Bioavailability

Bioavailability is understood to mean the rate and extent to which the active substance is absorbed from a pharmaceutical formulation and becomes available in the general circulation. It is further defined as the extent to which the active moiety enters systemic circulation, thereby gaining access to the site of action.

Differences in bioavailability among formulations of a given drug or supplement can have clinical significance. When a drug or supplement rapidly dissolves and readily crosses membranes, absorption tends to be complete, but absorption of orally administered drugs is not always complete. Before reaching the vena cava, a drug must move down the GI tract and pass through the gut wall and liver, common sites of drug metabolism. Many drugs have low oral bioavailability because of extensive first pass metabolism.

Low bioavailability is most common with oral dosage forms of poorly water-soluble, slowly absorbed drugs. More factors can affect bioavailability when absorption is slow or incomplete than when it is rapid and complete, so slow or incomplete absorption leads to variable therapeutic responses.

Assessment of bioavailability from plasma concentration-time data usually involves determining the maximum (peak) plasma drug concentration, the time at which maximum plasma drug concentration occurs, and the area under the plasma concentration-time curve (AUC). AUC is the most reliable measure of bioavailability, and is directly proportional to the amount of unchanged drug that reaches the systemic circulation.