Stealth Liposomes (PEGylated Liposomes) as Drug Carrier System for Drug Delivery Lipids are amphiphilic molecules with water-loving (hydrophilic) and water-hating (hydrophobic) parts of the molecule. When lipids are placed in contact with water, the unfavorable interactions of the hydrophobic segments of the molecule with the solvent result in the self assembly of lipids, often in the form of liposomes. Liposomes consist of an aqueous core surrounded by a lipid bilayer, similar to a membrane, separating the inner aqueous core from the bulk outside. They were first discovered by Alec D Bangham in the 1960s at the Babraham Institute, University of Cambridge. They are valued for their biological and technological advantages, and are considered to be the most successful drug-carrier system known to date.
Representation of the steric organization of a micelle (left), a liposome (center), and a lipid bilayer (right).
Liposomes have been used to improve the therapeutic index of new or established drugs by modifying drug absorption, reducing metabolism, prolonging biological half-life or reducing toxicity. Drug distribution is then controlled primarily by properties of the carrier and no longer by physico-chemical characteristics of the drug substance only. Despite all the hopes invested in conventional liposomes, they have presented various problems and pharmacological implications over the years. A major drawback of conventional liposomes is their quick capture by the RES. Liposomes are mainly accumulated in the liver and the spleen, due to their generous blood irroration and the abundance of tissue-resident phagocytic cells. This is extremely advantageous in the case of local infections: the high concentration of antimicrobial