First Pass Metabolism

Question 1

Define first pass metabolism

Answer 1

First pass metabolism refers to metabolism of a drug during its passage from site of absorption to systemic circulation

Question 2

All ____ administered drugs are exposed to _____________ in the intestinal wall and liver ( where they first reach through the ________)

Answer 2

Orally
Drug metabolising enzymes
Portal vein

Question 3

Presystemic drug metabolism can be avoided in the intestine and liver by administering the drug through

Answer 3

Sublingual, transdermal or parenteral routes

Question 4

Limited pre systemic metabolism can the occure in

Answer 4

Skin( Transdermally administered drugs)
Lungs( For drug reaching venous blood through any route)

Question 5

The extent of first pass metabolism differs for same / different drugs
What is the determinant of oral bioavailability.

Answer 5

The extent of
first pass metabolism differs for different drugs
and is an important determinant of
oral bioavailability.

Question 6

What is hepatic extraction ratio( ERliver) of drug?

Answer 6

Fraction of the absorbed drug prevented by liver from reaching systemic circulation. Both presystemic metabolism as well as direct excretion into bile determine ERliver
ER=CL liver/ hepatic blood flow

Question 7

Attributes of drug with high first pass metabolism

Answer 7

(a) Oral dose is considerably higher than sub-
lingual 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. chlorpro-
mazine and propranolol.