Bioavailability

Bioavailability is a pharmacological measure of the proportion of a pharmaceutical that is administered extravasally (p.o., s.c., i.m.), is available unchanged in the systemic circulation (especially in the bloodstream) and thus at the site of action. Bioavailability is also an important parameter of pharmacokinetics in the context of drug approval.

Absolute bioavailability (Fabs) is the ratio of AUC values after extravasation (AUC) and after i.v. administration (AUC i.v.). The bioavailability observed after oral administration is also called oral bioavailability. For drugs that are eliminated renally, the (Fabs) can also be determined from the urine.

If the absolute bioavailability (Fabs) after p.o. administration is <100%, either the absorption of the drug may be incomplete or the drug is eliminated on the way to systemic circulation. This is called the pre-systemic or "first pass effect".

The relative bioavailability (_Frel) compares 2 oral dosage forms with each other.

The amount of pharmacological substances absorbed into the systemic circulation cannot be measured directly. However, since it is known that the area under the curve (Area under the curve = AUC), which describes the pharmacoconcentration in venous blood as a function of time, is directly proportional to the concentration in the systemic circulation.

According to the above definition, intravascularly applied pharmaceuticals (i.v., i.a.) are 100% bioavailable. The bioavailability (BF) indicates how quickly and to what extent pharmaceuticals are absorbed and thus the site of action is available. Bioenhancers can increase the bioavailability of a substance.