Pharmacokinetics Flashcards
Define the term pharmacokinetics?
The description of a drug concentration profile against time
In other words… pharmacokinetics describes what the body does to the drug
What are the four principle components of pharmacokinetics?
Absorption,
Distribution,
Metabolism and,
Excretion
(ADME)
What is meant by the term ‘absorption’ in the context of pharmacokinetics?
The transportation of an un-metabolised drug from the site of administration to the systemic circulation. All drugs (except those administered intravenously) must cross the cell membrane to access the systemic circulation.
What are the main two mechanisms through which drugs cross cell membranes?
Passive Diffusion Through the Lipid Bilayer - The drug molecule moves across the lipid cell membrane according to the concentration gradient (from an area of high concentration to an area of low concentration) however the drug must be non-ionised (non-charged) in order to do this as ionised molecules (positively charged) are non lipid soluable
Carrier Mediated Transport/ Facilitated Transport - Numerous carrier-mediated membrane transport systems are utilised in the body to regular the transport of ions and nutrients across the cell membrane, the GI tract and blood brain barrier are two areas where carrier mediated transport occurs commonly. Certain drugs exploit this by being a similar structure to the carrier’s substrate and binding to the carrier molecule
What is meant by the term ‘distribution’ in the context of pharmacokinetics?
Drug distribution refers to the movement of a drug to and from the blood and tissues of the body, and the relative proportions of drug in the tissues.
Only ‘free’/ unbound drug is active within the body, many drugs bind to plasma proteins, and accumulate in the plasma, or are lipid soluble, and can be found in all compartments but accumulate in fatty deposits. Drugs with low lipid solubility often accumulate in the plasma and extracellular fluid
What are the three main physiological factors that affect drug distribution?
Ease of Penetration of Organ - Certain tissues, such as the brain and placenta, are harder to access than others as they are protected by the blood-brain barrier and placental barrier.
Plasma Protein Binding - Some drugs bind to plasma proteins, the bound drug is not pharmacologically active. So highly protein bound drugs often have a higher volume if distribution. So conditions like hypoalbumnuria can cause an excessive amount of the unbound drug to be present within the body.
Blood Perfusion Rate
