Drug Movement Flashcards
What are the fates of drugs in the body?
Absorption
Distribution
Metabolism
Excretion
What is absorption?
The process by which the drug enters the body from its site of administration and enters the general circulation
What is distribution?
The transport of drugs by the general tissues. This usually involves the drug leaving the blood and entering tissues.
What is metabolism?
The process by which tissue enzymes catalyze the chemical conversion of a lipid soluble drug into a less active and more polar form that can be readily excreted
What is excretion?
The process that removes the drug or its metabolites from the body
What factors affect drug absorption?
Solubility
Chemical stability
Lipid to water partition coefficient
How does solubility affect drug absorption?
The drug must be able to dissolve in an aqueous solution in order to be absorbed
How does chemical stability affect drug absorption?
Some drugs are destroyed by acid in the stomach or enzymes in the GI tract. In some cases, a compound is modified within the gut to release an active drug
How does lipid to water partition coefficient affect drug absorption?
Absorption usually occurs via simple diffusion across membranes. For a given drug concentration gradeint across a membrane, the rate of diffusion increases with the lipid solubility of the drug.
What is the partition coeffcient?
The ratio of the drug concentration in the membrane and concentration in water at equilibrium
What does the degree of ionisation depend on?
The pKa of the drug and the local pH
What is pKa?
The pH at which 50% of the drug is ionised and 50% of the drug is unionised
How can the proportions of ionised and unionised drugs be calculated?
Henderson-Hasselbalch equations:
Acid: pKa - pH = log(AH/A-)
Base: pKa - pH = log(BH+/B)
Where is the absorption of weak acids facilitated?
The pH of the stomach lumen.
Where does the majority of absorption occur?
In the small intestine due to the large surface area, due to the presence of villi