Enhanced Diagnostic Tools
What are liposomes?
Liposomes were first described by Bangham in 1965 while studying cell membranes. He found that lipsomes are vesicular structures consisting of hydrated bilalyers which form spontaneously when phospholipids are dispersed in water. Since this, further studies into liposomes and their application in various fields such as medicine and research have been explored.
Phospholipids are the main component of naturally occurring bilayers. These phospholipids include phosphatidylcholines (PC), phosphatidylethanolamines (PE) and phosphatidylserines (PS) . The key common feature that bilayer-forming compounds share is their amphiphilicity ie., they have defined polar and non-polar regions. This is the reason the non-polar regions orientate themselves towards the interior away from the aqueous phase, the polar regions being in contact with it.
Liposomes can be classified by either their structural properties or the basis of their preparation. The main types are listed and their characteristics are outlined in the table below.
Number of lipid bilayers
Small unilamellar vesicles
Diameter of 20-100nm.
One lipid bilayer
Large unilamellar vesicles
Diameter of >100nm.
Diameter of >0.5mm.
Five to twenty lipid bilayers
Diameter of 0.1-1mm.
Approximately five lipid bilayers
Diameter of >1mm.
Multicompartmental structure 
Preparation of liposomes
The Tc can vary between 15°C for egg yolk phosphatidylcholine (high degree of unsaturation) to over 50°C for fully saturated distearolylphosphatidylcholine (DSPC) .
The raw material for liposome formation depends on the intended use of the liposome. Several companies supply reasonable grade and priced lipids which usually contain at least 98% phospholipid and less than 1% lysophospholipid, low endotoxin and microbial load and trace metals. It is up to the individual investigator to purify the lipid to acceptable standards.
There are five main groups of phospholipids that are available that can be used for liposome preparation.
1. Phospholipid from natural sources.
2. Phospholipid modified from natural sources.
3. Semi-synthetic phospholipid.
4. Fully-synthetic phospholipid,
5. Phospholipid with non-natural headgroups .
Phosphatidylcholine (PC), phosphatidylenolamine (PE) and phosphatidyserine (PS) are commonly used phospholipids for liposome preparation. Cholesterol can be added to the bilayer mixture to reduce the permeability of fluid crystalline state bilayers.
There are many different strategies for the preparation of liposomes, which can be classified into 3 main groups.
1. Mechanical methods
A. Film method.
The original method of Bangham et al. is still the simplest procedure for the liposome formation but is limited because of its low encapsulation efficiency. This technique produces liposomes by hydrating thin lipid films deposited from an organic solution on a glass wall by shaking at temperatures above the Tc. The solvent is removed at reduced pressure in a rotary evaporator. The dry film of lipids which has been deposited onto the wall of a round-bottom flask is hydrated by adding a buffer with a water soluble marker. As the lipid becomes hydrated and starts to form into closed vesicles only a small amount of the solute becomes entrapped. This method yields a heterogeneous sized population of MLVs over 1mm in diameter. Further procedures must be employed to achieve a homogeneous population, which will be discussed later .
B. Ultrasonication method.
Ultrasonication of an aqueous dispersion of phospholipids with a strong bath sonicator or a probe sonicator will usually yield SUVs with diameters down to 15-25nm.
2. Methods based on replacement of organic solvent.
A. Reverse-phase evaporation.
In this method, several phospholipids (pure/mixed with cholesterol) can be used. The lipid mixture is added to a round bottom flask and the solvent is removed under reduced pressure by a rotary evaporator. The system is purged with nitrogen and the lipids are re-dissolved in the organic phase. This is the phase that the reverse phase vesicles will form. Diethly ether and isopropyl ether are the usual solvents of choice.
After the lipids are re-dissolved in this phase the aqueous phase (contains compound to be encapsulated) is added. The system is kept under continuous nitrogen and the two-phase system is sonicated until the mixture becomes a clear one-phase dispersion. The mixture is then placed on the rotary evaporator and the organic solvent removed until a gel is formed. Non-encapsulated material is removed. The resulting liposomes are are called reverse-phase evaporation vesicles (REV). The large unilamellar and oligolamellar vesicles formed have the ability to encapsulate large macromolecular vesicles with high efficiency .
B: Ether vaporisation method.
In this method a mixture of lipids in an organic solvent (diethyl ether, ethanol, etc.) is slowly injected into a warm aqueous solution. This results in osmotically active, unilamellar vesicles with a well defined size distribution and high volume trapping efficiency (about ten times that of sonicated and hand shaken preparations .
3. Methods based on size transformation or fusion of preformed vesicles.
A: Freeze-thaw extrusion method.
Liposomes formed by the film method are vortexed with the solute to be entrapped until the entire film is suspended and the resulting MLVs are frozen in a dry ice/acetone bath, thawed in lukewarm water and vortexed again. After two additional cycles of freeze-thaw and vortexing the sample is extruded three times. This is followed by six freeze-thaw cycles and an additional eight extrusions. The resulting liposomes are called large unilamellar vesicles by extrusion technique (LUVET) and they typically contain internal solute concentrations which are much higher than external solute concentrations ie. , they have entrapment ratios greater than one . Proteins can be effectively encapsulated using this technique .
B: The dehydration-rehydration method.
This method begins with empty buffer containing SUVs (handshaken MLVs can be also be used but are usually not preferred). These are mixed with the component to be entrapped, after which they are dried. Freeze-drying is often the method of choice but other methods such as by vacumn or under a stream of nitrogen can be used. The vesicles are then rehydrated. A mechanism has been proposed whereby as the vesicles become more concentrated during dehydration, they flatten and fuse forming multilamellar planes where the solute is sandwiched. Therefore on hydration, larger vesicles are formed. This technique is mild and simple, the main limitation being the heterogeneity of the size of the size of the liposomes .
Sizing of the liposomes.
Size characteristics of liposomes have a major effect on their fate ie, what applications they can be used for. Therefore, liposome production procedures must generate predictable and reproducible particle size distributions within a certain size range.
Sizing of liposomes (if the population is heterogeneous) is usually performed by sequential extrusion at relatively low pressures through polycarbonate membranes. It is easy, reproducible, no detectable degradation of the phospholipids takes place and it can double the encapsulation efficiency of the liposome preparation. Membranes of pore size 0.2mm will yield liposomes of ~0.27mm. It is necessary to seal the membrane holder tightly to avoid leaks .
Gel chromatography can also be used to size liposomes but more typically used to remove un-encapsulated components by seperation . Sonication is another process that is widely applied when sizing liposomes. Probe sonication is used rather than bath sonication and it produces small unilamellar vesicles of ~20nm. There are many disadvantages associated with this technique:
These problems can be avoided with the use of bath sonicators but reproducible results are difficult because of the number of varying parameters associated with such baths ( level and temperature of water, position of liposome in the bath, etc.) .
Liposomes in immunoassays.
The liposome immunoassay system is usually based on membrane immunochemistry and the release of a detectable marker.
In an enzyme linked immunosorbent assay (ELISA) the enzyme label generates a measurable amount of product which is proportional to the unknown concentration of an antigen. These ELISAs can be either competitive or non-competitive, direct or indirect of which there are many variations.
In a non-competitive assay an antigen is sandwiched between two antibodies. The solid phase antibodies attach onto the plate surface where they capture the antigen (usually the component being measured) and the detecting antibody is added which is labelled with enzyme. A substrate is then added which generates a signal. The amount of antigen is proportional to the signal produced
In a competitive assay the two reactants are competing to bind a third, the third of which is limited. The reactants can be two antibodies (one labelled) or two antigens (one labelled). The assay is called an immunoassay is the antibody is labelled and an immunometric assay if the antigen is labelled.
In a direct ELISA, the primary antibody is labelled whereas in an indirect ELISA the primary antibodies are not labelled, secondary antibodies which are directed against the antigen . A substrate is usually added as the last step to develop the colour and results in detection of the unknown component. There are a number of limitations with ELISAs. An indirect sandwich assay can take up to six hours complete and the sensitivity of the assay is limited to one label per molecule to be detected.
It is possible to create a liposome immunoassay similar to ELISA by replacing the enzyme which is conjugated to the antibody with a liposome which has encapsulated many marker molecules. In this way the amount of maker molecules can be multiplied and the sensitivity of the assay can be increased .
As aforementioned the liposomal interior aqueous pocket can encapsulate many molecules of interest, drugs, genes and in the case of analytical applications marker molecules such as enzymes or flurophores. In immunoassays antibodies can be incorporated into the lipid bilayer and in this way when they attach to their target the liposome is typically lysed by detergent, releasing the encapsulated molecule.
Complement-mediated lysis assays  comprise of the largest group of liposome immunoassays however it is advantageous if they can self-lyse, eliminating the need for addition of a lysing agent. This has been studied in the case of temperature sensitive liposomes, where the immunoliposomes undergo a rapid destabilisation reaction at an elevated temperature . In these liposomes additional reagents are added when preparing the liposomes to make them stable in all conditions except when heated.
These liposome assays are useful, nevertheless there is a final type of immunoliposomes that can be used in immunoassays, these are target sensitive liposomes. In this design the antibodies are used as stabilisers in the phospholipid bilayer. The orientation of the antibody in the bilayer is a major factor in stabilisation. Target specific binding to the antigen is sufficient to induce bilayer destabilisation ie. The liposomes are destabilised by antigen binding . When the marker molecule is released, for example the enzyme, the substrate for the enzyme may be immobilised on the surface, which would make the assay reagentless, washless and target specific. This type of immunoassay is very fast and easy to use , untrained personnel can perform it as one easy step.
The main markers of interest in immunoassays are enzymes and flurophores, both of which can be easily incorporated in to the aqueous interior of liposomes. Enzymes are the traditional marker molecules, examples of which are alkaline phosphatase (AP), horseradish peroxidase (HRP) and glucose oxidase (GOX). These molecules are quite large therefore the maximum that can be encapsulated is between 10-100 molecules in 200nm LUVs.
Fluorophores such as carboxyfluorescein (CF), FITC, sulforhodamine and calcein are widely used as markers and because of their small molecular weight many thousands may be encapsulated leading to an extremely high sensitivity of detection. However their size can lead to one main limitation which enzymes do not encounter. Smaller fluorophore molecules tend to leak through the liposome membrane more so than larger enzyme molecules .
Conjugation of antibodies to the liposome.
In all applications of immunoliposomes in immunassays, the amount of liposome uncapsulated marker must be related to the amount of antigen of unknown concentration. Though antibody molecules are known to bind non-specifically to liposomes and can be sonicated into the liposomal membrane, covalent attachment should prove a better method for antibody association with vesicles since the process should be controllable and should yield more stable liposomes that could displace non-specifically adsorbed immunoglobulins  There is one main strategy for the coupling of antibodies to liposomes.
The functional groups on the liposomes are conjugated with a heterobifunctional cross-linker to a functional group on the antibody. The functional group can be an amino in PE, DPPE and most proteins, a thiol group in Fab´ fragments or a hydroxyl group in glycolipids and glycoproteins. In an antibody a thiol group can be introduced by first performing a pepsin digestion yielding Fab´ dimmers followed by reduction with dithiothreitol (DTT). This step generates thiol-terminated Fab´ monomer fragments. Following removal of the DTT the Fab´ fragments are mixed with the liposome vesicles resulting in a stable thioether cross-linkage .
Another method of covalently attaching antibodies to liposomes is by using dehydration-rehydration vesicles to covalently attach antibodies to liposomes under conditions in which contact of solutes fated for entrapment with reagents used in the coupling process is avoided. In the first step the antibody is covalently attached to empty buffer containing SUVs. Next the solute to be entrapped is added to the antibody bearing empty SUVs and the mixture is freeze-dried. On rehydration, dehydration-rehydration vesicles (DRV) are formed with most of the antibody exposed on the outer bilayer capable of interacting with the target antigen. Following conversion of the SUVs to DRVs most (84.9%) of SUV bound antibody is recovered in the DRVs .
As mentioned there are also non-covalent methods of attaching antibodies to liposomes which can offer milder approaches to antibody-liposome association. One method exploits the high affinity of streptavidin for biotin. It is a two step protocol which involves the initial attachment of streptavidin to liposomes containing biotin PE followed by the coupling of biotinated antibodies to the strepavidin liposomes. The association is a swift two step method of attacking biotinated antibodies to biotinated liposomes in a mild incubation procedure. The main advantage is that it can be readily extended to couple any biotinated antibody or protein to lioposomal systems .
Characterisation of liposomes.
After preparation and before use in an immunoassay the liposomes must be characterised. The most important things to be characterised are size, stability, captured volume and organic phosphorous content.
Starting with size characterisation, this can be done by dynamic light scattering (DLS). In this method the scattering of laser light is influenced by the Brownian movement of the liposomes which is used as a measure for the diffusion co-efficient, this can then be used to calculate the liposome size. Size can also be determined by measuring an elelectrical current across a flow of liposomes. The current is propotional to the size of the liposomes. Electron microscopy can also be used to visualise the size of liposomes .
The stability of the liposomes is also of great importance and needs to be considered when characterising the batch of liposomes. There are many aspects of stability namely chemical and physical stability. The chemical instability mainly concerns two degradation pathways, oxidative and hydrolytic. Oxidation of phospholipids in liposomes mainly takes place in unsaturated fatty acyl chain-carrying phosphlipids. These chains are oxidised via a free radical chain mechanism in the absence of particular oxidants. Storage at low temperatures and protection from light and oxygen will reduce the chance of oxidation. Further protection could be enhanced with the addition of antioxidants such as a-tocopherol and butyl hydroxy toluene. Working under nitrogen or argon also minimises the oxidation of lipids during preparation. The hydrolysis of ester bonds can also be reduced by optimising the pH, temperature, ionic strength, chain length and headgroup and the amount of cholesterol incorporated into the bilayer .
There are many aspects to physical instability. Stabilisation may be achieved by careful selection of the bilayer components, for example cholesterol is added to permeable bilayers to decrease leakage rates .
The captured volume of the liposome is another parameter that needs to be characterised. The most common approach has been to incorporate an impermeable aqueous marker in the hydrating buffer solution. Once the liposomes are formed the distribution of the solute is measured by comparing the solid/lipid ratio before and after removal of the unencapsulated marker. The internal volume can be found if Vtotal and Vout are determined.
Vtotal=Vin +Vout +Vlipid .
The phosphate content needs to be determined to characterise the amount of phospholipid in the liposomal bilayer. Most protocols for this assay have been taken from the work of Chen et al. where the phosphate is first digested by concentrated acis and then is complexed by ascorbic acids and ammonium molybdate which yields a coloured compound that is readily detected spectrophotometrically .
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