ABSTRACT
Assessment of the absorption of a given compound from the gastrointestinal (GI) tract should, in fact, simply mean establishing the permeation of this compound across the wall of the gastrointestinal tract, but there are more factors involved in the net result. Apart from the permeation resulting in translocation, there are also factors that have a negative influence on the absorption of drugs from the gastrointestinal tract. These include the dissolution properties of the compound, which may be affected by physiological components such as gastric juice and bile, precipitation at the absorption site, adsorption to components in the gastrointestinal tract, chemical and/or bacterial degradation, and the metabolism of the compound in the lumen, at the brush border, or in the intestinal wall. No method can assess all these aspects simultaneously, with the exception of studies in intact animals or in humans. Whole animal or human studies suffer two drawbacks: first, they are unsuitable for screening large numbers of compounds at an experimental stage. Second, they pose ethical difficulties if the pharmacological effects and side effects are insufficiently well defined. As a result, a great variety of alternative methods have been developed to assess the permeation characteristics of new drugs . It should be kept in mind that each of these methods covers only some of the foregoing aspects, and does not take into account other factors that may be important
to the bioavailability of the compound. These can be to assess or predict the availability after oral administration and are based on methods ranging from the calculational, through in vitro uptake studies to perfusions in humans 1).
