CYP450 inhibition and induction

CYP450 System - The Body's Detox Crew

• A superfamily of heme-containing monooxygenase enzymes, named for their characteristic 450 nm light absorption peak when bound to carbon monoxide.
• Primary Function: Central to Phase I drug metabolism. They catalyze reactions like oxidation, reduction, and hydrolysis, converting lipophilic compounds into more polar, water-soluble metabolites for excretion.
• Primary Location: Predominantly embedded in the smooth endoplasmic reticulum membrane of hepatocytes, with some presence in extrahepatic tissues (e.g., small intestine).

⭐ The CYP3A4 isozyme is the most abundant form, responsible for metabolizing approximately 50% of all clinically used drugs.

CYP450 Inducers - Stepping on the Gas

CYP450 inducers increase the synthesis of cytochrome enzymes, accelerating the metabolism of co-administered drugs. This leads to a ↓ in their plasma concentration and potential therapeutic failure.

• Mechanism: Upregulation of gene expression via activation of nuclear receptors (e.g., PXR, CAR).
• Clinical Consequences:
  • ↓ plasma levels of other drugs (e.g., warfarin, OCPs) → subtherapeutic effect.
  • ↑ activation of prodrugs → ↑ active metabolite levels & potential toxicity.

📌 Mnemonic: "CRAP GPS"

• Carbamazepine
• Rifampin
• Alcohol (chronic)
• Phenytoin
• Griseofulvin
• Phenobarbital
• St. John's Wort

⭐ Exam Favorite: St. John's Wort, an over-the-counter supplement for depression, is a potent CYP3A4 inducer, significantly reducing the efficacy of drugs like oral contraceptives and cyclosporine.

CYP450 Inhibitors - Hitting the Brakes

CYP450 inhibitors block the metabolic activity of one or more CYP enzymes through direct competition or irreversible inactivation. This slows the metabolism of other drugs, leading to ↑ plasma concentrations and risk of toxicity.

• Mechanism: Competitive or non-competitive binding to the enzyme active site.
• Clinical Effect: ↑ substrate drug levels, ↑ risk of adverse effects.
  • Exception (Prodrugs): Inhibition ↓ conversion of prodrugs to their active form, leading to ↓ efficacy.

📌 Mnemonic: SICKFACES.COM

• Sodium valproate
• Isoniazid
• Cimetidine
• Ketoconazole
• Fluconazole
• Alcohol (acute)
• Chloramphenicol
• Erythromycin
• Sulfonamides
• .Ciprofloxacin
• Omeprazole
• Metronidazole
• Grapefruit juice

⭐ Exam Favorite: Co-administration of omeprazole (a potent CYP2C19 inhibitor) with clopidogrel (a prodrug activated by CYP2C19) can significantly reduce the antiplatelet effect of clopidogrel, increasing the risk of stent thrombosis.

Clinical Cross-Talk - Warfarin's Wild Ride

High‑Yield Points - ⚡ Biggest Takeaways

• CYP450 inducers (Rifampin, Phenobarbital, Carbamazepine, St. John's Wort) accelerate metabolism, leading to ↓ drug effect or prodrug activation.
• CYP450 inhibitors (Grapefruit juice, Azoles, Macrolides, Protease Inhibitors) slow metabolism, causing ↑ drug levels & toxicity.
• Classic interaction: With Warfarin, inducers ↓ INR (clot risk), while inhibitors ↑ INR (bleed risk).
• Remember the kinetics: Inhibition is rapid, while induction is a slow process requiring new enzyme synthesis.