Small Molecule Drug Metabolism

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III. What happens to drugs once they are metabolized?  


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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