Drug nanocrystals for the formulation of poorly soluble drugs and its ...

J Nanopart Res (2008) 10:845–862
DOI 10.1007/s11051-008-9357-4
TECHNOLOGY AND APPLICATIONS
Drug nanocrystals for the formulation of poorly soluble
drugs and its application as a potential drug delivery system
Lei Gao Æ Dianrui Zhang Æ Minghui Chen
Received: 13 April 2007 / Accepted: 24 December 2007 / Published online: 22 March 2008
Ó Springer Science+Business Media B.V. 2008
Abstract Formulation of poorly soluble drugs is a
general intractable problem in pharmaceutical field,
especially those compounds poorly soluble in both
aqueous and organic media. It is difficult to resolve
this problem using conventional formulation
approaches, so many drugs are abandoned early in
discovery. Nanocrystals, a new carrier-free colloidal
drug delivery system with a particle size ranging
from 100 to 1000 nm, is thought as a viable drug
delivery strategy to develop the poorly soluble drugs,
because of their simplicity in preparation and general
applicability. In this article, the product techniques of
the nanocrystals were reviewed and compared, the
special features of drug nanocrystals were discussed.
The researches on the application of the drug
nanocrystals to various administration routes were
described in detail. In addition, as introduced later,
the nanocrystals could be easily scaled up, which was
the prerequisite to the development of a delivery
system as a market product.
L. Gao D. Zhang (&)
Department of Pharmaceutics, College of Pharmacy,
Shandong University, 44 Wenhua Xilu, Jinan 250012,
China
e-mail: zhangdianrui2006@163.com
M. Chen
Department of Pharmacology, College of Pharmacy,
Shandong University, 44 Wenhua Xilu, Jinan 250012,
China
Keywords Poorly soluble drugs
Nanocrystals Bioavailability Precipitation
Pearl milling High-pressure homogenization
Colloids Nanomedicine
Introduction
It is estimated that approximately 40% or more of the
new chemical entities (NCE) generated through drug
discovery programs are poorly soluble in water
(Lipinski 2002). Formulation of this class of compounds
is a challenging problem faced by the
pharmaceutical researcher, because typical problems
associated with these drugs are a too low oral
bioavailability and erratic absorption due to their too
low saturation solubility and dissolution velocity. For
oral administration, the low concentration gradient
between the gut and blood vessel due to the poor
solubility of the drug leads to a limited transport,
consequently influences the oral absorption. Parenteral
administration as microsuspensions (e.g. i.m. or
i.p.) frequently cannot lead to sufficient drug levels
due to the limited solute volume at the injection site.
To obtain a sufficiently high bioavailability, intravenous
injection is necessarily formed by using a
solvent mixture (e.g. water/ethanol), solubilizing
agents (e.g. surfactants like Cremophor EL). The
problems associated with these solubilizing agents
are their limited ability to increase the drug solubility
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846 J Nanopart Res (2008) 10:845–862
(often show a large injection volume) or/and some
side effects or toxic reaction to the body. For
example, to increase the bioavailability of paclitaxel,
an injectable solution Taxol Ò was produced using
Cremophor EL as solubilizing agent, however, some
adverse effects such as allergic shock caused by the
Cremophor EL were reported. (Muller and Peters
1998).
Therefore, the urgent need for poorly soluble drugs
is to efficiently and safely increase their saturation
solubility in the body fluid. Generally, it is less cost
effective to chemically modify the molecules than
resolve it with the formulation methods, because the
pharmacological activity may be changed when the
chemical structure is altered, and it is costly to
redemonstrate the efficacy and safety of the newly
generated chemical species. So in many cases
screening an ideal formulation is a preferred way to
develop poorly soluble drugs.
Some conventional formulation approaches are
used to tackle the problems, such as generating a salt,
using solvent mixture or solubilizing agents, incorporating
into complexing agents, using some drug
carrier (e.g. o/w emulsions delivery). But the successful
application of all these approaches is limited
because they required the drugs to have processspecific
properties, for instance possessing sufficient
ionizable ability, possessing sufficient solubility in
oils or other hydrophobic mediums, having a suitable
molecular size and shape to incorporate in the
cyclodextrine ring. In the 1990s, liposomes achieved
reasonable success in formulating poorly soluble
drugs (Mohammed et al. 2004), but the poor physical
and chemical stability and expensive costs were
major obstacles for launching liposomes to the
market. So these approaches are not suitable for all
poorly soluble drugs, especially those drugs that are
not soluble in both aqueous and organic media.
Therefore, there is an imminent need for an economic
and a universal approach applicable to all drugs.
Micronization is a classic method applied to the
formulation of poorly soluble drugs; for many years
mechanical milling (e.g. jet milling or colloid milling)
is a commonly used technique to enhance dissolution
velocity of poorly soluble drugs (Rasenack and
Muller 2003). When the particles are comminuted
into micrometer range, the surface-to-volume ratio
increases significantly which leads to an increase of
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dissolution velocity according to the Noyes–Whitney
equation, and subsequently an improvement of
absorption in the gastrointestinal tract (GIT). If,
however, the compound has a very poor solubility,
e.g. below the mg/mL level, the definite increase of
dissolution velocity is not sufficient to obtain a high
bioavailability.
Consequently, the next step is nanosizing, i.e. the
transformation of the micronized drug particles to
nanoparticles (Liversidge et al. 1991; Muller et al.
1998). In 1990s a new drug solid nanoparticle
technology came about, i.e. drug nanocrystals. Nanocrystals,
consisting of pure drugs and a minimum of
surface active agents required for stabilization, are a
carrier-free submicron colloidal drug delivery with a
mean particle size in the nanometer range, typically
between 10 and 1000 nm. The drug nanocrystals not
only further increase the dissolution velocity but also
the saturation solubility (for details see section
‘Characterization and evaluation of nanocrystal suspensions’),
so they could more efficiently improve
the oral absorption of the drugs and achieve a higher
bioavailability compared to the microparticles. Following
some simple post process, more convenient
dosage forms could be obtained, such as tablets,
pellets, capsules. Apart from these, due to the
sufficiently small size and safe composition, nanocrystals
can be injected parenterally, even injected
intravenously; then a 100% bioavailability can be
obtained. At present, several production techniques
are applied to produce drug nanocrystals, such as
precipitation (Gassmann et al. 1994), pearl milling
(Liversidge et al. 1991), and high-pressure homogenization
(Krause and Muller 2001). Basically, these
techniques produce a suspension (so-called nanosuspensions)
of drug nanocrystals dispersed in liquid
medium and stabilized by surface active agents. The
use of drug nanocrystals is a universal approach
generally applicable to all poorly soluble drugs
because all these drugs could be directly disintegrated
into nanometer-sized drug particles.
This paper introduces different techniques preparing
drug nanocrystals, especially the high-pressure
homogenization methods. The special properties and
general applications of the nanocrystals are discussed
at length. The possibilities of large-scale production,
the prerequisite to bring a formulation to market, are
also discussed.

J Nanopart Res (2008) 10:845–862 847
Overview of production techniques of drug
nanocrystals
Generally, the drug nanocrystals techniques are
classified as ‘‘bottom up’’ methods and ‘‘top down’’
methods (Xing 2004). The ‘‘bottom up’’ methods,
such as the precipitation technique, mean that nanocrystals
can be constructed from the molecules;
however, the ‘‘top down’’ methods, such as the pearl
milling and homogenization techniques, mean that
the nanocrystals can be disintegrated step by step
from the coarse powder.
Precipitation techniques
Precipitation has been applied to prepare small
particles for many years,but until the 1980s it was
used in the preparation of nanoparticles for drug
delivery (Siostrom et al. 1993). Sucker and coworkers
prepared drug nanoparticles using a precipitation
technique (Gassmann et al. 1994).
Basically, the poorly water-soluble drug is dissolved
in a solvent (in general organic solvent), and the
solution is added into a miscible non-solvent (aqueous
solvent) under agitation. This leads to a sudden high
supersaturation, resulting in rapid nucleation and
precipitation. This technique simply takes advantage
of the variation in the solubility of the same drugs in
different but miscible liquids. However, the tendency
of the pharmaceutical particles to grow and being hard
to inhibit the growth brings problems to the production.
Stabilizers should be added to avoid formulation
of the microparticles due to the spontaneous aggregation
of the particles. To obtain uniform nanoparticles,
several parameters should be optimized:
1. Stirring rate
2. The volume ratio of antisolvent to solvent
3. Drug content
4. Temperature
The increase in stirring rate favors decrease of the
particle size. With the increasing stirring rate the
micromixing (i.e. mixing on the molecular level)
between the two phases is intensified. High micromixing
efficiency enhanced the rate of diffusion of
drugs between the two phases, which induces a rapid
and high homogenous supersaturation and increases
the nuclei number to produce smaller drug particles
(Douroumis and Fahr 2006). A larger volume ratio of
antisolvent to solvent also contributes to a higher
supersaturation in the interface of two phases leading
to a rapid nucleation. A moderate drug content is
better for the precipitation progress. The increased
viscocity due to a higher drug concentration will
hinder the diffusion between the two phases consequently,
thereby leading to a non-uniform
supersaturation. On the other hand, higher drug
content will increase the probability of aggregation
of particles (Zhang et al. 2006). A lower temperature
also favors the reduction of particle size. A lower
saturation solubility at a lower temperature makes the
supersaturation easier to reach. In addition the
nucleating process was a process of free-energy
decrease and heat release, so a lower temperature will
make for the nucleation.
The precipitation technique is simple and cost
effective; no expensive equipments are required,
compared with milling and high-pressure homogenization
technique. Furthermore, this method avoids the
use of high energy like in disintegration technique,
which prevents denaturation of drug due to high
energy input (Zhong et al. 2005). Amorphous drug
state often forms in this progress (Kipp 2004),
enabling an increase in the solubility and dissolution
velocity of the drug. However, drugs in this highenergy
state tend to convert to more stable crystalline;
so this metastable state needs to be maintained
during both the production progress and shelf time to
maintain good stability. For precipitation technique
the following prerequisites should be satisfied:
1. the drug should be soluble at least in one solvent.
2. the solvent is miscible with a non-solvent.
3. the solvents used in this techniques should be
eliminated to an acceptable level in the end
products.
The above mentioned all limit the application of the
precipitation technique; it cannot be generally used to
all poorly soluble drugs.
Pearl milling techniques
The first-generation disintegration technique is pearl
milling developed by Liversidge, leading to the
product Nanocrystals Ò in 1990 (Liversidge et al.
1991). In this method a nanocrystal suspension is
produced using a high shear pearl mill consisting of
milling chamber, milling pearls, milling motor, and a
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848 J Nanopart Res (2008) 10:845–862
recirculation chamber. Basically, the drug, surfactant
and water are filled into the milling chamber charged
with milling pearls (typically made from glass, zircon
oxide, or polystyrene resin); then the pearls are
rotated at a high rate driven by the motor and the drug
was comminuted into nanosized crystals by the high
shear forces. A fine nanocrystal nanosuspension can
be obtained through a pearl milling progress for hours
to several days, depending on the drug hardness,
quantities, and requested particle size for different
administration routes. The progress can be performed
in either batch or recirculation mode. A narrow
particle size distribution with a very little batch-tobatch
variation can be obtained once the optimal
formulation and progress is achieved. In general the
following process parameters should be investigated
to obtain an optimal formulation:
1. Drug amount
2. Number of milling pearls
3. Milling speed
4. Milling time
5. Temperature
One should bear in mind that particle size would
simultaneously exhibit two opposite effects in the
milling chamber: decrease occurring through fragmentation
to smaller particles and particle growth
occurring through collisions between particles
(Annapragada and Adjei 1996). To comminute particles
into nanopowders mean that the probability of
collisions between particles in milling chamber
should be reduced to a minimum. Drug amount and
milling time are two main parameters involved in the
two opposite effects: the dispersion and the formation
of secondary aggregates. Larger drug amount and
longer milling time mean a larger probability of
collisions between particles, leading to aggregation.
So it should be necessary to use a moderate time
associated with a sufficient drug amount to reduce
particle size (Geze et al. 1999). Milling speed also
should be compromised to reduce the occurrence of
secondary aggregation, while excessive speed will
favor the particle agglomeration.
The number of milling pearls is another important
parameter; more milling pearls lead to more contact
points between pearls and drugs, but too many
milling pearls in the chamber can increase the weight
loading of machine and collisions between pearls
which will consume the mechanical energy. Also
123
moderate number of milling pearls will be beneficial
for production.
It was reported that low milling temperature
slowed down the process of aggregation and provided
high efficiency (Gubskaya et al. 1995). An increase
in friability of particle and a low heat energy of
substance at a low temperature are both favorable for
grinding. In many case, cooling was carried out by
the addition of liquid nitrogen in milling chamber
before powder deposition. It was observed that this
short cooling step had an important effect on particle
size reduction and narrowed the particle size
distribution.
In fact, all these parameters are known to affect
particle size, but the interrelation among them is still
not clear; so their adjustment is often empirical at
present.
This technique is simple and applicable to drugs
simultaneously insoluble in aqueous and non-aqueous
media. Product quantity can be flexibly adapted,
ranging from milliliters to liters; so this technique
cannot only provide a new avenue to improve
screening efforts in the lab scale but also provide a
large-scale procedure suitable for commercialization
of various dosage forms. The first product based on
this technique is Rapamune Ò , an immunosuppressant
agent, marketed by Wyeth Research Laboratories in
2002. Compared with precipitation technique, highenergy
input is needed and the equipment expenditure
is higher. A major concern associated with pearl
milling is potential erosion of material from the
pearls, leading to product contamination (Keck and
Muller 2006). The product contamination by erosion
may be problematic, especially the final product is
intended to be administrated for a chronic therapy.
Another problem is that pearl milling is often a timeconsuming
progress, ranging from hours to several
days. It not only means a low product efficiency but
also bears a risk of microbiological pollution, especially
in the case of the progress being performed at
room temperature and a dispersion media providing
nutrition to bacteria being used in the procedure.
High-pressure homogenization techniques
In the mid-1990s, Müller et al. invented the secondgeneration
disintegration technique for production of
drug nanocrystals, that is nanosuspensions (Disso-
Cubes Ò ) produced by high-pressure homogenization

J Nanopart Res (2008) 10:845–862 849
with a piston-gap homogenizer (Muller et al. 1998).
Basically, macrosuspensions consisting of drug, surfactant,
and water passes a very thin gap with a high
velocity; the energy caused by cavitation forces and
crystal collision is high enough to comminute the
drug particles into nanosized crystals. Also, this
technique is applicable to all drugs poorly soluble in
both aqueous and non-aqueous media. The first
technology DissoCubes Ò was based on homogenization
of particles in water, because of universal
application and appealing advantages; nanocrystal
technology based on the high-pressure homogenization
was developed rapidly within the last decade. In
the first few years of 2000 a new homogenization
technique was developed, which means homogenization
of drug particles in non-aqueous media (e.g.
propylene glycol) or mixtures of water with watermiscible
liquids (e.g. PEG, glycerol). The registered
trade name is Nanopure Ò . To resolve the problem in
the precipitation technique, a homogenization progress
is performed following the precipitation to
avoid the growth of drug nanocrystals, i.e. NANO-
EDGE Ò (Rabinow 2004), developed recently by the
company Baxter. This paper briefly introduces the
principle of the homogenization progress and reviews
the piston-gap homogenization technique. The possibility
of large-scale production and the application of
the products based on this technique are also covered.
At present, the high-pressure homogenizer lines
have been widely applied in food and cosmetic
industry, and have already been approved for the
parenteral production (Muller and Peters 1998). The
homogenization principle is not a nascent concept;
for many years cavitation has been considered as the
most important effect to diminish particles in a
piston-gap homogenizer. Figure 1 (Muller and Keck
2004) shows the progress and principle of producing
drug nanocrystals by the piston-gap homogenizer. In
this progress the raw material suspensions (in microsize)
is suddenly pumped from a pipe into a thin gap
(generally 25 lm at 1500 bar). The high energy
provided by the pump converts to the increased
kinetic energy as the suspension passes through the
narrow gap. According to Bernoulli’s equation the
static pressure of the fluid simultaneously decreases
to maintain constant energy. The static pressure falls
below the vapor pressure of water causing the boiling
of the fluid. As the suspension leaves the gap, the
pressure suddenly rises to normal pressure and the
vapor bubbles implode vigorously, and the cleavage
along the crystal dislocation is caused by cavitation,
fluid shear, and particle collision against each other.
The energy generated in this instantaneous progress
(generally within milliseconds) is high enough to
break down the drug microparticles into nano-sizing
particles (Liedtke et al. 2000).
Several tapes of piston-gap homogenizers have
been reported to be used on a lab scale production of
nanosuspension, for example, the APV Micron LAB
40 (APV Deutschland GmbH, Lubeck, Germany)
(Jacobs et al. 2000), Avestin EmulsiFlex-C5 (Avestin
Inc., Ottawa, Canada) (Hecq et al. 2005), Niro Soavi
NS1001L (Niro Soavi, S.P.A., Italy) (Zhang et al.
2007). Figure 2 shows the transmission electron
microscope photos of the formulation progress of
the azithromycin nanosuspensions produced by a
NS1001L piston-gap homogenizer (Zhang et al.
2007).
To obtain an optimized formulation, the effects of
the following process parameters should be
investigated
1. Applied pressure
2. Number of homogenization cycles
3. Temperature
Usually, the homogenizer can handle varying pressures,
ranging from 100 to 1500 bar for most labscale
ones, so the effect of the homogenization
pressure on the particle size should be investigated to
optimize the final formulation. The pressure provided
by the pump converts the kinetic energy of the fluid
in the gap. The higher the homogenization pressure
is, the higher velocity of the fluid in the gap is, and
the static pressure will drop to a larger extent leading
to generation of more bubbles and then higher energy
to comminute the particles. This is consistent with the
law of conservation of energy. So it is anticipated that
the higher the homogenization pressure, the smaller
the particle size obtained. Usually for the production
of the drug nanocrystals, a maximum pressure (for
most lab homogenizers this value is 1500 bar) is
needed.
The progress that the fluid passes the gap is
performed instantaneously, in general within several
milliseconds. The energy generated in such short time
is not sufficient to comminute all particles into
uniform drug nanocrystals even at the highest applied
pressure 1500 bar, so more homogenization cycles
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850 J Nanopart Res (2008) 10:845–862
Fig. 1 Schematic
representation of
diminution mechanism in
homogenization gap of a
piston-gap homogenizer
(Muller and Keck 2004)
Fig. 2 Formulation progress of azithromycin nanosuspensions
by high-pressure homogenization with the increase of applied
pressure and cycles. (1% azithromycin, 3% Pluronic F68)
(Zhang et al. 2007). (a) TEM Micrographs of the azithromycin
nanosuspensions after 2 Cycles at 100 bar (10,0009). (b) TEM
123
Micrographs of the azithromycin nanosuspensions after 5
Cycles at 500 bar (10,0009). (c) TEM Micrographs of the
azithromycin nanosuspensions after 15 Cycles at 1500 bar
(20,0009)

J Nanopart Res (2008) 10:845–862 851
are needed to be performed on the suspensions. The
increased cycle number provided more energy to
break down the crystalline. Therefore, homogenization
often is performed in five, ten, or more cycles,
that depends on the hardness of the drug and the
desired particle size. The studies carried out on
RMKP 22 revealed that an inverse relationship exists
between the number of homogenization cycles and
the particle size (Muller and Peters 1998).
Apart from reducing the particle size, more cycles
lead to a more homogenous nanocrystal suspension,
i.e. a narrower size distribution. Because the flow rate
of fluid in the gap is not identical among different
zones, i.e. fluid on central zone of the pipe has a
higher velocity than the fluid near the wall, the
energy dispersed among the fluid is not uniform,
leading to an inhomogenous particle size distribution.
With increasing number of cycles, the probability that
larger particles pass the zone of high-power density in
the middle of the gap increases; thus these particles
are also diminished.
So the particle size is a function of the pressure
and number of cycles, and a desired particle size can
be achieved by adjusting the procedure parameters,
pressure and cycle number. The optimal particle size
of drug nanocrystals depends on the therapeutic
purpose of the drug. For example, a relatively small
particle size, about 100–200 nm, is required for a fast
dissolution; for a prolonged dissolution, the particle
size should be adjusted to a larger level, e.g. 800–
1000 nm.
Temperature is an important parameter which
should be strictly controlled when the drug is
temperature sensitive. There is an increase of temperature
in the homogenization progress, which is not
favorable to the temperature-sensitive drugs. The
temperature can be promptly reduced by placing a
heat exchanger ahead of the homogenizer valve, so
the sample temperature can be maintained at about
10 °C or even below. For the DissoCubes Ò and
Nanopure Ò technology production at temperatures of
0 °C and even below (e.g. -20 °C, so-called deep
freeze homogenization technique) was described to
process chemically labile compounds. For example,
to avoid the degradation of omeprazole during
production the nanosuspensions were prepared at
0 °C, and the samples were cooled down to 0 °C
between each cycle (Schwitzera et al. 2004). Hecq
et al. (2005) produced nifedipine nanosuspensions
using an Avestin EmulsiFlex-C5 homogenizer
equipped with a heat exchanger to maintain a lower
temperature. The fact that high-pressure homogenization
is applicable to the temperature-sensitive drugs
also shows that this is a universal technique in some
respects.
To sum up, high-pressure homogenization is a
simple technique. When an optimized procedure was
achieved following the adjustment of the production
parameters, high-quality nanosuspensions with little
patch-to-patch variation can be obtained, i.e. the
production of nanosuspensions by high-pressure
homogenization possesses a high reproducibility
(Grau et al. 2000). An important advantage of this
technique is that a considerably high productivity can
be obtained with a very low microparticle content in
the product, which is favorable for implementation of
industrial products. In addition, compared with pearl
milling technique, the contamination due to the
erosion from the wall of the homogenizer is at a
lower level. Muller et al. investigated the metal
contamination of the nanosuspensions under a harsh
production condition, i.e. 20 cycles at a maximum
pressure of 1500 bar. The most dominant ion iron in
steel is analyzed in nanosuspensions and was found to
be below 1 ppm; it is an uncritical level and safe even
for a chronic therapy (Borner et al. 1999; Krause
et al. 2000).
Characterization and evaluation of nanocrystal
suspensions
The essential characterization parameters for nanocrystal
suspensions are as follows:
Size and size distribution
Size and size distribution are important characterizations
of the nanosuspensions because they govern the
other characterizations, such as saturation solubility
and dissolution velocity, physical stability, or even
biological performances. The mean particle size of
nanosuspensions is typically analyzed by photon
correlation spectroscopy (PCS) (Muller and Muller
1984). Apart from the mean particle diameter, PCS
can also yield the width of the particle size distribution,
i.e. polydispersity index (PI). The PI value
ranges from 0 (monodisperse particles) to 0.500
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852 J Nanopart Res (2008) 10:845–862
(broad distribution), and is an important index that
governs the physical stability. For a long-term
stability the PI should be as low as possible.
However, due to a narrow measuring range of PCS,
approximately from 3 nm to 3 lm, it shows an
incapability in the detection of the larger microparticles.
Then the laser diffractometry (LD) is needed to
investigate the content of particles in the micrometer
range or aggregates of drug nanoparticles, especially
for the nanosuspensions that are meant for parenteral
and pulmonary delivery. The LD yields a volume
distribution and possesses a measuring range of
approximately 0.05–80 lm up to a maximum of
2000 lm, depending on the type of equipment
employed. Typical characterization parameters of
LD are diameters 50, 90, 99%, represented by D50,
D90, and D99, respectively (i.e. the D50 means that
50% of the volume of the particles is below the given
size). It should be noted that the particle size data of a
nanosuspension obtained by LD and PCS are not
identical, because the LD data are volume based and
the PCS mean particle size is a light-intensityweighted
size (Fig. 3).
A Coulter counter analysis is essential for nanosuspensions
to be administrated intravenously.
Compared with a volume distribution of the LD
analysis, the Coulter counter data give an absolute
value, that is the absolute number of particles per
volume unit for the different size classes. The size of
the smallest blood capillary is about 5 lm, so even a
Fig. 3 The D90 (-m-) and D99 (-j-) of RMKP 22 as a
function of cycle number (9% drug, 0.6% Phospholipon 90,
2.2% glycerol 85%, pressure 1500 bar, n = 3) (Muller and
Peters 1998)
123
small content of particles greater than 5 lm may
cause capillary blockade or emboli formation. So the
content of microparticles in nanosuspensions should
be controlled strictly by Coulter counter analysis.
Shape and morphous
Typically, the shape or the morphous of the nanocrystals
can be determined using a transmission
electron microscope (TEM) and/or a scanning electron
microscope (SEM). A wet sample of suitable
concentration is needed for the TEM analysis. When
the original nanosuspensions are required to be
processed into dried powder (e.g. by spray drying
or lyophilization), a SEM analysis is essential to
monitor changes of the particle size before and
following the progress of the water removal. In
general, agglomeration phenomenon may occur following
water removal, leading to an increase of the
particles’ size which can be viewed through SEM. To
minimize the extent of the increase of particle size,
some excipients should be added as a protectant. For
example, mannitol, generally used as a cryoprotectant
in lyophilization, can recrystallize around nanocrystals
during the water-removal operation, thus
preventing particle interaction and agglomeration.
An agglomeration within a certain extent is permitted
when the particle size still in an accepted range,
particularly the dried powder can be well redispersed
into stable nanosuspensions. The shape of the drug
crystals depends on their crystalline structure; different
crystal shapes of different drugs were viewed
under SEM (For details see ‘Scale up and
commercialization’).
Zeta potential
The measurement of the zeta potential allows the
prediction about the storage stability of submicron
colloidal dispersion (Li et al. 2006). In general,
particle aggregation is less likely to occur if particles
possess enough zeta potential providing sufficient
electric repulsion, or enough steric barrier providing
sufficient steric repulsion between each other.
According to the literature, a zeta potential of at
least -30 mV for electrostatic and -20 mV for
sterically stabilized systems is desired to obtain a
physically stable nanocrystal suspensions (Jacobs
et al. 2000).

J Nanopart Res (2008) 10:845–862 853
Crystalline state
The evaluation of crystalline state is necessary to
understand the polymorphic changes that a drug might
undergo when subjected to nanosizing. Differential
scanning calorimetry (DSC) and X-ray diffraction can
be used to evaluate the crystalline structure of the drug
nanocrystals. It was reported that some drugs retained
their crystalline state during homogenization, such as
danazol nifedipine, and ucb-35440-3 (Hecq et al.
2006). However, for some drugs, for example azithromycin,
the results of DSC and X-ray showed that
amorphous state was generated during homogenization
(Zhang et al. 2007). Though it increases the
saturation solubility, the amorphous state is a metastable
state with a higher energy and leads to an
instability during shelf time. Transforming the nanosuspension
into stable dried solid powder by water
removal can resolve this problem.
Saturation solubility and dissolution velocity
Determination of the saturation solubility and dissolution
velocity is very important as it not only can
help to assess the benefits compared to the microparticle
formulation but also help to anticipate the
in vivo performance (e.g. blood profiles, plasma
peaks, and bioavailability). The nanosuspensions
should be transferred into a dried powder before the
investigation of the dissolution behavior. In the
shakering experiments a different temperature can
be used to determined the saturation solubility of a
dried powder in a different artificial medium (i.e.
physiologic saline, artificial gastric juice, or artificial
intestinal juice). To determine the dissolution velocity,
the methods described in the Pharmacopoeia can be
used; also experiments in various physiological
buffers should be performed.
Surface properties
The research on the surface parameters of nanosuspensions
is very important, especially for the
nanosuspensions to be administrated intravenously.
The fate of the nanocrystals in vivo following injection,
such as organ distribution, depends on its surface
properties, such as surface hydrophobicity and interactions
with plasma proteins (Schmidt and Muller
2003; Goppert and Muller 2005). Therefore, some
special techniques have to be used to evaluate the
surface properties to give an idea of in vivo behavior.
Hydrophobic interaction chromatography has been
used to determine the surface hydrophobicity (Wallis
and Muller 1993), and 2-D PAGE can be performed to
measure the protein absorption after intravenous
injection of nanosuspensions (Blunk et al. 1996).
Physical and chemical properties of nanocrystals
Increase in saturation solubility and dissolution
velocity
An important feature of drug nanocrystals is the
increase of the saturation solubility and dissolution
velocity. This advantage makes drug nanocrystals
amenable to many applications. In textbooks, the
saturation solubility in a given solvent is defined as a
compound-specific constant depending only on the
temperature. However, the saturation solubility is
also a function of the crystalline structure (i.e. lattice
energy) and particle size. In general, solubility is best
for the polymorphic modification that is characterized
by highest energy and lowest melting point. Amorphous
fraction generated in homogenization progress
is also conducive to an increased solubility due to
high inner energy of substance in this state (Hancock
et al. 2002). The reason why saturation solubility is
also a function of particle size can be explained by
the Kelvin and the Ostwald–Freundlich equations
(Böhm and Müller 1999).
The Kelvin equation (Eq. 1) is originally used to
describe the vapor pressure over a curved surface of a
liquid droplet in gas; it is also applicable to explain
the relation between the dissolution pressure and the
curvature of the solid particles in liquid:
Ln Pr
¼
P1
2cMr
ð1Þ
rRTq
where Pr is the dissolution pressure of a particle with
the radius r, P ? is the dissolution pressure of an
infinitely large particle, c is the surface tension, R is
the gas constant, T is the absolute temperature, r is
the radius of the particle, Mr is the molecular weight,
and q is the density of the particle.
According to the Kevin equation, the dissolution
pressure increases with increasing curvature, which
means decreasing particle size. The curvature is
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854 J Nanopart Res (2008) 10:845–862
enormous when the particle size is in the nanometer
range; then a large dissolution pressure can be
achieved leading to a shift of the equilibrium
toward dissolution. The Ostwald–Freundlich (Eq. 2)
directly describes the relation between the saturation
solubility of the drug and the particle size:
log CS
¼
Ca
2rV
2:303RTqr
ð2Þ
where C S is the saturation solubility, C a is the
solubility of the solid consisting of large particles, r
is the interfacial tension of substance, V is the molar
volume of the particle material, R is the gas constant,
T is the absolute temperature, q is the density of the
solid, and r is the radius.
It is obvious that the saturation solubility (CS) of
the drug increases with a decrease of particle size (r).
However, this effect is pronounced for materials that
have mean particle size of less than 2 lm.
The increase of nanocrystals in the dissolution
velocity can be explained by the Noyes–Whitney
equation (Eq. 3) (Dressman et al. 1998). For drug
nanocrystals, the increased saturation solubility (CS)
and surface area (A) lead to an increase in the
dissolution velocity (dX/dt).
dX
dt
¼ DA
hD
ðCS CtÞ ð3Þ
where dX/dt is the dissolution velocity, D is the
diffusion coefficient, A is the surface area, h D is the
diffusional distance, CS is the saturation solubility,
and Ct is the concentration around the particles.
Another important factor is the diffusional distance
hD, which, as a part of the hydrodynamic boundary
layer hH, is also strongly dependent on the particle
size, as the Prandtl equation shows (Mosharraf and
Nystrom 1995):
hH ¼ k (L 1=2 =V 1=2 ) ð4Þ
where hH is the hydrodynamic boundary layer, k
denotes a constant, L is the length of the particle
surface, and V is the relative velocity of the flowing
liquid surrounding the particle.
According to the Prandtl equation, the reduced
particle size leads to a decreased diffusional distance
hD and consequently an increased dissolution velocity,
as described by the Noyes–Whitney equation. Therefore,
to sum up, a reduction in the drug particle size in
the nanometer range leads to an increase in solubility
123
as well as the dissolution velocity. Both are very
important factors with regard to the aim of improving
the bioavailability of poorly soluble drugs.
Increased adhesiveness
Another outstanding feature of nanosuspensions is
the distinct increased adhesiveness compared to
microparticles. The high adhesiveness, a general
characteristic of nanoparticles (Duchene and Ponchel
1997), can be considered as another factor improving
the oral absorption of poorly soluble drugs apart from
the increased saturation solubility and dissolution
velocity. In section ‘Oral administration route’
positive effects due to this feature are discussed at
length.
A good long-term stability
A long-term stability is another special feature of the
nanosuspensions, which is also one of prerequisites
for a qualified formulation product. The good physical
stability of nanosuspensions is mainly reflected
through the absence of aggregation and Ostwald
ripening phenomenon.
The absence of the aggregation phenomenon is
realized by the addition of suitable stabilizer. It is
well known that nanoparticles dispersed in a medium
have a nature of aggregation (Drews and Tsapatsis
2007). The physical instability of the system varies
from the natural tendency of the nanoparticles to
reduce the high surface energy created by the large
interface between the solid and the medium. Physical
stability may be achieved by using the surface-active
agents as a stabilizer, such as the ionic surfactants,
non-ionic surfactants, and amphiphilic copolymers
(Kocbek et al. 2006). During the homogenization
progress, the surfactant diffuse rapidly and cover the
surface of the crystals, stabilizing the system by
providing an electrostatic and static repulsion
between the crystals (Muller and Jacobs 2002). To
sufficiently stabilize special drug nanocrystals, the
used surfactants should meet the following requests:
firstly, the stabilizer should have a sufficient affinity
for the particle surface of the special drug to stabilize
the nanocrystals (Lee et al. 2005). For example, the
ionic surfactants such as sodium dodecyl sulfate
(SDS) and lecithin could migrate to the solid–liquid
intersurface and provide an electrostatic barrier

J Nanopart Res (2008) 10:845–862 855
against aggregation of the particles. The non-ionic
surfactants, such as Pluronic F68 and Birj 78, have
multiple attachments of hydrophobic domains at
surface which could adequately interact with the
hydrophobic functional groups of the compounds and
then stabilize the system by providing the steric
barrier between particles (Kipp 2004). Secondly, the
surfactant should possess an adequately high diffusion
velocity to cover the generated surface rapidly,
because the homogenization is a very fast progress.
Lastly, the amount of the stabilizer should be
sufficient for full coverage on the particle surface to
provide enough electronic or steric repulsion between
the particles. However, it not true that the more the
stabilizer, the better the excess surfactants tend to
form micelles containing a small number of dissolved
drug molecules (Choi et al. 2005). So screening for
an optimal surfactant and its amount is very important
for the product quality. It was reported that a
binary or ternary mixture of electrostatic and static
stabilizer usually had a better effectiveness. At
present the choice of the type and amount of the
surfactant are mainly based on experiences because
of the lack of systematic understanding of how the
stabilizer and drugs interact with each other.
The capital principle that must be observed to
choose a stabilizer is that it must be acceptable and
safe to the body. For the nanosuspensions to be
administrated intravenously, the stabilizers should be
more strictly chosen. In addition, the choice of the
stabilizers is limited by the procedure, e.g. the
surfactant which is in the liquid state (e.g. Tween
80) cannot be used if the nanosuspensions need to be
transformed into a dried powder.
The absence of Ostwald ripening phenomenon is
mainly attributed to the narrow size distribution of
nanosuspensions (Mantzaris 2005). It was reported
that the particles in the highly dispersed systems tend
to grow, which is caused by the differences in
saturation solubility in the vicinity of different sized
particles. The phenomenon was called Ostwald
ripening (Jacobs et al. 2000). The solute concentration
is higher in the vicinity of the smaller particles
than that of the larger ones due to the higher
saturation solubility of the small particles. So the
molecules will diffuse from the surrounding of the
small particles to the surrounding of the large
particles driven by the concentration gradient and
recrystallize on the surface of the larger particles. The
continual dissolution of the small particles and
recrystallization of the solute on the surface of the
large particles lead to the formulation of the microparticles.
A narrow size distribution of the
nanosuspensions created by adequate homogenization
progress avoids the different saturation solubility
between particles in different sizes and conduces to
the absence of the Ostwald ripening and a long-term
stability.
For the chemically labile drugs, the nanosuspensions
can also ensure a good chemical stability both
in vitro and in vivo. These drugs, containing a certain
functional group sensitive to water, hydrogen ions,
oxygen, or various enzymes in the body, are easy to
degrade as a form of molecule both in vitro and
in vivo which leads to a short half-time. The surface
layer of the drug nanocrystals can protect the drug
below the surface against degradation, just like the
formation of an oxide layer on aluminum. A good
example is paclitaxel, a water-sensitive drug. Figure 4
(Muller and Keck 2004) shows the HPLC diagram of
an aqueous paclitaxel solution (methanol 10 mL,
water (demineralized) 5 mL, paclitaxel 20.8 mg)
degraded for 48 h at room temperature; the main
peak degradation products are clearly visible. However,
only the paclitaxel peak occurs in the HPLC
diagram of the nanosuspension (stabilized with poloxamer
188) after 4 years’ storage at 4–8 °C, with no
optically visible peaks of degradation products.
Improved biological performance
All the advantages of nanosuspensions mentioned
above lead to an improvement in the in vivo performance
of drugs irrespective of the administration
route used. The improved biological performance in
various delivery form is discussed later in detail.
Administrations of nanosuspensions in different
routes
Oral administration route
The oral route is the first choice for drug delivery
because of its numerous advantages, such as safety,
convenience, etc. At present, most nanosuspension
products on the market are for oral delivery.
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856 J Nanopart Res (2008) 10:845–862
Fig. 4 HPLC diagram of an aqueous paclitaxel solution
degraded for 48 h at room temperature; the main peak
degradation products are clearly visible (a) However, only
the paclitaxel peak occurs in the HPLC diagram of the
nanosuspension (stabilized with poloxamer 188) after 4 years’
storage at 4–8 °C, with no optically visible peaks of
degradation products (b) (Muller and Keck 2004)
Following oral administration, the drug is absorbed
from the gut and enters into blood circulation, then
distributed to various tissues. The performance of the
drug depends on its solubility in digestive juice and
transportation through the gastrointestinal tract. The
poor solubility and/or dissolution rate of poorly
soluble drugs limit in vivo absorption and cannot
reach an effective therapeutic concentration in the
blood. In addition, it is well known that poorly
soluble drugs often exhibit increased or accelerated
absorption when they are administered with food.
This positive food effect would be attributed to the
enhancement of the dissolution rate in the gastrointestinal
tract (GIT) caused by many factors such as
delayed gastric emptying, increased bile secretion,
larger volume of the gastric fluid, increased gastric
pH (for acidic drugs), and/or increased splanchnic
blood flow (Jinno et al. 2006). Thus there is often an
absorption variability resulting from the presence or
123
absence of food. To sum up, the problems associated
with these drugs are as follows:
1. a low/variable bioavailability
2. a large oral dose
3. a variation in bioavailability resulting from fed/
fasted state
4. a retarded onset of action
Many examples verify that the nanosuspensions have
a lot of positive effects on the oral drug delivery and
can be applied to overcome the above issues. When
the drug are administered as nanosuspensions, a high
drug concentration gradient between the gastrointestinal
tract and blood vessel, resulting from the
increased saturation solubility and dissolution velocity
in digestive juice, improves absorption to a large
extent and leads to a high bioavailability. A classic
example is danazol, a poorly soluble gonadotropin
inhibitor (Liversidge and Cundy 1995). The absolute
bioavailability of marketed danazol microsuspension
Danocrine was only 5.2%. When administered as a
nanosuspension, an absolute bioavailability of 82.3%
could be achieved. Amphotericin B, a highly effective
polyene antibiotic, shows a low oral
bioavailability due to its low solubility. However,
when administered as nanosuspensions, the absorption
of Amphotericin B was significantly improved
compared to orally administered conventional commercial
formulations such as Fungizone Ò ,
AmBisome Ò , and micronized amphotericin B. The
oral Amphotericin B nanosuspensions significantly
reduced parasite numbers in the liver of infected
female Balb/c mice by 28.6%, whereas all liposomal
Amphotericin B (Ambisome Ò ), micronized Amphotericin
B and Fungizone Ò did not show any curative
effect at all upon oral administration (Fig. 5) (Kayser
et al. 2003).
The adhesiveness to the gut wall is also conducive
to a high bioavailability by prolonging residence and
contact time in the GIT. Buparvaquone, a naphthoquinone
antibiotic, is used for the treatment of
Cryptosporidium parvum (C. parvum). Because of
its low solubility, the bioavailability is very low when
given orally. What makes the matter worse is that the
infection with C. parvum usually causes watery
diarrhea leading to a fast clearance from the GIT.
The buparvaquone nanosuspensions cannot only
increase drug solubility but also show a mucoadhesiveness
to the gut wall to overcome drug loss

J Nanopart Res (2008) 10:845–862 857
Fig. 5 Comparison of percentage reduction of Leishmania
donovani parasite load in livers of infected Balb/c mice
following oral administration of untreated Amphotericin B (as
control), AmBisome Ò , Fungizone Ò , micronized Amphotericin
B and Amphotericin B nanosuspensions (Kayser et al. 2003)
resulting from the diarrhea (Jacobs et al. 2001).
Furthermore, to enhance the mucoadhesive, the
buparvaquone nanosuspensions are incorporated into
a mucoadhesive polymer to get a mucoadhesive
nanosuspension. The data illustrate that the mucoadhesive
nanosuspensions can more effectively clear
C. parvum from the gastrointestinal tract than
unmodified nanosuspensions due to the prolonged
gastrointestinal residence time resulting from mucoadhesiveness
(Kayser 2001).
When poorly soluble drugs are formulated as a
uniform nanosuspension, the variation in bioavailability
resulting from the fed/fasted state can be
minimized (Hecq et al. 2006; Jinno et al. 2006;
Liversidge and Cundy 1995). The reason is that the
dissolution rate of nanocrystals, enhanced significantly
due to the increase in solubility and enormous
particle surface, is fast enough even under the fasted
condition. Then, the absorptions both in fasted and
fed state might be a permeability-limited progress,
and the difference in the absorption due to different
dissolution between the two conditions will be
eliminated.
Nanosuspensions are also advantageous in achieving
a quick onset of action for drugs with a slow dissolution
rate. It can be illustrated by a study involving a
comparison of the pharmacokinetic profiles of
naproxen after oral administration in the form of
nanosuspensions (mean size is 270 nm) and unwilling
suspensions (Liversidge and Conzentino 1995).
The data showed that Cmax was achieved at 33.5 min
for the unwilling naproxen. However, the time for
naproxen nanosuspensions to reach an equivalent
mean plasma concentration to this Cmax was only
about 8 min. It suggested that the nanosuspensions
appeared to be absorbed approximately 4-fold faster
than the unwilling suspension. The increase in
absorption rate for the nanosuspensions was attributed
to the increased solubility and dissolution rate
compared to the unwilling microparticles.
Because the increase in bioavailability for nanosuspensions
is attributed to small particle size and
large particle surface, an unchanged disparity of
nanosuspensions in GIT following oral administration
is the prerequisite for a high bioavailability. If
aggregation of particles occurs in the GIT, the
positive effects of the nanosuspensions on the oral
bioavailability may be reduced. For instance, the
cyclosporine A nanosuspensions showed a disappointing
oral absorption and bioavailability due to
the aggregation of the drug nanocrystals in the GIT
(Muller et al. 2006). So it is important that the
stabilizer used is able to prevent aggregation of the
nanoparticles in both in vitro and in vivo circumstances.
Thus, many physiological factors in the GIT
should be considered an ideal stabilizer for oral
nanosuspensions. For example, the variation in pH
during transit through the GIT may preclude the use
of a charge stabilizing suspending agent, such as
sodium lauryl sulfate (Liversidge and Cundy 1995).
Parenteral administration route
For many cases, intravenous injection is requested to
meet some treatment purpose, such as immediate
effect, targeting effect, overcoming the first pass
effect, and so on. Through intravenous administration,
drugs can reach a 100% bioavailability in the
body. However, as known to us, the parenteral
administration is a critical route, for which many
requirements should be achieved. For example, the
products should be sterile, and the ingredients in them
should not cause any biological problems such as
toxic reactions and allergic reactions. As for the
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858 J Nanopart Res (2008) 10:845–862
submicron delivery system, the particle size is also a
crucial factor in determining whether or not it can be
used in parenteral route. For intravenous injection the
content of particles larger than 5 lm should be
controlled strictly, because the smallest size of blood
capillaries is about 5 lm, the existence of a high
content of particles larger than 5 lm can lead to
capillary blockade and embolism.
Nanosuspensions can be made into an absolutely
safe and injectable product for parenteral administration.
As mentioned above, nanosuspensions contain
only pure drug and slight stabilizers, and many
surface active agents that have ever been approved to
be used for intravenous injection can be used as
stabilizers. Therefore, besides reducing the injection
volume and increasing the tolerated dose, nanosuspensions
can also avoid biological problems caused
by some excipients. For example, compared to
Taxol Ò , paclitaxel nanosuspension showed a higher
LD50 (100 mg/kg), and no allergic reaction occurred
(Böhm and Müller 1999).
Section ‘Increase in saturation solubility and
dissolution velocity’ has mentioned that nanosuspensions,
that had undergone strict control over the larger
particles by Coulter counter analysis, can be safely
administrated intravenously. Injections of 0.3 mL
RMKP 23 nanocrystals (2.5% solid content) in
C57B1/6 mice were well tolerated without any signs
of acute toxicity (Muller and Peters 1998). When
judging the tolerability one should bear in mind that
the blood volume of the mice is about 2.0 mL, i.e.
15% of the blood volume was injected as nanosuspensions
(Muller and Peters 1998).
At present, several sterilization approaches have
successfully been applied to the nanosuspensions,
such as gamma irradiation, filtration sterilization, and
thermal sterilization (only in case the drug itself is not
sensitive to temperature and the stabilizers are suitable)
(Na et al. 1999). An aseptic production line for
nanosuspension products has been realized recently
by the Baxter company. Such a production line can
even been successfully applied to the production of
cytotoxics, such as paclitaxel. In addition, the most
noteworthy point is that the high-pressure homogenization
process itself has a germ-reducing effect. In the
homogenization progress, the bacteria as well as the
crystals are simultaneously disintegrated.
According to the required treatment purpose, the
fate of nanocrystals in vivo following parenteral
123
administration varies in different cases. This can be
realized by adjusting several parameters, such as
particle size and surface properties. In some case a
relatively small particle size is needed for nanocrystals
to achieve a rapid dissolution. For example, for
cyclosporine, it has been reported that the same
pharmacokinetic parameters were obtained after
intravenous injection of a solution and nanosuspensions
(Muller and Keck 2004). In other cases,
however, it is not necessary to make the nanocrystals
dissolve rapidly. That is the drug nanocrystals can
circulate in the blood circulation as submicron
particles for a certain time period. Then the nanocrystals
may be recognized as foreign matter and
rapidly cleared by phagocytic cells of mononuclear
phagocyte system (MPS) which abounds in special
tissues and organs, such as liver, lung and spleen.
This is what is termed as passive targeting progress. It
is important for some cases, for example, in the
treatment of infections caused by parasites residing
the macrophages of the MPS and cancers in special
parts, such as liver and lung. It was reported that
following i.v. administration of clofazimine nanosuspensions
and a control liposomal at a dose of 20 mg
drug/kg to mice, drug concentration in livers, spleens,
and lungs reached comparably high concentrations,
well in excess of the MIC for most Mycobacterium
avium strains, and nanosuspensions showed a better
targeting effect to liver, compared to the liposomal
(Peters et al. 2000).
However, the clearance of the MPS is not desirable
when a comparatively enduring and high drug
concentration in the circulation is requested. At
present studies are in progress to identify how best
to manipulate the surface properties, size, and shape
of the nanocrystals to eliminate, when desirable,
uptake by the phagocytic cells of MPS-enriched
organs (Liversidge et al. 2003).
Ocular administration route
For ocular delivery of poorly soluble drugs, preparations
such as suspensions and ointments have been
developed to meet the treatment request. Although
both two approaches show some advantageous such
as a relative higher drug dose and a prolonged
residence time in site of action, their actual performance
is greatly constrained by the limited intrinsic
solubility of drug in lachrymal fluids. The low drug

J Nanopart Res (2008) 10:845–862 859
intrinsic solubility in the lachrymal fluids leads to a
low level of drug concentration in the site of action,
so an effective performance can hardly be obtained.
However, when the drug is disintegrated into nanosized
crystals the remarkably increased saturation
solubility of the drug proves to be a benefit for ocular
delivery of poorly soluble drugs. Moreover, the
inherent adhesive features of nanocrystals not only
reduce the loss of drug caused by the outflow of
lachrymal fluids but also are conducive to a sustained
release of the drug. In addition, nanocrystals can be
incorporated into a suitable mucoadhesive base or
ocular inserts to achieve a sustained release of the
drug for a stipulated time period. These all ensure a
high bioavailability and effective performance. At
present, the approach of a formulation of polymeric
nanosuspensions loaded with drug is being investigated
to achieve the desired duration of action. Some
bioerodible polymers possessing ocular tolerability
can be used to produce sustain-release nanosuspensions.
A good example is the polymeric
nanosuspension of ibuprofen for ocular delivery
using acrylate polymers such as Eudragit RS 100 Ò .
The nanosuspensions have been characterized for
particle size, drug loading, zero potential, in vitro
drug release and ocular tolerability, and have proven
to be safely used for ocular delivery (Rosario et al.
2002). The rabbit in vivo experiment indicated that
the polymeric nanosuspensions revealed superiority
in in vivo performance over the existing eye drop
formulations in the market and could sustain drug
release for 24 h (Ponchel et al. 1997).
Pulmonary administration route
Nanosuspensions also show a great potential for the
pulmonary delivery of poorly soluble drugs. Some of
these drugs have been successfully made into aerosols
in the form of microparticles that can be
nebulized for pulmonary administration. But such
microparticle aerosol products are often accompanied
with some disadvantages. An unwanted deposition of
the microparticles in the mouth and pharynx, and
clearance of the drug by cilia movement in the lungs
are the main causes leading to the loss of the drug.
Moreover, a low level of drug concentration in the
action site due to a poor drug saturation solubility to a
great extent limits the absorption of the drug in lungs.
All these lead to a lower bioavailability greatly
affecting the performance. However, these problems
can be overcome by application of nanosuspensions
for pulmonary delivery. The increased dissolution
velocity and saturation solubility can rapidly realize a
larger concentration of drug in the lung, leading to
higher local drug levels at the absorption site. An
additional advantage is the natural tendency of the
nanoparticles to stick to mucosal surfaces at the
absorption site over an extended period of time. The
longer residence time on the mucosal surface not only
reduces the loss of the drug due to the clearance by
cilia movement (Ponchel et al. 1997) but also contributes
to a larger extent of absorption because of an
increased dissolution time compared with microparticles.
In addition, nanosuspensions generally possess
a very low fraction of microparticles (Gassmann
et al. 1994), which reduces the deposition of particles
in respiratory passage. Furthermore, compared to the
microparticles, nanoparticles increase the number of
particles in each aerosol droplet (see Fig. 6); then a
more homogenous distribution of the drug in the
aerosol can be obtained. This leads to a more efficient
delivery to the lungs and in this way decreases local
and systemic side effects of drug microparticles. A
long-term stable budesonide nanosuspension which
could be nebulized for pulmonary administration was
prepared by Claudia Jacobs by the high-pressure
homogenization technique (Jacobs and Muller 2002).
Mean PCS particle size of this nanosuspension was
about 500–600 nm, and analysis by laser diffraction
showed that the diameters of 95% and 99% were
below 3 lm. The PCS diameter before and after
aerosolization did not change and the LD diameters
increased negligibly, showing the suitability for
pulmonary delivery. The scale-up from 40 mL up to
Fig. 6 Comparison of particle distribution in the droplets, left
as a 3-lm particle, right as 500-nm particles (Jacobs and
Muller 2002)
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860 J Nanopart Res (2008) 10:845–862
300 mL could be performed successfully through this
technology (Jacobs and Muller 2002).
Surface modification and targeted delivery
Nanosuspensions can be used for targeted delivery as
their surface properties and in vivo behavior can be
easily be altered by changing the used stabilizer. That
is what Muller (Muller and Keck 2004) raised as the
concept of ‘‘differential protein absorption.’’ After
intravenous injection, the particles absorb proteins
from the blood; these absorbed proteins determine the
in vivo fate of the particles. The qualitative and
quantitative composition of the proteins adsorption
pattern depends on the physico-chemical surface
properties of the particle, which can be adjusted by
choosing a different stabilizer (Goppert and Muller
2005). In the PathFinder Ò technology proposed by
Muller (Muller and Keck 2004), atvaquone nanocrystals
were produced by high-pressure
homogenization, the surface was modified with the
surfactant Tween 80 leading to in vivo preferential
absorption of apolipoprotein E, the particles accumulated
to a sufficient amount in the blood brain
barrier (BBB), where the receptors of apolipoprotein
E abound and then a therapeutic level was achieved.
Natural targeting of MPS by nanosuspensions has
already been described. However, the natural targeting
progress could pose an obstacle when
macrophages are not the desired targets. Then, to
bypass the phagocytic uptake of the drug, its surface
properties need to be altered, as in the case of stealth
liposomes (Allen 1997). To sum up, for the targeted
delivery of nanosuspensions, the development of
stealth nanocrystals and active targeting nanocrystals
modified with functionalized surface coatings will be
the next focal point.
Scale-up and commercialization
The vitality of a pharmaceutical technique to a great
extent relies on the possibility of its commercialization,
or in other words, the large-scale production.
The time period between invention of the conception
and the first products on the market is much shorter
for the drug nanocrystals, compared to the other
submicron drug delivery system. For example the
liposomes were raised by Bingham in 1968, but the
123
first pharmaceutical products appeared on the market
at the beginning of 1900s. However, only about
10 years after the appearance of the concept of the
drug nanocrystals, the first product Rapamune Ò was
placed in the market by the company Wyeth. This
rapid commercialization verifies the simple realization
of the large scale production. First, the highpressure
homogenization was a well-developed
procedure in the pharmaceutical field before it was
applied to the production of the drug nanosuspensions.
At present, the homogenizers with a large yield
are successfully brought to operation for scale-up
purpose, for example, an APV Rannie 118 with a
capacity of 1 ton/h and an Avestin 1000 with a
capacity of 1000 L/h. In addition, for economical
purposes, a continuous model has successfully been
carried out in the production of the nanosuspensions,
i.e. placing several homogenizations in series; this
increases the production rate by many folds. For
example, assuming a homogenizer with a capacity of
1000 kg/h, the production of one-ton nanosuspension
would require 20 h and 20 cycles. In case of four
homogenizers in series, this would reduce to 5 h for
the production of 1000-kg nanosuspensions.
Conclusion and perspective
Drug nanocrystals, regardless of their production
method, can be applied to all the poorly soluble drugs
to overcome the solubility and bioavailability problems,
because all the poorly soluble drugs can be
disintegrated into nanocrystals. In some cases,
besides improving drug dissolution, drug nanocrystals
also show other biological activities, such as
realization of a sustained release and targeting to the
special tissues or organs. An important advantage of
the drug nanocrystals is that they can be applied to
various administration routes, such as oral, parenteral,
ocular, and pulmonary delivery, and have
shown great superiority over the counterparts of the
traditional formulation products in every administration
route. In addition, combined with the traditional
formulation approaches, drug nanocrystals also can
be transformed into various dosage forms, such as
tablets, capsules, injections, aerosols, and so on.
Production techniques such as high-pressure homogenization
have been successfully employed for
large-scale production of drug nanocrystals. So far,

J Nanopart Res (2008) 10:845–862 861
many nanocrystals products have been placed in the
market (Keck and Muller 2006). Due to the unique
advantage and pharmaeconomical value, drug nanocrystals
are paid increasingly more attention as a
promising approach. In the future, the development of
stealth nanocrystals and active targeting nanocrystals
modified with functionalized surface coatings will be
the next focal point.
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