Lipids may be broadly described as amphiphilic but water-insoluble small molecules. The amphiphilic character of some lipids allows them to self-organize in aqueous solution where they assemble to micelles and bilayer membranes. Liposomes are artificially prepared vesicles composed of lipid membranes that can act as carriers for the delivery of hydrophilic and hydrophobic drugs. Liposome design may include surface modifications with PEGs to alter their pharmacokinetic properties and targeting ligands to home them towards specific tissues.
Liposomal formulations for drug delivery usually contain different lipid molecules, each one complying with a specific task:
Cationic lipids (e.g. DOTAP) are routinely used as transfection agents for oligonucleotides in vitro and recently also in clinical studies for the delivery of short interfering RNA (siRNA). Natural phospholipids like DOPE act as helper lipids, which increase the transfection potency of liposomal formulations containing cationic lipids. For pharmaceutical applications it is essential to work with highly defined, characterized and ultra pure compounds. Stability can be an issue.