Page 105 - Interactive Theoritical Notes of Bioharmaceutics and pharamcokinetics.docx compressed

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

(B). Physiological factors affecting dissolution rate

The environment of the GIT can affect the parameters of the Noyes-Whitney

equation and hence the dissolution rate of a drug .

:
Examples
1. The diffusion coefficient, D, of the drug in the GIT fluids may be decreased by

the presence of substances that increase the viscosity of the fluids .Hence the

presence of food in the GIT may cause a decrease in dissolution rate of a drug

by reducing the rate of diffusion of the drug molecules away from the diffusion

layer
.
2. Surfactants in gastric juice and bile salts will increase both the wettability and

the solubility of the drug in the GIT (via micellization).

▪ Increasing the wettability will increase the effective surface area, A, exposed

to GIT fluids (thus increasing the dissolution rate).

▪ Increasing solubility will increase the concentration of the drug in the diffusion

layer, Cs (thus increasing the dissolution rate).

3. The thickness of the diffusion layer, h, will be influenced by the degree of

agitation experienced by each drug particle in the GIT Hence an

increase in gastric and/or intestinal motility may increase the dissolution
rate by decreasing the thickness of the diffusion layer.

4. The concentration, C, of drug in solution in the bulk of the GI fluids will be
:
influenced by such factors as

A. The rate of removal of dissolved drug by absorption
.
B. The volume of fluid available for dissolution.

• A low value of C will favor more rapid dissolution of the drug by virtue of

increasing the value of the term (Cs – C).
• In the case of drugs whose absorption is dissolution-rate limited; the value of C

.
is normally kept very low by absorption of the drug
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