3.5:
Bioavailability: Overview
After a drug is administered, the amount of administered drug that enters the systemic circulation and is available for distribution to its site of action is called bioavailability.
The bioavailability of an orally administered drug is affected by GI absorption and biotransformation by the liver.
In the case of a drug that is intravenously administered, the drug directly enters the systemic circulation, and the bioavailability is assumed to be 100%, by definition.
For an orally administered drug, a comparison of the plasma concentration of the drug administered through the IV route helps to determine the bioavailability of that drug.
A plot of the drug's plasma concentration versus time for the same drug administered in equivalent doses, both orally and intravenously, can be generated.
The total area under the curve for each route of administration represents the extent of drug absorption for that route.
The ratio of the areas calculated gives an estimate of the bioavailable fraction of the orally administered drug.
3.5:
Bioavailability: Overview
Bioavailability refers to the proportion of an unaltered drug that, after administration, enters the systemic circulation and can be distributed to the desired action site. Factors such as gastrointestinal (GI) absorption and liver biotransformation influence the bioavailability of a drug when it is administered orally. When a drug is administered intravenously, it enters the systemic circulation directly; by definition, its bioavailability is assumed to be 100%. The bioavailability of an orally administered drug can be determined by comparing its plasma concentration with that of the same drug administered intravenously. A graph can be plotted that shows the plasma concentration of a drug over time, given in equivalent doses both orally and intravenously. The total area under this graph for each method of administration signifies the degree of drug absorption via that particular route. By calculating the ratio of these areas, an estimate of the bioavailable fraction of an orally administered drug can be obtained.