Liposomes - properties and bursting

Introduction
Liposomes are synthetically constructed phospholipid vesicles that consist of at least one phospholipid bilayer. These are obtained by methods based on lipid diffusion in water.

Because of their characteristic bilayered structure, liposomes have the ability to cross cell membranes and therefore, can be used as a means of transport for the administration of nutrients as well as certain types of drugs. Upon reaching a specific pre-determined site within the body, the liposomes can burst or be broken down to release potentially lifesaving drugs.

The name liposome is derived from two Greek words: 'Lipos' meaning fat and 'Soma' meaning body. Liposomal vesicles are made up of a lipid bilayer surrounding the inner volume, that contains an aqueous solution. The vesicle membrane allows water to be exchanged between the outer and inner parts of the liposome. As the liposome is of enormous theoretical interest, being based around the simplest figure of a biological cell, they also play an important role in drug delivery.

Why Liposomes are Important in Clinical Medicine
Currently, clinical medicine contains a vast range of drug molecules in use, and the clinical usefulness of most of these therapeutics is limited by the inability of these drugs to reach their target tissues or by the harmful effects that are caused on healthy tissues and organs. Although various approaches have been tried to surmount these problems by attempting to create site-specific drugs that incorporate the idea of “selective delivery”; the real idyllic solution would be to create medication that targets only those cells or tissues that are affected by disease (1).

The study of liposomes has been particularly fascinating in the field of drug delivery and the reason lies in the liposomes’ characteristic properties, which makes them both biocompatible as well as biodegradable. The structure of a liposome consists of an aqueous core encapsulated by one or several bilayers that are composed of either natural or synthetic phospholipids. Liposomes that are composed of naturally occurring phospholipids are biologically unreactive and indistinguishably immunogenic, and possess a low intrinsic toxicity. As a result of this, drugs with different lipophilicities can be entrapped into liposomes - strongly lipophilic drugs are incorporated almost completely into the lipid bilayer, whereas strongly hydrophilic drugs are incorporated completely in the aqueous interior (2).

Literature Relating to the Properties of Liposomes
Table 1 shows the classification of different liposomes (3)

Liposome-entrapped drugs are divided within the body in a different way than free drugs. In most living organisms, vesicles accumulate in the liver, spleen, lung, bone marrow and lymph nodes when they are injected. This is unlike liposomes, which accumulate at the sites of inflammation and infection and in some solid tumors.

Prospects in the Use of Liposomes

New research in the field of liposomes has allowed liposomes the ability to avoid detection by the immune system. These are now known as stealth liposomes.

Liposomes can have various molecules attached to their surface. The most common surface modification is PEGylation; in which the polymer polyethylene glycol is covalently linked to the surface of a liposome. Small PEGylated liposomes have a longer circulatory life in the bloodstream than plain liposomes as the PEG coating is unreactive in the body.

In addition to the PEG, stealth liposomes may also have a biological species attached to them, thus allowing site-specific drug delivery. One such example could be antibodies, which can then be attached to these liposomes for targeting purposes. Many studies now show that targeting is more effective if the antibody is attached to a spacer (like PEG) rather than directly to the liposome surface. Another advantage of this is that naturally toxic drugs may be less toxic if they are delivered only to the sites of disease (5).

Conclusion
In conclusion, the studies into the properties and further uses of liposomes will continue to grow. As more advances are made into the use of liposomes as drug carriers, we will slowly but surely begin to see the use of this method of drug delivery to target even the most difficult of diseases. There is research into the use of liposomes in the treatment of tumours as well as the possibility of using liposomes containing positively charged lipids (such as the DOTAP liposome) as transport vehicles for negatively charged DNA and RNA through electrostatic interactions in the form of lipoplexes to cells both in vitro and in vivo.

The way in which liposomes enclose drugs represents a new drug delivery system that appears to offer important, therapeutic advantages over pre-existing methods of drug delivery.