Pharmacokinetics Quiz

See First-Pass Metabolism. Option B is correct. First-pass metabolism is metabolism of a drug before it reaches the systemic circulation, meaning the general circulation of the body. Drugs that are swallowed are absorbed from the small intestine and carried in the hepatic portal vein, the vein that takes blood from the gut to the liver. The liver is the main metabolic organ and may remove and inactivate much of the dose on its first pass, and the gut wall also contributes. Only the remaining fraction reaches the rest of the body, so the bioavailability (the fraction of the dose that reaches the circulation) can be low. An intravenous dose goes straight into the systemic circulation, so it avoids the portal route and has complete bioavailability. Other routes that largely bypass first pass include intramuscular, subcutaneous, inhaled, sublingual (under the tongue), buccal, transdermal and, to a large extent, rectal. This is why glyceryl trinitrate is given under the tongue or through the skin, since an oral dose would be mostly destroyed.

• First-pass metabolism: metabolism of a drug before it reaches the systemic circulation.
• Oral route: a swallowed drug travels in the hepatic portal vein to the liver, which may inactivate much of the dose, so bioavailability can be low.
• Intravenous route: the dose goes straight into the systemic circulation, so it avoids first pass and has complete bioavailability.