First-pass metabolism
First-pass metabolism
First-pass metabolism (pronounced: fɜrst pæs mɛtəbəlɪzəm), also known as presystemic metabolism or hepatic first pass, is a phenomenon of drug metabolism whereby the concentration of a drug is greatly reduced before it reaches the systemic circulation. It is the fraction of lost drug during the process of absorption which is generally related to the liver and gut wall.
Etymology
The term "first-pass" refers to the initial process through which a drug is metabolized, specifically in the liver, before it circulates throughout the body. "Metabolism" is derived from the Greek word "metabole" which means change, referring to the chemical changes that drugs undergo.
Process
After oral administration, drugs must pass through the gastrointestinal (GI) tract, where they may be metabolized in the gut wall. They then enter the hepatic portal vein, which carries them to the liver before they reach the rest of the body. The enzymes in the liver, particularly the cytochrome P450 enzymes, metabolize the drugs, reducing their concentration before they reach the systemic circulation.
Related Terms
- Bioavailability: The fraction of an administered drug that reaches the systemic circulation.
- Cytochrome P450: A family of enzymes involved in drug metabolism.
- Liver: The organ primarily responsible for drug metabolism.
- Drug metabolism: The process by which the body breaks down and converts medication into active chemical substances.
- Gastrointestinal tract: The structures from the mouth to the anus that form the human digestive system.
External links
- Medical encyclopedia article on First-pass metabolism
- Wikipedia's article - First-pass metabolism
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