Page 86 - Interactive Theoritical Notes of Bioharmaceutics and pharamcokinetics.docx compressed

PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

✓ It is determined by the physicochemical properties of the substance and its
interaction with the intestinal cells and maintain a so-called ‘ ‘ sink’’ condition.

✓ The process initially involves the partitioning of the drug between the aqueous

fluids within the GIT and the lipoidal-like membrane of the lining of the

epithelium.

The rate of drug transfer is determined by:

o Physicochemical properties of the drug

o Nature of the membrane

o Drug concentration gradient across the membrane.

Fick's law of diffusion

According to Fick's law of diffusion, drug molecules diffuse from a region of high

drug concentration to a region of low drug concentration.

• Because the drug distributes rapidly into a large volume after entering the blood, the

concentration will be lower in blood than the site of absorption.

= ( − )

Where:

dQ/dt = rate of diffusion
D = diffusion coefficient,
K = partition coefficient of drug (lipid-water)
A = surface area of membrane;

h = membrane thickness,
CGI – Cp = difference between the concentrations of drug in the gastrointestinal tract and in
the plasma.

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