Drug absorption from the GI tract is generally dependent on dissolution characteristics of a product which in turn is dependent on the aqueous solubility of the drug. In general, it is assumed that the higher the solubility, the higher the expected drug absorption will be, and vice versa.
Before considering the link between absorption and solubility, it should be prudent to define and establish the solubility characteristics of a drug for absorption purposes. In this regard, it is a well-known fact that drugs are mostly absorbed from the intestinal part of the GI tract (link). The liquid phase in the intestine is aqueous-based having a pH in the range of 5 to 7. For all practical purposes one may consider a pH of 6 (average of 5 to 7) for the intestinal fluid. Thus, to represent intestinal fluid, for dissolution testing, one may use water itself, which usually has pH around 6 or a (phosphate) buffer having a pH of 6. Therefore, in the following discussion, solubility of drugs in water will only be considered.
