Nanoparticles

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Nanoparticles
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Nanoparticles are the most extensively
investigated drug delivery systems. This
includes Polymeric nanoparticles
Liposomes
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Polymeric nanoparticles
Nanoparticles are solid colloidal particles
ranging in size from 10 to 1,000 nm. They are
made of a macromolecular material which can be of
synthetic or natural origin. Depending on the
process used for their preparation, two different
types of nanoparticles can be obtained, nanosphere
s and nanocapsules. Nanospheres have a
matrix-type structure in which a drug is
dispersed, whereas nanocapsules exhibit a
membrane-wall structure with a core containing
the drug. Because these systems have very high
surface areas, drugs may also be adsorbed on
their surface.
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MANUFACTURE OF NANOPARTICLES
Methods of manufacturing nanoparticles The
choice of the manufacturing method depends on the
raw material intended to be used and on the
solubility characteristics of the active compound
to be associated to the particles. The raw
material, biocompatibility, the degradation
behavior, choice of the administration route,
desired release profile of the drug, and the type
of biomedical application determine its
selection. Thus nanoparticle formulation requires
an initial and very precise definition of the
needs and objectives to be achieved.
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1- In situ polymerization of a monomer
Nanospheres
Two different approaches have been considered for
the preparation of nanospheres by in situ
polymerization, depending on whether the monomer
to be Polymerized is emulsified in a nonsolvent
phase (emulsification polymerization), or
dissolved in a solvent that is a nonsolvent for
the resulting polymer (dispersion polymerization)
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Emulsification Polymerization. Depending on the
nature of the continuous phase in the emulsion,
whether, the continuous phase is aqueous (o/w
emulsion), or organic (w/o emulsion). In both
cases the monomer is emulsified in the nonsolvent
phase in presence of surfactant molecules,
leading to the formation of monomer-swollen
micelles and stabilized monomer droplets.
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The polymerization reaction takes place in the
presence of a chemical or physical initiator.
The energy provided by the initiator creates
free reactive monomers in the continuous phase
which then collide with the surrounding
unreactive monomers and initiate the
polymerization chain reaction. The reaction
generally stops once full consumption of monomer
or initiator is achieved.
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The mechanism by which the polymeric particles
are formed during emulsification polymerization
is by micellar polymerization, where the
swollen-monomer micelles act as the site of
nucleation and polymerization. Swollen micelles
exhibit sizes in the nanometer range and thus
have a much larger surface area in comparison
with that of the monomer droplets. Once
generated in the continuous phase, free reactive
monomers would more probably initiate the
reaction within the micelles.
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As the monomer molecules are slightly soluble in
the surrounding phase, they reach the micelles by
diffusion from the monomer droplets through the
continuous phase, thus
allowing the polymerization to be followed within
the micelles. So, in this case, monomer droplets
would essentially act as monomer reservoirs.
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The drug to be associated to the nanospheres may
be present during polymerization or can be
subsequently added to the preformed nanospheres,
so that the drug can be either incorporated into
the matrix or simply adsorbed at the surface of
the nanospheres.
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Micellar polymerization mechanism.
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Dispersion Polymerization
The monomer is no more emulsified but dissolved
in an aqueous medium which acts as a precipitant
for the polymer to be formed. The nucleation is
directly induced in the aqueous monomer
solution. For Production of Polymethacrylic
Nanospheres, water soluble methyl methacrylate
monomers are dissolved in an aqueous medium and
polymerized by y-irradiation or by chemical
initiation (ammonium or potassium
peroxodisulfate) combined with heating to
temperatures above 650C.
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In the case of chemical initiation, the aqueous
medium must be previously flushed with nitrogen
for 1 h in order to remove its oxygen content,
which could inhibit the polymerization by
interfering with the initiated radicals.
Oligomers (primary polymer) are formed and above
a certain molecular weight precipitate in the
form of primary particles. Finally, nanospheres
are obtained by the growth or the fusion of
primary particles in the aqueous phase
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The removal of detergents is very important that
produce very slowly biodegradable and
biocompatible nanoparticles The technique can be
used for vaccination purposes. where initiation
by y-irradiation can be useful for the production
of nanospheres by polymerization in the presence
of antigenic material at room temperature, thus
preventing its destruction. Examples of antigenic
materials used to produce nanoparticulate were
different influenza antigens.
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Nanocapsules
Nanocapsules is a colloidal carrier with a
capsular structure consisting of a polymeric
envelope surrounding an oily central cavity
containing lipophilic drugs. Interfacial
Polymerization Mechanism The monomer (isobutyl
cyanoacrylate) and a lipophilic drug
(progesterone) are dissolved in an ethanolic
phase containing an oil (Myglyol, Lipiodol) or
a non-miscible organic solvent (benzylic
alcohol).
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This mixture is slowly injected through a needle
into a magnetically stirred aqueous phase (pH
410) containing an nonionic surfactant
(poloxamer 188). Upon mixing with the aqueous
phase, ethanol rapidly diffuses out of the
organic phase giving rise to spontaneous
emulsification of the oil/monomer/drug mixture.
Immediately the monomer molecules polymerize at
the water-oil interface, leading to the formation
of solid wall-structured particles.
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The mixture immediately becomes milky and
nanocapsules with a mean diameter of 200-300 nm
are formed. The colloidal suspension can then be
concentrated by evaporation under reduced
pressure and filtered.
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For encapsulate hydrophilic compounds such as
doxorubicin and fluorescein, inverse
emulsification polymerization Technique can be
used. In this procedure, the drug was dissolved
in a small volume of water and emulsified in an
organic external phase ( hexane) containing a
surfactant. An organic solution of cyanoacrylate
monomers is added to the w/o emulsion. Nanocapsule
s are formed, resulting from an interfacial
polymerization process around the nanodroplets.
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Disadvantages of preparation by in situ
polymerization of a monomer

• Most of the carriers produced by polymerization
  have inadequate biodegradability properties
  preventing their use for regular therapeutic
  administration.
• Only for vaccination purposes is being
  suitable when
• achievement of a very prolonged immune
  response is desired.
• 2. The possible inhibition of drug activity due
  to interactions with activated monomers present
  in polymerization processes.

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3. It is very difficult to calculate the
molecular weight of the resulting polymerized
material due to the multicomponent nature of the
polymerization media. However, the determination
of molecular weight is very important as it
influences the biodistribution and release of
the polymeric carrier. 4. The presence of toxic
residues due to the unreacted monomer, initiator,
and surfactant molecules whose elimination
requires time-consuming and not always efficient
procedures.
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In order to avoid those limitations and produce
biodegradable, well-characterized, and nontoxic
nanoparticles., already polymerized materials
have been used. These materials include natural
macromolecules (biopolymers) and synthetic
polymers.
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2- Dispersion of a Preformed Polymer
Nanospheres Prepared From Natural Macromolecules
Due to their biodegradability and
biocompatibility, example of natural
macromolecules used for the manufacture of
nanospheres are Proteins as albumin, gelatin,
(the most widely used) Polysaccharides as
alginate or agarose
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Two manufacturing techniques are used to produce
nanospheres from natural macromolecules. 1. The
first technique is based on the formation of
a w/o emulsion followed by heat denaturation or
chemical cross-linking of the macromolecule.
2.The second technique is a phase separation
process in an aqueous medium followed by
chemical crosslinking.
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Emulsification-Based Methods

• An aqueous solution of albumin is emulsified at
  room temperature in a vegetal oil (cottonseed
  oil) and homogenized either by a homogenizer or
  an ultrasonication.
• Once a high degree of dispersion is achieved,
  the emulsion is added drop wise to a large volume
  of preheated oil (gt120C) under stirring.
• This leads to the immediate vaporization of the
  water contained in the droplets and to the
  irreversible denaturation of the albumin which
  coagulates in the form of solid nanospheres.

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• The suspension is then cooled at room temperature
  or in an ice bath.
• For complete removal of the oil, wash the
  particles using large amounts of organic solvent
  (e.g., ether, ethanol, acetone).

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• Disadvantages of this technique
• The purification step may cause particle wastes.
• The hardening step by heat denaturation may be
  harmful to heat-sensitive drugs. This can be
  avoided by the use of a crosslinking agent.

• Large amounts of organic solvents are required
  to obtain nanospheres free of any oil or
  residues.
• It is very difficult to produce small
  nanospheres (lt500 nm) with narrow-size
  distributions, due to the instability of the
  emulsion prior to hardening by heat or
  crosslinking.

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Preparation of nanospheres by thermal
denaturation of albumin
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Phase Separation-Based Methods in an Aqueous
Medium

• The particles are formed in an aqueous medium by
  a phase
• separation process and are stabilized by cross
  linking with
• glutaraldehye.
• Gelatin and albumin nanospheres can be produced
  by the slow addition of a desolvating agent
  (neutral salt or alcohol) to the protein solution
  to the form protein aggregates
• The nanospheres are obtained by crosslinking of
  these aggregates with glutaraldehyde.

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Preparation of nanospheres by desolvation of
albumin.
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The major disadvantage of this technique is the
necessity for using hardening agents
(glutaraldehyde) that may react with the drug and
may cause toxicity to the nanoparticle
formulations.
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Nanospheres Prepared From Synthetic Polymers
Examples of synthetic polymers used for the
preparation of nanospheres Polylactic acid
(PLA), poly(glycolic acid) (PGA), polylactic-co-gl
ycolic acid) (PLGA), poly(e-caprolactone) (PCL),
and poly(Polyhydroxybutyrate) (PHB). These
polyesters polymers exhibit biodegradability and
biocompatibility. Under physiological
conditions, they are degraded into safe products
as glycolic acid and lactic acid.
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Polyesters nanoparticles can be produced using
two different methods. The method is based on
the emulsification of an organic solution of the
polymer (chloroform, methylene chloride, ethyl
acetate), in an aqueous phase (o/w emulsion)
containing surfactants (e.g., polysorbate,
poloxamer, sodium dodecyl sulfate). The
extraction of the solvent from the nanodroplets
is achieved by evaporation of the organic solvent
at room temperature under stirring.
Emulsification-Based Methods.
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Emulsification-solvent evaporation method
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A second method is based on the direct
precipitation of the solubilized polymer by
salting out process Two different salting-out
agents, magnesium chloride and magnesium acetate,
were used, providing an acidic or a basic aqueous
phase, respectively. Although the salting-out
process has proved suitable for the production of
large quantities of highly drug-loaded nanospheres
, the use of large amounts of salts may raise a
problem concerning compatibility with active
compounds.
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Salting-out process
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Direct Precipitation-Based Method
This method allows nanospheres to be obtained
without prior emulsification. This technique
involves the use of an organic solvent that is
completely miscible with the aqueous phase,
typically (acetone, ethanol or methanol). In
this case, the polymer precipitation is directly
induced in an aqueous medium (non solvent), by
progressive addition under stirring of the
polymer solution. This method is limited to drugs
that are highly soluble in polar solvents, but
only slightly soluble in water (e.g., indomethacin
).
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Direct precipitation method
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Pharmaceutical Aspects

• There are some requirements for nanoparticles
  intended to be used as pharmaceutical dosage
  forms in humans
• to be free of any potentially toxic impurities
• to be easy to store and to administer
• to be sterile if for parenteral administration.

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Purification.
Depending on the preparation method, various
toxic impurities can be found in the
nanoparticulate suspensions including organic
solvents, residual monomers, polymerization
initiators, electrolytes, surfactants,
stabilizers, and large polymer aggregates. The
necessity for and degree of purification are
dependent on the final purpose of the formulation
developed.
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For example, the stabilizer PVA, frequently used
to prepare polyester nanoparticles, is not
acceptable for parenteral administration, whereas
it is not so critical for oral and ocular
administration.
Polymer aggregates can be easily removed by
simple filtration. The removal of other
impurities requires more complicated procedures
as gel filtration, dialysis, and
ultracentrifugation.
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However, these methods are incapable of
eliminating molecules with high molecular weight.
Using cross-flow filtration technique, the
nanoparticle suspension is filtered through
membranes, with the direction of the fluid being
tangential to the surface of the membranes to
avoid the clogging of the filters. It was shown
that by using a microfiltration membrane
(porosity of 100 nm), nanoparticles produced by
the salting out process could be purified of the
salts.
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Main Methods for the Purification of
Nanoparticles on the Laboratory Scale
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Freeze-drying.
Freeze-drying (lyophilization) represents one of
the most useful methodologies to ensure the
long-term conservation of polymeric
nanoparticles This technique involves the
freezing of the suspension and the elimination of
its water content by sublimation under reduced
pressure, where nanoparticles are obtained in the
form of a dry powder that is easy to handle and
to store. Freeze-dried nanoparticles are usually
readily redispersible in water without
modification of their physicochemical properties
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Nanocapsules composed of an oily core surrounded
by a tiny polymeric wall tend to aggregate during
the freeze-drying process. This problem can be
solved by desiccating these systems in the
presence of an appropriate lyoprotective and
cryoprotection agent such as mono- or
disaccharides (e.g., lactose, sucrose,
glucose). The mechanisms by which sugars protect
nanoparticles during freeze-drying is that during
freezedrying sugars may interact with the solute
of interest (e.g., liposome, protein) through
hydrogen-bonding.
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As a result, the solute might be maintained in a
"pseudo-hydrated state during the dehydrating
step of freeze-drying, and would therefore be
protected from damage during dehydration and
subsequent rehydration.
It has to be kept in mind that the addition of
sugar may affect the isotonicity of the final
nanoparticulate suspension, and that a subsequent
step of tonicity adjustment may be required prior
to any parenteral or ocular administration.
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Sterilization

• Nanoparticles intended to be used parenterally
  are required to be sterile and apyrogenic.
• Filtration on 0.22 µm filters is not adequate
  for nanoparticle suspensions because
  microorganisms and nanoparticles are generally
  similar in size (0.25-1 µm).
• Sterilization may be achieved, either by using
  aseptic conditions throughout formulation, or by
  sterilizing treatments such as autoclaving or
  ?-irradiation.

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• The choice of the sterilizing treatment depends
  on the physical susceptibility of the system.
• Autoclaving (moist heat sterilization) and ? -
  irradiation
• May alter the physicochemical properties of
  the particles in several systems.
• These modifications occur as a consequence of
  the cleavage or cross-linking of the polymeric
  chains.
• The final formulation would therefore result from
  a rational balance between conditions maintaining
  the formulation integrity upon sterilization and
  the final purpose of the formulation.

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THANKS