ABSTRACT
The oral route is the preferred way of dosing, because this is the easiest and most convenient way of noninvasive administration. Most drug substances that are applied orally today are small molecules that can permeate the intestinal gut membrane by transcellular passive diffusion. This process is determined by physicochemical laws and by the properties of the intestinal cells. In addition to its permeability through the gut wall, the availability of a drug in the body depends on its ability to dissolve in the gastrointestinal (GI) fluids . This too can be influenced by both physiological and physicochemical factors. This chapter will focus on the prediction of fraction dose absorbed, based on solubility, for drugs that are transported across the gut wall by passive mechanisms.
