ABSTRACT
Drugs administered for their systemic effect have to be absorbed to the systemic circulation and become available at the site of action before they can produce their therapeutic effect. This means that the amount of the drug absorbed to the systemic circulation, rather than the labeled amount of the drug, is what produces the therapeutic effect. The concept of BA, which is a measure of the extent of drug absorption to the systemic circulation, is important in the drug development process and also in evaluating pharmaceutical products. The physiological, physicochemical, and formulation factors that can affect the rate and extent of drug absorption after extravascular drug administration have been discussed in previous chapters. So it
is possible that multisource drug products, meaning different drug products for the same active drug manufactured by different manufacturers, can produce different degrees of disease control. The cause of this difference in therapeutic effect can be due to variation of the extent of drug absorption to the systemic circulation and the rate at which the drug reaches the systemic circulation after administration of the different products. This has added a new dimension to the issue of quality of drug products, which should include the BA and BE of drug products. Inclusion of BA determination and proof of BE as regulatory requirements are important to ensure efcacy of drug products and stability of the therapeutic effect when the patient switch between products for the same active drug.
