Bioavailability

Definition

Bioavailability (F) is a pharmacokinetic parameter representing the fraction of a drug dose that reaches the systemic circulation unaltered.

Characteristics

• The greater the F, the greater the amount of drug that reaches the systemic circulation, and thus the greater the drug concentration in plasma and vice versa
• The route with the highest F (F=1) is via intravenous (I.V.) administration.
• The second highest F is achieved through inhalation.
• The parenteral routes of drug administration have low F because all the drug absorbed by the GIT, passes through the hepatic circulation undergoing *first pass metabolism, prior entering to the systemic circulation.

Types

Absolute bioavailability

It is the comparison of the per os F to the intravenous F

${F_{abs}}={AUC_{P.O.}\times D_{I.V.} \over AUC_{I.V.}\times D_{P.O.}}$

where

AUC_P.O.: area under the curve for the oral route

AUC_I.V.: area under the curve for the intravenous route

D_P.O.: dose administered from oral route

D_I.V.: dose administers from intravenous route

Relative bioavailability

It is the comparison of the per os F to another administration route, other than intravenous.