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Drug Metabolism: Conversion of Parent Drug to Metabolites

After having achieved the desired effects, drug molecules need to be deactivated and eliminated from the body. The body performs this function through a process termed metabolism or biotransformation. The function of drug metabolism is to convert lipophilic compounds (the parent drug) into more polar, hydrophilic compounds (metabolites) that can easily be dissolved in an aqueous environment such as blood, bile, or urine to thus efficiently be excreted from the body. Without biotransformation, the parent drug could not be eliminated and would accumulate in the body, eventually resulting in toxicity.
In general, drug metabolites are less active and less toxic than the parent drug. However, there are exceptions. Morphine, for example, is the active metabolite of the less active parent drug, codeine.
The chemical reactions of metabolism (biotransformation) that result in a change to the drug’s structure are catalyzed by enzymes. These enzymes reside predominately in the liver and small intestine but are widely distributed throughout the body in tissues of the kidney and lungs as well as in red blood cells, plasma, and saliva. All cells contain these enzymes to some extent and have some capacity to metabolize drugs. These enzyme-catalyzed reactions can be divided into Phase I and Phase II metabolic reactions.