The bioavailability (F) is the extent to which an extravascular (i.e., oral, subcutaneous, intraperitoneal, etc.) dose is available to the systemic circulation in relation to an intravenous dose. Bioavail-ability is expressed as a percentage. The calculation of bioavailabil-ity is shown in the following equation:
FAUCExtravascular/DoseExtravascular 1 nn 1\
220.127.116.11 Sample Calculation
Intravenous dose — 1 mg/kg Intravenous AUC — 50 mg xh/mL_
For oral dosing, a high bioavailability does not necessarily translate to great oral exposure. For compounds with very high clearance (CL), often you may observe high F% estimates due to a very low AUCIV. In addition, pharmacokinetic study designs often use high oral doses when compared to the intravenous dose. Since an oral dose is absorbed into the portal vein and must pass through the liver prior to entering the blood, saturation of hepatic first pass metabolism may occur. This scenario can result in a high F value that on occasion may be >100%, especially when high oral doses are given. Oral exposure of compounds should be ranked based upon both F% and oral AUC and interpreted in the context of the oral dose given.
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