Bioavailability studies are intended to measure the rate and extent to which an active drug ingredient or therapeutic moiety is absorbed from a drug product and becomes available at the site of action.
Bioavailability is usually assessed by determining the area under the plasma concentration–time curve or total amount excreted in the urine after drug administration. AUC is directly proportional to the total amount of unchanged drug that reaches systemic circulation. Drug products are generally considered bioequivalent in extent and rate of absorption if their plasma concentration curves are essentially super imposable.
Plasma drug concentration increases with extent of absorption and the maximum (peak) plasma concentration is reached when drug elimination rate equals absorption rate. Bioavailability determinations based on the peak plasma concentration can be misleading because drug elimination begins as soon as the drug enters the bloodstream. Peak time is the most widely used general index of absorption rate; slower the absorption, later the peak time.
AUC is inversely proportional to the clearance of the drug. That is, the higher the clearance, the less time the drug spends in the systemic circulation and the faster the decline in the plasma drug concentration. Therefore, in such situations, the body exposure to the drug and the area under the concentration-time curve are smaller.
For drugs excreted primarily unchanged in urine, bioavailability can be estimated by measuring the total amount of drug excreted after a single dose. Ideally, urine is collected over a period of 7 to 10 elimination half-lives for complete urinary recovery of the absorbed drug. After multiple dosing, bioavailability may be estimated by measuring unchanged drug recovered from urine over a 24-h period under steady-state conditions.