Enzyme Induction and Drug Metabolism
Learn how drugs induce metabolic enzymes, altering their own or other drugs' clearance.
## Introduction
Enzyme induction is the increased synthesis of drug-metabolizing enzymes—primarily cytochrome P450s, UDP-glucuronosyltransferases, and drug transporters—in response to xenobiotic exposure. Induction accelerates drug metabolism, reduces plasma levels of co-administered drugs, and can cause serious therapeutic failure.
## Molecular Mechanism
Most induction occurs at the transcriptional level through ligand-activated nuclear receptors. The inducing drug binds a nuclear receptor, which heterodimerizes with RXR and binds xenobiotic response elements in CYP gene promoters:
- **PXR (Pregnane X Receptor)**: The master regulator of CYP3A4 (metabolizes ~50% of drugs), CYP2C9, CYP2C19, and the P-gp efflux transporter. Activated by rifampin (the most potent clinical inducer), St. John’s wort, dexamethasone, and phenytoin.
- **CAR (Constitutive Androstane Receptor)**: Regulates CYP2B6, CYP2C9, and Phase II UGT enzymes. Activated by phenobarbital and phenytoin through an indirect mechanism involving nuclear translocation.
- **AhR (Aryl Hydrocarbon Receptor)**: Induces CYP1A1, CYP1A2, and CYP1B1. Activated by polycyclic aromatic hydrocarbons in cigarette smoke, chargrilled food, and by omeprazole at high doses.
## Time Course
Induction is gradual because it requires transcription, translation, and accumulation of new enzyme protein. Onset takes 2–3 days, and maximal effect occurs at 1–2 weeks (approximately 3–5 enzyme protein half-lives). After removing the inducer, enzyme levels return to baseline over a similar timeframe as existing enzyme protein is degraded.
## Auto-Induction
Some drugs induce their own metabolism. Carbamazepine's half-life decreases from ~36 hours after the first dose to ~16 hours at steady state due to CYP3A4 auto-induction. This means doses that are adequate initially may become subtherapeutic after 2–4 weeks, requiring upward titration.
## Major Clinical Interactions
- **Rifampin + oral contraceptives**: CYP3A4 induction reduces ethinyl estradiol levels, causing contraceptive failure
- **Rifampin + warfarin**: CYP2C9 induction accelerates warfarin metabolism, dropping INR and risking thrombosis
- **Smoking + clozapine**: CYP1A2 induction lowers clozapine levels; smoking cessation reverses induction and can cause toxicity
- **St. John’s wort + cyclosporine**: PXR-mediated CYP3A4 induction risks transplant rejection
## Key Takeaways
- Enzyme induction increases CYP synthesis via PXR, CAR, and AhR nuclear receptors
- Onset takes days; maximal effect requires 1–2 weeks of continuous exposure
- Rifampin is the most potent clinical inducer, affecting CYP3A4 and many transporters
- Induction can cause therapeutic failure of oral contraceptives, warfarin, and immunosuppressants