Enterohepatic circulation can affect pharmacokinetics by which mechanism?

• A. Direct renal excretion of unchanged drug with no reabsorption.
• B. Re-circulation of drug from the liver into the gut and back to systemic circulation, causing secondary peaks and prolonged half-life.
• C. Metabolism in the liver leading to rapid clearance.
• D. Absorption is unaffected by enterohepatic cycling.

Answer: (B)

Enterohepatic circulation is recycling of a drug through the liver and the gut. The drug is secreted into bile, enters the intestine, and can be reabsorbed back into the bloodstream. This re-entry into circulation can happen again and again, which often produces a secondary rise in plasma concentration and can prolong the drug’s apparent half-life and overall exposure (AUC). The cycle depends on the drug being excreted into bile and on mechanisms in the gut that allow reabsorption, sometimes aided by bacterial deconjugation of metabolites. Because of this recycling, the drug can linger longer in the body and may show a delayed or multiple-peak concentration–time profile.

The other scenarios don’t fit this mechanism. Direct renal excretion with no reabsorption wouldn’t create re-circulation or secondary peaks. Metabolism in the liver leading to rapid clearance describes faster elimination, not recycling through the gut. And absorption being unaffected by enterohepatic cycling ignores the possibility that reabsorption after biliary excretion can alter timing and amount absorbed, sometimes yielding secondary peaks or prolonged exposure.