Intestinal transport as a potential determinant of drug bioavailability.

Orally administered drugs are generally absorbed by the small intestine and transported either to the lymphatic system or to the hepatic portal system. In general, lipid soluble drugs and vitamins are transported by the small intestine to the lymphatics, and water-soluble drugs are transported to the hepatic portal system. By avoiding the early hepatic first pass effect, the lymphatic transport system may increase drug bioavailability. In addition to its transport systems, the small intestine may affect drug bioavailability through drug uptake, intestinal first pass effect, recruitment of drugs by chylomicrons, formation and secretion of chylomicrons, and enterohepatic circulation. All of these factors should be considered when formulating orally administered lipophilic drugs. Our data also suggest that Caco-2 cells may serve as a valuable in vitro model to study the intestinal transport of orally administered drugs.