Bioavailability

In pharmacology, bioavailability is used to describe the fraction of an administered dose of medication that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered intravenously, its bioavailability is 100%. However, when a medication is administered via other routes (such as by mouth), its bioavailability decreases (due to incomplete absorption and first-pass metabolism). Bioavailability is one of the essential tools in pharmacokinetics, as bioavailability must be considered when calculating dosages for non-intravenous routes of administration.

1 Definition
2 Absolute bioavailability
3 Relative bioavailability
4 Factors influencing bioavailability
5 See also
6 References

Definition

Bioavailability is a measurement of the rate and extent of therapeutically active drug that reaches the systemic circulation and is available at the site of action. (Shargel & Yu, 1999)

It is expressed as the letter F.

Absolute bioavailability

Absolute bioavailability measures the availability of the active drug in systemic circulation after non-intravenous administration (i.e. after oral, rectal, transdermal, subcutaneous, etc administration).

In order to determine absolute bioavailability of a drug, a pharmacokinetic study must be done to obtain a plasma drug concentration vs time plot for the drug after both intravenous and extravascular administration. The absolute bioavailability is the dose-corrected area under curve (AUC) extravascular divided by AUC intravenous.

$F = \frac{[AUC]_{po}*dose_{IV}}{[AUC]_{IV}*dose_{po}}$

Note here that a drug given by the intravenous route will have an absolute bioavailability of 1 (F=1). Drugs given by other routes usually have an absolute bioavailability of less than one.

Relative bioavailability

This measures the bioavailability of the a certain drug when compared with another formulation of the same drug, usually an established standard, or through administration via a different route. When the standard consists of intravenously administered drug, this is known as absolute bioavailability.

$\mathit{relative\ bioavailability} = \frac{[AUC]_{A}*dose_{B}}{[AUC]_{B}*dose_{A}}$

Factors influencing bioavailability

As mentioned above the absolute bioavailability of a drug, when administered by an extravascular route, is usually less than one. This means that there are factors at work which reduce the availability of the drug prior to it entering the systemic circulation.

Such factors may include, but are not limited to:

poor absorption from the gastrointestinal tract
hepatic first-pass effect
degradation of the drug prior to reaching system circulation

References

Shargel, L.; Yu, A.B. (1999). Applied biopharmaceutics & pharmacokinetics (4th ed.). New York: McGraw-Hill. ISBN 0-8385-0278-4.

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