Bioavailability is a key pharmacokinetic parameter that measures the rate and extent to which the active substance of a drug is absorbed from its pharmaceutical form and becomes available at the site of action. For intravenously administered drugs, bioavailability is 100% by definition. For orally administered medications, however, it is often lower due to incomplete absorption and first-pass metabolism in the intestinal wall and the liver.
Absolute bioavailability compares oral with intravenous administration, while relative bioavailability compares two different formulations (e.g., test vs. reference), which forms the basis for bioequivalence studies. For CROs conducting early-phase clinical trials, determining bioavailability is a crucial step in establishing the correct dosage for later phases and optimizing the formulation. It requires precise pharmacokinetic sampling and validated bioanalytical methods to measure the concentration of the active substance in the bloodstream over time.