Drug X is more active when given orally than when given IV. What can this indicate?

• A. It is rapidly absorbed IV causing overdose
• B. It has no active metabolites and first-pass is not involved
• C. It is a prodrug with active metabolites requiring first-pass activation
• D. It has poor oral bioavailability but is more active orally due to slower elimination

Answer: (C)

If a drug is more active when given by mouth than by IV, it points to metabolic activation during first-pass metabolism, which is typical of a prodrug. When taken orally, the drug travels through the gut and then the liver via the portal vein, where enzymes can convert an inactive or less active form into active metabolites. If those metabolites are responsible for the therapeutic effect, oral administration can yield greater activity than IV, where the parent compound bypasses this first-pass activation. An example is a prodrug that is designed to be activated in the liver after absorption from the gut.

This explains why the drug would be more potent orally: the first-pass metabolism generates the active form that drives the response. The other scenarios don’t fit as well. For instance, rapid IV absorption causing overdose would imply stronger IV activity, not greater oral activity; a drug with no active metabolites and no first-pass involvement would not show increased activity with oral dosing; and a situation with poor oral bioavailability but supposedly slower elimination would affect duration more than the amount of active drug produced through first-pass activation.