CYP3A4 inhibitors are drugs or substances that decrease the metabolic activity of the CYP3A4 enzyme, one of the most important cytochrome P450 enzymes in the liver and intestine.
Clinical relevance (key points)
- CYP3A4 metabolizes ~30–50% of commonly used drugs.
- Inhibition → reduced clearance of CYP3A4 substrates → increased plasma concentrations, higher AUC, and greater risk of dose-related toxicity.
- Effects can be immediate with reversible inhibitors; delayed and prolonged with mechanism-based (irreversible) inhibitors.
Common inhibitors (high yield)
- Strong: Clarithromycin, erythromycin, ketoconazole, itraconazole, ritonavir/cobicistat, grapefruit juice
- Moderate: Diltiazem, verapamil, fluconazole
- Weak: Cimetidine
Clinical consequences
- Increased toxicity of narrow-therapeutic-index substrates (e.g., tacrolimus, cyclosporine, midazolam, certain statins like simvastatin, DOACs).
- Often requires dose reduction, alternative therapy, or enhanced monitoring.
Pearl: Intestinal CYP3A4 inhibition (e.g., grapefruit juice) can significantly increase oral bioavailability without affecting IV dosing.
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