First-pass metabolism

First-pass metabolism (also known as presystemic metabolism or phase I drug metabolism) 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. Notable drugs that experience a significant first-pass effect are propranolol, morphine, pethidine, lidocaine, glyceryl trinitrate, and alcohol.

Mechanism

First-pass metabolism occurs when the drug's bioavailability is compromised. Some drugs can be metabolically activated by this process. This presystemic metabolism occurs in the gastrointestinal tract, the liver, and the lung.

Factors affecting first-pass metabolism

Several factors can affect the first-pass metabolism of drugs. These include:

• Physiological factors: Age, sex, nutritional status, and circadian rhythm can influence drug metabolism.
• Pathological factors: Diseases like liver disease or heart failure can affect drug metabolism.
• Genetic factors: Genetic polymorphisms can lead to "poor" or "rapid" metabolizers of drugs.
• Drug interactions: Some drugs can affect the metabolism of other drugs by inhibiting or inducing their metabolism.

Clinical significance

Understanding the first-pass metabolism is important in pharmacokinetics. It can help in determining the route of drug administration. For drugs that undergo extensive first-pass metabolism, alternative routes of administration like intravenous, transdermal, or sublingual might be preferred.

See also

• Drug metabolism
• Bioavailability
• Pharmacokinetics

References

First-pass metabolism Resources
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