First-Pass Metabolism

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First-pass metabolism (also known as presystemic metabolism) refers to the action of metabolic enzymes, in the liver and gut wall, on drugs [1]. These enzymes metabolise drugs before they enter the systemic circulation, often rendering a fraction of the drug inactive [1]. This lowers the proportion of an ingested drug that is acquired by the systemic circulation in its active form (its bioavailability).[1]

This form of metabolism is therapeutically relevant as...

  1. It implies that the efficacy of a drug is lower when its route of administration is oral rather than intravenous.[2] This frequently means that a higher dose is used when a drug is administered orally [1]. First-pass metabolism can reduce the bioavailability of some drugs to the extent that it renders them completely ineffective and makes oral administration unsuitable (insulin is an example of this) [3]
  2. First-Pass Metabolism differs between people [1].

References

  1. 1.0 1.1 1.2 1.3 1.4 Rang H, Dale M, Ritter J, Flower R and Henderson G. (2012) Rang and Dale's Pharmacology, 7th Edition, Spain: Elsevier Inc.
  2. Grahame-Smith D, Aronson J. (2002) Oxford Textbook of Clinical Pharmacology and Drug Therapy, 3rd Edition, Hong Kong: Oxford University Press.
  3. Grahame-Smith D, Aronson J. (2002) Oxford Textbook of Clinical Pharmacology and Drug Therapy, 3rd Edition, Hong Kong: Oxford University Press.

 

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