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Concept of Bioavailability

If a drug is given intravenously, it is administered directly into the blood and therefore we can be sure that all of the drug reaches the systemic circulation. The drug is therefore said to be 100% bioavailable.

However, if a drug is given by another route there is no guarantee that the whole dose will reach the systemic circulation intact. The amount of an administered dose of the drug that does reach the systemic circulation in the unchanged form is known as the bioavailable dose. 

The percentage of an administered dose of a particular drug that reaches the systemic circulation intact is known as the bioavailability.

Bioavailability is defined in the FDA’s regulations as ‘the rate and extent to which the active ingredient or active moiety is absorbed from a drug product and becomes available at the site of action’. 

Absolute bioavailability compares the bioavailability of the unchanged drug in the systemic circulation following a non-intravenous dose, e.g. oral, rectal, transdermal, sub-lingual, intramuscular, subcutaneous, with the bioavailability of the same drug following intravenous administration. 

The bioavailability exhibited by a drug is thus very important in determining whether a therapeutically effective concentration will be achieved at the site(s) of action.

It should also be noted that, in the context of bioavailability, the term ‘systemic circulation’ refers primarily to venous blood (excluding the hepatic portal vein, which carries blood from the gastrointestinal tract to the liver in the absorption phase) and the arterial blood, which carries the blood to the tissues.

Therefore, for a drug which is administered orally to be 100% bioavailable, the entire dose must move from the dosage form to the systemic circulation. The drug must therefore:

be completely released from the dosage form

be fully dissolved in the gastrointestinal fluids

be stable in solution in the gastrointestinal fluid

pass through the gastrointestinal barrier into the mesenteric circulation without being metabolized

pass through the liver into the systemic circulation unchanged.

Anything which adversely affects either the release of the drug from the dosage form, its dissolution into the gastrointestinal fluids, its permeation through and stability in the gastrointestinal barrier or its stability in the hepatic portal circulation will influence the bioavailability exhibited by that drug from the dosage form in which it was administered.

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