Def: Bioavailability (F) is the fraction of drug in circulation compared to dose.
Use: not commonly used in IV drugs as bioavailability = 1 (all drug absorbed into circulation). Used in oral doses because some of drug will not be absorbed and taken up into circulation. So, we calculated bioavailability to find out how much of drug is actually absorbed.
Calculation: Give equal oral & iv dosages & plot conc of drug in plasma over time -> Then measure area under curve (AUC) for oral & area under the curve for iv -> AUC oral/AUC iv
Rules:
Iv = 1 // All other administration routes < 1 (e.g. F = 0.1 -> 10% bioavailability)
If bioavailability low, we +dosage so we get effective circulation conc. or change drug structure if still in development or finally consider a different metabolism route.
Low bioavailability is caused by: Poor absorption, chemistry of delivery site, first-pass metabolism
Choice of route is a compromise, guided by: bioavailability, chemistry of the drug, convenience, specificity of action, desired onset