Unilamelar liposome.

Definition Liposomes are extremely small vesicles (in the nanometres range) mainly composed by phospholipids organized in a bilayer. These vesicles contain an internal watery phase and are suspended in an external watery phase.

Composition
Lipids are used for its preparation, basically natural or related compouds. These molecules are characterized by having a hydrophilic region and a lipophilic region that once hydrated tend to associate forming bilayers.

Classification
Liposomes are classified by their size –small and big- and by the number of bilayers – uni-, oligo-, or multilamellar. Each type has its own potential application. In pharmaceutical preparations the most used are the unilamellar liposomes (40-250 nanometres). To get the different types of liposomes, distintc preparation techniques and lipidic composition are used. Due to their composition and structure liposomes can incorporate –separately or altogether- liposoluble and hydrosoluble active principles. The liposoluble one are incorporated in the external lipidic surface while hydrosoluble ones are incorporated basically in the interior watery phase.

Comparative study of the influence of the composition and type of liposome in "leakage" of carboxifluoresceina .

Uses
Liposomes are basically used to transport active principles in a very selective manner to in increased its efficacy and ti decrease the non-desired effects of them (toxicity). In other cases liposomes are utilised to prolong the action, to improve the absorption, to change the route of administration or simply to stabilise an active principle.

Liposomes have also been utilised in gene transference and in diverse diagnostic and industrial applications. Their size and physicochemical characteristics make these structures circulate, penetrate and diffuse in the tissue liberating the encapsulated product in a different way compared to the free product or other galenic formulations.

Different strategies to improve the specificity and type of interleave of the liposome.	Specificity The characteristics and properties of the different liposomal formualtions are determined by the preparation technique, lipidic composition, charge, active product, etc. All these variables have an important influence on the behaviour of the liposome final active product.
The size as well as the molecular interactions between the cell and the liposome must be considered in order to achieve the most effective targeting. For instance, ultra-specific targeting has been further achieved by attaching monoclonal antibodies to the surface of liposomes.

Membrane interaction
Liposomes may interact with the cells in different ways: fusion, adsortion, endocytosis and phagocytosis. Depending on the liposome and the cellular type one or the other interaction will take place. Different kind of interaction between liposomes and cells have demonstrated that are highly dependant on the liposome structure and composition.

Different kinds of interaction between liposomes and cells:

Different kinds of interaction between liposomes and cells.

Image: Endocytosis. Degradation in the lysosome. Adsortion. Fusion. “Lipidic change”.

Routes of administration
Liposomes have been administered by all possible medical routes: topical, transdermal, oral, intraperitoneal, ocular, intramuscular, subcutaneous, inhaled, etc., having achieved positive results in each one of them.

More frequent applications
In an attempt to improve the therapeutic properties the most frequent encapsulated active principles so far have been: anti-fungal, anti-cancer, immunosupressants, antibiotics, hormones, anti-inflammatory and analgesics.

Empty liposome
Additionally, one ought to take into account the possibility of the liposomes as a structure without active principles. In this way multiple activities have been studied: anti-inflammatory, anti-hypertensive, anti-viral, semen capacitation, etc. Liposomes inteding to emulate organules and cell (artifical blood) have also been developed. It m ay be in this field that the future is most promising and results encouraging.

In section "Practical examples with Transtechnics Liposomes" some of the results obtained using liposomes with different active principles through various ways of administration are presented. In all of the test a clear improvement in the liposomal formulation compared to the free form may be appreciated.