Definition and properties
When we take a tablet or capsule, it contains a defined amount of an active pharmaceutical ingredient. Usually, the full dose does not enter the bloodstream. Some active ingredients are not fully released from the dosage form (liberation), others are only partially absorbed from the intestine (absorption), and some are metabolized in the intestine and during the first liver passage (first-pass metabolism). Oral bioavailability refers to the fraction of the dose that appears in the systemic bloodstream. It varies between 0 (0%) and 1 (100%). It is calculated as follows: Absolute oral bioavailability F = AUCoral / AUCi.v. Area Under the Curve (AUC) refers to the area under the plasma concentration curve for intravenous or peroral administration. For reference, the AUCi.v. is used. It is always 100% because the whole dose appears in the blood when administered intravenously. The definition of bioavailability usually includes not only the (AUC) but also the (e.g., tmax, curve progression).
Dependence on formulation
Bioavailability is not exclusively a substance property. It also depends significantly on the formulation of the drug. A steel tablet that does not dissolve in the stomach and intestines has a bioavailability of 0%. Therefore, generic drugs must also meet a test for what is known as bioequivalence (see under generic drugs).
Drug Interactions
Drugs with low bioavailability are prone to drug-drug interactions. For example, the bisphosphonate ibandronate, which is used for osteoporosis therapy, has a deep bioavailability of just 0.6%. If calcium is taken at the same time, the bioavailability deteriorates even further to the point of loss of efficacy. Drugs with a high first-pass metabolism, which can also reduce bioavailability, are also susceptible to interactions. If biotransformation is inhibited, the dose reaching the circulation may increase. And this favors adverse effects.
Deep oral bioavailability
Some agents have such deep bioavailability that they cannot be administered perorally. This is true, for example, of nitroglycerin, which is therefore given sublingually. Also, many modern drugs, such as antibodies and other biologics, are not orally available and are therefore often given as infusions or injections.
