Absorption Flashcards
What is drug absorption?
Absorption is the process of incorporation of drug into the blood stream.
This process takes place after administration of the drug, which is then found solubilised in the blood ready to reach its target. Once dissolved, the drug molecules must cross a number of membrane barriers on their route to the bloodstream and must also traverse out of the bloodstream and through the membranes leading to the cell targets.
How do drugs cross membrane barriers?
Drugs (usually small molecules) pass through cell membranes (lipids) by 3 mechanisms:
1) Direct diffusion through the lipid
2) Carrier-mediated transport
3) Diffusion through aqueous pores (BUT most drugs are too large)
4) Pinocytosis (not important for small molecules but is for macromolecules).
What factors affect diffusion?
- surface area
- concentration difference (gradient)
- molecular size
- lipid solubility
is the molecular size important?
Smaller molecules are more mobile in lipid membranes than larger ones and thus diffuse faster.
But because variation in molecular size between drugs is generally not large, molecular size is generally less important than lipid solubility.
What properites can be predicted from the lipid solubility?
- rate of absorption from the GIT
- penetration into the brain and other tissues
- Extent of renal elimination.
What is a weak acid drug?
Neutral molecule that can reversibly dissociate into an anion (a negatively charged molecule) and a hydrogen ion (positively charged).
What is a weak base drug?
Neutral molecule that can form a cation (positively charged) by combining with a hydrogen ion (proton).
What is the effect of pH on acidic drugs?
Acidic drugs become more unionised in acidic pH.
Weakly acidic drugs such as aspirin are less ionised at low pH. thus an acidic environment would favour their absorption.
What is the effect of pH on basic drugs?
Basic drugs becomes unionised in alkaline pH.
Weakly basic drugs such as opiates are less ionised at high pH. thus an alkaline environment favours their absorption.
What is gastric emptying rate?
Gastric emptying rate is the time during which the drug is retained in the stomach before it passes into the small intestine.
What drugs increase gastric emptying rate?
Prokinetic agents, e.g.
metoclopramide, prucalopride, erythromycin.
What drugs decrease gastric emptying rate?
Antimuscarinic agents (atropine-like-drugs) and opiods. Fatty foods also slow the gastric emptying rate.
