ABSTRACT
However, the impaired systemic availability of many drugs following oral administration is not always fully understood. Not infrequently, impaired availability is attributed to poor permeability or incomplete dissolution. Because oral formulations play such a dominant role in drug therapy, qualitative explanations of this kind are unsatisfactory. Research should show and quantify the reasons for reduced availability. Special attention should be focused on extended-release (ER) formulations and drug substances with low solubility or slow dissolution rates. For these, it becomes a crucial factor whether the drug is absorbed along the intestinal tract at the same rate and extent or whether either the rate or the extent differs for the various segments of the intestinal tract. The period during which a formulation or drug substance resides in a particular segment of the gastrointestinal tract (GIT) will become a decisive factor for drugs with a limited rate of absorption or that are absorbed from only a particular segment of the GIT.
