Which of the following is the best description of pharmacokinetics?
Pharamcokinetics is the study of the mechanisms of drug actions10%Pharamcokinetics is the study of absorption, distribution, metabolism, and excretion of drugs78%Pharamcokinetics is the study of drugs using randomised control trials4%Pharamcokinetics is the study of illicit drugs3%Pharamcokinetics is the study of the therapeutic effects of drugs5%
Pharmacokinetics
Pharmacokinetics is concerned with the body's effect on the drugs. This concerns how a drug gains access to the plasma (absorption), if it stays in the plasma or prefers to rest in other areas of the body (distribution), what the body does to it once it recognises it (metabolism), and how it gets rid of it once it does (elimination/excretion).
An acronym used to remember this is ADME
Absorption
Drug absorption is the movement of a drug into the blood stream. Hydrophobic (lipid soluble) drugs are absorbed better than hydrophilic ones.
Hydrophilic drugs:-
- pass through the liver unchanged (as they can't pass into hepatocytes)
- show little protein binding
- are not subject to tubular reabsorption
- are excreted rapidly
Distribution
Drug distribution refers to the process whereby an absorbed drug moves from the blood (vascular compartment) to other spaces of the body (such as fat, cells, interstitial space).
Metabolism
Metabolism refers to the process where a drug is changed from one form to another. One important function of metabolism is to convert lipid soluble drugs into a water soluble form so that they can then be excreted by the kidneys.
Drug metabolism is divided into two phases.
Phase 1 involves reduction or hydrolysis of the drug, but the most common biochemical process that occurs is oxidation.
Phase 2 involves conjugation, which is the attachment of an ionised group (which is very hydrophilic) to the drug.
The most common method is conjugation with glucuronic acid. Other added groups include sulphate, amino acids, acetate and methyl.
Drug metabolism takes place not only in the liver but also in the kidneys, intestine, and the lungs.
Excretion
Drugs are excreted mainly by the kidneys and the biliary system.
