prodrug

A prodrug is a medicine that arrives switched off and must be turned on inside the body. Picture a packaged gift that is useless until the body unwraps it: the swallowed compound has little or no activity until metabolism converts it into the real working drug.

Prodrugs are designed deliberately, usually to overcome a problem with the active molecule — poor absorption, bad taste, rapid breakdown, or lack of targeting. Enzymes (often cytochrome P450 or esterases) chemically transform the prodrug into the active species at the right place and time. Enalapril, for example, is hydrolyzed in the liver to its active form enalaprilat.

The reliance on metabolism is also a limitation. If a patient lacks or has reduced activity of the activating enzyme — as with clopidogrel in people with low CYP2C19 activity, or codeine in poor CYP2D6 metabolizers — the prodrug may fail to work or, conversely, produce too much active drug in ultra-rapid metabolizers.