The oral bioavailability is the function of the fraction of absorption, the fraction undergoing first-pass metabolism, the fraction of loss due to efflux, and the fraction of degradation
(Fig. 6.5) . The amount of the drug reaching the site of measurement is the bioavailable portion of a dose administered.
It is important to distinguish between the terms bioavailability (BA) and bioequivalence (BE). Bioavailability determines the amount of drug that is absorbed in the bloodstream as compared with a standard (i.e., after intravenous administration). Absolute oral bioavailability is usually calculated as the ratio of the exposure as determined by area under the concentration-time curve (AUC) compared with the same parameter after intravenous administration that is assumed to be 100%. The relative bioavail-abilty is the AUC ratio of a test formulation to a reference formulation. The comparison could be of a tablet versus a capsule or a solution and so on. If the 90% confidence interval of the ratio (ratio of the least square means using log transformed data) is contained within the limits of 0.8-1.25, then the formulations are
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