Oral administration of medication is the first option when patient compliance is considered.
However, many barriers face oral absorption of drugs that limit bioavailability in about 90% of therapeutic
agents. Utilization of nanoparticulate drug delivery systems is a major strategy for increasing oral
absorption. They can improve oral bioavailability through mechanisms such as protection of the drug in
the GI tract, increasing cellular contact and residence time of the drug, protection of the drug from presystemic
metabolism and efflux and increasing diffusion across the mucosal and epithelial layers.
Liposomes are biocompatible carriers employed to improve oral bioavailability of drugs and in addition
to the general advantages of nanocarriers for oral delivery, they offer benefits derived from their lipidic
bilayer structure. They can better adhere to biomembranes, form mixed-micelle structures with bile salts
to increase the solubility of poorly-soluble drugs and are suitable candidates for lymphatic uptake. They
have been successful in improving oral bioavailability of a variety of compounds including peptide and
proteins, hydrophilic and lipophilic drugs. Stability under GI conditions is the main concern for oral
liposomes, however, promising approaches have been suggested to increase the stability of oral
liposomes. These include: using appropriate lipid compositions, polymer coating, addition of stabilizing
lipids to liposomal structures, preparation of double liposomes and proliposomes and some other innovative
methods. The present review focuses on the role of liposomes in improving oral absorption of
drugs, the problems encountered, and the types of liposomes designed to overcome these issues. Barriers
to oral delivery will be discussed and examples of bioavailability enhancement upon encapsulation
in various types of liposomes investigated.