Liposomes as drug carriers

Liposomes as a drug delivery system

Liposomes are microscopic spherical vesicles. These vesicles have a great impact when it comes to the field of medicine. Liposomes have been used as a way of delivering drugs, which could possibly make it the way of the future. However, there are some risks that may prevent it from achieving that potential.

A liposome containing a certain drug, or genetic material, can be delivered past the cellular lipid bilayer by fusing with the cell’s bilayer. To do this both membranes need to come in very close contact of each other. The liposome fuses with the outer layer of the plasma membrane, then the two fused membranes unite as the inner layers of both membranes get closer to each other as the drug is delivered as both layers fuse.

Basic structure

Cholesterol in a conventional liposome

The outer part of a liposome, the membrane, is composed of a phospholipid bilayer enclosing in an aqueous volume. Phospholipids are the main building block of liposomes. They have tubular shape and two acyl chains attached to a polar head, which with hydration results in a bilayer, with a hydrophilic head and two hydrophobic tails. This combination means that liposomes are amphiphilic. Liposomes can either be naturally derived phospholipids or of pure surfactant components like DOPE.

Cholesterol is important for liposomes as it is used as a membrane additive to fill up the empty spaces between the phospholipids. Cholesterol increases the fluidity of the cell’s membrane and provides an increase in the order of the bilayer as it anchors the components of the bilayer more strongly. This increases the transition temperature of the system and provides stability.  

  === Inroducing the drug === Depending on the molecule we wish to introduce, it can either be introduced into an aqueous space or intercalated into the lipid bilayer. This depends on whether the molecule is hydrophobic or hydrophilic. Unilamellar vesicles, which are liposomes with one lipid bilayer, are optimally used with water soluble drugs as they contain the largest aqueous core. However, when lipid soluble drugs are introduced, a multilamellar vesicle is used, as it can passively entrap the drug between its multiple lipid bilayers structured like an onion skin.   

Hydrophobic and hydrophilic drug placement inside the liposome

Liposomes containing a certain drug, or genetic material can be delivered past the cellular lipid bilayer by fusing with the cell’s bilayer. To do this, both membranes need to come in very close contact of each other. The liposome fuses with the outer layer of the plasma membrane, then the two fused membranes unite as the inner layers of both membranes get closer to each other, the drug is delivered as both layers fuse.

The method of preparing liposomes varies depending on the intended use, and the type of drug being used. A general method of preparation includes hydrophilic materials, which are are entrapped to be used as the hydrating fluid. More simply, the drug could be added during some stage of the lipid formation the lipophilic materials are solubilized in an organic solution pertaining to the lipid. The solvent is evaporated, leaving behind the solute. It is then dispersed it into an aqueous solution for hydration. It is somewhere during these stages that the drug is introduced. However, remote loading of ionizable groups can be introduced to the lipid. Finally, the resulting liposome is purified and analyzed. Also, liposomes of varying sizes be used to target certain endocytosis events. This way, the drug is delivered once the liposome has been digested. This could be made more effective by adding opsonins and ligands to the liposome.  Also, the drug can be used to naturally diffuse through the cell by naturally neutralizing it. A liposome with high or low pH is used and a charged aqueous drug is introduced into the lipid to be neutralized.