First-pass metabolism

Core Concept - The Liver's Tollbooth

• Orally administered drugs are absorbed from the GI tract and travel first to the liver via the portal vein before reaching systemic circulation.
• The liver metabolizes a fraction of the drug, reducing its active concentration. This is the First-Pass Effect.
• This process significantly lowers a drug's bioavailability (F).
  • Drugs with high first-pass metabolism (e.g., lidocaine, morphine, propranolol) require much larger oral doses than parenteral doses.

⭐ High first-pass metabolism is why drugs like nitroglycerin are given sublingually-bypassing the liver for rapid, direct systemic absorption and higher bioavailability.

Clinical Significance - Dose & Route Roulette

• First-Pass Effect: Orally administered drugs are absorbed from the GI tract and enter portal circulation, passing through the liver before reaching systemic circulation. The liver metabolizes a portion of the drug, reducing its concentration.
• Bioavailability (F): The fraction of administered drug that reaches systemic circulation unchanged. For oral drugs, it's reduced by the first-pass effect.
  • Calculated as: $F = f \times (1 - ER)$, where ER is the hepatic extraction ratio.

• Dose Discrepancy: Drugs with high first-pass metabolism have a much larger oral dose than parenteral dose to achieve equivalent therapeutic effect.

  • Examples: Lidocaine, Morphine, Propranolol, Nitroglycerin.

• Bypassing the Liver: Alternative routes avoid the portal vein and thus, first-pass metabolism.

  • Routes: IV, IM, Subcutaneous, Sublingual (SL), Transdermal, Inhalational, and a portion of Rectal (PR) administration.

⭐ Exam Favorite: Nitroglycerin has nearly 100% first-pass metabolism, making oral administration ineffective. It is given sublingually or transdermally for rapid absorption directly into systemic circulation, bypassing the liver for acute angina treatment.

Influencing Factors - The Metabolic Modulators

• Enzyme Inducers: ↑ rate of drug metabolism → ↓ drug concentration & efficacy.

  • 📌 Mnemonic: Guinness, Corona, & PBRS induce Chronic Alcoholism
  • Griseofulvin, Carbamazepine, Phenytoin, Barbiturates, Rifampin, St. John's Wort, Chronic alcoholism.

• Enzyme Inhibitors: ↓ rate of drug metabolism → ↑ drug concentration & risk of toxicity.

  • 📌 Mnemonic: CRACK AMIGOS
  • Cimetidine/Ciprofloxacin, Ritonavir, Amiodarone, Chloramphenicol, Ketoconazole, Alcohol (acute), Macrolides, Isoniazid, Grapefruit juice, Omeprazole, Sulfonamides.

⭐ Grapefruit juice irreversibly inhibits intestinal wall CYP3A4, markedly ↑ bioavailability of drugs like statins (e.g., atorvastatin, simvastatin) and non-dihydropyridine calcium channel blockers.

High‑Yield Points - ⚡ Biggest Takeaways

• First-pass metabolism is the pre-systemic elimination of a drug, primarily by the liver and gut wall, before reaching systemic circulation.
• It significantly ↓ decreases the oral bioavailability of susceptible drugs.
• Drugs with high extraction ratios (e.g., lidocaine, nitroglycerin) have very low oral bioavailability.
• Routes like IV, sublingual, and transdermal bypass the liver's first pass.
• Liver disease like cirrhosis can reduce first-pass metabolism, increasing drug toxicity risk.
• This necessitates higher oral doses than IV doses for drugs like propranolol.