inhibition act faster than induction?

Inhibition takes effect as soon as the inhibitor reaches the enzyme, but induction requires the cell to make new enzyme protein, so its onset and offset take days to weeks.

What is the difference between competitive and mechanism-based inhibition?

Competitive inhibition is reversible and fades as the inhibitor is cleared, whereas mechanism-based inhibition permanently inactivates the enzyme, so normal activity returns only after new enzyme is synthesized.