Absorption Flashcards
What factors determine the rate of absorption of a drug?
Route of administration, formulation and physiochemical properties.
What are the two routes drugs can be absorbed into the body when administered orally?
- Oral mucosa
- GI tract
Discuss absorption through oral mucosa
The oral mucosa is thin epithelium and rich vascularity which favours absorption. This is good for sublingual administration which is retained longer near oral mucosa, enhancing absorption. However, contact is too brief here for other oral routes.
What are the 4 factors affecting absorption from the GI tract?
- Lipid solubility and ionisation
- Drug formulation
- First pass effect
- Absorptive capacity
How does lipid solubility and ionisation affect absorption from GI tract?
Drugs must cross several semipermeable cell membranes before it reaches the circulation. Therefore dissolution rate determines availability of drug for absorption.
If small and lipid soluble can passively diffuse.
If simliar to endogenous biological molecules can use facilitated diffusion.
Pinocytosis.
How does drug formulation affect absorption from GI tract?
Drugs may be chemically altered once in the body through the action of gut contents. The acidic pH and enzymes may degrade the drug, reducing absorption.
What is first pass effect and how does it affect absorption from GI tract? Give an example of a drug which is prone to this.
When the concentration of drug is greatly reduced before it reaches the systematic circulation due to venous drainage from the stomach and intestine to the liver for hepatic extraction so little is left for absorption.
Propanolol is an orally taken beta block which is subject to large first pass effect, where as sublingual and rectal administration tend to avoid this.
How does absorptive capacity of the stomach and small intestine affect absorption from GI tract?
Once orally delivered, drug travels through GI tract.
The stomach has a large epithelial surface but a thick mucous layer which limits absorption. Quicker absorption on an empty stomach.
The small intestine has the largest surface area for drug absorption in the GI tract and a more permeable membrane than stomach. Therefore orally administered drugs are primarily absorbed through the small intestine into the blood where it can travel to the CNS.
