First Pass (Presystemic) Metabolism

FIRST PASS (PRESYSTEMIC) METABOLISM

This refers to metabolism of a drug during its passage from the site of absorption into the systemic circulation. All orally administered drugs are exposed to drug metabolizing enzymes in the intestinal wall and liver (where they first reach through the portal vein). Presystemic metabolism of limited magnitude can also occur in the skin (transdermally administered drug) and in lungs (for drug reaching venous blood through any route). The extent of first pass metabolism differs for different drugs (Table 3.1) and is an important determinant of oral bioavailability.

Attributes of drugs with high first pass metabolism:

a)  Oral dose is considerably higher than sublingual or parenteral dose.

b)  There is marked individual variation in the oral dose due to differences in the extent of first pass metabolism.

c) Oral bioavailability is apparently increased in patients with severe liver disease.

d) Oral bioavailability of a drug is increased if another drug competing with it in first pass metabolism is given concurrently, e.g. chlorpromazine and propranolol.