CYP450 Drug Interactions: What Pharmacy Techs Need to Know

What Are CYP450 Enzymes?

The cytochrome P450 (CYP450) enzyme system is a family of liver enzymes responsible for metabolizing approximately 75% of all drugs. When one drug affects the activity of a CYP enzyme that metabolizes another drug, a drug interaction occurs. Understanding CYP450 interactions is fundamental to pharmacy practice and a recurring PTCE topic.

Key Concepts

• Substrate - A drug metabolized by a specific CYP enzyme
• Inhibitor - A drug that slows or blocks a CYP enzyme, causing substrate drug levels to rise (increased effect and toxicity)
• Inducer - A drug that speeds up a CYP enzyme, causing substrate drug levels to fall (decreased effect)

Major CYP Enzymes and Common Interactions

CYP3A4 (Metabolizes ~50% of drugs)

Substrates: simvastatin, atorvastatin, midazolam, cyclosporine, many calcium channel blockers. Inhibitors: ketoconazole, itraconazole, clarithromycin, grapefruit juice, ritonavir. Inducers: rifampin, carbamazepine, phenytoin, St. John's Wort.

CYP2D6

Substrates: codeine, tramadol, tamoxifen, many antidepressants, metoprolol. Inhibitors: fluoxetine, paroxetine, bupropion, quinidine. Note: CYP2D6 has no significant inducers.

CYP2C9

Substrates: warfarin, phenytoin, NSAIDs. Inhibitors: fluconazole, amiodarone. Inducers: rifampin.

CYP2C19

Substrates: omeprazole, clopidogrel, citalopram, diazepam. Inhibitors: omeprazole, fluoxetine, fluvoxamine. Inducers: rifampin.

Practical Application

When you see a new prescription for a CYP3A4 inhibitor like clarithromycin added to a patient already taking simvastatin, you should recognize the potential for increased statin levels and myopathy risk. This awareness helps you flag potential problems for the pharmacist to review.

CYP450 interactions are tested on the PTCE and encountered daily. Study these pathways with PTCB flashcards organized by enzyme.