Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Lipid Metabolism
Liposomes

Liposomes are artificially prepared lipid vesicles consisting of one or more phospholipid bilayers separated by an aqueous phase. The diameter of liposomes can range from 25 to 10000 nm.

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Fig. 11.8. Model of a multilamellar liposome with encapsulated Water- and lipid-soluble drugs (according to Gregoriadis).

1 - molecules soluble in the aqueous layer; 2 - molecules soluble in the lipid layer; 3 - molecules soluble in the aqueous layer with hydrophobic radicals penetrating into the lipid layer.

Liposomes are typically obtained by shaking or sonicating aqueous Suspensions of Phospholipids. They can be formed from individual phospholipids, either natural or synthetic, as well as from mixtures of phospholipids.

Initially, liposomes were used exclusively as models of Introduction/36.html">Biological Membranes. Later, it was established that they can serve as microcontainers capable of delivering various medicinal agents to different Organs and Tissues. Liposomes can encapsulate Enzymes, Hormones, Vitamins, Antibiotics, cytostatics, Cyclic NUCLEOTIDES, etc.

Fig. 11.8 presents a model of a multilamellar liposome and illustrates the distribution of water- and lipid-soluble drugs encapsulated within the liposome.

The Use of liposome-drug complexes offers several advantages over the administration of free drugs: liposomes allow the delivery into Cells of substances that would otherwise fail to penetrate them; attaching appropriate Antibodies (vectors) to liposomes can ensure targeted delivery to specific cells; drugs encapsulated in liposomes provide an enhanced therapeutic effect (prolonging the duration of action while allowing a significant dose reduction); liposomes can be effectively used as adjuvants, i.e., substances that stimulate immune responses. It is hypothesized that there are at least two pathways for liposome cellular uptake. The first involves endocytosis, whereby the liposome is engulfed by The Cell to form a vacuole that subsequently fuses with Lysosomes. Lysosomal phospholipases hydrolyze the Lipids of the liposomal membrane, releasing the drug into the Cell Cytoplasm. If the liposome consists of multiple lipid membranes, their gradual Hydrolysis ensures a sustained release of the drug into the cell. The second pathway involves fusion with The cell membrane, where the lipid component of the liposome integrates into the cell membrane while the water-soluble drug enters the cytoplasm. Thus, in both cases, the substance encapsulated in the liposome successfully crosses the membrane barrier to enter cells.

Animal experiments have demonstrated that upon intravenous, intramuscular, and intraperitoneal administration, liposomes rather rapidly leave the bloodstream as they are captured by Cells of the macrophage system, primarily in The Liver and Spleen. Other organs and tissues take up a certain amount of administered liposomes, though their fraction remains relatively small. Currently, research is underway to develop new systems (approaches) for targeted drug delivery to the body using liposomes.