Liposomes and Vesicular Carriers

Liposomes are spherical vesicles composed of one or more phospholipid bilayers enclosing an aqueous core, most commonly formulated from phosphatidylcholine...

Liposomes are spherical vesicles composed of one or more phospholipid bilayers enclosing an aqueous core, most commonly formulated from phosphatidylcholine and cholesterol, with the cholesterol serving to modulate bilayer fluidity and stability. Conventional liposomes, typically 100 to 400 nanometres in diameter, are rapidly recognised and cleared by the mononuclear phagocyte system, limiting their circulation time in systemic circulation and constraining their utility for applications requiring sustained exposure. PEGylated liposomes address this limitation through the incorporation of a polyethylene glycol-conjugated phospholipid, most commonly DSPE-PEG2000, into the bilayer, creating a hydrophilic steric barrier that reduces opsonisation and phagocytic uptake, extending circulation half-life to approximately forty-five hours in the well-characterised case of the anticancer product Doxil, a landmark example that established the clinical and commercial viability of the stealth liposome platform.

Targeted liposomes extend this concept further through the covalent conjugation of a targeting ligand — an antibody fragment, a folate moiety, or a transferrin molecule, among others — to the liposomal surface, enabling receptor-mediated endocytosis by cells overexpressing the corresponding receptor and thereby achieving active, cell-specific drug delivery beyond the passive accumulation afforded by PEGylation alone. Niosomes represent a related but chemically distinct vesicular platform, formed from non-ionic surfactants such as Span 60 in combination with cholesterol rather than phospholipids; their principal advantages over conventional liposomes are substantially lower raw material cost and superior chemical stability, since non-ionic surfactants are less prone to the oxidative degradation that can affect unsaturated phospholipids.

Transfersomes and ethosomes are specialised vesicular systems engineered specifically for transdermal delivery. Transfersomes incorporate an edge activator, commonly Tween 80, that confers exceptional bilayer deformability, enabling the vesicle to squeeze through pores in the stratum corneum substantially smaller than its own diameter under the influence of the natural transdermal hydration gradient. Ethosomes instead achieve enhanced skin penetration through a high ethanol content, typically twenty to forty-five per cent, which disrupts the tightly packed lipid organisation of the stratum corneum, fluidising the intercellular lipid domains and thereby facilitating vesicle and drug passage into and through the skin. Both platforms have found particular application in the transdermal delivery of molecules that would otherwise be excluded by the stratum corneum's formidable permeability barrier.