Definition:
An artificial microscopic vesicle consisting of an aqueous core enclosed in one
or more phospholipids layers, used to convey vaccines, drugs, enzymes, or other
substances to target cells or organs.
Introduction
¡ Liposome was
found by Alec Bingham of Babraham Institute in Cambridge, England in 1965.
¡ In
1990, drugs with liposome and Amphotericin B were approved by Ireland.
¡ In
1995 America F.D.A approved liposor doxodubicin.
¡ Liposome is
a lipid vesicle suspending in the hydro-phase with a diameter around
0.0025~3.5um. The membrane of liposome is made of phospholipids, which have
phosphoric acid sides to form the liposome bilayers.
Composition
of liposomes:
A. Phospholipids
A. Phospholipids
The
most common natural phospholipid is the phospatidylcholine (PC) is the
amphipathic molecule and also known as lecithin.
Naturally occurring phospholipids
used in liposomes are:
▪
Phosphatidylcholine
▪
Phosphatidylethanolamine
▪
Phosphatidylserine
Synthetic phospholipids used in the
liposomes are:
▪
Dioleoyl phosphatidylcholine
▪
Disteroyl phosphatidylcholine
▪
Dioleoyl phosphatidylethanolamine
▪
Distearoyl phosphatidylethanolamine
B.
Cholesterol:
¡ Cholesterol
can be incorporated into phospholipids membrane in very high concentration up
to 1:1 or 2:1 molar ratios of cholesterol to phospatidylcholine.
¡ Being
an amphipathic molecule, cholesterol inserts into the membrane with its
hydroxyl group of cholesterol oriented towards the aqueous surface and
aliphatic chain aligned parallel to the acyl chains in the center of the
bilayers and also it increase the separation between choline head groups and
eliminates the normal electrostatic and hydrogen bonding interaction.
¡
The
phospholipids are arranged in such a way that the hydrophilic head is exposed
outside and the lipophilic tails are aliened inside. This makes the liposomes
water soluble molecules.
Advantages
:
¡ Biocompatible,
completely biodegradable, non-toxic, flexible and non-immunogenic.
¡ Liposomes
supply both a lipophilic environment and aqueous “milieu interne” in one
system.
¡ Liposomes
have the ability to protect their encapsulated drug from the external
environment.
¡ Liposomes
help to reduce exposure of sensitive tissues to toxic drugs.
¡ Alter
the pharmacokinetic and pharmacodynamic property of drugs (reduced elimination,
increased circulation life time).
¡ Flexibility
to couple with site-specific ligands to achieve active targeting (Anticancer
and Antimicrobial drugs).
¡ Liposomes
can be formulated into multiple dosage forms.
¡ Liposomes
can encapsulate both micro and macromolecules such as hemoglobin,
erythropoeitin, interferon g etc.
Disadvantages
:
¡ Production
cost is high
¡ Leakage
and fusion of encapsulated drug / molecules.
¡ Sometimes
phospholipids undergoes oxidation and hydrolysis like reaction
¡ Short
half-life
¡ Low
solubility
Classification
:
¡ Based
on Structural Parameters:
- Multi-laminar vesicles (MLV): made up of series of concentric bi-layer of lipid enclosing a small internal volume with size range > 0.5um.
- Oligolamelar vesicles (OLV): constitutes 2 to 10 bi layer of lipids surrounding a large internal volume with size range of 0.1 – 1um.
- Unilamellar vesicle (ULV): single layer of lipids. Based on the size of the single layer they are further divide into the following types with in ULV as
- Small unilaminar vesicle: size of 20 to 40 nm
- Medium unilaminar vesicle: size of 40 to 80 nm
- Large unilaminar vesicle: size of 100 to 1000 nm
- Gaint unilaminar vesicle: size of more than 1000 nm
- Multivesicular Vesicle(MV): constitutes for multiple vesicles and size range >1um.
General
Structure of various types of liposomes:
General
Method of Liposome Preparation:
Different
Methods of Preparation
The
methods are broadly classified into three classes according to basic modes of dispersion:
- Physical Dispersion
- Solvent dispersion
- Detergent solubilisation.
Physical
Dispersion:
There are
four basic methods of physical dispersion:
a) Hand
shaken multilamellar vesicles.
b) Non
shaking vesicles.
c) Pro
– liposomes.
d) Freeze
drying.
Hand
Shaking Method:
Non
Shaking Method & Pro - Liposomes:
Buchi
Rotatory Evaporator
Freeze
Thaw Method:
Dried Reconstituted Vesicles (DRV) and Freeze Thaw
Sonication (FTS):
Ethanol / Ether Injection Method:
Reverse
Phase Evaporation Vesicles:
Mechanism of incorporation of drug in liposomes:
- Encapsulation
- Partitioning
- Reverse loading
Characterisation
of liposomes:
1.Physical Characterization
1.Physical Characterization
S.no
|
Characterization
parameters
|
Analytical
method/Instrument
|
1
|
Vesicle shape
and surface morphology
|
Transmission
electron microscopy,
Freeze-fracture
electron microscopy
|
2
|
Mean vesicle size and size distribution
(submicron and micron range)
|
Dynamic light
scattering, zetasizer,
Photon correlation
spectroscopy, laser
light scattering, gel permeation and gel
exclusion
|
3
|
Surface charge
|
Free-flow
electrophoresis
|
4
|
Electrical
surface potential and surface pH
|
Zetapotential
measurements & pH sensitive probes
|
5
|
Lamellarity
|
Small angle X-ray
scattering, 31P-NMR,
Freeze-fracture
electron microscopy
|
6
|
Phase behavior
|
Freeze-fracture
electron microscopy, Differential
scanning colorimetery
|
7
|
Percent of
free drug/ percent capture
|
Minicolumn
centrifugation, ion-exchange
chromatography, radiolabelling
|
8
|
Drug release
|
Diffusion
cell/ dialysis
|
Chemical
Characterization
S.No
|
Characterization
parameters
|
Analytical
method/Instrument
|
1
|
Phospholipid
concentration
|
Barlett assay,
stewart assay, HPLC
|
2
|
Cholesterol
concentration
|
Cholesterol
oxidase assay and HPLC
|
3
|
Phopholipid
peroxidation
|
UV absorbance,
Iodometric and GLC
|
4
|
Phospholipid
hydrolysis, Cholesterol auto-oxidation.
|
HPLC and TLC
|
5
|
Osmolarity
|
Osmometer
|
Biological
Characterization
S.No
|
Characterization
parameters
|
Analytical
method/Instrument
|
1
|
Sterility
|
Aerobic or
anaerobic cultures
|
2
|
Pyrogenicity
|
Limulus
Amebocyte Lysate (LAL) test
|
3
|
Animal
toxicity
|
Monitoring
survival rates, histology and pathology
|
Therapeutic
applications of liposomes
List
of marketed products
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