Stomach, selective permeability

Central to the osmosis phenomenon is the semipermeable membrane (SPM), whose physical properties and species-selectivity directly govern the kinetics and thermodynamics of osmotic flow. Naturally occurring biomembranes of high selectivity, permeable to water but not to other solutes, are ubiquitous, for example, in macroscopic stomach linings and blood vessels, as well as in the microscopic cell membranes that encapsulate all known cell types. Some common synthetic membranes, such as Gore-Tex and cellophane, also exhibit selective permeability and osmotic activity. 

Aqueous solubility of ionizable molecules at different pH values is an important characteristic because it indicates the potential substance behavior in the stomach and intestinal tract and its potential impact on bioavailability. Moreover, it also provides important information for formulation scientists to define the class of a drug substance in the Biopharmaceutics Classification System (BCS), a regulatory guidance for bioequivalence studies. The BCS is a scientific framework proposed by the FDA to classify drug substances based on their aqueous solubility and intestinal permeability and defines important parameters in the selection of drug candidates into development. According to the BCS, drug substances are classified as shown in Table 12-4. 

---