DRUG EXSORPTION FROM BLOOD INTO THE GASTROINTESTINAL-TRACT

Drugs are exsorbed from the blood across the gastrointestinal membrane s by passive or active processes. In the case of a passive transport m echanism, the exsorption of drugs depends on the concentration gradien ts between the serosal and mucosal sides. The extent of secretion (exs orption) is determined by numerous factors such as extent of binding t o serum proteins, distribution volume, lipophilicity, pKa and molecula r size of drugs, and the blood flow rate in the gut. Specific transpor t systems such as P-glycoprotein (P-gp), organic cation and organic an ion transporters are found to be involved in active intestinal secreti on of drugs. Intestinal secretory transport systems reduce the extent of drug absorption sometimes resulting in low oral bioavailability. It is, therefore, important to know whether poor drug absorption is due to the involvement of specialized secretory transport systems. Modulat ion of intestinal secretory transport can be a means to enhance absorp tion of drugs with low oral bioavailability if exsorption of drugs is based on active secretion pathways that are open for control from the ''outside.''