Liposomes - A Drug Delivery System

Liposomes are spherical vesicles composed of one or more phospholipid bilayers that can encapsulate a variety of substances, such as drugs, genes, vaccines, enzymes, and cosmetics. Liposomes were first discovered in 1961 by Alec Bangham, who observed that phospholipids spontaneously formed closed structures when hydrated in water. Since then, liposomes have been widely used in small molecule drugs, proteins, nucleic acids and imaging agents. In order to enhance the therapeutic effect and improve patient compliance, various administrations for liposomes have been developed, such as parenteral, pulmonary, oral, transdermal, ophthalmic and nasal administration. medicine etc. In addition, liposomes are also widely used in food, cosmetics and other fields.

Fig. 1 Structural illustration of liposome composition (Liu, P.; et al. 2022).

How are Liposomes Classified?

Liposomes can be classified according to different criteria, such as size, number of bilayers, composition, and preparation method.

Liposome as a Drug Delivery System

Liposomes have several advantages as delivery systems, such as biocompatibility, biodegradability, low toxicity, high stability, and versatility. Liposomes can protect the encapsulated substances from degradation or elimination by the body's defense mechanisms, and can also target specific tissues or cells by modifying their surface properties. Liposomes can also enhance the solubility, bioavailability, and efficacy of poorly soluble or unstable drugs.

Fig. 2 Structural and design considerations for liposomal drug delivery (Çağdaş, M.; et al. 2014).

To design a liposome drug delivery system, several aspects need to be considered, such as:

• The choice of lipids and their ratio, which affect the stability, fluidity, charge, and permeability of the liposomal membrane.
• The method of liposome preparation and drug loading, which influence the size, shape, lamellarity, encapsulation efficiency, and drug release kinetics of the liposomes. There are various techniques for liposome production, such as sonication, extrusion, reverse-phase evaporation, freeze-thawing, solvent injection, and microfluidics.
• The characterization of liposomes, which involves measuring their physical, chemical, and biological properties, such as size distribution, polydispersity index, zeta potential, morphology, phase behavior, drug content, and in vitro drug release. Different methods can be used for liposome characterization, such as dynamic light scattering, electron microscopy, differential scanning calorimetry, Fourier transform infrared spectroscopy, and chromatography.
• The functionalization and targeting of liposomes, which involve modifying the surface of liposomes with ligands or antibodies that can recognize specific receptors or antigens on the target cells or tissues. This can enhance the specificity and selectivity of liposome delivery and reduce unwanted side effects.
• The stability and limitations of liposomes, which include factors such as oxidation, hydrolysis, aggregation, leakage, and clearance of liposomes in vivo. These factors can affect the shelf-life and performance of liposome formulations and require optimization of storage conditions and formulation parameters.

Several liposome products have been approved by regulatory agencies and used in clinical practice for different indications. Some examples are doxorubicin, ciprofloxacin, and cytarabine. These products demonstrate the potential and versatility of liposomes as a drug delivery system.

References

1. Liu, P.; et al. A review of liposomes as a drug delivery system: current status of approved products, regulatory environments, and future perspectives. Molecules 27.4 (2022): 1372.
2. Çağdaş, M.; et al. Liposomes as potential drug carrier systems for drug delivery. Application of Nanotechnology in Drug Delivery 1 (2014): 1-50.