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Bioavailability and factors affecting bioavailability-    • Bioavailability | Factor Affecting Bioavai...  

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Bioavailability refers to the extent and rate at which the active moiety (drug or metabolite) enters systemic circulation, thereby accessing the site of action.
Bioavailability of a drug is largely determined by the properties of the dosage form, which depend partly on its design and manufacture. Differences in bioavailability among formulations of a given drug can have clinical significance; thus, knowing whether drug formulations are equivalent is essential.Bioavailability is usually assessed by determining the area under the plasma concentration–time curve The most reliable measure of a drug’s bioavailability is AUC. AUC is directly proportional to the total amount of unchanged drug that reaches systemic circulation. Drug products may be considered bioequivalent in extent and rate of absorption if their plasma concentration curves are essentially superimposable. Plasma drug concentration increases with extent of absorption; the maximum (peak) plasma concentration is reached when drug elimination rate equals absorption rate. Bioavailability determinations based on the peak plasma concentration can be misleading because drug elimination begins as soon as the drug enters the bloodstream. Peak time (when maximum plasma drug concentration occurs) is the most widely used general index of absorption rate; the slower the absorption, the later the peak time.

When the drug is administered orally the bioavailability depends on several factors:

Physicochemical properties of the drug and its excipients that determine its dissolution in the intestinal lumen and its absorption across the intestinal wall.
Decomposition of the drug in the lumen.
pH and perfusion of the small intestine.
Surface and time available for absorption.
Competing reactions in the lumen (for example of the drug with food).
Hepatic first-pass effect.

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