The most convenient route of drug administration is oral. Along with the presence of proteolytic enzymes in the gastrointestinal tract, the intestinal mucosa represents the main barrier to the absorption of orally administered drugs into the systemic circulation. To overcome this biological barrier, drugs may cross the intestinal wall via transcellular or paracellular route. The transcellular pathway involves the passage of drugs through the cells, while the paracellular pathway refers to the passage of drugs between the adjacent cells, which is mostly restricted by tight junction proteins. The preferred pathway for absorption or transport of a specific drug depends on its physicochemical properties as well as the biological membrane features. Generally, lipophilic drugs pass through biological membrane by transcellular pathway, while hydrophilic drugs tend to path through the membrane paracellularly. Since absorption potential has become an important criterion in the discovery process, there is a need for reliable screening methods to assess compound permeability. Intestinal absorption screening assays should be highly predictive, fast, reliable, cost-effective, and require a small amount of compound.
Drug discovery scientists use many techniques to evaluate the intestinal permeability of drug candidates during the drug selection process. The most common preclinical methods currently used throughout the industry are: in vitro methods, for example, Ussing chamber or membrane vesicles based on animal tissue; cell-based assay systems such as Caco-2 cells and Mardin-Darby canine kidney (MDCK); artificial lipid-based systems such as parallel artificial membrane permeability assay (PAMPA) or immobilized artificial membranes (IAM); in vivo methods (animal pharmacokinetic studies); in situ methods (single-pass perfusion); and in silico (computer-aided drug design) methods. One, or a combination of these models, is commonly used in permeability assessment in drug discovery. A tiered approach is often used, which involves a high-throughput (but less predictive) model for primary screening, and then a low-throughput (but more predictive) model for secondary screening and mechanism study. The cell culture models strike the right balance between predictability and throughput, and are therefore the method of choice for permeability assessment across the pharmaceutical industry.