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Small Molecule Drug Metabolism Return to
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III. What happens to drugs once they are metabolized?
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| Drugs are ultimately excreted from the body through various routes.
The
kidney is the major organ for drug excretion. It excretes hydrophilic drug and
drug metabolites through glomerular filtration. Macromolecules such as proteins
are retained. Lipophilic drug molecules are not directly excreted from the kidney.
Only after they are metabolized into more hydrophilic molecules,
can they be excreted through the kidneys into the urine. Drugs and their metabolites are also
excreted into bile. This is usually mediated by protein transporters.
Drugs and
their metabolites in bile are eventually released into the intestinal tract.
The
drugs may be reabsorbed into the body from the intestine. Drug metabolites such
as glucuronide conjugates, may be converted back to the parent drug in the intestine
through glucuronidase enzyme, and then reabsorbed into systemic circulation.
This drug recycling process is called enterohepatic recycling. This process, if
extensive, may prolong the half-life of the drug. The bile drugs and drug
metabolites, if not reabsorbed by intestine, are excreted from the body through
feces. Also, a variety of orally administrated drugs are excreted through
feces because they are not absorbed through the intestine. Oral bioavailability
constitutes a major challenge for drug developers. Other routes of excretion,
such as sweat, tears, and saliva, are quantitatively less important. Excretion
through breast milk is not important to the mother, but may be of key importance
to the baby, because the drug may be toxic to the baby. Pulmonary excretion is
important for anesthetic gases and vapor drugs.
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