INTRODUCTION:

Bioavailability is defined as the rate and extent to which the active ingredient is absorbed from a drug product and becomes available at the site of action.¹From a pharmacokinetic perspective, bioavailability data for a given formulation provide an estimate of the relative fraction of the orally administered dose that is absorbed into the systemic circulation when compared to the bioavailability data for a solution, suspension or intravenous dosage form. In addition, bioavailability studies provide other useful pharmacokinetic information related to distribution, elimination, effects of nutrients on absorption of the drug, dose proportionality and linearity in pharmacokinetics of the active and inactive moieties. Bioavailability data can also provide information indirectly about the properties of a drug substance before entry into the systemic circulation, such as permeability and the influence of pre-systemic enzymes and/or transporters.

Bioavailability of a drug is largely determined by the properties of the dosage form, rather than by the drug's physicochemical properties, which determine absorption potential. Differences in bioavailability among formulations of a given drug can have clinical significance; thus, knowing whether drug formulations are equivalent is essential.

Poorly water soluble drugs are increasingly becoming a problem in terms of obtaining satisfactory dissolution within the gastrointestinal tract that is necessary for good oral bioavailability. It is not only existing drugs that cause problems but it is the challenge to ensure that new drugs are not only active pharmacologically but have enough solubility to ensure fast enough dissolution at the site of administration, often the gastrointestinal tract.²

FACTORS AFFECTING BIOAVAILABILITY:

Low bioavailability is most common with oral dosage forms of poorly water-soluble, slowly absorbed drugs. Solid drugs need to dissolve before they are exposed to be absorbed. If the drug does not dissolve readily or cannot penetrate the epithelial membrane (eg, if it is highly ionized and polar), time at the absorption site may be insufficient. In such cases, bioavailability tends to be highly variable as well as low³. Age, sex, physical activity, genetic phenotype, stress, disorders (eg, achlorhydria, malabsorption syndromes), or previous GI surgery (eg, bariatric surgery) can also affect drug bioavailability.

IMPROVEMENT OF BIOAVAILABILITY:

Improvement of bioavailability of poorly water soluble drug remains one of the most challenging aspects of drug development. By many estimates up to 40% of new chemical entities discovered by the pharmaceutical industry today are poorly water soluble compounds.⁴

Together with the permeability, the solubility behavior of a drug is a key determinant of its bioavailability. There have always been certain drugs for which solubility has presented a challenge to the development of a suitable formulation for oral administration. Examples are griseofulvin, digoxin, phenytoin, sulphathiazole etc. With the recent arrival of high throughput screening of potential therapeutic agents, the number of poorly soluble drug candidates has risen sharply and the formulation of poorly soluble compounds for delivery now presents one of the most frequent and greatest challenges to formulation scientists in the pharmaceutical industry.

Consideration of the modified Noyes-Whitney equation provides some hints as to how the dissolution rate of even very poorly soluble compounds might be improved to minimize the limitations to oral availability.⁵The main possibilities for improving dissolution according to this analysis are:

· To increase the surface area available for dissolution by decreasing the particle size of the solid compound,

· By optimizing the wetting characteristics of the compound surface,

· To decrease the diffusion layer thickness,

· To ensure sink conditions for dissolution and,

· To improve the apparent solubility of the drug under physiologically relevant conditions.⁶

A fundamental step in the solubilization of drug compound is the selection of an appropriate salt form, or for liquid drugs, adjustment of pH of the solution. Traditional approaches to drug solubilization include either chemical or mechanical modification of the drug molecule, or physically altering the macromolecular characteristics of aggregated drug particles.

Improvement of bioavailability can be obtained by following measures:

· Addition of solubilizing excipients

· Inclusion complexes

· Polymorphism

· Lipid-based emulsion systems

· Salt form

· Solid dispersions

· Particle size reduction etc.

NEED OF NANOSUSPENSION FOR BIOAVAILABILITY ENHANCEMENT:

Nevertheless, pharmacokinetic studies of BCS class – II drugs showed that they have a low oral bioavailability, which may be due to poor water solubility of drug. There are many classical pharmaceutical ways to improve drug dissolution rate such as dissolution in aqueous mixtures with an organic solvent,⁷ formation of ß-cyclodextrin complexes,⁸solid dispersions⁹ and drug salt form.¹⁰

During last 20 years a new technology, reducing drug particle size, has been developed to increase drug dissolution rate. According to Noyes–Whitney equation, drugs with smaller particle size have enlarged surface areas which lead to increase dissolution velocity. Higher the dissolution rate together with the resulting higher concentration gradient between gastrointestinal lumen and systemic circulation could further increase oral bioavailability of drugs.¹¹ A nanosuspension is a submicron colloidal dispersion of drug particles which are stabilized by surfactants. A pharmaceutical nanosuspension is defined as very finely dispersed solid drug particles in an aqueous vehicle for oral, topical, parenteral or pulmonary administration. The particle size distribution of the solid particles in nanosuspensions is usually less than one micron with an average particle size ranging between 200 and 600 nm.¹² In nanosuspension technology, the drug is maintained in the required crystalline state with reduced particle size, leading to an increased dissolution rate and therefore improved bioavailability. An increase in the dissolution rate of micronized particles (particle size < 10 μm) is related to an increase in the surface area and consequently the dissolution velocity. Nanosized particles can increase solution velocity and saturation solubility because of the vapor pressure effect. In addition; the diffusional distance on the surface of drug nanoparticles is decreased, thus leading to an increased concentration gradient. Increase in surface area as well as concentration gradient lead to a much more pronounced increase in the dissolution velocity as compared to a micronized product. Another possible explanation for the increased saturation solubility is the creation of high energy surfaces when disrupting the more or less ideal drug microcrystals to nanoparticles. Dissolution experiments can be performed to quantify the increase in the saturation solubility of a drug when formulated into a nanosuspension.¹³

The stability of the particles obtained in the nanosuspension is attributed to their uniform particle size which is created by various manufacturing processes. The absence of particles with large differences in their size in nanosuspensions prevents the existence of different saturation solubilities and concentration gradients; consequently preventing the Oswald ripening effect. Ostwald ripening is responsible for crystal growth and subsequently formation of microparticles. It is caused by a difference in dissolution pressure/saturation solubility between small and large particles. Molecules diffuse from the higher concentration area around small particles which have higher saturation solubility to an area around larger particles possessing a lower drug concentration. This leads to the formation of a supersaturated solution around the large particles and consequently to drug crystallization and growth of the large particles.

ADVANTAGES OF NANOSUSPENSIONS:

The major advantages of nanosuspension technology are:¹⁴

· Provides ease of manufacture and scale-up for large scale production,

· Long-term physical stability due to the presence of stabilizers,

· Oral administration of nanosuspensions provide rapid onset, reduced fed/fasted ratio and improved bioavailability,

· Rapid dissolution and tissue targeting can be achieved by IV route of administration,

· Reduction in tissue irritation in case of subcutaneous/intramuscular administration,

· Higher bioavailability in case of ocular administration and inhalation delivery,

· Drugs with high log P value can be formulated as nanosuspensions to increase the bioavailability of such drugs,

· Improvement in biological performance due to high dissolution rate and saturation solubility of the drug,

· Nanosuspensions can be incorporated in tablets, pellets, hydrogels and suppositories are suitable for various routes of administration,

· The flexibility offered in the modification of surface properties and particle size, and ease of post-production processing of nanosuspensions enables them to be incorporated in various dosage forms for various routes of administration, thus proving their versatility.

INTERESTING SPECIAL FEATURES OF NANOSUSPENSIONS: ¹⁵

· Increase in saturation solubility and consequently an increase in the dissolution rate of the drug.

· Increase in adhesive nature, thus resulting in enhanced bioavailability.

· Increasing the amorphous fraction in the particles, leading to a potential change in the crystalline structure and higher solubility.

· Absence of ostwald ripening, producing physical long term stability as an aqueous suspension.

· Possibility of surface-modification of nanosuspensions for site specific delivery.

CRITERIA FOR SELECTION OF DRUG FOR NANOSUSPENSIONS:

Nanosuspension can be prepared for the API that is having either of the following characteristics:¹⁶

ü Water insoluble but which are soluble in oil (high log P) OR API are insoluble in both water and oils

ü Drugs with reduced tendency of the crystal to dissolve, regardless of the solvent

ü API with very large dose

METHODS OF PREPARATION FOR NANOSUSPENSIONS:

Milling techniques (Nanocrystals or Nanosystems)

Media milling:

Media milling is a technique used to prepare nanosuspensions.^{11,12, 17-19} Nanocrystal is a patent protected technology developed by Liversidge et al. In this technique, the drug nanoparticles are obtained by subjecting the drug to media milling. High energy and shear forces generated as a result of impaction of the milling media with the drug provide the necessary energy input to disintegrate the microparticulate drug into nanosized particles. In the media milling process, the milling chamber is charged with the milling media, water or suitable buffer, drug and stabilizer. Then the milling media or pearls are rotated at a very high shear rate. The major concern with this method is the residues of milling media remaining in the finished product could be problematic for administration.¹⁷

Nanosuspensions are produced by using high-shear media mills or pearl mills. The mill consists of a milling chamber, milling shaft and a recirculation chamber. An aqueous suspension of the drug is then fed into the mill containing small grinding balls/pearls. As these balls rotate at a very high shear rate under controlled temperature, they fly through the grinding jar interior and impact against the sample on the opposite grinding jar wall. The combined forces of friction and impact produce a high degree of particle size reduction. The milling media or balls are made of ceramic-sintered aluminium oxide or zirconium oxide or highly cross-linked polystyrene resin with high abrasion resistance. Planetary ball mill is one example of the equipment that can be used to achieve a grind size below 0.1 μm.

Dry co-grinding:

Nanosuspensions prepared by high pressure homogenization and media milling using pearl-ball mill are wet–grinding processes. Recently, nanosuspensions can be obtained by dry milling techniques. Successful work in preparing stable nanosuspensions using dry-grinding of poorly soluble drugs with soluble polymers and copolymers after dispersing in a liquid media has been reported.^20-22

Itoh et al reported the colloidal particles formation of many poorly water soluble drugs; griseofulvin, glibenclamide and nifedipine obtained by grinding with polyvinyl pyrrolidone (PVP) and sodium dodecyl sulfate (SDS). Many soluble polymers and co-polymers such as PVP, polyethylene glycol (PEG), hydroxyl propyl methylcellulose (HPMC) and cyclo-dextrin derivatives have been used.^23-25 Physico-chemical properties and dissolution of poorly water soluble drugs were improved by co-grinding because of an improvement in the surface polarity and transformation from a crystalline to an amorphous drug.^26,27Dry co-grinding can be carried out easily and economically and can be conducted without organic solvents. The co-grinding technique can reduce particles to the submicron level and a stable amorphous solid can be obtained.

Advantages:

· Media milling is applicable to the drugs that are poorly soluble in both aqueous and organic media.

· Very dilute as well as highly concentrated nanosuspensions can be prepared by handling 1mg/ml to 400mg/ml drug quantity.

· Nanosize distribution of final nanosized products.

Disadvantages:

· Nanosuspensions contaminated with materials eroded from balls may be problematic when it is used for long therapy. (Wet milling technique)

· 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.

High Pressure Homogenization:

Homogenization in Aqueous media (Dissocubes)

Homogenization involves the forcing of the suspension under pressure through a valve having a narrow aperture. Dissocube technology was developed by Muller et al. in which, the suspension of the drug is made to pass through a small orifice that results in a reduction of the static pressure below the boiling pressure of water, which leads to boiling of water and formation of gas bubbles. When the suspension leaves the gap and normal air pressure is reached again, the bubbles shrink and the surrounding part containing the drug particles rushes to the center and in the process colloids, causing a reduction in the particle size. Most of the cases require multiple passes or cycles through the homogenizer, which depends on the hardness of drug, the desired mean particle size and the required homogeneity.

Scholer et al. prepared atovaquone nanosuspensions using this technique.²⁸ To produce a nanosuspension with a higher concentration of solids, it is preferred to start homogenization with very fine drug particles, which can be accomplished by pre-milling.

Homogenization in Non Aqueous Media (Nanopure):

Nanopure is the technology in which suspension is homogenized in water-free media or water mixtures.²⁹ 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 vapor 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 thermo labile compounds. In nanopure technology, the drug suspensions in the non- aqueous 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 thermo labile substances at milder conditions.

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.³⁰

· Narrow size distribution of the nanoparticulate drug present in the final product ³¹

· Allows aseptic production of nanosuspensions for parenteral administration.

· Flexibility in handling the drug quantity, ranging from 1 to 400mg mL^-1, thus enabling formulation of very dilute as well as highly concentrated nanosuspensions.

Disadvantages:

· Prerequisite of micronized drug particles.

· Prerequisite of suspension formation using high-speed mixers before subjecting it to homogenization.

Precipitation Method:

Using a precipitation technique, the drug is dissolved in an organic solvent and this solution is mixed with a miscible anti-solvent. In water-solvent mixture the solubility is low and the drug precipitates. Mixing processes vary considerably. Precipitation has also been coupled with high shear processing. The nanoedge process (is a registered trademark of Baxter International Inc. and its subsidiaries) relies on the precipitation of friable materials for subsequent fragmentation under conditions of high shear and/or thermal energy.³²

Nanoedge:

The basic principles of Nanoedge are the same as that of precipitation and homogenization. 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. Rapid addition of a drug solution to an anti-solvent leads to sudden super-saturation of the mixed solution, and generation of fine crystalline or amorphous solids. Precipitation of an amorphous material may be favored at high super-saturation when the solubility of the amorphous state is exceeded. The success of drug nanosuspensions prepared by precipitation techniques has been reported.^32-35

In this technique, the precipitated suspension is further homogenized, leading to reduction in particle size and avoiding crystal growth. Precipitation is performed in water using water-miscible solvents such as methanol, ethanol and isopropanol. It is desirable to remove those solvents completely, although they can be tolerated to a certain extent in the formulation. For an effective production of nanosuspensions using the Nanoedge technology, an evaporation step can be included to provide a solvent-free modified starting material followed by high-pressure homogenization.

Nanojet technology:

This technique, called opposite stream or nanojet technology, uses a chamber where a stream of suspension is divided into two or more parts, which colloid with each other at high pressure. The high shear force produced during the process results in particle size reduction. Equipment using this principle includes the M110L and M110S microfluidizers (Microfluidics).

The major disadvantage of this technique is the high number of passes through the microfluidizer and the product obtained contains a relatively larger fraction of microparticles.

Emulsions as templates:

Apart from the use of emulsions as a drug delivery vehicle, they can also be used as templates to produce nanosuspensions. The use of emulsions as templates is applicable for those drugs that are soluble in either volatile organic solvent or partially water-miscible solvent. Such solvents can be used as the dispersed phase of the emulsion. There are two ways of fabricating drug nanosuspensions by the emulsification method. In the first method, an organic solvent or mixture of solvents loaded with the drug is dispersed in the aqueous phase containing suitable surfactants to form an emulsion. The organic phase is then evaporated under reduced pressure so that the drug particles precipitate instantaneously to form a nanosuspension stabilized by surfactants. Since one particle is formed in each emulsion droplet, it is possible to control the particle size of the nanosuspension by controlling the size of the emulsion droplet. Optimizing the surfactant composition increases the intake of organic phase and ultimately the drug loading in the emulsion. Originally, organic solvents such as methylene chloride and chloroform were used.³⁶

However, environmental hazards and human safety concerns about residual solvents have limited their use in routine manufacturing processes. Relatively safer solvents such as ethyl acetate and ethyl formate can still be considered for use.^37,38

The emulsion is formed by the conventional method and the drug nanosuspension is obtained by just diluting the emulsion. Dilution of the emulsion with water causes complete diffusion of the internal phase into the external phase, leading to instantaneous formation of a nanosuspension. The nanosuspension thus formed has to be made free of the internal phase and surfactants by means of di-ultrafiltration in order to make it suitable for administration. However, if all the ingredients that are used for the production of the nanosuspension are present in a concentration acceptable for the desired route of administration, then simple centrifugation or ultracentrifugation is sufficient to separate the nanosuspension.

Advantages:

· Use of specialized equipment is not necessary.

· Particle size can easily be controlled by controlling the size of the emulsion droplet.

· Ease of scale-up if formulation is optimized properly.

Disadvantages:

· Drugs that are poorly soluble in both aqueous and organic media cannot be formulated by this technique.

· Safety concerns because of the use of hazardous solvents in the process.

· Need for di-ultrafiltration for purification of the drug nanosuspension, which may render the process costly.

· High amount of surfactant / stabilizer is required as compared to the production techniques described earlier.

The production of drug nanosuspensions from emulsion templates has been successfully applied to the poorly water-soluble and poorly bioavailable anti-cancer drug mitotane, where a significant improvement in the dissolution rate of the drug (five-fold increase) as compared to the commercial product was observed.³⁹

Microemulsions as templates:

Microemulsions are thermodynamically stable and iso-tropically clear dispersions of two immiscible liquids, such as oil and water, stabilized by an interfacial film of surfactant and co surfactant.⁴⁰

Their advantages, such as high drug solublization, long shelf life and ease of manufacture, make them an ideal drug delivery vehicle. Recently, the use of microemulsions as templates for the production of solid lipid nanoparticles⁴¹ and polymeric nanoparticles⁴² has been described. Taking advantage of the micro emulsion structure, one can use microemulsions even for the production of nanosuspensions.⁴³The drug can be either loaded in the internal phase or preformed microemulsions can be saturated with the drug by intimate mixing. The suitable dilution of the microemulsion yields the drug nanosuspension by the mechanism described earlier. The influence of the amount and ratio of surfactant to co surfactant on the uptake of internal phase and on the globule size of the microemulsion should be investigated and optimized in order to achieve the desired drug loading. The nanosuspension thus formed has to be made free of the internal phase and surfactants by means of di-ultrafiltration in order to make it suitable for administration. However, if all the ingredients that are used for the production of the nanosuspension are present in a concentration acceptable for the desired route of administration, then simple centrifugation or ultracentrifugation is sufficient to separate the nanosuspension. The advantages and disadvantages are the same as for emulsion templates. The only added advantage is the need for less energy input for the production of nanosuspensions by virtue of microemulsions.

Supercritical fluid method:

Supercritical fluid technology can be used to produce nanoparticles from drug solutions. The various methods attempted are rapid expansion of supercritical solution process (RESS), supercritical anti-solvent process and precipitation with compressed anti-solvent process (PCA). The RESS involves expansion of the drug solution in supercritical fluid through a nozzle, which leads to loss of solvent power of the supercritical fluid resulting in precipitation of the drug as fine particles. In the PCA method, the drug solution is atomized into a chamber containing compressed CO₂. As the solvent is removed, the solution gets supersaturated and thus precipitates as fine crystals. The supercritical anti- solvent process uses a supercritical fluid in which a drug is poorly soluble and a solvent for the drug that is also miscible with the supercritical fluid. The drug solution is injected into the supercritical fluid and the solvent gets extracted by the supercritical fluid and the drug solution gets supersaturated. The drug is then precipitated as fine crystals. The disadvantages of the above methods are use of hazardous solvents and use of high proportions of surfactants and stabilizers as compared with other techniques, particle nucleation overgrowth due to transient high supersaturation, which may also result in the development of an amorphous form or another undesired polymorph.⁴⁴

POST-PRODUCTION PROCESSING:

Post-production processing of nanosuspensions becomes essential when the drug candidate is highly susceptible to hydrolytic cleavage or chemical degradation. Processing may also be required when the best possible stabilizer is not able to stabilize the nanosuspension for a longer period of time or there are acceptability restrictions with respect to the desired route. Considering these aspects, techniques such as lyophillization or spray drying may be employed to produce a dry powder of nano-sized drug particles. Rational selection has to be made in these unit operations considering the drug properties and economic aspects.¹⁷

CHARACTERIZATION OF NANOSUSPENSION:

Mean particle size and particle size distribution

The mean particle size and particle size distribution are important characterization parameters as they influence the saturation solubility, dissolution velocity, physical stability as well as biological performance of nanosuspensions. It has been indicated by Muller and Peters (1998) that saturation solubility and dissolution velocity show considerable variation with the changing particle size of the drug.¹³Photon correlation spectroscopy (PCS) can be used for rapid and accurate determination of the mean particle diameter of nanosuspensions. Moreover, PCS can even be used for determining the width of the particle size distribution (polydispersity index, PI). The PI is an important parameter that governs the physical stability of nanosuspensions and should be as low as possible for the long-term stability of nanosuspensions. A PI value of 0.1–0.25 indicates a fairly narrow size distribution whereas a PI value greater than 0.5 indicates a very broad distribution. No logarithmic normal distribution can definitely be attributed to such a high PI value. Although PCS is a versatile technique, because of its low measuring range (3nm to 3µm) it becomes difficult to determine the possibility of contamination of the nanosuspension by micro particulate drugs (having particle size greater than 3µm). Hence, in addition to PCS analysis, laser diffractometry (LD) analysis of nanosuspensions should be carried out in order to detect as well as quantify the drug microparticles that might have been generated during the production process.

Various methods are available for particle size measurement.⁴⁵ Laser diffractometry yields a volume size distribution and can be used to measure particles ranging from 0.05–80 µm and in certain instruments particle sizes up to 2000µm can be measured. The typical LD characterization includes determination of diameter 50% LD (50) and diameter 99% LD (99) values, which indicate that either 50 or 99% of the particles are below the indicated size. The LD analysis becomes critical for nanosuspensions that are meant for parenteral and pulmonary delivery. Even if the nanosuspension contains a small number of particles greater than 5–6 µm, there could be a possibility of capillary blockade or emboli formation, as the size of the smallest blood capillary is 5–6 µm. It should be noted that the particle size data of a nanosuspension obtained by LD and PCS analysis are not identical as LD data are volume based and the PCS mean diameter is the light intensity weighted size. The PCS mean diameter and the 50 or 99% diameter from the LD analysis are likely to differ, with LD data generally exhibiting higher values. The nanosuspensions can be suitably diluted with deionized water before carrying out PCS or LD analysis.

Crystalline state and particle morphology:

The assessment of the crystalline state and particle morphology together helps in understanding the polymorphic or morphological changes that a drug might undergo when subjected to nano sizing. Additionally, when nanosuspensions are prepared drug particles in an amorphous state are likely to be generated. Hence, it is essential to investigate the extent of amorphous drug nanoparticles generated during the production of nanosuspensions. The changes in the physical state of the drug particles as well as the extent of the amorphous fraction can be determined by X-ray diffraction analysis^30,31 and can be supplemented by differential scanning Calorimetry.⁴⁶ In order to get an actual idea of particle morphology, scanning electron microscopy is preferred.³¹

Particle charge (zeta potential):

The particle charge is of importance in the study of the stability of the suspensions. Usually the zeta potential of more than ±40mV will be considered to be required for the stabilization of the dispersions. For electrostatically stabilized nanosuspension a minimum zeta potential of ±30mV is required and in case of combined steric and electrostatic stabilization it should be a minimum of ± 20mV of zeta potential is required.

Surface charges can arise from (i) ionization of the particle surface or (ii) adsorption of ions (such as surfactants) onto the surface. Typically, the surface charge is assessed through measurements of the zeta potential. Zeta potential is the potential at the hydrodynamic shear plane and can be determined from the particle mobility under an applied electric field.⁴⁷ The mobility will depend on the effective charge on the surface. Zeta potential is also a function of electrolyte concentration.

Solubility study:

The solubility can also define as the ability of one substance to form a solution with another substance. The substance to be dissolved is called as solute and the dissolving fluid in which the solute dissolve is called as solvent, which together form a solution.

The main advantage associated with the nanosuspensions is improved saturation solubility. This is studied in different physiological solutions at different pH. Kelvin equation and the Ostwald-Freundlich equations can explain increase in saturation solubility. Determination of this parameter is useful to assess in vivo performance of the formulation also.⁴⁸

In vitro dissolution study:

Dissolution rate may be defined as amount of drug substance that goes in the solution per unit time under standard conditions of liquid/solid interface, temperature and solvent composition. It can be considered as a specific type of certain heterogeneous reaction in which a mass transfer results as a net effect between escape and deposition of solute molecules at a solid surface.⁴⁹

In vitro dissolution screening should be the first line of biopharmaceutical evaluation of nano-formulations. Since oral nano-formulations are designed to disperse in the stomach contents, dissolution in Simulated Gastric Fluid (SGF) should provide an initial estimate of the dissolution rate enhancement. For insoluble compounds, where dissolution is expected to mainly occur in the intestinal region, further in vitro testing in simulated intestinal media will provide additional insight on expected bio-performance. Several reports in the literature report an increased in vitro dissolution rate for nanosized APIs. However one should keep in mind that the small particle size for nano-formulations may pose additional needs in terms of analytical sample handling and processing to ensure that no undissolved API is assayed during the dissolution test. Filtering through smaller pore size filters or (ultra)centrifugation to separate un-dissolved API has been implemented in the literature to address this issue.⁵⁰

Stability of Nanosuspensions:

Stability of the suspensions is dependent on the particle size. As the particle size reduces to the nanosize the surface energy of the particles will be increased and they tend to agglomerate. So stabilizers are used which will decrease the chances of Ostwald ripening effect and improving the stability of the suspension by providing a steric or ionic barrier. Typical examples of stabilizers used in nanosuspensions are cellulosics, poloxamer, polysorbates, lecithin, polyoleate and povidones etc.⁵¹

In-vivo biological performance:

The establishment of an in-vitro/in-vivo correlation and the monitoring of the in-vivo performance of the drug is an essential part of the study, irrespective of the route and the delivery system employed. It is of the utmost importance in the case of intravenously injected nanosuspensions since the nanosuspensions: a promising drug delivery strategy in-vivo behaviour of the drug depends on the organ distribution, which in turn depends on its surface properties, such as surface hydrophobicity and interactions with plasma proteins.^52-55 In fact, the qualitative and quantitative composition of the protein absorption pattern observed after the intravenous injection of nanoparticles is recognized as the essential factor for organ distribution.^52-56 Hence, suitable techniques have to be used in order to evaluate the surface properties and protein interactions to get an idea of in-vivo behaviour. Techniques such as hydrophobic interaction chromatography can be used to determine surface hydrophobicity,⁵⁷ whereas 2-D PAGE⁵² can be employed for the quantitative and qualitative measurement of protein adsorption after intravenous injection of drug nanosuspensions in animals.

APPLICATIONS OF NANOSUSPENSIONS IN DRUG DELIVERY:

Parenteral administration:

From the formulation perspective, nanosuspensions meet almost all the requirements of an ideal drug delivery system for the parenteral route. Since the drug particles are directly nanosized, it becomes easy to process almost all drugs for parenteral administration. Hence, nanosuspensions enable significant improvement in the parenterally tolerable dose of the drug, leading to a reduction in the cost of the therapy and also improved therapeutic performance. The maximum tolerable dose of paclitaxel nanosuspension was found to be three times higher than the currently marketed Taxol, which uses Cremophore EL and ethanol to solubilize the drug.⁵⁸

Nanosuspensions can be administered via different parenteral administration routes ranging from intra-articular via intra peritonal to intravenous injection. For administration by the parenteral route, the drug either has to be solubilized or has particle/globule size below 5μm to avoid capillary blockage. In this regard, liposomes are much more tolerable and versatile in terms of parenteral delivery. However, they often suffer from problems such as physical instability, high manufacturing cost and difficulties in scale-up. Nanosuspensions would be able to solve the problems mentioned above. In addition, nanosuspensions have been found to increase the efficacy of parenterally administered drugs.²⁹

Oral administration:

The oral route is the preferred route for drug delivery because of its numerous well-known advantages. The efficacy or performance of the orally administered drug generally depends on its solubility and absorption through the gastrointestinal tract. Hence, a drug candidate that exhibits poor aqueous solubility and / or dissolution rate limited absorption is believed to possess slow and/or highly variable oral bioavailability. Danazol is poorly bioavailable gonadotropin inhibitor, showed a drastic improvement in bioavailability when administered as a nanosuspension as compared to the commercial danazol macrosuspension Danocrine. Danazol nanosuspension led to an absolute bioavailability of 82.3%, where as the marketed danazol suspension Danocrine was 5.2% bioavailable.¹¹

Nanosizing of drugs can lead to a dramatic increase in their oral absorption and subsequent bioavailability. Improved bioavailability can be explained by the adhesiveness of drug nanoparticles to the mucosa, the increased saturation solubility leading to an increased concentration gradient between gastrointestinal tract lumen and blood as well as the increased dissolution velocity of the drug. Aqueous nanosuspensions can be used directly in a liquid dosage form and a dry dosage form such as tablet or hard gelatin capsule with pellets. The aqueous nanosuspension can be used directly in the granulation process or as a wetting agent for preparing the extrusion mass pellets. A similar process has been reported for incorporating solid lipid nanoparticles into pellets. Granulates can also be produced by spray drying of nanosuspensions.²⁹

Ophthalmic drug delivery:

Nanosuspensions could prove to be vital for drugs that exhibit poor solubility in lachrymal fluids. Suspensions offer advantages such as prolonged residence time in a cul-de-sac, which is desirable for most ocular diseases for effective treatment and avoidance of high tonicity created by water soluble drugs. Their actual performance depends on the intrinsic solubility of the drug in lachrymal fluids. Thus the intrinsic dissolution rate of the drug in lachrymal fluids controls its release and ocular bioavailability. However, the intrinsic dissolution rate of the drug will vary because of the constant inflow and outflow of lachrymal fluids. One example of a nanosuspension intended for ophthalmic controlled delivery was developed as a polymeric nanosuspension of ibuprofen.⁵⁹ This nanosuspension is successfully prepared using Eudragit RS100 by a quasi-emulsion and solvent diffusion method.

Nanosuspensions of glucocorticoid drugs; hydrocortisone, prednisolone and dexamethasone enhance rate, drug absorption and increase the duration of drug action.⁶⁰ To achieve sustained release of the drug for a stipulated time period, nanosuspensions can be incorporated in a suitable hydro-gel base or mucoadhesive base or even in ocular inserts. The bio-erodible as well as water soluble/permeable polymers possessing ocular tolerability⁶¹ could be used to sustain the release of the medication. The polymeric nanosuspension of flurbiprofen has been successfully formulated using acrylate polymers such as Eudragit RS 100 and Eudragit RL 100.^62-64 The polymeric nanosuspensions have been characterized for drug loading, particle size, zeta potential, in-vitro drug release, ocular tolerability and in-vivo biological performance. The designed polymeric nanosuspensions revealed superior in-vivo performance over the existing marketed formulations and could sustain drug release for 24 h. The scope of this strategy could be extended by using various polymers with ocular tolerability.

Pulmonary drug delivery:

Nanosuspensions may prove to be an ideal approach for delivering drugs that exhibit poor solubility in pulmonary secretions. Currently such drugs are delivered as suspension aerosols or as dry powders by means of dry powder inhalers. The drugs used in suspension aerosols and dry powder inhalers are often jet milled and have particle sizes of microns.

Because of the microparticulate nature and wide particle size distribution of the drug moiety present in suspension aerosols and dry powder inhalers, some disadvantages are encountered: like limited diffusion and dissolution of the drug at the site of action, rapid clearance of the drug from the lungs, less residence time for the drugs, unwanted deposition of the drug particles in pharynx and mouth.^65,66

The ability of nanosuspensions to offer quick onset of action initially and then controlled release of the active moiety is highly beneficial and is required by most pulmonary diseases. Moreover, as nanosuspensions generally contain a very low fraction of microparticulate drug, they prevent unwanted deposition of particles in the mouth and pharynx, leading to decreased local and systemic side-effects of the drug. Additionally, because of the nanoparticulate nature and uniform size distribution of nanosuspensions, it is very likely that in each aerosol droplet at least one drug nanoparticle is contained, leading to even distribution of the drug in the lungs as compared to the microparticulate form of the drug. In conventional suspension aerosols many droplets are drug free and others are highly loaded with the drug, leading to uneven delivery and distribution of the drug in the lungs. Nanosuspensions could be used in all available types of nebulizer. However, the extent of influence exerted by the nebulizer type as well as the nebulization process on the particle size of nanosuspensions should be ascertained.

Bioavailability enhancement:

Drug with poor solubility or permeability in gastrointestinal tract leads to poor oral bioavailability. Nanosuspension resolves the problem of poor bioavailability by solving the problem of poor solubility, and poor permeability across the membranes. Dissolution rate was increased in diclofenac when formulated in nanosuspension form from 25% to 50% in SGF and H₂O while in case of SIF it was increased from 10% to 35% as compared to coarse suspension.⁶⁷

Bioavailability of poorly soluble, a COX-2 inhibitor, celecoxib was improved using a nanosuspension formulation. The crystalline nanosized celecoxib alone or in tablet showed a dramatic increase of dissolution rate and extent compared to micronized tablet. Spironolactone and budesonide are poorly soluble drugs. The higher flux contributes to the higher bioavailability of nanosuspension formulation. The bioavailability of poorly soluble fenofibrate following oral administration was increased compared to the suspensions of micronized fenofibrate.⁶⁸

Significant difference (p< 0.05) was observed between the fluxes from saturated solution Vs nanosuspension at all concentrations of surfactant. Oral administration of micronized Amphotericin B did not show any significant effect. However administration in nanosuspension form, showed a significant reduction (P < 0.5%) of the liver parasite load by 28.6%, it indicates that the nanosuspension of amphotericin B has high systemic effect and superior oral uptake in nanosuspension form.⁶⁹

The poor oral bioavailability of the drug may be due to poor solubility, poor permeability or poor stability in the gastrointestinal tract (GIT). Nanosuspensions resolve the problem of poor bioavailability by solving the twin problems of poor solubility and poor permeability across the membrane. Bioavailability of poorly soluble oleanolic acid, a hepato-protective agent, was improved using a nanosuspension formulation. The therapeutic effect was significantly enhanced, which indicated higher bioavailability. This was due to the faster dissolution (90% in 20 min) of the lyophilized nanosuspension powder when compared with the dissolution from a coarse powder (15% in 20 min).²⁹

Target drug delivery:

Nanosuspensions can also be used for targeted delivery as their surface properties and in vivo behavior can easily be altered by changing either the stabilizer or the milieu. Their versatility, ease of scale up and commercial product enable the development of commercial viable nanosuspensions for targeted delivery. The engineering of stealth nanosuspensions by using various surface coatings for active or passive targeting of the desired site is the future of targeted drug delivery systems. Targeting of Cryptosporidium parvum, the organism responsible for cryptosporidiosis, was achieved by using surface modified mucoadhesive nanosuspensions of bupravaquone.^70,71 Similarly, conditions such as pulmonary aspergillosis can easily be targeted by using suitable drug candidates, such as amphotericin B, in the form of pulmonary nanosuspensions instead of using stealth liposomes.⁷²(Review 8)

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 intracellularly.⁷³

Topical formulations:

Drug nanoparticles can be incorporated into creams and water-free ointments. The nanocrystalline form leads to an increased saturation solubility of the drug in the topical dosage form, thus enhancing the diffusion of the drug into the skin.^74-78

Mucoadhesion of the nanoparticles:

Nanosuspension containing drug nanoparticles orally diffuse into the liquid media and rapidly encounter the mucosal surface. The particles are immobilized at the intestinal surface by an adhesion mechanism referred to as "bioadhesion." From this moment on, the concentrated suspension acts as a reservoir of particles and an adsorption process takes place very rapidly. The direct contact of the particles with the intestinal cells through a bioadhesive phase is the first step before particle absorption.⁶⁶ The adhesiveness of the nanosuspensions not only helps to improve bioavailability but also improves targeting of the parasites persisting in the GIT.

MARKETED PRODUCTS BASED ON NANOSUSPENSION:

All the products based on nanosuspension have been approved by the FDA from the year 2000 on. All listed products are based on top-down approaches, eight relying on media milling and one on high- pressure homogenization. Although the bottom-up approaches hold tremendous potential with respect to improving bioavailability in obtaining smaller particle sizes (< 100nm) and amorphous drug particles, no commercial application of these systems has yet been realized. A third remarkable point is that all commercial products are intended for oral delivery. This is an illustration of the general preference of the oral route, since it avoids the pain and discomfort associated with injections and is more attractive from a marketing and patient compliance perspective. Finally, the major advantage of nanocrystals for oral delivery is generally regarded as being on the increased specific surface area of the particles. However, EMEND^® and Triglide^TM are formulated as nanosuspension to reduce fed/fasted variability.⁷⁹

CONCLUSION:

Nanotechnology is an incredible field in the medicine. Since solubility is a crucial factor for drug effectiveness, it is a challenging task to formulate any poorly soluble drug in the industry in conventional dosage forms. Nano-technique is simple; fewer requirements of excipients are there for formulation of dosage form. Attractive features, such as reduction of particles size up to submicron level lead to a significant increase in dissolution velocity as well as saturation solubility. Improved bio-adhesiveness, versatility in surface modification and ease of post-production processing have widened the applications of nanosuspensions for various routes. Nanosuspension technology can be combined with traditional dosage forms: tablets, capsules, pellets and also can be used for parenteral products. Production techniques such as media milling and high-pressure homogenization have been successfully employed for large scale production of nanosuspensions. The advances in production methodologies using emulsions or micro emulsions as templates and precipitation method have provided still simpler approaches for production but with limitations. Further investigation in this regard is still essential. Some of the patented commercially productive technologies have been reviewed and if the patent period ends for such techniques there would be a revolutionary advancement in formulation of poorly water soluble drugs.

REFERENCES:

1. Makoid CM, Vuchetich PJ, Banakar UV. Basic Pharmacokinetics. First Edition. The Virtual University Press. 1999.

2. Aulton ME. Pharmaceutics - The Science and Dosage Form Design. Second Edition. Churchill Livingstone. 2007.

3. Russell TL et al. Influence of gastric pH and emptying on dipyridamole absorption. Pharm Res. 1994;11:136-143.

4. Lipinski CA. Avoiding investment in doomed drugs, is poor solubility an industry wide problem? Curr Drug Discov. 2001;17-19.

5. Noyes AA, Whitney WR. The rate of solution of solid substances in their own solutions. J Am Chem Soc. 1897;19:930–934.

6. Galia E et al. Evaluation of various dissolution media for predicting in vivo performance of class I and II drugs. Pharm Res. 1998;15:698-705.

7. Stovall DM et al. Solubility of crystalline nonelectrolyte solutes in organic solvents: mathematical correlation of 4-chloro-3- itrobenzoic acid and 2-chloro-5-nitrobenzoic acid solubilities with the Abraham solvation parameter model. Phys Chem Liq. 2005;43:351–360.

8. Makhlof A et al. Cyclodextrins as stabilizers for the preparation of drug nanocrystals by the emulsion solvent diffusion method. Int J Pharm. 2008;357:280–285.

9. Park YJ, Hyun CK. Revaprazan-containing solid dispersion and process for the preparation thereof. WO Patent 078922. 2008.

10. Tao T et al. Preparation and evaluation of Itraconazole dihydrochloride for the solubility and dissolution rate enhancement. Int J Pharm. 2009;367:109–114.

11. Liversidge GG, Conzentino P. Drug particle size reduction for decreasing gastric irritancy and enhancing absorption of naproxen in rats. Int J Pharm. 1995;125:309–313.

12. Muller RH, Jacobs C, Kayer O. Nanosuspensions for the formulation of poorly soluble drugs. In: F Nielloud, G Marti-Mestres (ed). Pharmaceutical emulsion and suspension. Marcel Dekker, New York. 2000,383-407.

13. Müller RH, Peters K. Nanosuspensions for the formulation of poorly soluble drug I: Preparation by size reduction technique. Int J Pharm. 1998;160:229-37.

14. Nagaraju P et al. Nanosuspensions: Promising Drug Delivery Systems. International Journal of Pharmaceutical Sciences and Nanotechnology. 2010;2(4):679-684.

15. Dhiman S., Thakur GS, Dharmila. Nanosuspension: A recent approach for nano drug delivery system. International Journal of Current Pharmaceutical Research. 2011;3(4):96-101.

16. Patil SA, Rane BR, Bakliwal SR, Pawar SP. Nano Suspension: At a glance. Int J Ph Sci. 2011; 3(1): 947-960.

17. Patravale VB, Date AA, Kulkarni RM. Nanosuspension: a promising drug delivery strategy. J Pharm Pharmacol. 2004; 56:827-40.

18. Rabinow BE. Nanosuspensions in drug delivery. Nat Rev Drug Discov. 2004;3:785-96.

19. Shah T et al. Nanosuspensions as a drug delivery system: A comprehensive review. Drug Deliv Tech. 2007;7:42-53.

20. Wongmekiat A et al. Formation of fine drug particles by co-grinding with cyclodextrin. I. the use of β-cyclodextrin anhydrate and hydrate. Pharm Res. 2002;19:1867-72.

21. Itoh K et al. Nanoparticle formation of poorly water soluble drugs from ternary ground mixtures with PVP and SDS. Chem Pharm Bull. 2003;51:171-4.

22. Mura P et al. Investigation of the effects of grinding and co-grinding on physicochemical properties of glisentide. J Pharm Biomed Anal. 2002;30:227-37.

23. Mura P et al. The influence of polyvinylpyrrolidone on naproxen complexation with hydroxyl propyl- β-cyclodextrin. Eur J Pharm Sci. 2001;13:187-94.

24. Otsuka M and MatsudaY. Effect of co-grinding with various kinds of surfactants on the dissolution behavior of phenytoin. J Pharm Sci. 1995;84:1434-37.

25. Sugimoto M et al. Improvement of dissolution characteristics and bioavailability of poorly water-soluble drugs by novel co-grinding method using water soluble polymer. Int J Pharm. 1998;160:11-9.

26. Yonemochi E et al. Physicochemical properties of amorphous clarithromycin obtained by grinding and spray drying. Eur J Pharm Sci. 1999;7:331-8.

27. Watanabe T et al. Stabilization of amorphous indomethacin by co-grinding in a ternary mixture. Int J Pharm. 2002;241:103-11.

28. Scholer N et al. Atovaquone nanosuspensions show excellent therapeutic effect in a new murine model of reactivated toxoplasmosis. Antimicrob Agents Chemother. 2001;45:1771–1779.

29. Venkatesha T et al. Nanosuspensions: Ideal Approach for the Drug Delivery of Poorly Water Soluble Drugs. Der Pharmacia Lettre. 2011;3(2):203-213.

30. Muller RH, Bohm BHL, Grau J. Nanosuspensions: a formulation approach for poorly soluble and poorly bioavailable drugs. In D. Wise (Ed.) Handbook of pharmaceutical controlled release technology, 2000, 345- 357.

31. Jahnke S. The theory of high-pressure homogenization. In: Muller RH, Benita S, Bohm BHL, Emulsions and nano suspensions for the formulation of poorly soluble drugs, Medpharm Scientific Publishers, Stuttgart, 1998:177–200.

32. Kipp JE et al. Microprecipitation method for preparing submicron suspensions. US Patent 6,607,784 2003.

33. Zili Z, Sfar S and Fessi H. Preparation and characterization of poly-ε- carprolactone nanoparticles containing griseofulvin. Int J Pharm. 2005;294:261-7.

34. Trotta M et al. Emulsions containing partially water-miscible solvents for the preparation of dry nanosuspensions. J Control Rel. 2001;76:119-28.

35. Zhang X, Xia Q and Gu N. Preparation of all-trans retinoic acid nanosuspensions using a modified precipitation method. Drug Dev Ind Pharm. 2006;32:857-63.

36. Bodmeier R, McGinity JM. Solvent selection in the preparation of poly (DL-lactide) microspheres prepared by solvent evaporation method. Int J Pharm. 1998;43:179–186.

37. Sah H. Microencapsulation technique using ethyl acetate as a dispersed solvent: effects on its extraction rate on the characteristics of PLGA microspheres. J Control Rel. 1997;47:233–245.

38. Sah H. Ethyl formate–alternative dispersed solvent useful in preparing PLGA microspheres. Int J Pharm. 2000;195:103–113.

39. Trotta M, Gallarate M, Pattarino F, Morel S, Emulsions containing partially water miscible solvents for the preparation of drug nanosuspensions, J Control Rel. 2001;76:119–128.

40. Eccleston GM, Microemulsions. In: Swarbrick S, Boylan CJ, (eds) Encyclopedia of pharmaceutical technology, Vol.9, Marcel Dekker, New York. 1992:375–421.

41. Gasco MR, Solid lipid nanospheres form warm micro-emulsions, Pharm Technol Eur. 1997;9:32–42.

42. Rades T et al. Effects of formulation variables on characteristics of poly (ethylcyanoacrylates) nanocapsules prepared from w/o micro-emulsions, Int J Pharm. 2002;235:237– 246.

43. Trotta M et al. Preparation of Griseofulvin nanoparticles from water-dilutable microemulsions. Int J Pharm. 2003;254:235–242.

44. Kamble V et al. Nanosuspension a novel drug delivery system. International Journal of Pharma and Bio Sciences. 2010;1(4):352-360.

45. Allen T, Particle Size Measurement, 5th edition. Springer, 2004.

46. Shanthakumar TR et al. Comparative pharmacokinetic data of DRF-4367 using nanosuspension and HP-β-CD formulation. Proceedings of the International Symposium on Advances in Technology and Business Potential of New Drug Delivery Systems, Mumbai. 2004; 5:75.

47. Hunter RJ. Foundations of Colloid Science. 2^nd Edition. Oxford University Press, New York. 2001.

48. Banavath H et al. Nanosuspension: an attempt to enhance bioavailability of poorly soluble drugs International Journal of Pharmaceutical Sciences and Rresearch. 2010;1(9):1-11.

49. Prasanna L, Giddam AK. Nanosuspension Technology: A Review. International Journal of Pharmacy and Pharmaceutical Sciences. 2010;2(4):35-40.

50. Keck CM, Muller RH. Drug nanocrystals of poorly soluble drugs produced by high pressure homogenization. Eur J Pharm Biopharm. 2006;62(1):3–16.

51. Tejal S et al. Nanosuspensions as a drug delivery system: A comprehensive review. Drug Deliv Tech. 2007; 7:42-53.

52. Blunk T et al. Colloidal carriers for intravenous drug targeting: Plasma protein adsorption patterns on surface-modified latex particles evaluated by two-dimensional polyacrylamide gel electrophoresis. Electrophoresis 1993;14:1382–1387.

53. Blunk T et al. Kinetics of plasma protein adsorption on model particles for controlled drug delivery and drug targeting. Eur J Pharm Biopharm. 1996;42:262–268.

54. Luck M et al. Identification of plasma proteins facilitated by enrichment on particulate surfaces: Analysis by two-dimensional electrophoresis and N-terminal micro sequencing. Electrophoresis 1997a;18:2961–2967.

55. Luck M et al. Analysis of plasma protein adsorption on polymeric nanoparticles with different surface characteristics. J Biomed Mater Res. 1997b;1:478–485.

56. Muller RH. Differential opsonization: A new approach for the targeting of colloidal drug carriers. Arch Pharm. 1989;322:700.

57. Wallis KH and Muller RH. Determination of the surface hydrophobicity of colloidal dispersions by mini-hydrophobic interaction chromatography. Pharm Ind. 1993;55:1124–1128.

58. Merisko L et al. Formulation and anti-tumor activity evaluation of nanocrystalline suspensions of poorly soluble anti-cancer drugs. Pharm Res. 1996;13:272–278.

59. Pignatello R et al. Eudragit RS100® nanosuspensions for the ophthalmic controlled delivery of ibuprofen. Eur J Pharm Sci. 2002;16,53-61.

60. Kassem MA et al. Nanosuspension as an opthamic delivery system for certain glucocorticoid drugs. Int J Pharm. 2007;340,126-33.

61. Pignatello R et al. Flurbiprofen-loaded acrylate polymer nanosuspensions for ophthalmic application. Biomaterials. 2002a;23:3247–3255.

62. Bucolo C et al. Enhanced ocular anti-inflammatory activity of ibuprofen carried by an Eudragit RS 100 nanoparticle suspension. Ophthalmic Res. 2002;34:319–323.

63. Pignatello R et al. Eudragit RS100 nanosuspensions for the ophthalmic controlled delivery of ibuprofen. Eur. J. Pharm. Sci. 2002b;16:53–61.

64. Pignatello R, Bucolo C, Puglisi G. Ocular tolerability of Eudragit RS 100 and RL 100 nanosuspensions as carrier for ophthalmic controlled delivery. J Pharm Sci. 2002c;91:2636–2641.

65. Muller RH, Jacobs C. Production and characterization of a budesonide nanosuspension for pulmonary administration. Pharm Res. 2002b;19:189–194.

66. Ponchel M et al. Mucoadhesion of colloidal particulate systems in the gastrointestinal tract. Eur J Pharm Biopharm. 1997;4:25–31.

67. Francesco L et al. Diclofenac nanosuspensions. Influence of preparation procedure and crystal form on drug dissolution behavior. Int J Pharm 2009;373:124–132.

68. Hanafy A et al. Pharmacokinetic evaluation of oral fenofibrate nanosuspension and SLN in comparison to conventional suspension of micronized drug. Adv Drug Del Rev. 2007;59(6):419‐426.

69. Kayser O et al. Formulation of amphotericin B as nanosuspension for oral administration. Int J Pharm. 2003;254:73–75.

70. Müller RH and Jacobs C. Buparvaquone mucoadhesive nanosuspension: preparation, optimization and long-term stability. Int J Pharm. 2002;237:151-61.

71. Kayser O. A new approach for targeting to Cryptosporidium parvum using mucoadhesive nanosuspensions: research and applications. Int J Pharm. 2001;214:83-5.

72. Kohno S et al. Amphotericin B encapsulated in polyethylene glycol immunoliposomes for infectious diseases. Adv Drug Del Rev. 1997;24:325-9.

73. Kayser O et al. The impact of Nanobiotechnology on the development of new drug delivery systems. Current Pharm Biotech. 2005;6:3-5.

74. Müller RH, Böhm BHL and Grau MJ. Nanosuspensions-Formulations for poorly soluble drugs with poor bioavailability /2nd communication: Stability, biopharmaceutical aspects, possible drug forms and registration aspects. Pharm Ind. 1999;61:175-8.

75. Shim J et al. Transdermal delivery of mixnoxidil with block copolymer nanoparticles. J Control Rel. 2004;97:477-84.

76. Kohli AK and Alpar HO. Potential use of nanoparticles for transcutaneous vaccine delivery: Effect of particle size and charge. Int J Pharm. 2004;275:13-7.

77. Yamaguchi Y et al. Successful treatment of photo-damaged skin of nano-scale at RA particles using a novel transdermal delivery. J Control Rel. 2005;104:29-40.

78. ChenX et al. Ketoprofen nanoparticle gels formed by evaporative precipitation into aqueous solution. AIChE J. 2006;52:2428-35.

79. Eerdenbrugh BV, Mooter GV and Augustijns P. Top-down production of drug nanocrystals: Nanosuspension stabilization, miniaturation and transformation into solid products. Int J Pharm. 2008;364: 64-75

80. Mauludin R. Nanosuspension of poorly soluble drugs for oral administration. Ph D Thesis. Free University of Berlin.

Received on 28.10.2012 Accepted on 12.11.2012

Asian J. Pharm. Tech. 2(4): Oct. - Dec. 2012; Page 157-168

Excipients	Function	Examples
Stabilizers	Wet the drug particles thoroughly, prevent Ostwald’s ripening and agglomeration of nanosuspensions, providing steric or ionic barrier	Lecithins, Poloxamers, Polysorbate, Cellulosics, Povidones
Co-surfactants	Influence phase behavior when micro emulsions are used to formulate nanosuspensions	Bile salts, Dipotassium Glycerrhizinate, Transcutol, Glycofurol, Ethanol, Isopropanol,
Organic solvent	Pharmaceutically acceptable less hazardous solvent for preparation of formulation.	Methanol, Ethanol, Chloroform, Isopropanol, Ethyl acetate, Ethyl formate, Butyl lactate, Triacetin, Propylene carbonate, Benzyl alcohol.
Other additives	According to the requirement of the route of administration or the properties of the drug moiety	Buffers, Salts, Polyols, Osmogens, Cryoprotectant etc.

Product	Drug Compound	Company	Manufacturing Technique	Technology
RAPAMUNE^®	Sirolimus	Wyeth	MM	Elan Nanocrystals^®
EMEND^®	Aprepitant	Merck	MM	Elan Nanocrystals^®
TriCor^®	Fenofibrate	Abbott	MM	Elan Nanocrystals^®
MEGACE^®ES	Megestrol Acetate	PAR Pharmaceutical	MM	Elan Nanocrystals^®
Avinza^®	Morphine Sulphate	King Pharmaceutical	MM	Elan Nanocrystals^®
Focalin^®XR	Dexmethylphenidate Hydrochloride	Novartis	MM	Elan Nanocrystals^®
Ritalin^®LA	Methylphenidate Hydrochloride	Novartis	MM	Elan Nanocrystals^®
Zanaflex Capsules^TM	Tizanidine Hydrochloride	Acorda	MM	Elan Nanocrystals^®
Triglide^TM	Fenofibrate	First Horizon Pharmaceutical	HPH	Skye Pharma IDD^{® -}P Technology