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LIPOSOMES: NOVEL DRUG DELIVERY CARRIER


2.      Based on Size and Number of Lamellae
A.   
Multi Lamellar Vesicles (M.L.V.)
Multi lamellar vesicles have more than one bilayer; moderate aqueous volume to lipid ratio 4: 1 mole lipid. Greater encapsulation of lipophilic drug, mechanically stable upon long term storage, rapidly cleared by R.E.S, useful for targeting the cells of R.E.S, simplest to prepare by thin film hydration of lipids in presence of an organic solvent.
a.     
Oligo lamellar vesicles or Paucilamellar vesicles: Intermediate between L.U.V. & M.L.V.
b.     
Multi vesicular liposomes: Separate compartments are present in a single M.L.V.
c.      
Stable Pluri lamellar vesicles: Have unique physical and biological properties due to osmotic compression.

B.     Large Unilamellar Vesicles (L.U.V.)
Large unilamellar vesicles have single bilayer, high aqueous volume to lipid ratio (7: 1 mole lipid), useful for hydrophilic drugs, high capture of macro molecules; rapidly cleared by R.E.S. Prepared by detergent dialysis, ether injection, reverse phase evaporation or active loading methods.

C.    Small Unilamellar Vesicles (S.U.V.)
Single bilayer, homogeneous in size, thermodynamically unstable, susceptible to aggregation and fusion at low or no charge, limited capture of macro molecules, low aqueous volume to lipid ratio (0.2 : 1.5 : 1 mole lipid) prepared by reducing the size of M.L.V. or L.U.V. using probe sonicator or gas extruder or by active loading or solvent injection technique.

METHOD OF PREPARATION2, 5, 9, 10
The preparation of all types of vesicular systems requires the input of energy. Generally all the methods of liposome preparation involve three basic stages:
1.      Dissolved cholesterol and lecithin in organic solvent.
2.      Dispersion of lipids in aqueous media.
3.      Separation and purification of resultant liposomes.

Various methods used for the preparation of liposome:

1.      Passive loading techniques

Passive loading techniques include three different methods:
A.   
Mechanical dispersion method
a.       Lipid film hydration by hand shaking, non-hand shaking or freeze drying
b.      Micro-emulsification
c.       Sonication
d.      French pressure cell
e.       Membrane extrusion
f.       Dried reconstituted vesicles
g.      Freeze-thawed liposomes

B.     Solvent dispersion method
a.       Ether injection
b.      Ethanol injection
c.       Double emulsion vesicles
d.      Reverse phase evaporation vesicles
e.       Stable plurilamellar vesicles

C.    Detergent removal method
a.       Detergent (cholate, alkylglycoside, Triton X-100) removal form mixed micelles
b.      Dialysis
c.       Column chromatography
d.      Dilution

2.      Active loading

CHARACTERIZATION OF LIPOSOMES11, 12
Liposome prepared by one of the preceding method must be characterized. The most important parameters of liposome characterization include visual appearance, turbidity, size distribution, lamellarity, concentration, composition, presence of degradation products, and stability.
1.     
Visual appearance
Liposome suspension can range from translucent to milky, depending on the composition and particle size. If the turbidity has a bluish shade this means that particles in the sample are homogeneous; a flat, gray color indicates that presence of a nonliposomal dispersion and is most likely a dispersed, inversed hexagonal phase or dispersed microcrystallites. An optical microscope (phase contrast) can detect liposome > 0.3 μm and contamination with larger particles.
2.     
Determination of liposomal size distribution
Size distribution is normally measured by dynamic light scattering. This method is reliable for liposomes with relatively homogeneous size distribution. A simple but powerful method is gel exclusion chromatography, in which a truly hydrodynamic radius can be detected. Sephacryl-S100 can separate liposome in size range of 30-300nm. Sepharose -4B and -2B columns can separate SUV from micelles.
3.     
Surface charge
Liposomes are usually prepared using charge imparting constituting lipids and hence it is imparting to study the charge on the vesicle surface. In general two methods are used to assess the charge, namely free flow electrophoresis and zeta potential measurement. From the mobility of the liposomal dispersion in a suitable buffer, the surface charge on the vesicles.
4.     
Determination of lamillarity
The lamellarity of liposomes is measured by electron microscopy or by spectroscopic techniques. Most frequently the nuclear magnetic resonance spectrum of liposome is recorded with and without the addition of a paramagnetic agent that shifts or bleaches the signal of the observed nuclei on the outer surface of liposome. Encapsulation efficiency is measured by encapsulating a hydrophilic marker.
5.     
Entrapped volume
The entrapped volume of a population of liposome (in μL/ mg phospholipid) can often be deduced from measurements of the total quantity of solute entrapped inside liposome assuring that the concentration of solute in the aqueous medium inside liposomes is the same after separation from unentrapped material. For example, in two phase method of preparation, water can be lost from the internal compartment during the drying down step to remove organic solvent.
6.     
Liposome stability
Liposome stability is a complex issue, and consists of physical, chemical, and biological stability. In the pharmaceutical industry and in drug delivery, shelf life stability is also important. Physical stability indicates mostly the constancy of the size and the ratio of lipid to active agent. The cationic liposomes can be stable at 4°C for a long period of time, if properly sterilized.

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