On Nano suspension

1. DEVELOPMENT AND EVALUATION OF NANOSUSPENSION
By Vikram Mishra

2. CHAPTER-1 INTRODUCTION
INTRODUCTION
Almost more than 90% of the drugs are orally administered. Drug absorption, sufficient
& reproducible bioavailability, pharmacokinetic profile of orally administered drug substances is
highly dependent on solubility of that compound in aqueous medium1-2
. Drug absorption,
sufficient and reproducible bioavailability and/or pharmacokinetic profile in humans are
recognized today as one of the major challenges in oral delivery of new drug substances. Orally
administered drugs on the Model list of Essential Medicines of the World Health Organization
(WHO) are assigned BCS classifications on the basis of data available in the public domain. Of
the 130 orally administered drugs on the WHO list, 61 could be classified with certainty. 84% of
these belong to class I (highly soluble, highly permeable), 17% to class II (poorly soluble, highly
permeable), 24 (39%) to class III (highly soluble, poorly permeable) and 6 (10%) to class IV
(poorly soluble, poorly permeable). The rate and extent of absorption of class II & class IV
compounds is highly dependent on the bioavailability which ultimately depends on solubility.
Due to this major reason Solubility enhancement is one of the important parameters which
should be considered in formulation development of orally administered drug with poor aqueous
solubility .Solubility is the characteristic physical property referring to the ability of a given
substance, the solute, to dissolve in a solvent.
Fig.1.BCS classification
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3. CHAPTER-1 INTRODUCTION
Table 1. Solubilitydefinitions
Definition Parts of solvent required for one part of solute
Very Soluble < 1
Freely soluble 1-10
Soluble 10 – 30
Sparingly soluble 30 – 100
Slightly soluble 100 – 1000
Very slightly soluble 1000 - 10,000
Insoluble > 10,000
PROCESS OF SOLUBLISATION
The process of solubilisation involves the breaking of inter-ionic or intermolecular bonds
in the solute, the separation of the molecules of the solvent to provide space in the solvent for the
solute, interaction between the solvent and the solute molecule or ion3-4
. The step involved in
process of solubilisation are depicted as follows:
Step 1: Holes opens in the solvent
Step2: Molecules of the solid breaks away from the bulk
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4. CHAPTER-1 INTRODUCTION
Step 3:The freed solid molecule is intergrated into the hole in the solvent
TECHNIQUES TO OVERCOME POOR SOLUBILITY4-5
The techniques that are used to overcome poor drug solubility are discussed under
following major headings;
I. Chemical Modifications:
1. Salt Formation
2. Co-crystallization
3. Co-solvency
4. Hydrotropic
5. Solubilizing agent
6. Nanotechnology
II. Physical Modifications:
1. Particle size reduction
a. Micronization
b. Nanosuspension
2. Modification of the crystal habit
a. Polymorphs
b. Pseudo polymorphs
3. Complexation
a. Use of complexing agents
4. Solubilisation by surfactants:
a. Micro emulsions
b. Self micro emulsifying drug delivery system
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5. CHAPTER-1 INTRODUCTION
surface, which means decreasing particle size. Analogously to the vapour pressure, the
dissolution pressure of a substance increases with decreasing particle size (Fig. 1.5).
Fig. 1.4 Mechanisms of increasing saturation solubility (Cs) and dissolution velocity in
nanosuspensions. (Δp, dissolution pressure; CSN, saturation solubility nanoparticles; CSM,
saturation solubility microparticles; hM, diffusional distance microparticles; hN, diffusional
distance nanoparticles (Muller et al; 1998).
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6. CHAPTER-1 INTRODUCTION
Fig.1.5 Differences in diffusion and absorption by both micro and nano particles
Formulation theory:
Nanoparticles can be formed by building particles up from the molecular state, as in
precipitation, or by breaking larger micron-sized particles down, as in milling. In either case, a
new surface area, ΔA, is formed, which necessitates a free-energy (ΔG) cost as defined by ΔG =
γs/l •ΔA, in which γs/l is the interfacial tension. This arises because water molecules incur fewer
attractive forces with other water molecules when located at a free surface. The system prefers to
reduce this increase in surface area by either dissolving incipient crystalline nuclei, in the case of
precipitation, or by agglomerating small particles, regardless of their formation mechanism. This
tendency is resisted by the formulator through the addition of surface-active agents, which
reduce the γs/l and therefore the free energy of the system (Fig. 1.6)
These agents confer immediate protection and are more effective when present at the
time of creation of the new, fresh surface than if added afterwards. By virtue of their
complementary properties, surfactants of two classes are utilized: charged or ionic surfactants,
which effect an electrostatic repulsion among the particles; and non-ionic polymers, which
confer a steric repulsion that is, they resist compression. If the particles approach each other too
closely, they will agglomerate. This must be prevented to ensure a stable system. Energetically,
this requires the placement of a sufficiently high energy barrier at relatively long separation
distances, to prevent the particles coming too close together. Therefore, a non-ionic polymeric
surfactant is also used that coats the surface with a hydrophobic chain, and permits a hydrophilic
tail to project into the water.
Compression of the polymeric coating, as by the approach of a similarly coated particle,
causes loss of entropy and is therefore unfavorable. This provides the necessary repulsive barrier
between two neighboring particles. The polymeric coating performs a dual role: inhibiting
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7. CHAPTER-1 INTRODUCTION
crystal growth and reducing particle size (Ziller K H; 1990) Both electrostatic and steric
mechanisms are enabled by combining polymers and ionic surfactants, which therefore
complement each other. Entropic steric interactions are inherently more sensitive to temperature
fluctuations than is electrostatic repulsion.
Therefore, temperature cycling could disrupt a suspension stabilized only by polymer.
To prevent this, ionic surfactants are used as well. There is a synergy between the two, because
adding a neutral polymer to a surface stabilized with an ionic surfactant permits greater coverage
by the ionic surfactant. This occurs because self-repulsion of the charged surfactant molecules is
minimized, which therefore permits closer packing.
The repulsive energy of two similarly charged particles is given by the equation
VR= (εaψ0/κ2) ln [1+ exp(-κHo)]
Where,
a is the particle radius;
H0 is the distance of separation between the two particles,
ε is the dielectric constant of the medium,
ψ0 is the electrostatic surface potential, and
K is related to the thickness of the diffuse electric double layer.
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8. CHAPTER-1 INTRODUCTION
Fig.1.9 Flow chart : methods of preparation of nanosuspensions.
1. Media milling (NanoCrystals):
This patent-protected technology was developed by Liversidge et al (1992). Formerly, the
technology was owned by the company NanoSystems but recently it has been acquired by Elan
Drug Delivery. In this method the nanosuspensions are produced using high-shear media mills or
pearl mills. The media mill consists of a milling chamber, a milling shaft and a recirculation
chamber(Fig.1.10). The milling chamber is charged with the milling media, water, drug and
stabilizer, and the milling media or pearls are then rotated at a very high shear rate. The milling
process is performed under controlled temperatures.
Principle:
The high energy and shear forces generated as a result of the impaction of the milling
media with the drug provide the energy input to break the microparticulate drug into nano-sized
particles. The milling medium is composed of glass, zirconium oxide or highly cross-linked
polystyrene resin. The process can be performed in either batch or recirculation mode. In batch
mode, the time required to obtain dispersions with unimodal distribution profiles and mean
diameters<200nm is 30–60 min. The media milling process can successfully process micronized
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9. CHAPTER-1 INTRODUCTION
and non-micronized drug crystals. Once the formulation and the process are optimized, very little
batch-to-batch variation is observed in the quality of the dispersion.
Advantages:
 Drugs that are poorly soluble in both aqueous and organic media can be easily formulated
into nanosuspensions.
 Ease of scale-up and little batch-to-batch variation.
 Narrow size distribution of the final nano-sized product.
 Flexibility in handling the drug quantity, ranging from 1 to 400 mg/mL, enabling
formulation of very dilute as well as highly concentrated nanosuspensions.
Disadvantages:
 Nanosuspensions contaminated with materials eroded from balls may be problematic
when it is used for long therapy.
 The media milling technique is time consuming.
 Some fractions of particles are in the micrometer range.
 Scale up is not easy due to mill size and weight.
Fig.1.10 Schematic diagram of nanosizing process using media milling (Merisko-Liversidge
And Liversidge; 2011).
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10. CHAPTER-1 INTRODUCTION
Fig.1.11 Schematic Cartoon of the High-Pressure Homogenization Process17
.
3. Homogenization in nonaqueous media (Nanopure)
Nanopure is suspensions homogenized in waterfree media or water mixtures18
. In the
Dissocubes technology, the cavitation is the determining factor of the process. But, in contrast to
water, oils and oily fatty acids have very low vapour pressure and a high boiling point. Hence,
the drop of static pressure will not be sufficient enough to initiate cavitation. Patents covering
disintegration of polymeric material by high- pressure homogenization mention that higher
temperatures of about 80°C promoted disintegration, which cannot be used for thermolabile
compounds. In nanopure technology, the drug suspensions in the nonaqueous media were
homogenized at 0°C or even below the freezing point and hence are called "deep-freeze"
homogenization. The results obtained were comparable to Dissocubes and hence can be used
effectively for thermolabile substances at milder conditions.
4. Combined precipitation and homogenization (Nanoedege)
The basic principles of Nanoedge are the same as that of precipitation and
homogenization19
. A combination of these techniques results in smaller particle size and better
stability in a shorter time. The major drawback of the precipitation technique, such as crystal
growth and long term stability, can be resolved using the Nanoedge technology. In this
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11. CHAPTER-1 INTRODUCTION
technique, the precipitated suspension is further homogenized, leading to reduction in particle
size and avoiding crystal growth.
Fig.1.12 Method for preparation of nanoedge
Fig.1.13 Manufacturing techniques employed towards fabricating nanocrystals. Top-down
methods process primary drug dispersions and scale down the size of drug particles by milling,
cavitation, grinding, impaction, shearing, and attrition. Bottom-up techniques work on the
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12. CHAPTER-1 INTRODUCTION
Fig.1.14 Attributes
STABILITY OF NANOSUSPENSIONS
Stability is a very important property for nanosuspensions. Nanoparticles can for example
agglomerate, aggregate or sinter during the production or storage. Agglomerates are clusters of
primary particles held together by weak physical interactions. Aggregation forms stronger
particle clusters and it is often irreversible process. In sintering, individual particles are merged
irreversibly to larger particles. Increased particle size can change the apparent saturation
solubility and dissolution rate, and consequently change the blood plasma concentration and
bioavailability for oral drug delivery; change the tissue distribution or cause the vascular
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13. CHAPTER-1 INTRODUCTION
stronger than the Van Der Waal’s attraction forces and hydrogen bonds26
. Depending on the
particle size, size deviation and surface properties, the importance of different forces can vary.
The situation is further complicated, when nanosuspensions are dried for formulation purposes.
Fig.1.15 The curves of Van Der Waals attraction, electrostatic repulsion and total potential
energy against distance of surface separation (H) for interaction between two particles.
In nanocrystalline systems the balance between attractive and repulsive forces are
adjusted by adding stabilizers on the particle surfaces.
1. Stabilizers
Stabilizer plays a very important role in particle size reduction and stability of
nanosuspensions. Stabilizers can spontaneously adsorb on and cover the newly formed particle
surface to (a) decrease the free energy of system and interfacial (surface) tension of particles; (b)
form a dense hydrophilic layer around hydrophobic particles, provide steric hindrance and steep
repulsions between the particles (steric stabilization); (c) charge the particle surface if the
stabilizer has ionizable groups, which increase the repulsive force (electrostatic stabilization); (d)
combine the steric and electrostatic stabilization (Fig. 1.16).
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14. CHAPTER-1 INTRODUCTION
Fig.1.16 The mechanism of nanocrystal stabilization during particle reduction (1): steric
stabilization (2), and electrostatic stabilization (3).
The stabilizers, including polymers, surfactants and their combinations, are widely used
in the nanosuspension preparation. Homopolymers used as stabilizers always contain hydrophilic
backbone chains, such as polyvidone (PVP), polyvinyl alcohol (PVA), hydroxypropylcellulose
(HPC), hydroxypropylmethylcellulose (HPMC) and methylcellulose (MC). These polymers can
adsorb on the particle surface by hydrogen bonds to form a hydrodynamic boundary layer.
Douroumis et al.27
screened various stabilizing agents for carbamazepine using the cosolvent
technique. Cellulose ethers (HPMC and MC) contain a high degree of substitution as methoxy or
hydroxypropoxy groups, which can form hydrogen bonds with the drug and inhibit the crystal
growth. The stability of the nanosuspensions is dependent on the degree of substitution. PVP has
only one hydrogen bonding carbonyl group per molecule unit, showing a weak crystal inhibition
because of less favorable drug-polymer association. Opposite to the above results, Choi et al.28
found out that hydrophobic drug surfaces without polar functional groups are ideal for HPC and
PVP to physically adsorb and produce steric stabilization, since the hydrogen bonding between
polymer and drug tends to interfere the stabilization activity of polymers.
Amphiphilic block/graft polymers comprising of hydrophobic and hydrophilic chains are
more promising stabilizers compared to homopolymers. The hydrophobic segments adsorb on
the surfaces of the drug crystals by hydrophobic interactions, while the hydrophilic segments
cover the drug particles and stick out into the solution, giving the steric hindrance and preventing
particle aggregation and growth. The most common amphiphilic polymers are poloxamer 407
(F127) and poloxamer 188 (F68).
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15. CHAPTER-1 INTRODUCTION
2. POST-PRODUCTION PROCESSING
Solidification Techniques
The nanosuspensions usually have the stability issues involved in the physical (e.g.
Ostwald ripening and agglomeration) and chemical (e.g. hydrolysis) processes. In this case, solid
dosage forms are considered more attractive, due to their patient convenience (marketing
aspects) and good stability. Therefore, transformation of nanosuspensions into the solid dosage
form is desirable. Solidification methods of the nanosuspensions include some unit-operations
such as pelletization, granulation, spray drying or lyophilization63
. As the primary objective of
the nanoparticulate system is rapid dissolution, disintegration of the solid form and redispersion
of the individual nanoparticles should be rather rapid, so that it does not impose a barrier on the
integrated dissolution process. Drying of nanoparticles can create stress on the particles that can
cause aggregation. For example, drying may lead to crystallization of the polymers such as
poloxamers, thereby compromising their ability to prevent aggregation. Drying can also create
additional thermal stresses that may destabilize the particles. Due to the above considerations,
adding matrix-formers to the suspension prior to solidification is necessary.
Van Eerdenbrugh et al. had successfully used microcrystalline cellulose to displace
sucrose as a matrix former during freeze-drying of itraconazole nanosuspensions64
and had again
evaluated four alternative matrix formers [Avicel®PH101, Fujicalin® (CaHPO4), Aerosil®200
SiO2) and Inutec®SP1] for their capability in preserving rapid dissolution after spray-drying of
nanosuspensions65
. In addition, the effect of surface hydrophobicity on drug dissolution
behaviour upon redispersion had been investigated, indicating the more intense hydrophobicity,
the more aggregation of the nanoparticles and the slower the drug’s dissolution after
solidification66
.
Surface Modification Techniques
Nanosuspensions have the particular characteristics to increase the saturation solubility
and dissolution rate for the poorly soluble drugs. But in some cases, the rapid or burst release of
nanosuspensions may result in the side effect and toxicity. As a colloid nanoparticle system,
nanosuspensions usually can target the monocyte phagocytic system (MPS), which can aid in the
treatment of lymphatic-mediated diseases67
, like Mycobacterium tuberculosis, Listeria
monogyna, Leishmania sp. The action is called as ‘passive targeting’. However, the passive
targeting process could pose an obstacle when either macrophages are not the desired targets or
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16. CHAPTER-1 INTRODUCTION
Fig.1.17 Fate of ophthalmic drug delivery systems (modified after reference77
).
5. Drug targeting
Nanosuspensions can also be used for targeting as their surface properties and changing
of the stabilizer can easily alter the in vivo behavior. The drug will be up taken by the
mononuclear phagocytic system to allow regional-specific delivery. This can be used for
targeting anti-mycobacterial, fungal or leishmanial drugs to the macrophages if the infectious
pathogen is persisting intracellularly78
.
Nanosuspension provide passive targeting which enhance permeability and
Fig.1.18
availability”
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17. CHAPTER-1 INTRODUCTION
ii. Consolidation stage: Various physicochemical interactions occur to consolidate and
strengthen the adhesive joint, resulting in prolonged adhesion and increased residence time of the
dosage form.
a) Contact stage
Initially the mucoadhesive polymer spreads over the mucus membrane and forms a deep
contact with the mucus layer resulting in the swelling of the formulation as is depicted in Figure
1.4. The dosage form is attached either mechanically by placing and holding it over the mucus
membrane (in case of oral, ocular and vaginal delivery) or by aerodynamics of the organ (in case
of nasal route). In GI tract the mucosal surface is not accessible for adhesion as the adhesive
material cannot be directly placed or be delivered to the mucosal surface by organ design83
.
A particle will experience two forces during its attachment to the mucosal surface,
one is repulsive force and another one is attractive force. Repulsive forces arise from osmotic
pressure effects as a result of the interpenetration of the electrical double layers, steric effects and
also electrostatic interactions in case when the surface and particle carry similar charges.
Attractive forces arise from Van Der Waals interactions, surface energy effects and electrostatic
interactions in case when the surface and particles carry opposite charges. For mucoadhesion to
occur, the attractive interaction should be larger than nonspecific repulsion.
b) Consolidation stage
After the attachment of the bioadhesive formulation to the mucus layer, bioadhesive
materials penetrate into the crevice of the tissue surface as is depicted in Fig.2. Mucoadhesive
materials adhere most strongly to solid dry surfaces as long as they are activated by the presence
of moisture84
. The moisture effectively plasticizes the system allowing mucoadhesive molecules
to become free, conform to the shape of the surface, and form weaker Van Der Waals and
hydrogen bonds. In the case of cationic materials, electrostatic interactions with the negatively
charged groups (such as carboxyl or sulphate) on the mucin or cell surfaces may occur85
.
Fig.2 The two steps of the mucoadhesion process.
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18. CHAPTER-1 INTRODUCTION
The consolidation step can be explained by two theories: the diffusion theory and the
dehydration theory.
Diffusion theory is based on macromolecular interpenetration effect86
. According to
diffusion theory, the mucoadhesive molecules and the glycoprotein of the mucus mutually
interact with each other by means of interpenetration of their chains forming secondary bonds.
For this to take place the mucoadhesive device must have the characteristics favoring both
chemical and mechanical interactions.
According to dehydration theory, materials that are capable of forming a gelled structure
in an aqueous environment when placed in contact with mucus can cause its dehydration due to
the difference of osmotic pressure. The difference in concentration gradient is responsible for the
entrapment of water into the formulation until the osmotic balance is reached. This process leads
to the mixture of formulation and mucus and can thus increase residence time with the mucous
membrane. Therefore, it is the movement of water that leads to the consolidation of the adhesive
bond, and not the interpenetration of macromolecular chains. However, the dehydration theory is
not applicable for solid formulations or highly hydrated forms.
On a molecular level, mucoadhesion can be explained on the basis of molecular
interactions. The interaction between two molecules is composed of attraction and repulsion.
Attractive interaction arise from van der waals forces, electrostatic attraction, hydrogen bonding
and hydrophobic interaction. Repulsive interaction occur because of electrostatic and steric
repulsion. For muco-adhesion to occur, the attractive interaction should be larger than non-
specific repulsion.
Fig.2.1 Dehydration theory of mucoadhesion
2.2 Factors Influencing Mucoadhesion/Bioadhesion
Mucoadhesive characteristics are a factor of both the bioadhesive polymer and the
medium in which the polymer will reside. A variety of factors affect the mucoadhesive
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19. CHAPTER-1 INTRODUCTION
Mucoadhesion strength can also be measured in terms of shear strength. This test measures
the force required to separate two parallel glass slides covered with the polymer and with a
mucus film (Bruschi, Freitas, 2005; Chowdary, Rao, 2004). This can also be done using
Wilhemy’s model (Fig.2.5), in which a glass plate is suspended by a microforce balance and
immersed in a sample of mucus under controlled temperature. The force required to pull the plate
out of the sample is then measured under constant experimental conditions (Ahuja, Khar, Ali,
1997). Although measures taken by this method are reproducible, the technique involves no
biological tissue and therefore does not provide a realistic simulation of biological conditions
(Wong, Yuen, Peh, 1999).
Fig.2.5 , Apparatus to determine mucoadhesion in vitro, using Wilhemy’s technique.
Fig.2.6 A schematic of the Wilhelmy plate method14
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