1. Nanocrystals
Pharmaceutical drug formulation development has for some time now experienced a
lot of issues especially when it comes to solubility of various compounds (Tuomela, 2015). In
fact, it is evident from various studies that new chemical entities from either the synthesizer or
the development pipelines are equally poorly soluble in not only the aqueous media but also
various organic solvents. As a result, such drugs have possed a lot of issues ranging from low
bioavailability to lack of dose proportionality (REF). Accordingly, the solubility issue has
brought about the retarded onset of action, high interpatient variations as well as high local
irritation. Based on the literature and studies on Pharmaceutical Nanoparticles, it is evident that
drug compounds that are either insoluble or poorly soluble in water pose a significant problem
when it comes to pharmaceutical formulation development. Statistically, around 60% of the
chemical entities used and about 40% of drugs in the development pipeline are poorly soluble
in water (REF). Accordingly, it is no doubt that dissolution is a major trait when it comes to
drug absorption. This is imperative especially in oral drug delivery which is outrightly the
major form or rather type of drug delivery. In that connection, various studies have always been
based on methods on how to improve solubility of prominent molecules such as (BCS class
(II) and Class (IV) because the solubility is limited by the strong intermolecular bond that exists
within the crystal structure (REF). Fortunately, scientists in the nanoscience field have
indicated a possibility that bioperformance and solubility of drug compounds can be improved
through the use of nanocrystal technology (Tuomela, 2015). The technology enhances bioperformance since it allows for diminution of the size of the relevant particle increasing its
surface area and thus improving saturation solubility. Also, the size of the particle increases its
adhesiveness and dissolution velocity while at the same time bringing about the optimally
acceptable bioavailability once administered (Tuomela, 2015). As much as the different
technologies have tried to solve the problem of dissolution, it is evident that such solutions
don’t provide a universal solution. However, it is important to note that novel possibilities are
being brought by nanotechnology which has been a wide area of research and development
over the past decades (REF). Consequently, the reduced particle size has the ability of
increasing the surface area while at the same time reducing diffusion layer thickness. In the
process, it is able to improve solubility while enhancing bioavailability (Tuomela, 2015).
As much as the different technologies have tried to solve the problem of dissolution, it is
evident that such solutions don’t provide a universal solution. However, it is important to note
that novel possibilities are being brought by nanotechnology which has been a wide area of
research and development over the past decades (Tuomela, 2015). In fact, the use of
nanotechnology to come up with reduced size drugs or rather drug formulations have proved
to be so advantageous. Hence by definition, nanocrystal is a subset of nanotechnology that is
aimed at reducing the size of a particle to a size that is smaller than 100 nanometres (Tuomela,
2015). Consequently, the reduced particle size has the ability of increasing the surface area
while at the same time reducing diffusion layer thickness. In the process, it is able to improve
solubility while enhancing bioavailability (Tuomela, 2015).
In that connection,
nanotechnology has been in the areas of nanomaterials, nanofabrication, nanomedicine and
nano particles. Conventionally, the formulation of poorly water-soluble actives such as the
nanometre-sized particles has proved to be successful when it comes to biomedical applications
(Tuomela, 2015). This is evident from the novel drug delivery systems, tissue engineering as
well as drug targeting.
Definition
In this technological era, nanotechnology is nearly becoming the norm in the field of
research. This is evident due to the fact that it is being used in not only the computing field but
also cosmetics research. In that case, nontechnology is also being applied in the medical
industry, and due to the fact that the number of poorly soluble drugs have increased over the
past ten years, there has been the need to base drug development on Nano sized particles
(Junghanns and Müller, 2008). Hence, by definition, drug nanocrystal is a term used to refer to
crystals that have a size that lies within the range of a manometer. Consequently, these are just
nanoparticles comprising of a crystalline character. Further definition based on colloid
chemistry believes or rather characterizes nanocrystal as nanoparticles that have a size of less
than 100mm or equal to 20nm. Pharmaceutically, nanoparticles are particles that have a size of
between a few nanometers to about 1000nm (Junghanns and Müller, 2008).
Besides the size of the particle, it is important to note that as compared to
polymeric nanoparticles drug nanocrystals comprise of 100% drug without any traces of career
material (Junghanns and Müller, 2008). Another important aspect to note is that in the process
of dispersing drug nanocrystals using dispersion media such as nonaqueous and water,
nanosuspensions come out as the by-product. This crucial process is undertaken because the
previously dispersed particles need to undergo dispersion. In that connection, depending on the
type of technology used in the dispersion process, the drug particles can either be an amorphous
or crystalline product (Junghanns and Müller, 2008). However, the amorphous drug should not
necessarily be referred to as nanocrystal. Eventually, the bioavailability and dissolution
velocity is likely to increase due to an increase in the surface area as well as saturation solubility
(Junghanns and Müller, 2008).
Properties of Nanocrystals
Evidently, Nanocrystals are the major alternatives when it comes to increasing the
dissolution rate of drugs especially when the drug is meant to be administered orally
(Junghanns and Müller, 2008). Before discussing the features of NCs, it is important to note
that drugs with poor solubility such as the BCS class (II) are likely to provide erratic and slow
dissolution that in most cases limits the absorption and achievement of an effective therapeutic
concentration. In that case, most investigations into the solubility of drugs are based on the
special features of NCs.
Increase of dissolution velocity by surface-area enlargement
According to Noyes-Whitney equation, nanoparticles have increased dissolution velocity
which is because of increased surface area.
According to the above equation, the increase in drug surface area in addition to an increase in
saturation solubility (e.g. due to nanosizing) will increase the dissolution rate. The dissolution process
can be further enhanced by making the drug nanoparticles amorphous (Sigfridsson et al. 2007).
In that case, when a drug particle moves from micronization to nanonization, the
particle’s surface increases leading to a significant increase in the dissolution velocity.
Comparatively, this property is based on the principle that small particles have large surface
curvature meaning that their thickness at the boundary is reduced which results to rapid drug
diffusion from the particle surface to the bulk.
Increase in saturation solubility
Ostwald–Freundlich equation (see equation 2) can also help to explain the
increased saturation solubility with particle size reduction (Kesisoglou et al. 2007):
(
𝑺 = 𝑺∞ 𝒆𝒙𝒑
𝟐𝜸𝑴
𝒓𝝆𝑹𝑻
where:
S : saturation solubility of the nanosized
drug.
S∞ : saturation solubility of an infinitely
large drug crystal.
γ : the crystal-medium interfacial tension.
)
Equation 2
M : the compound molecular weight.
r : the particle radius.
ρ : the density of drug particles.
R : the gas constant.
T : the absolute temperature.
According to the book statement, saturation solubility is a constant aspect in regard to the
dissolution medium, temperature as well as the compound. This sentiment is valid or rather
true for various powders of daily life that have sizes that lay within the micrometer range. It
is also important to note that when the powders size go below the size of 1-2 micrometers, the
saturation solubility comes in as a function of the respective particle size. Comparatively,
saturation solubility increases with the decrease in the particle size below 1000nm. In that
connection, drug nanocrystals are associated with increased saturation solubility which is
beneficial due to various reasons. One benefit is in accordance to Noyes and Whitney 1897
who argue that the dissolution velocity is enhanced when the saturation solubility is
increased. This is because the function dc/dt is proportional to the concentration gradient
donated by (cs/cx)/h. In this case, cs, refers to the saturation solubility, h-diffusion distance
and cx- the bulk concentration. Another benefit is the fact that increased saturation solubility
leads to increased concentration gradient between the gut lumen and the blood. As a result,
there is efficient absorption that takes place through passive diffusion.
Figure 1: Special features of nanocrystals: (1) increased saturation solubility (Cs) due to the
increased dissolution pressure of strongly curved small NCs, (2) increased dissolution
velocity (dc/dt) due to increased surface area (A) and decreased diffusional distance (h) and
(3) increased adhesiveness of NCs due to the increased contact area (modified after
Mauludin et al., 2009; Müller et al., 2011a).
In accordance to the Kelvin equation (Anger 2005) increased curvature of the surface is
likely to increase the vapor pressure of the lipid droplets in then gas phase while at the same
time decreasing the particle size. Since each liquid has its own vapor pressure, it is evident
that increase in vapor pressure is influenced by the compounds specific vapor pressure. In
which case, the transfer of molecules from the liquid phase to the gas stage is a principal that
can actually be compared to transfer of molecules from a solid state to a liquid or rather
dispersion medium. Also, it is crucial to note that the vapor pressure is likely to be equal or
similar to the dissolution pressure. Hence, when it comes to saturation solubility, there is
likely to be an equilibrium of molecules recrystallizing and dissolving. Consequently, the
equilibrium can be manipulated in the event of increased dissolution pressure which makes
the saturation solubility to increase. (Figure 6). In regard to liquids that have different vapor
pressures, each drug crystal is expected to have a specific dissolution pressure in micrometer
size. Shows the increase in saturation solubility for poorly soluble salt BaSO4.
Enhanced Mucoadhesion
One of the outstanding characteristics of drug nanocrystals is the fact that they have
increased adhesiveness to the biological mucosa which eventually increases the oral
bioavailability. In fact, Mucoadhesion at a given absorption site is likely to cause a high
concentration gradient as well as prolonged retention time. Additionally, it is important to note
that the mucoadhesive property of drug nanocrystals can be strengthened by modifying the
surface of nanosupensions by making use of mucoadhesive polymers such as cationic
polymers. In which case, the use of mucoadhesive polymers in the nanosupensions of
buparvaquone has proved to increase the retention time.
Figure 2: Adsorption isotherm shapes and corresponding adsorption models. A: case of
particles < 1 µm. B: case of particles > 1µm [20].
Enhanced Safety
When drugs are poorly soluble, their need for extreme pH or the use of co-solvents with
the main aim of increasing solubility. In which case, the formulation of nanosuspension in
aqueous dispersion helps in avoiding the use of such co-solvents. In that case, nanocrystals
possess increased safety since there is no use of cosolvents such as Sporanox which contain
toxic cyclodextrin agents.
Methods for Nanocrystallization-Production of Nanocrystals
There are various established methods to produce drug nanocrystals and are
schematically depicted in The selection of method of preparation depends on the nature of drug
molecule. Several detailed reviews are already published on production technologies of
nanocrystals; however, brief details of these technologies are included here [70, 73–75].
Techniques used in the process of preparing nanocrystals can be categorized into two namely
bottom-up and top-down techniques.
Table 1: Advantages and disadvantages of different techniques of drug nanonization.
(Junghanns and Muller 2008).
Technology
Advantages
- Proven by 4 FDA
approved drugs.
- Possible contaminations
from milling media.
- Long processing time.
- Needs to be stabilized
- Limited size of milling chamber
(difficult to process large
batches).
- Universal applicability.
- Large batches
can be
processed.
- Relatively fast method.
- High energy technique.
- Great experience is needed.
Milling
Top
down
Homogenization
Disadvantages
Bottom
up
Precipitation
- Finely dispersed drug.
-Good control of
desired size.
-Certain conditions of
pressure and temperature
may be needed as in CSF.
- Needs to be stabilized.
- Organic solvent residue.
- Drugs solubility is required.
The top down approach is a process that involves mechanical breakdown of large drug
particles using techniques such as homogenization and milling. The bottom up approach on the
other hand involves the dissolution of molecules which is then precipitated through a nonsolvent such as spray freezing into high liquid process, and liquid solvent change process (Li
et al., 2007). In that case, this section looks at the top down approach as well as the bottom up
approach as well as their associated advantages and challenges.
Top-Down Technology
The top down technology is a well-established nanocrystal technology that make use of
high energy levels. In fact, it involves a crucial process where drug crystals are introduced to
high pressure collisions or rather mechanical attrition that reduces their sizes. In which case,
this process makes use of high pressure homogenization as well as milling. However, this
approach faces challenges associated with high energy input as well as contamination due to
impurities from the grinding media. On the hand it is beneficial due to the fact that it facilitates
preparation of crystalline nanoparticles. Also, it involves no harsh solvents and it is very
flexible when it comes to scale up production.
Media milling
Figure 3 Diagram illustrates an example of a media milling process. Source (Merisko-Liversidge et
al. 2003)
Media milling is a top down approach introduced in 1992 by Liversidge et al. The
process takes in a mill that comprises of milling motor, milling chamber, milling pearls as well
as the recirculation chamber. The process consists of a milling media that comprises of glass,
polystyrene beads and Zirconium oxide. Later, surfactant, water, and the compositions of the
milling media are introduced into the milling chamber. The milling media is then rotated at a
very high speed an action that causes high energy and shear that results to breakdown of drug
particles. In that case, it is evident that the amount of milling media, time of rotation, rotation
speed are very important parameters in the process. The use of media milling results into drug
particles with low size distribution with ease in low batch to batch as well as scale up variations.
The method has been approved due to the fact that commercial products produced this method
have shown a desirable size of drug particles. However, it is important to note that the process
is associated with impurities of the milling media which are likely to remain the final drug
product even through special coatings such as ceria are used to avoid impurities. This is a major
concern especially when the produced or rather formulated drug has to administered
chronically.
High-pressure homogenization
High pressure homogenization works on the same principle of top down technology. In
which case, when a given drug suspension is passed through a narrow homogenization gap of
around 25 μm, the force results into cavitation, high shear force and collision among particles
causing a reducing in size. Accordingly, cavitation comes as a result of gas bubbles of water
vapor that explodes when the suspension reaches normal air pressure. In this process, the most
common parameters are homogenization pressure and the associated homogenization cycle.
Just like the media milling technique, homogenization can be used in the production of both
concentrated as well as dilute nanosupensions with low particle size distribution. Additionally,
it provides ease of scalability with less batch to batch variations. Since aseptic condition can
be maintained, the method comes in handy in the production of nanosupensions for parenteral
administration.
Figure 4: A diagram represents the HGCP process. Source (Hu et al. 2008 B).
Bottom-up technology (add Fig 5 a b and c)
Bottom up technology is the oldest approach used in the production of drug
nanocrystals. In this case, small drug particles are formed due to precipitation of drug particles
using agents in the presence of a stabilizer. The process later on leads to what is called induction
of crystal formation. Accordingly, this approach involves a process where drugs molecules are
dissolved into a solvent system that is miscible with the anti-solvent. In fact, water is usually
considered the appropriate anti-solvent system in the production of nanocrystals of poorly
soluble drugs.
Later on various improvements were made in the bottom up technology and to avoid
the use of toxic organic solvents, the process indulged the use of supercritical fluids. This fluid
was used either as the anti-solvent or the solvent depending on the solubility of the drug.
Consequently, the use of supercritical fluids leads to the development of supercritical solvents
method as well as rapid expansion of supercritical solution methods. On the other hand,
Evaporative precipitation into aqueous solution was a technique developed to take care of drugs
which are soluble in water immiscible solvents. In that connection, hydrosols and
NanomorphTM are the main patented technologies based on precipitation. Some of the
parameters that affect property of nanocrystals include the rate of addition of either the solvent
or the ant-solvent mixing time, type of mixing process, type and amount of stabilizers well as
the ratio of solvent and the ant-solvent. Although ultrafine particles can be produced using this
method, it is associated with challenges such as subsequent particle growth, which are likely
to affect scale-up, solid state stability as well as redispersibility.
Combination technology
Combination technology is also a nanocrystal production technique that involves the use of
both bottom-up and top-down approaches. In fact, it involves a crucial process where bottom-
up technique is carried out first followed by the top-down technique. Accordingly, nanocrystals
undergo the process of precipitation before being introduced to top-down approach that
involves ultrasounds, high energy mixing as well as HPH. The main advantage of this
technique is the fact that it centers around the production of much smaller nanocrystals.
However, it is imperative to note that the method is only used when it is extremely needed due
to the costs associated with the double procedure.
Figure 6: Schematic representation of the main methods for the production of drug
nanocrystals, using top-down and bottom-up approaches.