Bioavailability Extent Of Absorption

If a drug is administered by an extravascular route and acts systemically, its potency will be directly related to the amount of drug the dosage form delivers to the blood. Also, if the pharmacologic effects of the drug are related directly and instantaneously to its plasma concentration, the rate of absorption will be important because the rate will influence the height of the plasma concentration peak (response) and the time at which the peak occurs (onset). Thus, the bioavailability of a drug product is defined in terms of the amount of active drug delivered to the blood and the rate at which it is delivered.

Whenever a drug is administered through an extravascular route, there is a danger that part of the dose may not reach the blood (i.e., absorption may not be complete). When the IV route is used, the drug is placed directly in the blood; therefore an IV injection is, by definition, 100% absorbed. The absolute bioavailability of an extravascular dosage form is defined relative to an IV injection. If IV data are not available, the relative bioavailability may be defined relative to a standard dosage form. For example, the bioavailability of a tablet may be defined relative to an oral solution of the drug.

In the section "Kinetics Of Drug Absorption" we dealt with methods of determining the rate (and mechanism) of absorption. In this section we will deal with methods of determining the extent of absorption. In every example, the calculation will involve a comparison between two studies carried out in the same group of volunteers on different occasions. Usually, it will be necessary to assume that the volunteers behaved identically on both occasions, especially with regard to their pharmacokinetic parameters.

0 0

Post a comment