Liposomes

Abstract Liposomes are synthetic vesicles consisting of one or more phospholipid bilayers, able to accommodate water‐ and lipid‐soluble molecules. They are used as a delivery system for drugs, genes and vaccines in therapeutics. Liposomes may be formulated with a range of characteristics including different size, charge and drug retention, which can be tailored for a given drug and target site. There is a range of clinical products approved for use which exploit liposomes to passively target drugs or vaccines to the appropriate site of action thereby improving specificity and reducing toxicity. Liposomes can also be actively targeted to specific cells or subcellular regions using targeting ligands attached to their surface, or by modification of the bilayer to give triggered release under appropriate conditions. Key concepts: Liposomes are bilayer constructs formed when amphiphiles are exposed to water. Liposomes can be formulated in different sizes, with different bilayer melting points and with different surface charges allowing for optimization to suit a given drug and target site. The lipid head‐group dictates the surface charge of the liposomes and the acyl tail influences the melting point of the lipid bilayer and its permeability and therefore influences the drug release rates from liposomes. The presence of cholesterol within the bilayers can reduce their permeability and drug leakage. In commercial products, phosphatidylcholines in combination with cholesterol are commonly used to formulate the liposomes. Drugs can be loaded within the aqueous compartment of the liposomes or within the lipophilic region of the bilayers depending on the lipophilicity of the drug. Through electrostatic interactions, molecules can also be adsorbed on the surface of liposomes. Currently, liposomes are used for the delivery of drugs, for example cancer chemotherapy, systemic fungal infections and vaccines. Cationic liposomes are being investigated as potential delivery systems for a range of nucleic acid therapies and for subunit vaccines.