Liposomes detail topic explanation notes

LIST OF CONTENTS
 Introduction
 Advantages with use of liposomes as drug delivery system.
 Classification
 Manufacturing of liposomes
 Liposome characterization and control
 Stability consideration for liposomal formulations
 Drug release from liposomes
 Applications
 Recent innovations
 Approved liposome products

INTRODUCTION
 The preparation of liposomes, with entrapped solutes, was
demonstrated for the first time in 1965 by Prof. A.D. Bangham of
the United Kingdom.

 What is Liposomes
“Are simple microscopic, concentric bilayered vesicles in which
an aqueous material is entirely enclosed by a membranous lipid
bilayer mainly composed of natural or synthetic phospholipids.”
and range in size from 50 nanometers to several micrometers in
diameter”

How Liposomes Formed?
 Phosphlipids(amphiphilic molecules) dispersed in water, they
spontaneously form closed structure with internal aqueous
enviornment bounded by phospholipid bilayer membranes, this
vesicular system is called as Liposomes.

Structure Of Liposomes
 The structural main components of liposomes are
phospholipids and cholesterol.
 Phospholipids
Liposomes can be formed from a variety of phospholipids.
The lipid most widely used is
phosphatidyl choline,
phosphatidyl ethanolamime and
 phosphatidlyl serine
 Cholesterol
Condense the packing of phospholipids in bilayer, Thereby
reducing their permeability to encapsulated compounds.

 Phospholipid Bilayers are the core structure of liposome
and cell membrane.
 Thus the structure of liposomes is similar to the
structure of cell membranes.
Liposome Cell Membrane

Advantages with Liposomes
 Suitable for delivery of hydrophobic, hydrophilic and
amphipathic drugs and agents because its organized structure
could hold drugs, depending on their solubility characteristics,
in both the aqueous and lipid phases.

 Chemically and physically well characterized entities
 Biocompatible and biologically inert in nature.
 Suitable for controlled release.
 Suitable to give localized action in particular tissues.
 Suitable to administer via various routes.
 Drug carrier for variety of small molecules, proteins, nucleotides
and even plasmids.

Disadvantages of liposomes:
 Production cost is high.
 Leakage and fusion of encapsulated drug / molecules.
 Sometimes phospholipid undergoes oxidation and
hydrolysis like reaction.
 Short half-life.
 Low solubility.

Cross-section of liposomes:
H2O Layer
Water Soluble
ingredients
(Drugs,
Nutrients
& vitamins)
Polar Lipids
(Phospholipid)
Lipid Soluble
ingredients
(Drugs,
Nutrients
& vitamins)

Locus of drugs in liposomes: Hydrophilic (DOXORUBICIN)
Low entrapment
Low Leakage
Hydrolytic degradation
Lipophilic (CYCLOSPORINE)
High entrapment
Low leakage
Chemical stability
Amphiphilic (VINBLASTIN)
High entrapment
Rapid leakage
Biphasic insoluble
(ALLOPURINOL, 6-
MERCAPTOPURINE)
Poor loading & entrapment 43

Classification
 based on size of liposomes
 based on method of preparation
 based on composition and in vivo application

Classification based on structure &
size
1. Small unilamellar vesicles
2. Medium sized unilamellar vesicles
3. Large unilamellar vesicles
4. Giant unilamellar vesicles
5. Unilamellar vesicles
6. Oligolamellar vesicles
7. Multilamellar large vesicles
8. Multivesicular vesicles

Lamella :
Types of vesicles based on lamella

Unilamellar vesicles (UV) :
 UV refers to vesicles bounded by single bilayer membrane.
Their further types are:
 SUV( Small Unilamellar vesicles):
 Size: 25-50nm
 No. of lipid bi layer: 1
 MUV(Medium unilamellar vesicles):
 Size: >100nm

 LUV (Large unilamellar vesicles):
 Size: >1000nm
 GUV(Giant unilamellar vesicles):
 Size: >1um

 Advantages of Small unilamellar vesicles (SUV)
 Because of their small size, clearance from the systemic circulation
is reduced, so they remain circulating for longer and thus have a
better chance of exerting the desired therapeutic effect in tissues.
 Disadvantages of small unilamellar vesicles
(SUV)
 The small size cause lower capacity for drug entrapment, less than
1% of the material available.

 Advantages of Large unilamellar vesicles (LUV)
 There is a large space for incorporation of "drug.“
 Disadvantages of Large unilamellar vesicles (LUV)
 They are more fragile than MLV and have increased
Permeability to small solutes due to the absence of additional
lamellae.

Multilamellar large vesicles (MLV):
Refers to those vesicles bounded by two or more bilayer
membranes.
 Size: > 0.5um
 No. of lipid bi layer: 5-20
 As water added to the phosphholipid, the polar head groups
at the surface of the exposed amphiphile become hydrated
and start to reorganize into the lamellar form.

 The water diffuses through this surface bilayer causing the underlying
lipid to undergo a similar rearrangement, and the process is repeated
until all of the lipid is organized into a series of parallel lamellae, each
separated from the next by a layer of water.
 Mild agitation allows portions of close-packed, multilamellar lipid to
break away resulting large spherical liposomes, each consisting of
numerous concentric bilayers in close, alternating with layers of
water, which are known as multilamellar vesicles (MLV).

 Advantage of MLV:
 They are simple to make and
 have a relatively compact construction.
 Disadvantage of MLV:
 The volume available for solute incorporation is limited

 Their large size is a drawback for many medical applications
requiring parenteral administration, because it leads to rapid
clearance from the bloodstream by the cells of the RES.
On the other hand, this effect can be used for passive targeting
of substances to the fixed macrophages of the liver and
spleen.

Oligolamellar vesicles (OLV):
 Size: > 0.1-1um
 No. of lipid bi layer:~ 5
Multivesicular vesicles (MVV):
 Size: > 1um
 No. of lipid bi layer: Multi- compartmental structure

Liposome Function Depending on Size
 Large Multiple-layer liposome:
 Are liposomes within liposomes .They have a limited ability to
penetrate narrow blood vessels or into the skin.
 The materials that are entrapped in the inner layers of these liposomes
are practically less releasable .
 Large Unilamellar liposomes :
 Are easy to make by shaking phospholipids in water.
 These liposomes have very limited functions and are usually made of
commercial lecithin (Phospotidyicholine PC), commonly found in
food products.

 Commercial lecithin’s main function is as an emulsifying agent,
improving the ability of oil and water to remain mixed.
 Small Unilamellar liposomes (Nanosomes)
 Are constructed from the highest quality and high percentage of
phosphatidylcholine (PC), one of the essential components of cell
membranes.
 Thus, nanosomes can easily penetrate into small blood vessels by
intravenous injection; and into the skin by topical application.
 Their entrapped material can be easily delivered to desired targets
such as cells .

Classification based on composition and
in vivo application
 Conventional Liposomes (CL)
 Long circulating Liposomes (LCL)
 Immuno Liposomes (Antibody targeted) (CL) (LCL) With
attached monoclonal antibody or recognition sequence.
 Cationic Liposomes
 pH sensitive liposomes (Phospholipid such as PE or DOPE
with either CHEMS or OA.

Based on
composition
and
application:
20

Conventional Liposomes
 Composed of only phospholipids (neutral or negatively
charged) and/or cholesterol and used for passive targeting.

LONG CIRCULATING STEALTH LIPOSOMES
 These liposomes carry hydrophilic coatings(PEG coating)
 Also called sterically stabilized.
 Low permeability liquid matrix and internal aqueous buffer
system
 Used to obtain prolonged circulation times.

Immunoliposomes (targeted liposomes)
 Either conventional or sterically stabilized.
 Used for active targeting purposes.
 Target specific ligands, such as antibodies, immuno globulins,
lectins and oligosaccharides attached to the surface.

Cationic Liposomes
 Positively charged.
 Used for delivery of genetic material.
 Cationic lipid component interact with negatively- charged DNA,
Results into Lipid –DNA Complexes.

Passive
loading
technique
Active/remo
te loading
technique
Loading of the entrapped agents
before/ during the manufacture
procedure.
Certain types of compounds with
ionizable groups & those with both
lipid & water solubility can be
Introduced into liposomes after the
formationof intact vesicles.
Methods of Liposome Preparation
21

General Method Of Liposome
Preparation:
23

Methods of loading of liposome
Passive loading techniques Active loading techniques
Mechanical dispersion
methods
Solvent dispersion
methods
Detergentremoval
technique
22

LOADING OF DRUG IN LIPOSOMES
PASSIVE LOADINGTECHNIQUES
 Methods have been classified according to 3 basic modes of
dispersion:
1. Physical(Mechanical)dispersion
2. Chemical dispersion
3. Detergent Solubilization

1.PHYSICAL DISPERSION
METHOD:
 In this method, aqueous volume enclosed in lipid membrane is 5-
10%.
 So water soluble drugs are more wasted while lipid soluble drugs
encapsulated up to 100% efficiency.
 4 methods of physical dispersion:
1. Hand shaking method.
2. Non shaking method.
3. Pro – liposomes .
4. Freeze drying .

Liposome
Lipid spontaneously swell & Hydrate
Solid lipid mixture is hydrated by using aqueous buffer
Film deposition
Remove organic solvent under vacuum
Lipid dissolve in organic solvent/co-solvent
1. Physical( Mechanical )dispersion method:
24

1. Hand Shaken Method
 PROCEDURE:
1. Prepare lipid mixture by dissolving phospholipids and charge
components in chloroform: methanol mixture (2:1)
ratio in 250ml round bottom flask.
2. Attach flask to rotary evaporator & rotate at 60 r.p.m to
evaporate solvent at 30 ͦc.
3. Rotate for 15 min after dry residue appear.
4. Isolate evaporator from vacuum & introduce nitrogen till no
pressure difference b/w inside and outside flask.
5. Remove evaporator and attach lyophilizer.

 HYDRATION OF LIPIDS:
 After releasing the vacuum & Remove from lyophilizer, flush
with nitrogen & 5ml of phosphate buffer containing
solute(drug) to be entrapped is added.
Attach flask again with evaporator at 60 r.p.m or less at room
temp for 30 min until all lipid removed from walls of flask.
Homogeneous white milky suspension formed.
Allow milky suspension to stand for 2 hrs at room temp to
allow swelling process which yields MLVs.
At the end of this period, the loaded liposomes can be
separated from nonencapsulated solute using a process such as
centrifugation or dialysis.
The possibility of lipid oxidation can be minimized by working
in an inert atmosphere of nitrogen.

Lipids form stacks of film
from organic solution
Then film is treated with
aqueous medium
Upon hydration lipids
swell and peel out from
RB flask
vesiculate to form Multi
lamellar vesicles(MLVs)
26

2. Non shaking method.
 In this method, LUVs formed with high entrapment volume.
 Use same procedure but instead of round bottom flask we will use
flat bottom flask .
 More care over swelling process.
 Hydration and swelling process in two separate steps.
 Hydration:
Dried film of lipid + stream of water saturated with nitrogen
for 15 min.
 Swelling:
In aqueous medium without shaking.
 Chloroform: methanol in ratio of 1:2 by volume is used.

3. Pro-liposomes:
 To increase the surface area of dried lipid
film & to facilitate instantaneous hydration.
lipid Dried
over
lipid
Finely divided
particulate support
like powdered NACL/
sorbital
Pro - liposomes
Pro-
liposomes
water Dispersion of MLV’S
This Method overcome the stability problem. 27

 In this method, the lipids are dried down to a finely divided
particulate support, such as powdered sodium chloride, or
sorbitol or other polysaccharides.
 The lipids are swelled upon adding water to dried lipid coated
powder (pro-liposomes), where the support rapidly dissolves to
give a suspension of MLVs in aqueous solution.
 This method overcomes the problems encountered when storing
liposomes themselves in either liquid, dry or frozen form.
 This method is ideal for preparations where the material to be
entrapped incorporates into lipid membrane.

4. Freeze drying .
 Another method of dispersing lipid in finely divided form prior to
the addition of aqueous media.
 In it freeze dry the lipid dissolved in suitable organic solvent
(Tertiary butanol).
 After obtaining dry lipid, water or saline can be added with rapid
mixing to give MLVs.

Processing of Liposomes
1. Microemulsification:

 2. Sonicated UnilamellarVesicles:
a. Bath sonication
b. Probe sonication
Ultra sonicated sound( high frequency) waves travel through medium
containing liposomes. Because sound waves can not pass through
vaccum.
Through any media when sound waves pass they pass in 2 region one is
compressional region (air give it resistance) and this resistance region is
called rear fractional region.This second region back convert these
waves into smaller waves and spread in different regions.

3.French Pressure Cell liposomes:
 The ultrasonic radiation can degrade the lipids, other sensitive
compounds, macromolecules.
 For this extrusion of preformed larger liposomes in a French press
under very high pressure (20,000 -40,000 psi) is done.
 This tech. yields unit or oligo lamellar liposomes of size (30-80 nm
in dia)
 Includes high cost of press that consist of electric hydraulic press and
pressure cell.
 Liposome prepare by this method are less likely to suffer from
structural defects and instabilities as observed in sonication vesicles.

4. Membrane Extrusion Method:
 Use to process LUVs as well as MLV.
 Liposomes prepare by this is called membrane filter extrusion
liposomes.
 The 30% capture volume can be obtained using high lipid conc. The
trapped volume in this process is 1-2 liter /mole of lipids.
 It is due to their ease of production, readily selectable vesicle
diameter, batch to batch reproducibility and freedom from solvent or
surfactant contamination is possible.
 All vesicle Form by this will of same size and lamellarity.

DRIED RECONSTITUEDVESICLE(DRV) and FREEZETHAW
SONICATION:

2. SOLVENT SOLVENT(CHEMICAL)
DISPERSION METHODS:
 In this method, lipids are first dissolved in an organic solution, which
is then brought in to contact with aqueous phase containing material
to be entrapped within liposomes.
 At the interface between the organic and aqueous media, the
phospholipids align themselves.
 Methods employing solvent dispersion fall in to one of three
categories:
 Organic solvent miscible with aqueous phase.
 Organic solvent immiscible with aqueous phase and aq.
Phase in excess.
 Organic solvent in excess and immiscible with aqueous
phase.

 Its further 5 types.
A. Ethanol injection
B. Ether injection
C. Water in organic phase
D. Double emulsion vesicles
E. Reverse Phase evaporation vesicles.(REV)

Liposome
Formation of monolayer and bilayer of phospholipids
Lipids align at interface of aqueous and organic layer
Excess addition of aqueous phase
Lipid dissolve in organic solvent
Solvent(Chemical) dispersion methods:
36

A:ETHANOL INJECTION:
 In this method, an ethanol solution of lipids is injected rapidly into an
excess of saline or other aqueous medium (BUFFER), through fine
needle.
 The force of injection is usually sufficient to achieve complete
mixing, so that the ethanol is diluted almost instantaneously in water
and phospholipid molecules are dispersed evenly throughout the
medium.
 This procedure yields a high proportion of SUV (Dia. 25nm).
 Although lipid aggregates and larger vesicles may form if the mixing
is not thorough enough.
Disadvantages:
 Population is heterogeneous (30-110nm), liposomes are very dilute,
the removal of ethanol is difficult because it forms into Azeotrope ( a
mixture of two or more liquids whose components cannot be altered
by simple distillation) with water.

B: ETHER INJECTION (SolventVaporization).
 This method is very similar to ethanol injection method.
 Ether injection method contrasts markedly with ethanol injection in
many respects.
 It involve injecting the immiscible organic solution very slowly into
an aqueous phase through a narrow needle at the temperature of
vaporizing the organic solvent.
 This method may also treat sensitive lipids very gently and very little
risk of causing oxidative degradation.
Disadvantages of the technique are the long time taken to produce a
batch of liposomes.
 The efficiency of encapsulation is relatively low.

Solvent dispersion methods:
A:ETHANOL INJECTION/ B: ETHER INJECTION:
37

C. Reverse phase evaporation method:
 The novel key in this method is the removal of solvent from an
emulsion by evaporation.
 The droplets are formed by bath sonication of mixture of the two
phases, then the emulsion is dried down to a semi solid gel in a
rotatory evaporator under reduced pressure.
 The next step is to bring about the collapse of a certain proportion
of the water droplets by vigorous mechanical shaking with a vortex
mixer.
 In these circumstances, the lipid monolayer which enclosed the
collapsed vesicle, is contributed to adjacent intact vesicle to form
the outer leaflet of the bilayer of a large unilamellar liposomes.

 The aqueous content of the collapsed droplet provides the
medium required for suspension of these newly formed
liposomes.
 After conversion of the gel to a homogenous free flowing fluid
, the suspension is dialyzed in order to remove the last traces
of solvent.
 The vesicles formed are unilamellar and have a diameter of
0.5um.
 The encapsulation percentage is found to be not greater then
50%.

C. Reverse phase evaporation method:
39
Organic phase

Reverse phase evaporation technique.
Lipid in solvent
solution
Two-phasesystem Water in oil
emulsion
Solvent removal
Gel formation
REV liposomes

D.WATER IN ORGANIC PHASE:
 The common feature of method is formation of “ water in oil
emulsion’’ by introduction of a small quantity of aq. Medium
containing the material to be entrapped, into large volume of
immiscible organic solution of lipid followed by mechanical
agitation to break up aq. Phase into microscopic droplets.
 These droplets are solubilized by the presence of phospholipid
monolayer at the phase interface and form the central core of
the final liposomes.

E. DOUBLE EMULSIONVESICLES
 In this method, firstly water is emulsified in organic phase,
 Which is followed by emulsifying w/o emulsion in aq. Phase
resulting in a w/o/w system.
 The organic solvent is removed by evaporation and the product is
centrifuged to remove the lipid aggregates.
 The organic solution which already contains water droplets is
introduced into excess aqueous medium followed by mechanical
dispersion, a multi compartment vesicle is obtained, which may be
described as a w/o/w system (i.e double emulsion).

 These vesicles are suspended in aqueous medium and have an
aqueous core, the two aqueous compartments being separated
from each other by a pair of phospholipid monolayers whose
hydrophobic surfaces face each other across a thin film of organic
solvent.
 Removal of this solvent clearly results in an intermediate size
unilamellar vesicles.

Formation of Liposome by detergent removal
By addition optimized concentration of
detergent
Phospholipids brought into intimate contact
with aqueous phase
Note:- Liposome size and
shape depend on chemical
nature of detergent,
concentration and other
lipid involved
DETERGENT SOLUBILISATIOIN METHODS
40

3.DETERGENT SOLUBILISATIOIN
METHODS:
 Phospholipids can be solubilized in a aq. Medium by using detergent.
 The structures formed as a result of this association, are known as
micelles.
 The concentration of detergent in water at which micelles just start to
form is known as critical micelle concentration(CMC).
 Below CMC, detergent molecules exist in free soln. As the concentration is
increased,micelles are formed.
 Production of liposomes is dependent on the removal of detergent
molecule from aqueous dispersion of phospholipids / detergent mixed
micelles.
 As detergent is removed, the micelles progressively richer in
phospholipids and coalesce to form closed single bilayer vesicles (LUVs).

 DETERGENTS:
 BILE SALTS such as sodium cholate and sodium deoxy cholate.
 SYNTHETIC such as octylglucoside.
 Liposome size and shape depend on chemical nature of detergent,
concentration and other lipid involved.
 Sizes of liposomes can also control by controlling conditions of
detergent removal.
 Not efficient method in terms of %age of solute entrapped.

PURIFICATION OF LIPOSOMES
 Purification of liposomes involved
 The removal of unbound drug from liposomes.
 Removal of detergents from liposomes.
 To fractionate heterogeneous liposomal dispersions.
 When lipophilic drugs of appropriate structure are
encapsulated in the bilayer phase, the degree of
"encapsulation" is dependent upon the saturation of the lipid
phase.
 In this case degrees of encapsulation of over 90% is achieved.
Thus it is unnecessary to remove the unbound drug.

 However, in the case of water-soluble drugs, the encapsulated
drug is only a fraction of the total drug used.Thus, it is
required to remove the unbound drug from the drug-loaded
liposomes in dispersion.
 Liposomes are purified by either:
A. Dialysis
B. Centrifugation
C. Gel filtration or column chromatography.

A. Dialysis:
 Dialysis is the simplest procedure used for the removal of
the unbound drug, except when macromolecular compounds
are involved.
Advantages:
Dialysis Technique requiring no complicated or
expensive equipment.
Dialysis is effective in removing nearly all of
the free drug with a sufficient number of
changes of the dialyzing medium.
For large scale liposomes, HOLLOW
DIALYSIS CARTRIDGE may be used.
Liposome
dispersion

 Disadvantages:
• Dialysis is a slow process.
• Removal of over 95% of the free drug require a minimum
of 3 changes of the external medium over 10 to 24 hr at
room temperature.
• Care is taken to balance the osmotic strengths of the
liposomal dispersion and the dialyzing medium to avoid
leakage of the encapsulated drug.

B. Centrifugation:
 Centrifugation is an effective means of isolating liposomes
from the free drug in the suspending medium.
 Separation of liposomes depend on size as well as composition of
bilayer.
 SUVs Sediments by spinning at 200,000 g for 10-20 hrs in
ultracentrifuge.
 MLVs Sediments rapidly at 100,000g in less than one hr.

C. Gel Filtration
 Gel permeation chromatographic technique is used extensively
both to separate liposomes from unbound drug and also to
fractionate heterogeneous liposomal dispersions.
Advantages:
 The technique is very effective and rapid at the laboratory level.

Disadvantages:
Gel filtration is expensive.
Dilution of the liposomal dispersion with the eluting medium may necessitate
another concentration step.
Lipid losses on the column materials.

Active/remote loading technique:
The lipid bilayer membrane is impermeable to ions & hydrophilic
molecules. But, Permeation of hydrophobic molecules can be controlled
by concentration gradients.
Some weak acids or bases can be transported due to various
transmembrane gradients
Electrical gradients.
Ionic(pH) gradients.
Chemical potential gradients.
Weak amphipathic bases accumulate in aq phase of lipid vesicles
in response to difference in pH b/w Inside & outside of
liposomes 41

pH gradient is createdby preparing liposomes
withlow internal pH.
[Basic compds ( lipophilic (non ionic) at high
pH & hydrophilic(ionic) at low pH)]
Lipophilic (UNPROTONATED) drug diffuse
through the bilayer
At low pH side, the molecules are
predominantly protonated.
.
Weak bases like doxorubicine,
adriamycin and vincristine are
encapsulated.
Solute bearing no
chargeat neutral pH
Liposomes with low
internal pH
Neutral solute passes
easily through bilayer
membrane by diffusion
Charge aquired by solute
inside liposomes makes
them unable to exit
42

CHARACTERIZATION OF LIPOSOMES:
PHYSICAL CHARACTERISATION
→ Vesicles size/shape/morphology
→ Surface -charge/electrical potential
→ Phase behaviour/ lamellarity
→ Drug release
→ % capture /free drug
CHEMICA L CHARACTERISATION
→ Phospholipids /lipid concentration
→ Drug concentration
→ PH / Osmomolality
→Phospholipids / cholesterols –
peroxidation/oxidation/hydrolysis
BIOLOGICAL CHARACTERISATION
→ Sterility
→ Pyrogenisity
→ Animal toxicity
→Plasma Stability:
44

Characterization parameters Analytical method/Instrument
1. Vesicle shape and surfacemorphology Transmissionelectron microscopy, Freeze-
fractureelectron microscopy
2.Meanvesicle size and size distribution
(submicron and micron range)
Electronmicroscopy, Photon correlation
spectroscopy,laser light scattering,gel
permeation and gel exclusion
3. Surfacecharge Free-flowelectrophoresis
4. Electricalsurfacepotential and surfacepH Zetapotential measurements
5. Lamellarity Small angle X-ray scattering,31 P-NMR, Freeze-
fractureelectron microscopy
6. Phase behavior of Liposomes Freeze-fractureelectron microscopy, Differential
scanning calorimetery
7. Percent of freedrug/ percent
capture(entrapment)
Minicolumn centrifugation,ion-exchange
chromatography, radio labelling
8. Drug release Diffusioncell/ dialysis

1. Phospholipid concentration TLC & HPLC
Barlett assay, stewart assay,
2. Cholesterol concentration Cholesterol oxidase assayand HPLC
3. Phopholipid oxidation UV absorbance
4. Phospholipid hydrolysis,
Cholesterol auto-oxidation.
HPLC and TLC
5. Osmolarity Osmometer

3. BIOLOGICAL
CHARACTERIZATION:

1. Sterility Aerobic or anaerobic cultures
2. Pyrogenicity Limulus Amebocyte Lysate (LAL) test
3. Animal toxicity Monitoring survival rates, histology and
pathology

STABILITY OF LIPOSOMES:
 Liposomes stability problems are of course much more
severe .
 ~ Perform Long term & accelerated stability
 Liposomal phospholipid can undergo chemical
degradation such as
~ Lipid oxidation
~ Lipid Hydrolysis
 Either as a result of these changes or otherwise,
liposomes maintained in aqueous suspension may
aggregate, fuse or leak their contents.

PREVENTION OF CHEMICAL DEGRADATION:
 The following precautions may be taken to minimize chemical
degradation.
 Start with freshly purified lipids and freshly distilled solvents.
 Avoid procedures which involves high temperature.
 Carry out manufacturing in the absence of oxygen.
 Deoxygenate aqueous solution with nitrogen.
 Store all liposome suspension in inert atmosphere.
 Include an anti-oxidant as a component of the lipid membrane.

1. Endocytosis 2. Adsorption
3. fusion 4. Lipid transfer
48
5. Contact release

Liposomes can interact with cells by 5
different mechanims:
Lipid Exchange
Intermembrane
Transfer
Adsorption
Endocytosis
Fusion
Contact
Release

1) Endocytosis by phagocytic cells of the reticuloendothelial system
such as macrophages and neutrophils, that makes the liposomal content
available to the cell, where lisosomes break , and phospholipids hydrolysed to
fatty acids which can be incorporated into host phospholipids.

2) Fusion with the cell membrane
By insertion of the lipid bilayer of the liposome into the cell membrane to
become its part, with simultaneous release of liposomal contents into the
cytoplasm.

3) Adsorption to the cell surface either by nonspecific weak
hydrophobic or electrostatic forces, or by interactions of specific receptors
on cell surface to ligands on the vesicle membrane.
For water soluble components, vesicle contents are diffused through the
lipids of the cell.
For lipid soluble components, vesicle
contents are exchanged with the
cellular membrane along with the
lipid of the vesicle.

4) Inter-membrane Transfer (Lipid transfer):
WithTransfer of liposomal lipids to cellular or sub cellular
membranes, or vice versa.

5) Contact-Release:
This case can occur when the membranes of the cell and that of
liposomes exert perturbation (agitation) which increase the
permeability of liposomal membrane, and exposure of solute
molecule to be entrapped by cell membrane.

CLINICAL APPLICATIONS
 In cancer chemotherapy
 In immunology (As carrier of vaccines, antigens)
 Liposomes as drug carrier in oral treatment
 As targeted delivery system
 Parental Liposomes
 Liposomes for topical application
 Liposomes for pulmonary delivery
 Liposomes for ophthalmic delivery of drugs
 Cell biological applications
 Advanced applications of Liposomes

Cancer chemotherapy
 Liposomes entrapped anticancer drugs and enhance the
selectivity for tumor cells.
 Results in decreased toxicity to normal cells.
 Protects the metabolic degradation of drug.
 Increase circulation life time.
 EXAMPLE: (LipADM)
LipADM is encapsulation of doxorubicin in liposome
enhance the anti-tumor effects of drug and reduce side
effects such as cardiac toxicity.

Liposomes could serve as tumor specific vehicles
(even without special targeting)
Liposomes better penetrate into tissues
with disrupted endothelial lining 53

Liposomes in immunology
 As immunological adjuvant.
 Adjuvants may be incorporated with antigen into liposomes.
 Small amount of immunogen may be suitable as antigens.
 Hydrophobic antigen may be incorporated.
 Multiple antigens may be incorporated in to single
liposomes.
 Toxicity or allergic reactions of antigens may be reduced.

Liposomes as drug carrier in oral treatment
• Regular liposomes are typically unstable in intestine.
 Polymer coated liposomes have increased stability in GIT.
 Using Liposomes as a potential system for oral delivery of
insulin has been extensively studied.
As targeted delivery system:
• The encapsulation of drugs with liposomes alters drug
pharmacokinetics, and may be exploited to achieve
targeted therapies by the flexibility in alteration of the
liposome surface.

 ARTHRITIS:
 Drugs used in arthritis specially steroids are destroyed by
peripheral effects.
 This effect is minimized by local administration of drugs.
 Liposomes can be used in treatment of local diseases.
 Steroids can be entrapped in to large MLVs and administered
locally in arthritis.

Parentral (I/V) Liposomes
 The closed pack of liposome structure can encapsulate
aqueous soluble drugs within the central aqueous
compartment or lipid soluble drugs within the bilayer
membrane.
 ATRA-IV Liposomal retinoic acid:
is a liposomes encapsulated, intravenous form of all trans
retinoic acid (ATRA), a derivative of vitamin A.

Liposomes for topical application
 Liposomes have shown great potential as a topical drug
delivery system.
 EXAMPLES:
 Hydrocortisone ointment
Shows higher conc. Of drug in layers of human skin.
 Diclofenac gel
Shows higher conc. Of drug and increase permeation
through skin.
 Benzocaine gel
Shows prolonged anesthesia as compared to plain benzocaine
gel.

Liposomes for pulmonary delivery
 Liposomes are available in different sizes and provide the
opportunity of a controlled drug delivery for administration
to respiratory tract.
 Inhalation devices such as nebulizer will produce an aerosol
of droplets containing liposomes.
 Liposomes encapsulated drugs for pulmonary administration
are:
 Tobramycin
 Salbutamol
 Cytarabine

Cell biological applications
• DNA delivery of Genes by Liposomes.
• Cheaper than viruses.
• No immune response.
• Liposomes are also used to insert regulatory molecules
such as AMP, CGMP.

Ophthalmic delivery of drugs
 Liposomes improve bioavailability of ophthalmic drugs
after topical application due to lipophilisation of water
soluble drugs which can not penetrate the lipophilic
cornea.
 Liposomes have been used as vehicles for sub conjunctival inj.
of cytotoxic drug and antibiotics.
 Dihydro-streptomycin
 Benzyl penicillin

Advanced applications of Liposomes
 The recent research is on use of liposomes to deliver
hemoglobin act as red blood cell substitute.
 EXAMPLE:
Liposomes encapsulated Hb (LEH)

DRUG ROUTE OF
ADMINISTRATION
APPLICATION TARGETED DISEASES
Amphotericin B Oral delivery Ergosterol membrane Mycotic infection
Insulin Oral,ocular,pulmonary
And transdermal
Decrease glucose level Diabetic mellitus
Ketoprofen Ocular delivary Cyclooxygenase enzyme inhibitor Pain muscle condition
Pentoxyfyllin Pulmonary delivery phosphodiesterase Asthama
Tobramycin Pulmonary delivery Protein synthesis inhibitor Pseudomonas aeroginosai
nfection,
Salbutamol Pulmonary delivery ß2-adrenoceptor antagonist Asthama
Cytarabin Pulmonary delivery DNA-polymerase inhibition Acute leukameias
Benzocaine Transdermal Inhibition of nerve impulse from
sensory nerves
Ulcer on mucous surface
with pain
Ketaconazole Transdermal Inhibit ergosterol membrane Candida albicans
Levanogesterol Transdermal Rhamnose receptor skin disorder
hydroxyzine Transdermal H1-receptor antagonist Urtecaria,allergic skin
disease
Ibuprofen Oral delivery Chaemoceptor,free ending Rheumatoid arthritis
triamcilonone Ocular delivery,Transdermal Inhibition of prostaglandin Anti-inflammatory
Therapeutic application of liposomes:
54

NAME TRADE NAME COMPANY INDICATION
Liposomal
amphotericin B
Abelcet Enzon Fungal infections
Liposomal
amphotericin B
Ambisome Gilead Sciences Fungal and protozoal infections
Liposomal cytarabine Depocyt Pacira (formerly
SkyePharma)
Malignant lymphomatous meningitis
Liposomal
daunorubicin
DaunoXome Gilead Sciences HIV-related Kaposi’s sarcoma
Liposomal doxorubicin Myocet Zeneus Combination therapy with cyclophosphamide in
metastatic breast cancer
Liposomal IRIV vaccine Epaxal Berna Biotech Hepatitis A
Liposomal IRIV vaccine InflexalV Berna Biotech Influenza
Liposomal morphine DepoDur SkyePharma, Endo Postsurgical analgesia
Liposomal verteporfin Visudyne QLT, Novartis Age-related macular degeneration, pathologic
myopia, ocular
histoplasmosis
Liposome-PEG
doxorubicin
Doxil/Caelyx Ortho Biotech,
Schering-Plough
HIV-related Kaposi’s sarcoma, metastatic breast
cancer, metastatic
ovarian cancer
Micellular estradiol Estrasorb Novavax Menopausal therapy
55

NANO REVIEW Open Access
Liposome: classification, preparation, and
applications
Abolfazl Akbarzadeh1*
, Rogaie Rezaei-Sadabady1,2
, Soodabeh Davaran1
, Sang Woo Joo5*
, Nosratollah Zarghami1*
,
Younes Hanifehpour5
, Mohammad Samiei3
, Mohammad Kouhi4
and Kazem Nejati-Koshki1
Abstract
Liposomes, sphere-shaped vesicles consisting of one or more phospholipid bilayers, were first described in the mid-
60s. Today, they are a very useful reproduction, reagent, and tool in various scientific disciplines, including
mathematics and theoretical physics, biophysics, chemistry, colloid science, biochemistry, and biology. Since then,
liposomes have made their way to the market. Among several talented new drug delivery systems, liposomes
characterize an advanced technology to deliver active molecules to the site of action, and at present, several
formulations are in clinical use. Research on liposome technology has progressed from conventional vesicles to
‘second-generation liposomes’, in which long-circulating liposomes are obtained by modulating the lipid
composition, size, and charge of the vesicle. Liposomes with modified surfaces have also been developed using
several molecules, such as glycolipids or sialic acid. This paper summarizes exclusively scalable techniques and
focuses on strengths, respectively, limitations in respect to industrial applicability and regulatory requirements
concerning liposomal drug formulations based on FDA and EMEA documents.
Keywords: Liposomes, Glycolipids, Drug formulations, Drug delivery systems
Review
Introduction
Liposomes are small artificial vesicles of spherical shape
that can be created from cholesterol and natural non-
toxic phospholipids. Due to their size and hydrophobic
and hydrophilic character(besides biocompatibility),
liposomes are promising systems for drug delivery. Lipo-
some properties differ considerably with lipid compos-
ition, surface charge, size, and the method of
preparation (Table 1). Furthermore, the choice of bilayer
components determines the ‘rigidity’ or ‘fluidity’ and the
charge of the bilayer. For instance, unsaturated phos-
phatidylcholine species from natural sources (egg or
soybean phosphatidylcholine) give much more perme-
able and less stable bilayers, whereas the satu-
rated phospholipids with long acyl chains (for example,
dipalmitoylphos phatidylcholine) form a rigid, rather im-
permeable bilayer structure [1-3].
It has been displayed that phospholipids impulsively
form closed structures when they are hydrated in aque-
ous solutions. Such vesicles which have one or more
phospholipid bilayer membranes can transport aqueous
or lipid drugs, depending on the nature of those drugs.
Because lipids are amphipathic (both hydrophobic and
hydrophilic) in aqueous media, their thermodynamic
phase properties and self assembling characteristics in-
fluence entropically focused confiscation of their hydro-
phobic sections into spherical bilayers. Those layers are
referred to as lamellae [4]. Generally, liposomes are def-
inite as spherical vesicles with particle sizes ranging from
30 nm to several micrometers. They consist of one or
more lipid bilayers surrounding aqueous units, where
the polar head groups are oriented in the pathway of the
interior and exterior aqueous phases. On the other hand,
self-aggregation of polar lipids is not limited to conven-
tional bilayer structures which rely on molecular shape,
temperature, and environmental and preparation con-
ditions but may self-assemble into various types of col-
loidal particles [5].
* Correspondence: akbarzadehab@tbzmed.ac.ir; swjoo@yu.ac.kr; zarghami@
tbzmed.ac.ir
1
Department of Medical Nanotechnology, Faculty of Advanced Medical
Science, Tabriz University of Medical Sciences, Tabriz 51664, Iran
5
School of Mechanical Engineering, WCU Nanoresearch Center, Yeungnam
University, Gyeongsan 712-749, South Korea
Full list of author information is available at the end of the article
© 2013 Akbarzadeh et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.
Akbarzadeh et al. Nanoscale Research Letters 2013, 8:102
http://www.nanoscalereslett.com/content/8/1/102

Liposomes are extensively used as carriers for numer-
ous molecules in cosmetic and pharmaceutical indus-
tries. Additionally, food and farming industries have
extensively studied the use of liposome encapsulation
to grow delivery systems that can entrap unstable
compounds (for example, antimicrobials, antioxidants,
flavors and bioactive elements) and shield their func-
tionality. Liposomes can trap both hydrophobic and
hydrophilic compounds, avoid decomposition of the en-
trapped combinations, and release the entrapped at
designated targets [6-8].
Because of their biocompatibility, biodegradability, low
toxicity, and aptitude to trap both hydrophilic and lipo-
philic drugs [9] and simplify site-specific drug delivery
to tumor tissues [10], liposomes have increased rate both
as an investigational system and commercially as a drug-
delivery system. Many studies have been conducted on
liposomes with the goal of decreasing drug toxicity and/
or targeting specific cells [11-13].
Liposomal encapsulation technology (LET) is the
newest delivery technique used by medical investigators
to transmit drugs that act as curative promoters to the
assured body organs. This form of delivery system pro-
posal targeted the delivery of vital combinations to the
body. LET is a method of generating sub-microscopic
foams called liposomes, which encapsulate numerous
materials. These ‘liposomes’ form a barrier around their
contents, which is resistant to enzymes in the mouth
and stomach, alkaline solutions, digestive juices, bile
salts, and intestinal flora that are generated in the
human body, as well as free radicals. The contents of the
liposomes are, therefore, protected from oxidation and
degradation. This protective phospholipid shield or bar-
rier remains undamaged until the contents of the lipo-
some are delivered to the exact target gland, organ, or
system where the contents will be utilized [14].
Clinical medication keeps an enormously broad range
of drug molecules at this time in use, and new drugs are
added to the list every year. One of the main aims of any
cure employing drug is to increase the therapeutic index
of the drug while minimizing its side effects. The clinical
usefulness of most conservative chemotherapeutics is
restricted either by the incapability to deliver therapeutic
drug concentrations to the target soft tissue or by Spar-
tan and harmful toxic side effects on normal organs and
tissues. Different approaches have been made to over-
come these difficulties by providing the ‘selective’ deliv-
ery to the target area; the ideal solution would be to
target the drug alone to those cells, tissues, organs that
are affected by the disease. Selected carriers, for instance
colloidal particulates and molecular conjugates, can be
appropriate for this determination. Colloidal particu-
lates result from the physical incorporation of the drug
into a particulate colloidal system, for instance reverse
micelles, noisome, micro- and nano-spheres, erythro-
cytes, and polymers and liposomes. Among these car-
riers, liposomes have been most studied. Their
attractiveness lies in their composition, which makes
them biodegradable and biocompatible. Liposome
involves an aqueous core entrapped by one or more
bilayers composed of natural or synthetic lipids. They
are composed of natural phospholipids that are biologic-
ally inert and feebly immunogenic, and they have low in-
herent toxicity. Furthermore, drugs with different
lipophilicities can be encapsulated into liposomes:
strongly lipophilic drugs are entrapped almost totally in
the lipid bilayer, intensely hydrophilic drugs are located
entirely in the aqueous compartment, and drugs with
intermediary logP effortlessly partition between the lipid
and aqueous phases, both in the bilayer and in the aque-
ous core [15].
The present review will briefly explain the charac-
teristics of liposomes and explore the related problems
and solutions proposed, with a focus on liposome prep-
aration, characterizations, affecting factors, advantages,
and disadvantages. In particular, we return to the litera-
ture relating to high-stability, long-circulating liposomes
(stealth liposomes), and their field of application.
Classification of liposomes
The liposome size can vary from very small (0.025 μm)
to large (2.5 μm) vesicles. Moreover, liposomes may have
Table 1 Advantages and disadvantages of liposome [19]
Advantages of liposome Disadvantages of liposome
Liposomes increased efficacy and therapeutic index of drug (actinomycin-D) Low solubility
Liposome increased stability via encapsulation Short half-life
Liposomes are non-toxic, flexible, biocompatible, completely biodegradable, and non-
immunogenic for systemic and non-systemic administrations
Sometimes phospholipid undergoes oxidation
and hydrolysis-like reaction
Liposomes reduce the toxicity of the encapsulated agent (amphotericin B, Taxol) Leakage and fusion of encapsulated drug/
molecules
Liposomes help reduce the exposure of sensitive tissues to toxic drugs Production cost is high
Site avoidance effect Fewer stables
Flexibility to couple with site-specific ligands to achieve active targeting
Akbarzadeh et al. Nanoscale Research Letters 2013, 8:102 Page 2 of 9

one or bilayer membranes. The vesicle size is an acute
parameter in determining the circulation half-life of
liposomes, and both size and number of bilayers affect
the amount of drug encapsulation in the liposomes. On
the basis of their size and number of bilayers, liposomes
can also be classified into one of two categories: (1)
multilamellar vesicles (MLV) and (2) unilamellar vesi-
cles. Unilamellar vesicles can also be classified into two
categories: (1) large unilamellar vesicles (LUV) and (2)
small unilamellar vesicles (SUV) [16]. In unilamellar
liposomes, the vesicle has a single phospholipid bilayer
sphere enclosing the aqueous solution. In multilamellar
liposomes, vesicles have an onion structure. Classically,
several unilamellar vesicles will form on the inside of the
other with smaller size, making a multilamellar structure
of concentric phospholipid spheres separated by layers
of water [17].
Methods of liposome preparation
General methods of preparation
All the methods of preparing the liposomes involve four
basic stages:
1. Drying down lipids from organic solvent.
2. Dispersing the lipid in aqueous media.
3. Purifying the resultant liposome.
4. Analyzing the final product.
Method of liposome preparation and drug loading
The following methods are used for the preparation of
liposome:
1. Passive loading techniques
2. Active loading technique.
Passive loading techniques include three different
methods:
1. Mechanical dispersion method.
2. Solvent dispersion method.
3. Detergent removal method (removal of non-
encapsulated material) [18,19].
Mechanical dispersion method
The following are types of mechanical dispersion
methods:
1.1. Sonication.
1.2. French pressure cell: extrusion.
1.3. Freeze-thawed liposomes.
1.4. Lipid film hydration by hand shaking, non-hand.
shaking or freeze drying.
1.5. Micro-emulsification.
1.6. Membrane extrusion.
1.7. Dried reconstituted vesicles [18,19].
Sonication Sonication is perhaps the most extensively
used method for the preparation of SUV. Here, MLVs
are sonicated either with a bath type sonicator or a
probe sonicator under a passive atmosphere. The main
disadvantages of this method are very low internal vol-
ume/encapsulation efficacy, possible degradation of
phospholipids and compounds to be encapsulated, elim-
ination of large molecules, metal pollution from probe
tip, and presence of MLV along with SUV [18].
There are two sonication techniques:
a) Probe sonication. The tip of a sonicator is directly
engrossed into the liposome dispersion. The energy
input into lipid dispersion is very high in this
method. The coupling of energy at the tip results in
local hotness; therefore, the vessel must be engrossed
into a water/ice bath. Throughout the sonication up
to 1 h, more than 5% of the lipids can be de-
esterified. Also, with the probe sonicator, titanium
will slough off and pollute the solution.
b) Bath sonication. The liposome dispersion in a
cylinder is placed into a bath sonicator. Controlling
the temperature of the lipid dispersion is usually
easier in this method, in contrast to sonication by
dispersal directly using the tip. The material being
sonicated can be protected in a sterile vessel,
dissimilar the probe units, or under an inert
atmosphere [20].
French pressure cell: extrusion French pressure cell
involves the extrusion of MLV through a small orifice
[18]. An important feature of the French press vesicle
method is that the proteins do not seem to be signifi-
cantly pretentious during the procedure as they are in
sonication [21]. An interesting comment is that French
press vesicle appears to recall entrapped solutes signifi-
cantly longer than SUVs do, produced by sonication or
detergent removal [22-24].
The method involves gentle handling of unstable
materials. The method has several advantages over son-
ication method [25]. The resulting liposomes are rather
larger than sonicated SUVs. The drawbacks of the
method are that the high temperature is difficult to at-
tain, and the working volumes are comparatively small
(about 50 mL as the maximum) [18,19].
Freeze-thawed liposomes SUVs are rapidly frozen and
thawed slowly. The short-lived sonication disper-
ses aggregated materials to LUV. The creation of
unilamellar vesicles is as a result of the fusion of SUV
throughout the processes of freezing and thawing
[26-28]. This type of synthesis is strongly inhibited by

increasing the phospholipid concentration and by in-
creasing the ionic strength of the medium. The encapsu-
lation efficacies from 20% to 30% were obtained [26].
Solvent dispersion method
Ether injection (solvent vaporization) A solution of
lipids dissolved in diethyl ether or ether-methanol mix-
ture is gradually injected to an aqueous solution of the
material to be encapsulated at 55°C to 65°C or under
reduced pressure. The consequent removal of ether
under vacuum leads to the creation of liposomes. The
main disadvantages of the technique are that the popula-
tion is heterogeneous (70 to 200 nm) and the exposure
of compounds to be encapsulated to organic solvents at
high temperature [29,30].
Ethanol injection A lipid solution of ethanol is rapidly
injected to a huge excess of buffer. The MLVs are at
once formed. The disadvantages of the method are that
the population is heterogeneous (30 to 110 nm),
liposomes are very dilute, the removal all ethanol is
difficult because it forms into azeotrope with water, and
the probability of the various biologically active
macromolecules to inactivate in the presence of even
low amounts of ethanol is high [31].
Reverse phase evaporation method This method
provided a progress in liposome technology, since it
allowed for the first time the preparation of liposomes
with a high aqueous space-to-lipid ratio and a capability
to entrap a large percentage of the aqueous material
presented. Reverse-phase evaporation is based on the
creation of inverted micelles. These inverted micelles are
shaped upon sonication of a mixture of a buffered aque-
ous phase, which contains the water-soluble molecules
to be encapsulated into the liposomes and an organic
phase in which the amphiphilic molecules are
solubilized. The slow elimination of the organic solvent
leads to the conversion of these inverted micelles into
viscous state and gel form. At a critical point in this
process, the gel state collapses, and some of the inverted
micelles were disturbed. The excess of phospholipids in
the environment donates to the formation of a complete
bilayer around the residual micelles, which results in the
creation of liposomes. Liposomes made by reverse phase
evaporation method can be made from numerous lipid
formulations and have aqueous volume-to-lipid ratios
that are four times higher than hand-shaken liposomes
or multilamellar liposomes [19,20].
Briefly, first, the water-in-oil emulsion is shaped by
brief sonication of a two-phase system, containing
phospholipids in organic solvent such as isopropyl ether
or diethyl ether or a mixture of isopropyl ether and
chloroform with aqueous buffer. The organic solvents
are detached under reduced pressure, resulting in the
creation of a viscous gel. The liposomes are shaped
when residual solvent is detached during continued ro-
tary evaporation under reduced pressure. With this
method, high encapsulation efficiency up to 65% can be
obtained in a medium of low ionic strength for example
0.01 M NaCl. The method has been used to encapsulate
small, large, and macromolecules. The main drawback of
the technique is the contact of the materials to be
encapsulated to organic solvents and to brief periods of
sonication. These conditions may possibly result in the
breakage of DNA strands or the denaturation of some
proteins [32]. Modified reverse phase evaporation
method was presented by Handa et al., and the main
benefit of the method is that the liposomes had high en-
capsulation efficiency (about 80%) [33].
Detergent removal method (removal of non-encapsulated
material)
Dialysis The detergents at their critical micelle
concentrations (CMC) have been used to solubilize
lipids. As the detergent is detached, the micelles become
increasingly better-off in phospholipid and lastly com-
bine to form LUVs. The detergents were removed by
dialysis [34-36]. A commercial device called LipoPrep
(Diachema AG, Switzerland), which is a version of
dialysis system, is obtainable for the elimination of
detergents. The dialysis can be performed in dialysis
bags engrossed in large detergent free buffers (equilib-
rium dialysis) [17].
Detergent (cholate, alkyl glycoside, Triton X-100) re-
moval of mixed micelles (absorption) Detergent
absorption is attained by shaking mixed micelle solution
with beaded organic polystyrene adsorbers such
as XAD-2 beads (SERVA Electrophoresis GmbH,
Heidelberg, Germany) and Bio-beads SM2 (Bio-Rad
Laboratories, Inc., Hercules, USA). The great benefit of
using detergent adsorbers is that they can eliminate
detergents with a very low CMC, which are not entirely
depleted.
Gel-permeation chromatography In this method, the
detergent is depleted by size special chromatography.
Sephadex G-50, Sephadex G-l 00 (Sigma-Aldrich, MO,
USA), Sepharose 2B-6B, and Sephacryl S200-S1000
(General Electric Company, Tehran, Iran) can be used
for gel filtration. The liposomes do not penetrate into
the pores of the beads packed in a column. They perco-
late through the inter-bead spaces. At slow flow rates,
the separation of liposomes from detergent monomers is
very good. The swollen polysaccharide beads adsorb

substantial amounts of amphiphilic lipids; therefore, pre-
treatment is necessary. The pre-treatment is done by
pre-saturation of the gel filtration column by lipids using
empty liposome suspensions.
Dilution
Upon dilution of aqueous mixed micellar solution of de-
tergent and phospholipids with buffer, the micellar size
and the polydispersity increase fundamentally, and as
the system is diluted beyond the mixed micellar phase
boundary, a spontaneous transition from poly-dispersed
micelles to vesicles occurs.
Stealth liposomes and conventional liposomes
Although liposomes are like biomembranes, they are still
foreign objects of the body. Therefore, liposomes are
known by the mononuclear phagocytic system (MPS)
after contact with plasma proteins. Accordingly, lipo-
somes are cleared from the blood stream.
These stability difficulties are solved through the use
of synthetic phospholipids, particle coated with amphi-
pathic polyethylene glycol, coating liposomes with chitin
derivatives, freeze drying, polymerization, micro-encap-
sulation of gangliosides [17].
Coating liposomes with PEG reduces the percentage of
uptake by macrophages and leads to a prolonged pres-
ence of liposomes in the circulation and, therefore, make
available abundant time for these liposomes to leak from
the circulation through leaky endothelium.
A stealth liposome is a sphere-shaped vesicle with a
membrane composed of phospholipid bilayer used to de-
liver drugs or genetic material into a cell. A liposome
can be composed of naturally derived phospholipids with
mixed lipid chains coated or steadied by polymers of
PEG and colloidal in nature. Stealth liposomes are
attained and grown in new drug delivery and in con-
trolled release. This stealth principle has been used to
develop the successful doxorubicin-loaded liposome
product that is presently marketed as Doxil (Janssen
Biotech, Inc., Horsham, USA) or Caelyx (Schering-
Plough Corporation, Kenilworth, USA) for the treatment
of solid tumors. Recently impressive therapeutic impro-
vements were described with the useof corticosteroid-
loaded liposome in experimental arthritic models. The
concerning on the application of stealth liposomes has
been on their potential to escape from the blood circula-
tion. However, long circulating liposome may also act as
a reservoir for prolonged release of a therapeutic agent.
Pharmacological action of vasopressin is formulated in
long circulating liposome [37,38].
Drug loading in liposomes
Drug loading can be attained either passively (i.e., the
drug is encapsulated during liposome formation) or
actively (i.e., after liposome formation). Hydrophobic
drugs, for example amphotericin B taxol or annamycin,
can be directly combined into liposomes during vesicle
formation, and the amount of uptake and retention is
governed by drug-lipid interactions. Trapping effective-
ness of 100% is often achievable, but this is dependent
on the solubility of the drug in the liposome membrane.
Passive encapsulation of water-soluble drugs depends on
the ability of liposomes to trap aqueous buffer
containing a dissolved drug during vesicle formation.
Trapping effectiveness (generally <30%) is limited by the
trapped volume delimited in the liposomes and drug
solubility. On the other hand, water-soluble drugs that
have protonizable amine functions can be actively en-
trapped by employing pH gradients [39], which can re-
sult in trapping effectiveness approaching 100% [40].
Freeze-protectant for liposomes (lyophilization)
Natural excerpts are usually degraded because of oxida-
tion and other chemical reactions before they are
delivered to the target site. Freeze-drying has been a
standard practice employed to the production of many
pharmaceutical products. The overwhelming majority of
these products are lyophilized from simple aqueous
solutions. Classically, water is the only solvent that must be
detached from the solution using the freeze-drying process,
but there are still many examples where pharmaceutical
products are manufactured via a process that requires
freeze-drying from organic co-solvent systems [14].
Freeze-drying (lyophilization) involves the removal of
water from products in the frozen state at tremendously
low pressures. The process is normally used to dry
products that are thermo-labile and would be demo-
lished by heat-drying. The technique has too much
potential as a method to solve long-term stability diffi-
culties with admiration to liposomal stability. Studies
showed that leakage of entrapped materials may take
place during the process of freeze-drying and on recon-
stitution. Newly, it was shown that liposomes when
freeze-dried in the presence of adequate amounts of tre-
halose (a carbohydrate commonly found at high
concentrations in organism) retained as much as 100%
of their original substances. It shows that trehalose is an
excellent cryoprotectant (freeze-protectant) for lipo-
somes. Freeze-driers range in size from small laboratory
models to large industrial units available from pharma-
ceutical equipment suppliers [41].
Mechanism of transportation through liposome
The limitations and benefits of liposome drug carriers
lie critically on the interaction of liposomes with cells
and their destiny in vivo after administration. In vivo
and in vitro studies of the contacts with cells have
shown that the main interaction of liposomes with cells

is either simple adsorption (by specific interactions with
cell-surface components, electrostatic forces, or by non-
specific weak hydrophobic) or following endocytosis (by
phagocytic cells of the reticuloendothelial system, for ex-
ample macrophages and neutrophils).
Fusion with the plasma cell membrane by insertion of
the lipid bilayer of the liposome into the plasma mem-
brane, with simultaneous release of liposomal content into
the cytoplasm, is much rare. The fourth possible inter-
action is the exchange of bilayer components, for instance
cholesterol, lipids, and membrane-bound molecules with
components of cell membranes. It is often difficult to de-
termine what mechanism is functioning, and more than
one may function at the same time [42-44].
Fusogenic liposomes and antibody-mediated liposomes in
cancer therapy
It has been infrequently well-known that a powerful
anticancer drug, especially one that targets the cyto-
plasm or cell nucleus, does not work due to the low per-
meability across a plasma membrane, degradation by
lysosomal enzymes through an endocytosis-dependent
pathway, and other reasons. Thus, much attention on
the use of drug delivery systems is focused on overcom-
ing these problems, ultimately leading to the induction
of maximal ability of anti-cancer drug. In this respect, a
new model for cancer therapy using a novel drug deliv-
ery system, fusogenic liposome [45], was developed.
Fusogenic liposomes are poised of the ultraviolet-
inactivated Sendai virus and conventional liposomes.
Fusogenic liposomes effectively and directly deliver their
encapsulated contents into the cytoplasm using a fusion
mechanism of the Sendai virus, whereas conventional
liposomes are taken up by endocytosis by phagocytic
cells of the reticuloendothelial system, for example
macrophages and neutrophils. Thus, fusogenic liposome
is a good candidate as a vehicle to deliver drugs into the
cytoplasm in an endocytosis-independent manner [45].
Liposomal drug delivery systems provide steady
formulation, provide better pharmacokinetics, and make
a degree of ‘passive’ or ‘physiological’ targeting to tumor
tissue available. However, these transporters do not
directly target tumor cells. The design modifications that
protect liposomes from unwanted interactions with
plasma proteins and cell membranes which differed
them with reactive carriers, for example cationic
liposomes, also prevent interactions with tumor cells. As
an alternative, after extravasation into tumor tissue,
liposomes remain within tumor stroma as a drug-loaded
depot. Liposomes ultimately become subject to enzym-
atic degradation and/or phagocytic attack, leading to re-
lease of drug for subsequent diffusion to tumor cells.
The next generation of drug carriers under development
features directs molecular targeting of cancer cells via
antibody-mediated or other ligand-mediated interactions
[17,45].
Applications of liposomes in medicine and pharmacology
Applications of liposomes in medicine and pharmacol-
ogy can be divided into diagnostic and therapeutic
applications of liposomes containing various markers or
drugs, and their use as a tool, a model, or reagent in the
basic studies of cell interactions, recognition processes,
and mode of action of certain substances [43].
Unfortunately, many drugs have a very narrow thera-
peutic window, meaning that the therapeutic concentra-
tion is not much lower than the toxic one. In several
cases, the toxicity can be reduced or the efficacy can be
enhanced by the use of a suitable drug carrier which
alters the temporal and spatial delivery of the drug, i.e.,
its biodistribution and pharmacokinetics. It is clear from
many pre-clinical and clinical studies that drugs, for
instance antitumor drugs, parceled in liposome demon-
stration reduced toxicities, while retentive enhanced
efficacy.
Advances in liposome design are leading to new
applications for the delivery of new biotechnology
products, for example antisense oligonucleotides, cloned
genes, and recombinant proteins. A vast literature define
the viability of formulating wide range of conservative
drugs in liposomes, frequently resultant in improved
therapeutic activity and/or reduced toxicity compared
with the free drug. As a whole, changed pharmacokinet-
ics for liposomal drugs can lead to improved drug bio-
availability to particular target cells that live in the
circulation, or more prominently, to extravascular dis-
ease sites, for example, tumors. Recent improvements
include liposomal formulations of all-trans-retinoic acid
[46,47] and daunorubicin [48-51], which has received
Food and Drug Administration consent as a first-line
treatment of AIDS-related advanced Kaposi's sarcoma.
Distinguished examples are vincristine, doxorubicin, and
amphotericin B [38].
The benefits of drug load in liposomes, which can be
applied as (colloidal) solution, aerosol, or in (semi) solid
forms, such as creams and gels, can be summarized into
seven categories [44] (Table 2):
Liposomes in parasitic diseases and infections
From the time when conventional liposomes are
digested by phagocytic cells in the body after intraven-
ous management, they are ideal vehicles for the targeting
drug molecules into these macrophages. The best known
instances of this ‘Trojan horse-like’ mechanism are sev-
eral parasitic diseases which normally exist in the cell of
MPS. They comprise leishmaniasis and several fungal
infections.

Leishmaniasis is a parasitic infection of macrophages
which affects over 100 million people in tropical regions
and is often deadly. The effectual dose of drugs, mostly
different antimonials, is not much lower than the toxic
one. Liposomes accumulate in the very same cell popu-
lation which is infected, and so an ideal drug delivery ve-
hicle was proposed [52]. Certainly, the therapeutic index
was increased in rodents as much as several hundred
times upon administration of the drug in various
liposomes. Unexpectedly, and unfortunately, there was
not much interest to scale up the formulations and clin-
ically approve them after several very encouraging stud-
ies dating back to 1978. Only now, there are several
continuing studies with various anti-parasitic liposome
formulations in humans. These formulations use mostly
ionosphere amphotericin B and are transplanted from
very successful and prolific area of liposome formu-
lations in antifungal therapy.
The best results reported so far in human therapy are
probably liposomes as carriers foramphotericin B in
antifungal therapies. This is the drug of choice in
dispersed fungal infections which often in parallel work
together with chemotherapy, immune system, or AIDS,
and is frequently fatal. Unfortunately, the drug itself is
very toxic and its dosage is limited due to its ionosphere
and neurotoxicity. These toxicities are normally related
with the size of the drug molecule or its complex. Obvi-
ously, liposome encapsulation inhibits the accumulation
of drug in these organs and radically reduces toxicity [53].
Furthermore, often, the fungus exists in the cells of the
mononuclear phagocytic system; therefore, the encapsula-
tion results in reduced toxicity and passive targeting.
These benefits, however, can be associated with any
colloidal drug carrier. Certainly, similar improvements
in therapy were observed with stable mixed micellar
formulations and micro-emulsions [54]. Additionally, it
seems that many of the early liposomal preparations were
in actual fact liquid crystalline colloidal particles rather
than self-closed MLV. Since the lives of the first terminally
ill patients (who did not rely to all the conventional
therapies) were saved [53], many patients were very effect-
ively treated with diverse of amphotericin B formulations.
Comparable methods can be achieved in antiviral and
antibacterial therapies [55]. Most of the antibiotics, how-
ever, are orally available; liposome encapsulation can be
considered only in the case of very potent and toxic ones
which are administered parenterally. The preparation of
antibiotic-loaded liposomes at sensibly high drug-to-lipid
ratios may not be easy because of the interactions of
these molecules with bilayers and high densities of their
aqueous solutions which often force liposomes to float
as a creamy layer on the top of the tube. Several other
ways, for instance, topical or pulmonary (by inhalation)
administration are being considered also. Liposome-
encapsulated antivirals (for example ribavirin, azidothy-
midine, or acyclovir) have also shown to reduce toxicity;
currently, more detailed experiments are being per-
formed in relation to their efficacy.
Liposomes in anticancer therapy
Numerous different liposome formulations of numerous
anticancer agents were shown to be less toxic than the
free drug [56-59]. Anthracyclines are drugs which stop
the growth of dividing cells by intercalating into the
DNA and, thus, kill mainly rapidly dividing cells. These
cells are not only in tumors but are also in hair, gastro-
intestinal mucosa, and blood cells; therefore, this class of
drug is very toxic. The most used and studied is
Adriamycin (commercial name for doxorubicin HCl;
Ben Venue Laboratories, Bedford, Ohio). In addition to
the above-mentioned acute toxicities, its dosage is
limited by its increasing cardio toxicity. Numerous di-
verse formulations were tried. In most cases, the toxicity
was reduced to about 50%. These include both acute
and chronic toxicities because liposome encapsulation
reduces the delivery of the drug molecules towards those
tissues. For the same reason, the efficiency was in many
cases compromised due to the reduced bioavailability of
the drug, especially if the tumor was not phagocytic or
located in the organs of mononuclear phagocytic system.
Table 2 Benefits of drug load in liposomes
Benefits of drug load in liposome Examples
1. Improved solubility of lipophilic and amphiphilic drugs Amphotericin B, porphyrins, minoxidil, some peptides, and anthracyclines,
respectively; hydrophilic drugs, such as anticancer agent doxorubicin or acyclovir
2. Passive targeting to the cells of the immune system,
especially cells of the mononuclear phagocytic system
Antimonials, amphotericin B, porphyrins, vaccines, immunomodulators
3. Sustained release system of systemically or locally
administered liposomes
Doxorubicin, cytosine arabinoside, cortisones, biological proteins or peptides such
as vasopressin
4. Site-avoidance mechanism Doxorubicin andamphotericin B
5. Site-specific targeting Anti-inflammatory drugs, anti-cancer, anti-infection
6. Improved transfer of hydrophilic, charged molecules Antibiotics, chelators, plasmids, and genes
7. Improved penetration into tissues Corticosteroids, anesthetics, and insulin

In some cases, such as systemic lymphoma, the effect of
liposome encapsulation showed enhanced efficacy due to
the continued release effect, i.e., longer presence of
therapeutic concentrations in the circulation [60-62],
while in several other cases, the sequestration of the
drug into tissues of mononuclear phagocytic system ac-
tually reduced its efficacy.
Applications in man showed, in general, reduced tox-
icity and better tolerability of administration with not too
encouraging efficacy. Several different formulations are in
different phases of clinical studies and show mixed results.
Conclusions
Liposomes have been used in a broad range of pharmaceut-
ical applications. Liposomes are showing particular promise
as intracellular delivery systems for anti-sense molecules,
ribosomes, proteins/peptides, and DNA. Liposomes with
enhanced drug delivery to disease locations, by ability of
long circulation residence times, are now achieving clinical
acceptance. Also, liposomes promote targeting of particular
diseased cells within the disease site. Finally, liposomal
drugs exhibit reduced toxicities and retain enhanced effi-
cacy compared with free complements. Only time will tell
which of the above applications and speculations will prove
to be successful. However, based on the pharmaceutical
applications and available products, we can say that
liposomes have definitely established their position in mod-
ern delivery systems.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
SWJ conceived the study and participated in its design and coordination. NZ
participated in the sequence alignment and drafted the manuscript. AA, RRS,
SD, YH, MS, MK, and KNK helped in drafting the manuscript. All authors read
and approved the final manuscript.
Acknowledgments
The authors thank the Department of Medical Nanotechnology, Faculty of
Advanced Medical Science of Tabriz University for all the support provided.
This work is funded by the 2012 Yeungnam University Research Grant.
Author details
1
Department of Medical Nanotechnology, Faculty of Advanced Medical
Science, Tabriz University of Medical Sciences, Tabriz 51664, Iran.
2
Department of Biology, Science and Research Branch, Islamic Azad
University, Tehran, Iran. 3
Department of Endodontics, Dental School, Tabriz
University of Medical Sciences, Tabriz, Iran. 4
Department of Physics, Tabriz
Branch, Islamic Azad University, Tabriz, Iran. 5
School of Mechanical
Engineering, WCU Nanoresearch Center, Yeungnam University, Gyeongsan
712-749, South Korea.
Received: 5 January 2013 Accepted: 22 January 2013
Published: 22 February 2013
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Medical nanoparticles for next generation
drug delivery to the lungs
Sabine H. van Rijt1
, Thomas Bein2
and Silke Meiners1
Affiliations:
1
Comprehensive Pneumology Center (CPC), University Hospital, Ludwig-Maximilians University, Helmholtz
Zentrum München, Member of the German Center for Lung Research (DZL), Munich, Germany.
2
Dept of Chemistry, Center for NanoScience (CeNS), Ludwig-Maximilians University, Munich, Germany.
Correspondence: Sabine H. van Rijt, Comprehensive Pneumology Center, Ludwig-Maximilians University,
Helmholtz Zentrum München, Max-Lebsche-Platz 31, 81377 Munich, Germany.
E-mail: sabine.vanrijt@helmholtz-muenchen.de
ABSTRACT Respiratory diseases are an increasing burden for the ageing population. Although our
understanding of these diseases has improved significantly over the past decades, diagnostic and therapeutic
options for treating lung diseases, such as chronic obstructive pulmonary disease, idiopathic pulmonary
fibrosis and lung cancer, remain limited. Multidisciplinary approaches that bridge the gap between
medicinal and materials sciences will likely contribute to promising new therapeutic and diagnostic
solutions. One such multidisciplinary approach is the use of nanoparticles as carriers for the delivery of
drugs. The advantages of using nanoparticles to deliver drugs include: increased drug concentration at the
disease site; minimised drug degradation and loss; ease of creating inhalable formulations; and the
possibility of specific cell targeting. This article gives a brief overview on the emerging field of nanocarriers
as drug delivery vehicles for the treatment of lung diseases.
@ERSpublications
This article provides an overview on the emerging field of nanocarriers as drug delivery vehicles for
lung therapy http://ow.ly/vkNCx
Received: Dec 06 2013 | Accepted after revision: March 26 2014 | First published online: May 2 2014
Support statement: The Alexander von Humboldt foundation (Bonn, Germany) provided funding for S.H. van Rijt.
Conflict of interest: None declared.
Copyright ßERS 2014
| BACK TO BASICS
MEDICAL NANOPARTICLES
Eur Respir J 2014; 44: 765–774 | DOI: 10.1183/09031936.00212813 765

Introduction
The lung is a unique organ for drug delivery. Its large surface area, thin epithelium layer and rich blood
supply ensure fast drug absorption. These properties offer not only the possibility to treat lung disease, but
also allow for the systemic application of drugs. Levels of metabolism in the lung are much lower compared
to the gastrointestinal tract and the liver. Therefore, drugs largely bypass metabolism when directly
delivered into the lung. Administration of drugs via the lung is made even more attractive by its
noninvasiveness and possibility for self-administration. Even though inhaled drugs have been popular for
over 50 years, drawbacks such as difficulties with formulation, administration, dosing, compliance and
stability of inhaled drugs have limited their clinical success. Only one inhalable therapeutic protein is
currently on the market for the treatment of cystic fibrosis (CF) (pulmozyme; Genentech Inc., San
Francisco, CA, USA). Inhalable insulin (exubera; Pfizer, New York, NY, USA) for systemic drug delivery was
approved in 2006 but has already been withdrawn from the market due to disappointing sales and adverse
effects. Another inhalable insulin product (AFREZZA; MannKind Corporation, Valencia, CA, USA) is
currently under review by the US Food and Drug Administration (FDA) [1].
Exactly where it is best to deposit drugs in the lungs is not completely clear and depends on the disease type.
In asthma, for example, drug delivery to the upper airways is needed while for alveolar lung diseases, such as
chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis, it is important to have
drug deposition in the deep lung rather than the upper airways. Depositing drugs in the lung sounds
straightforward but in reality it’s quite challenging, for example, due to the efficient clearance mechanisms
of the lung (fig. 1). Inhaled particles can be cleared out of the lungs by two systems. In the conducting zone
of the lung, particles can efficiently be removed by moving patches of mucus. This mechanism hinders drug
delivery to the underlying epithelium, a difficulty that is worsened in lung diseases such as CF and COPD,
which are characterised by increased mucus production and thicker mucus. In the deep lung, the monolayer
of the alveolar epithelium is extremely thin and the surface liquid lining the alveoli is much thinner than in
the bronchial tree and therefore represents less of a hindrance for particle uptake. However, the air side of
the alveolar cells is patrolled by macrophages, which engulf and digest insoluble particles that are deposited
there quite rapidly [7].
The deposition and distribution of inhaled particles in the lungs is very complex and depends on many
factors, such as breathing rate and lung volume of the individual. Additionally, depending on particle size,
air flow and desired location in the lung, particle deposition occurs via several different mechanisms.
A detailed summary of this is beyond the scope of this article and has been excellently reviewed elsewhere
[2, 3]. However, many studies suggest that the size of the inhaled particles is an important factor in dictating
their distribution and deposition site in the lung. There is some consensus in the literature that particles in
the range of 1 mm to 5 mm are deposited in the deep lungs, while particles .10 mm are generally deposited
in the oropharyngeal region [4, 5]. Only a few studies have examined deposition of nanosized particles in
lungs and these data suggest that nanosized particles are deposited to a larger extent and more evenly
through the lung compared to micron-sized particles [6].
In the alveolar region, the size of the particles appears to dominate the clearance rate. Although many
aspects concerning the interactions of particles with macrophages remain unknown, several studies agree on
the following statements. 1) Particles at the nanoscale (f200 nm) can cross the cellular barrier independent
of energy and are phagocytosed by macrophages to a minor extent. 2) Micron scale particles between 1 mm
and 5 mm are efficiently taken up by macrophages. 3) Particles .6 mm are generally not phagocytosed but
rather exhaled [2]. This suggests that particles that are small enough can evade macrophage clearance when
deposited in the deep lung. As a result, nanosized particles have ideal deposition characteristics for delivery
to alveolar regions of the lung (fig. 1).
The drawbacks associated with generating inhalable drugs, namely formulation and deposition difficulties,
can mostly be circumvented by using nanoparticles as almost any drug can be encapsulated. In addition,
formulating drugs in nanosized particles, rather than the currently used micron-sized particles, offers
additional benefits. Unlike the micron-sized particles, nanoparticles penetrate the lung more deeply and are
able to enter the alveolar region. In addition, as mentioned previously, nanosized particles evade
macrophage clearance more easily and penetrate the epithelium. At the same time, nanoparticles can be
modified on the surface to increase bioavailability or help in penetrating the mucus layers, as well as
targeting functionalities to aid drug delivery (discussed in the following sections).
Nano-sized drug carriers
Over the past decades, the application of nanotechnology in medicine has received much attention. In
particular, the use of nanoparticles as carriers for drug delivery is a rapidly growing field of research. The
advantages of using nanoscale drug carriers are numerous and include: increased drug circulation times;
MEDICAL NANOPARTICLES | S.H. VAN RIJT ET AL.
DOI: 10.1183/09031936.00212813
766

increased drug concentration at the disease site; minimised drug degradation and loss; and ease of
administration [8]. It is also important that the use of nanoparticles allows for controlled drug release at the
disease site, as well as the possibility for specific cell targeting. Nanocarriers are solid colloidal particles with
dimensions usually between 10 nm and 200 nm. Depending on the nanocarrier, the drug can be captured
inside the particle using different techniques and can be retained with different interactions (e.g. covalent
and electrostatic interactions). As such not every drug is compatible with every type of nanoparticle. To
date, there are many different kinds of known nanoparticles such as polymeric nanoparticles, liposomes,
carbon nanotubes, mesoporous silica, protein nanoassemblies, dendrimers, micelles and many more
(fig. 2). Depending on the material used and modifications on the surface, nanocarriers can have different
properties and drug release characteristics.
Nanoparticles have sizes similar to biological entities such as proteins and viruses (fig. 3) and can, therefore,
interact with biomolecules on the cell surface and within the cell. Biodegradable polymers have received a
lot of attention due to their biocompatibility, possibility for surface modifications and sustained release of
the drug [8]. Polymeric particles have also been extensively studied for the delivery of pulmonary medicines;
these include asthmatic drugs [9, 10], anti-tuberculosis drugs [11] and anti-cancer drugs [12, 13]. Many
types of liposomal nano-formulations have been investigated for their drug delivery properties as well.
Currently, there are several US FDA-approved liposomal drug delivery systems on the market and some
liposomal drug formulations are in clinical trials [14]. Liposomes seem particularly appropriate for delivery
to the lung since they can be prepared from compounds endogenous to the lungs (e.g. lung surfactants).
Encouragingly, some liposomal particles made from surfactants (e.g. exosurf (GlaxosSmithKline, Brentford,
UK) and alveofact (Lyomark Pharma, Oberhaching, Germany)) are currently on the market for the
treatment of acute respiratory distress syndrome [15]. Polymeric and liposomal nanoparticles can be
classified as ‘‘soft’’ organic particles. Recently, ‘‘hard’’, often inorganic-based, nanocarriers have received
much attention for their drug delivery properties, for example carbon nanotubes and mesoporous silica
nanoparticles. Many exciting new nanocarriers with different properties are being reported in this rapidly
evolving field of research, making it impossible to highlight all novel findings. However, several properties
that are important for all nanocarriers designed for drug delivery are highlighted in figure 4 and discussed
in the following sections.
Surface modifications
Most particles allow for surface modifications that can positively affect their biodistribution and circulation
times. Interestingly, nanocarrier hydrophobicity determines, to a large degree, the level of particle binding
to blood components. Indeed, nonsurface-modified hydrophobic particles are known to be cleared rapidly
[16]. To increase the blood circulation time of these particles, they can be coated with hydrophilic polymers
or surfactants. For example, low molecular weight poly(ethylene glycol) (PEG) chains that decorate the
particle in a brush-like fashion (PEGylation) are known to reduce phagocytosis [17]. Importantly for
chronic obstructive lung diseases, PEGylation of nanoparticles has also been shown to increase mucous
Largely
phagocytosed
>1–5 µm
<1 µm
>5 µm
Exhaled
Deposition and
uptake in alveoli
FIGURE 1 Schematic diagram showing the
size-dependent deposition of particles in
the lungs [2–6].
MEDICAL NANOPARTICLES | S.H. VAN RIJT ET AL.
DOI: 10.1183/09031936.00212813 767

Liposomes detail topic explanation notes

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