Approaches to improve intestinal and transmucosal absorption of peptide and protein drugs.

The oral bioavailability of hydrophilic and macromolecular drugs is generally poor owing to their poor membrane permeability. For example, peptide and protein drugs are poorly absorbed because of their low stability and poor membrane permeability in the gastrointestinal tract. Consequently, these drugs can be clinically administered only via injection. However, such frequent administration of injections subjects the patients to considerable pain, along with increasing the possibility of serious side effects. Several approaches have been examined to overcome the delivery problems associated with the poorly absorbed drugs. These include (1) use of additives such as absorption enhancers and protease inhibitors, (2) modification of the chemical structure of drugs to produce prodrugs and analogs, (3) application of dosage forms to entrap these poorly absorbed drugs, and (4) development of novel and alternative administration methods (apart from oral and parenteral administration). We examined these approaches and demonstrated their effectiveness in improving intestinal and transmucosal absorption of several poorly absorbed drugs. These approaches may provide useful and basic information to improve the intestinal and transmucosal absorption of poorly absorbed drugs including peptide and protein drugs.