INTRODUCTION:

A large proportion of new chemical entities coming from drug discovery are water insoluble, and therefore poorly bioavailable. Nanosuspension technology offers a novel solution for the poorly soluble drugs.

Nanosuspensions are sub-micron colloidal dispersion of pure particles of drug which are stabilized by surfactants. They can also be defined as a biphasic system consisting of pure drug particles dispersed in an aqueous vehicle in which the diameter of the suspended particle is less than 1 μm in size¹.

Nanosuspensions differ from nanoparticles, which are polymeric colloidal carriers of the drug and solid lipid nanoparticles, which are lipidic carrier of drug. Nanosuspension technology is applied to drugs which have high crystal energy i.e. high melting point, reduces the solubility of drug substances. By this technology the drug is maintained in required crystalline state with reduced particle size and this causes 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 increase in the dissolution velocity and saturation solubility because of the vapor pressure effect. Nanosuspension provides chemically and physically stable product^2-3.

Nanosuspensions can be used to enhance the solubility of drugs that are poorly soluble in aqueous as well as lipid media. As a result, the rate of flooding of the active compound increases and the maximum plasma level is reached faster (e.g., oral or intravenous (i.v.) administration of the nanosuspension). This is one of the unique advantages that it has over other approaches for enhancing solubility. It is useful for molecules with poor solubility, poor permeability or both, which possess a significant challenge for the formulators. The reduced particle size renders the possibility of intravenous administration of poorly soluble drugs without blockade of the blood capillaries. The nanosuspensions can be lyophilized or spray dried and the particles of a nanosuspension can also be incorporated in a solid matrix. Apart from this, it has all other advantages of a liquid dosage form over the solid dosage forms. The present review is focused on various methods of preparing nanosuspensions, critical parameters to be characterized and the application of nanosuspension formulations^4-5.

CHARACTERIZATION OF NANOSUSPENSIONS:

The various essential parameters to be characterized for nanosuspensions include:

o Size and size distribution

o Particle charge (zeta potential)

o Crystal morphology

o Dissolution velocity and saturation solubility

o Internal structure of nanosuspensions

o Physical long-term stability

For surface-modified nanosuspensions, a number of additional parameters have to be investigated to obtain a complete picture, especially with relevance for the in-vivo behavior:

o Adhesion properties (in case of mucoadhesive particles)

o Surface hydrophilicity / hydrophobicity

o Interaction with body proteins^6-7.

Particle size and size distribution:

Particle size distribution determines the physiochemical behavior of the formulation, such as saturation solubility, dissolution velocity, physical stability, etc. The particle size distribution can be determined by photon correlation spectroscopy (PCS), laser diffraction (LD) and coulter counter multisizer. The PCS method can measure particles in the size range of 3 nm to 3 μm and the LD method has a measuring range of 0.05-80 μm. The coulter counter multisizer gives the absolute number of particles, in contrast to the LD method, which gives only a relative size distribution. For i.v. use, particles should be less than 5 μm, considering that the smallest size of the capillaries is 5-6 μm and hence a higher particle size can lead to capillary blockade and embolism⁸.

Particle charge (Zeta potential):

Zeta potential is an indication of the stability of the suspension. For a stable suspension stabilized only by electrostatic repulsion, a minimum zeta potential of ±30 mV is required whereas in case of a combined electrostatic and steric stabilizer, a zeta potential of ±20 mV would be sufficient⁹.

Crystal morphology:

To characterize the polymorphic changes due to the impact of high-pressure homogenization in the crystalline structure of the drug, techniques like X-ray diffraction analysis in combination with differential scanning calorimetry or differential thermal analysis can be utilized. Nanosuspensions may undergo a change in the crystalline structure, which may be to an amorphous form or to other polymorphic forms because of high-pressure homogenization¹⁰.

Dissolution velocity and saturation solubility:

Nanosuspensions have an important advantage over other techniques, that it can increase the dissolution velocity as well as the saturation solubility. These two parameters should be determined in various physiological solutions. The assessment of saturation solubility and dissolution velocity helps in determining the in vitro behavior of the formulation. An increase in the dissolution pressure as well as dissolution velocity with a reduction in the particle size to the nanometer range.Size reduction leads to an increase in the dissolution pressure. An increase in solubility that occurs with relatively low particle size reduction may be mainly due to a change in the surface tension leading to increased saturation solubility and the energy introduced during the particle size reduction process leads to an increase in the surface tension and an associated increase in the dissolution pressure¹¹.

Dissolution of drug is increased due to increase in the surface area of the drug particles from micrometers to the nanometer size. According to Noyes-Whitney equation (equation no.1) dissolution velocity increase due to increase in the surface area from micron size to nanometer size.

Dx/dt = [(D x A/ h] [Cs-X/V] --------------- (1)

where D is diffusion coefficient, A is surface area of particle, dx/dt is the dissolution velocity, V is volume of dissolution medium and X is the concentration in surrounding liquid.

According to the Prandtl equation, for small particles the diffusional distance h decreases with decreasing particle size. The decrease in h increases Cs (saturation solubility) and leads to an increase in gradient (Cs-Cx)/h and thus to an increase in the dissolution velocity. According to Ostwald-Freunddlich equation, decrease in particle size below 1μm increases the intrinsic solubility or saturation solubility¹².

Internal structure of nanosuspensions:

The high-energy input during disintegration process causes structural changes inside the drug particles. When the drug particles are exposed to high-pressure homogenization particles are transformed from crystalline state to amorphous state. The change in state depends upon the hardness of drug, number of homogenization cycles chemical nature of drug and power density applied by homogeniser¹³.

Physical long-term stability:

Dispersed systems show physical instability due to Ostwald ripening which is responsible for crystal growth to form microparticles. Ostwald ripening is defined as the tendency for a particle dispersion to grow in diameter over time; by a process in which the smaller particles dissolve because of their higher solubility, with subsequent crystallization onto larger particles to form microparticles. Ostwald ripening is caused due to the difference in dissolution velocity/ saturation solubility of small and large particles. In nanosuspensions all particles are of uniform size hence there is little difference between saturation solubility of drug particles. The difference in the concentration of the saturated solutions around a small and large particle leads to the diffusion of dissolved drug from the outer area of the large particles. As a result the solution around large particles is supersaturated leading to the drug crystallization and growth of the large crystals or microparticles. Ostwald ripening is totally absent in nanosuspensions due to uniform particle size, which is also responsible for long-term physical stability of nanosuspensions^14-15.

FORMULATION CONSIDERATIONS:

Stabilizer:

Stabilizer plays an important role in the formulation of nanosuspensions. In the absence of an appropriate stabilizer, the high surface energy of nano-sized particles can induce agglomeration or aggregation of the drug crystals. The main functions of a stabilizer are to wet the drug particles thoroughly, to prevent ostwald’s ripening and agglomeration of nanosuspensions in order to yield a physically stable formulation by providing steric or ionic barriers. The type and amount of stabilizer has a pronounced effect on the physical stability and in-vivo behavior of nanosuspensions. In some cases, a mixture of stabilizers is required to obtain a stable nanosuspension. The drug-to-stabilizer ratio in the formulation may vary from 1:20 to 20:1 and should be investigated for a specific case. Stabilizers that have been explored so far include cellulosics, poloxamers, polysorbates, lecithins and povidones. Lecithin is the stabilizer of choice if one intends to develop a parenterally acceptable and autoclavable nanosuspensions^16-18.

Organic solvents:

Organic solvents may be required in the formulation of nanosuspensions if they are to be prepared using an emulsion or microemulsion as a template. As these techniques are still in their infancy, elaborate information on formulation considerations is not available. The acceptability of the organic solvents in the pharmaceutical arena, their toxicity potential and the ease of their removal from the formulation need to be considered when formulating nanosuspensions using emulsions or microemulsions as templates. The pharmaceutically acceptable and less hazardous water-miscible solvents, such as ethanol and isopropanol, and partially water-miscible solvents, such as ethyl acetate, ethyl formate, butyl lactate, triacetin, propylene carbonate and benzyl alcohol, are preferred in the formulation over the conventional hazardous solvents, such as dichloromethane. Additionally, partially water miscible organic solvents can be used as the internal phase of the microemulsion when the nanosuspensions are to be produced using a microemulsion as a template¹⁶.

Co-surfactants:

The choice of co-surfactant is critical when using microemulsions to formulate nanosuspensions. Since co-surfactants can greatly influence phase behavior, the effect of co-surfactant on uptake of the internal phase for selected microemulsion composition and on drug loading should be investigated. Although the literature describes the use of bile salts and dipotassium glycerrhizinate as co-surfactants, various solubilizers, such as transcutol, glycofurol, ethanol and isopropanol, can be safely used as co-surfactants in the formulation of microemulsions¹⁷.

Other additives:

Nanosuspensions may contain additives such as buffers, salts, polyols, osmogent and cryoprotectant, depending on either the route of administration or the properties of the drug moiety^{18, 35}.

METHODS OF PREPARATION:

Mainly there are two methods for preparation of nanosuspensions. The conventional methods of precipitation (hydrosols) are called 'Bottom Up Technology'. In Bottom Up Technology, the drug is dissolved in a solvent, which is then added to non-solvent to precipitate the crystals. The basic advantage of precipitation technique is the use of simple and low cost equipments. The basic challenge of this technique is that during the precipitation procedure, the growth of the drug crystals needs to be controlled by addition of surfactant to avoid formation of micro particles. The limitation of this precipitation technique is that the drug needs to be soluble in at least one solvent and this solvent needs to be miscible with the non-solvent. Moreover, precipitation technique is not applicable to drugs which are simultaneously poorly soluble in aqueous and non-aqueous media.

The 'Top Down Technologies' are the disintegration methods and are preferred over the precipitation methods. The 'Top Down Technologies' include: Media milling (nanocrystals), high-pressure homogenization in water (dissocubes), high-pressure homogenization in non-aqueous media (nanopure) and combination of precipitation and high-pressure homogenization (nanoedege). Few other techniques used for preparing nanosuspensions are emulsion as templates, micro emulsion as templates etc.

The principle techniques used in recent years for preparing nanosuspensions can be classified into four basic methods: (1) Wet milling, (2) Homogenization, (3) Emulsification-Solvent evaporation and (4) Supercritical fluid method^19-22.

Wet milling:

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 reduction²⁸.

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 mills (PM100 and PM200; Retsch GmbH and Co., KG, Haan, Germany) is one example of an equipment that can be used to achieve a grind size below 0.1 μm. A nanosuspension of Zn-Insulin with a mean particle size of 150 nm was prepared using the wet milling technique. The major drawbacks of this technology include the erosion of balls/pearls that can leave residues as contaminants in the final product, degradation of the thermolabile drugs due to heat generated during the process and presence of relatively high proportions of particles ≥5 μm^19,
28.

Homogenization:
Dissocubes:
Homogenization involves the forcing of the suspension under pressure through a valve having a narrow aperture.In this case, the suspension of the drug is made to pass through a small orifice those 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 implode 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.

An aqueous suspension of atovaquone was dispersed using an Ultra turrax T25, IKA-Werke GmbH and Co. KG, Staufen, Germany and was further homogenized in a Gaulin Micron Lab 40 high-pressure homogenizer. After subjecting to pressures of 1.5 x 10⁷ (two cycles), 5 x 10⁷ (two cycles) and 1.5 x 10⁸ (20 cycles) Pa, a nanosuspension of atovaquone with a mean diameter of 279 ± 7 nm and mean polydispersity index of 0.18 ± 0.001 was obtained. 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. The major advantage of high-pressure homogenization over media milling is that it can be used for both diluted as well as concentrated suspensions and also allows aseptic production^{20, 32}.

Nanopure:
Nanopure is suspensions homogenized in water-free media or water mixtures.In the Dissocubes technology, the cavitation is the determining factor of the process. But, in contrast to water, oils and oily fatty acids have very low vapour pressure and a high boiling point. Hence, the drop of static pressure will not be sufficient enough to initiate cavitation. Patents covering disintegration of polymeric material by high-pressure homogenization mention that higher temperatures of about 80 ⁰C promoted disintegration, which cannot be used for thermolabile compounds. In nanopure technology, the drug suspensions in the non-aqueous media were homogenized at 0 ⁰C or even below the freezing point and hence are called "deep-freeze" homogenization. The results obtained were comparable to Dissocubes and hence can be used effectively for thermolabile substances at milder conditions^{18, 21}.

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. 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^{10, 22}.

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). Dearn prepared nanosuspensions of atovaquone using the microfluidization process.The major disadvantage of this technique is the high number of passes through the microfluidizer and that the product obtained contains a relatively larger fraction of microparticles^22-23.

Emulsification-solvent evaporation technique:

This technique involves preparing a solution of drug followed by its emulsification in another liquid that is a non-solvent for the drug. Evaporation of the solvent leads to precipitation of the drug. Crystal growth and particle aggregation can be controlled by creating high shear forces using a high-speed stirrer^{15, 23}.

Hydrosol method:

This is similar to the emulsification- solvent evaporation method. The only difference between the two methods is that the drug solvent is miscible with the drug anti-solvent.Higher shear force prevents crystal growth and Ostwald ripening and ensures that the precipitates remain smaller in size^{23, 27}.

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^23-24.

APPLICATIONS OF NANOSUSPENSIONS:

Bioavailability enhancement:

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 hepatoprotective 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). The ocular anti-inflammatory activity of Ibuprofen-Eudragit RS100 nanosuspensions was greatly improved when compared with an aqueous solution of Ibuprofen lysinate. Further, the aqueous humor drug concentration was significantly higher in groups treated with Ibuprofen-Eudragit RS when compared with the Ibuprofen-treated group^25-26.

Parenteral administration:

The parenteral route of administration provides a quick onset of action, rapid targeting and reduced dosage of the drug. It is the preferred route for drugs undergoing first-pass metabolism and those that are not absorbed in the GIT or degraded in the GIT. One of the important applications of nanosuspension technology is the formulation of intravenously administered products. Intravenous administration results in several advantages, such as administration of poorly soluble drugs without using a higher concentration of toxic co-solvents, improving the therapeutic effect of the drug available as conventional oral formulations and targeting the drug to macrophages and the pathogenic microorganisms residing in the macrophages.
Injectable nanosuspensions of poorly soluble drug tarazepide have been prepared to overcome the limited success achieved using conventional solubilization techniques, such as use of surfactants, cyclodextrins, etc., to improve bioavailability.A stable intravenously injectable formulation of omeprazole has been prepared to prevent the degradation of orally administered omeprazole^27-28.

Pulmonary administration:

Aqueous nanosuspensions can be nebulized using mechanical or ultrasonic nebulizers for lung delivery. Because of their small size, it is likely that in each aerosol droplet at least one drug particle is contained, leading to a more uniform distribution of the drug in lungs. They also increase adhesiveness and thus cause a prolonged residence time. Budenoside drug nanoparticles were successfully nebulized using an ultrasonic nebulizer.

Other applications include ocular delivery of the drugs as nanosuspensions to provide a sustained release of drug. Eudragit retard nanosuspensions of cloricromene for ocular delivery.They observed that the drug showed a higher availability in rabbit aqueous humor and the formulation appeared to offer a promising means of improving the shelf-life and the bioavailability of this drug after ophthalmic application^29-30.

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 governs 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. This nanosuspension is successfully prepared using Eudragit RS100 by a quasi-emulsion and solvent diffusion method. Nanosuspensions of glucocorticoid drugs; hydrocortisone, prednisolone and dexamethasone, enhanced the rate of drug absorption and increase the duration of drug action^31-33.

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. 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^34-35.

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^36-38.

CONCLUSION:

Nanosuspensions of pure drug offer a method to formulate poorly soluble drugs and enhance the bioavailability of several drugs. It has many formulations and therapeutic advantages, such as simple method of preparation, less requirement of excipients, increased dissolution velocity and saturation solubility, improved adhesion, increases the bioavailability leading to a decrease in the dose and fast-fed variability and ease of large-scale manufacturing. A nanosuspension not only solves the problems of poor solubility and bioavailability but also alters the pharmacokinetics of drug and thus improves drug safety and efficacy. Nanosuspensions can be formulated for various routes of administration, such as oral, parenteral, ocular, topical and pulmonary routes. Compared with injectable solution dosage forms, nanosuspensions may exhibit reduced peak but prolonged drug levels, which can be advantageous for certain therapeutic indications.

REFERENCES:

1. Elaine Merisko-Liversidge, Gary G. Liversidge, Eugene R.Cooper. Nanosizing: a formulation approach for poorly water-soluble compounds. Eur.J.Pharm. Sci.2003; 18:113-120.

2. Mersiko-Liversidge E, MGurk SL, Liversidge GG. Insulin nanoparticles: A novel formulation approach for poorly water soluble Zn-Insulin. Pharm Res 2004; 21:1545-53.

3. S.M.Wong,I.W.Kellaway, S.Murdan. Enhancement of the dissolution rate and oral absorption of a poorly water soluble drug by formation of surfactant-containing microparticles. Int.J.Pharm.2006; 317:61-68.

4. True L.Rogers, Ian B. Gillespie, James E. Hitt, Kevin L.Fransen, Clindy A. Crowl,Chritoper J.Tucker, et.al. Development and characterization of a scalable controlled precipitation process to enhance the dissolution of poorly soluble drugs. Pharm. Res.2004; 21(11): 879-886

5. Moschwitzer J, Achleitner G, Promp.er H, Muller RH. Development of an intravenously injectable chemically stable aqueous omeprazole formulation using nanosuspension technology. Eur J Pharm Biopharm 2004; 58:615-9.

6. Dubey R. Pure drug nanosuspensions-Impact of nanosuspension technology on drug discovery and development. Drug Del Tech 2006; 6:65-71.

7. Shobha Rani, R.Hiremath and Hota A. Nanoparticles as drug delivery systems. Ind.J.Pharm.Sci.1999; 61(2): 69-75.

8. SchOler N, Krause K, Kayser O, Mόller RH, Borner K, Hahn H, et al. Atovaquone nanosuspensions show excellent therapeutic effect in a new murine model of reactivated toxoplamosis. Antimicrob Agents Chemother 2001; 45:1771-9.

9. R.H.Muller, B.H.L.Bohm and .J.Grau. Nanosuspensions : a formulation approach for poorly soluble and poorly bioavailable drugs. In D.Wise (Ed.) Handbook of pharmaceutical controlled release technology.2000; 345-357.

10. K.P. Krause, O. Kayser, K. Mader, R. Gust, R.H. Muller. Heavy metal intamination of nanosuspensions produced by high-pressure homogenisation. Int J. Pharm.2000; 196:169–172.

11. K. Peters, R.H. M üller, Nanosuspensions for the oral application of poorly soluble drugs, in: Proceeding European Symposium on Formulation of Poorly-available Drugs for Oral Administration, APGI, Paris, 1996, pp. 330–333.

12. Young TJ, Mawson S, Johnston KP, Henriska IB, Pace GW, Mishra AK. Rapid expansion from supercritical to aqueous solution to produce submicron suspension of water insoluble drugs. Biotechnol Prog 2000; 16:402-7.

13. Pignatello R, Ricupero N, Bucolo C, Maugeri F, Maltese A, Puglisi G. Preparation and characterization of Eudragit retard nanosuspensions for the ocular delivery of cloricromene. AAPS Pharmscitech 2006; 7:E27.

14. M.J. Grau, R.H. M üller, Increase of dissolution velocity and solubility of poorly soluble drugs by formulation as nanosuspension, in: Proceedings 2nd World Meeting APGI/APV, Paris, 1998, pp. 623–624.

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

16. Shah T, Patel D, Hirani J, Amin AF. Nanosuspensions as drug delivery systems-A comprehensice review. Drug Del Tech 2007; 7:42-53.

17. Rabinow BE, White RD, Glosson J, Sun CS, Papadopoulos P. Enhanced efficacy of NANOEDGE itraconazole nanosuspension in an immunosuppressed rat model infected with an itraconazole-resistant C. albicans strain. Proceedings of American Association of Pharmaceutical Scientists (AAPS) Annual Meeting and Exposition, Abstract #R6184, Salt Lake City, Utah, 2003.

18. Rabinow BE. Nanosuspensions in drug delivery. Nature Reviews Drug Discovery 3:785-796, 2004.

19. M. Luck, B.R. Paulke,W. Schröder, T. Blunk, R.H. M üller, Analysis of plasma protein adsorption on polymeric nanoparticles with different surface characteristics, J. Biomed. Mater. Res. 1 (1997) 478–485.

20. K Peters, S Leitzke, JE Diederichs, K Borner, H Hahn, RH Müller and S Ehlers.Preparation of a clofazamine nanosuspension for intravenous use and evaluation of its therapeutic efficacy in murine Mycobacterium avium infection. J. Antimicrob. Chemother. 2000; 45, 77-83.

21. M Trotta, M Gallarete, F Pattarino and S Morel. Emulsions containing partially water-miscible solvents for the preparation of dry nanosuspensions. J. Control. Rel. 2001; 76, 119-28.

22. P Kocbek, S Baumgartner and J Kristl. Preparation and evaluation of nanosuspensions for enhancing the dissolution of poorly soluble drug. Int. J. Pharm. 2006; 312, 179-86.

23. O Kayser. Nanosuspensions for the formulation of aphidicolin to improve drug targeting effects against Leishmania infected macrophages. Int. J. Pharm. 2000; 196, 253-6.

24. N Hernández-Trejo, O Kayser, H Steckel and RH Müller. Characterization of nebulized bupravaquone nanosuspensions-Effect of nebulization technology. J. Drug. Target. 2005; 13, 499-507.

25. Barret ER, Nanosuspensions in drug delivery, Nat. rev, 3, 2004, 785-796.

26. Patravale VB, Abhijit AD, and Kulkarni RM, Nanosuspensions: a promising drug delivery strategy. J. Pharm. Pharmcol, 56, 2004, 827-840.

27. Pignatello R, Bucolo C, Ferrara P, Maltese A, Puleo A, Puglisi G , Eudragit RS100 nanosuspensions for the ophthalmic controlled delivery of ibuprofen, Eur. J. Pharm. Sci, 16,2002, 53–61.

28. Grau, M. J., Kayser, O., Mu ller, R. H. (2000) Nanosuspensions of poorly soluble drugs – reproducibility of small-scale production. Int. J. Pharm. 196: 155–157.

29. Jahnke, S. (1998) The theory of high-pressure homogenization. In: Mu ller, R. H., Benita, S., Bo¨ hm, B. H. L. (eds) Emulsions and nanosuspensions for the formulation of poorly soluble drugs. Medpharm Scientific Publishers, Stuttgart, pp 177–200.

30. Kim, C. K., Gao Gao, Z., Choia, H. G., Shin, H. J., Park, K. M., Lim, S. J., Hwang, K. J. (1998) Physicochemical characterization and evaluation of a microemulsion system for oral delivery of cyclosporin A. Int. J. Pharm. 161: 75–86.

31. Krause, K., Mu¨ ller, R. H. (2001) Production and characterization of highly concentrated nanosuspensions by high pressure homogenization. Int. J. Pharm. 214: 21–24.

32. Muller, R. H., Bohm, B. H. L. (1998) Nanosuspensions. In: Muller, R. H., Benita, S., Bohm, B. H. L. (eds) Emulsions and nanosuspensions for the formulation of poorly soluble drugs. Medpharm Scientific Publishers, Stuttgart, pp 149–174.

33. Arun kumar et al, Nanosuspension technology and its applications in drug delivery; Asian j Pharm; 2009; 168-173.

34. Rao G.C.S., Satish Kumar M., Mathivnan N.and Rao .E.B. Advances in nanoparticulate drug delivery systems. Ind. Drugs. 2004; 41(7):389-395.

35. Jan Moschwitzer,Georgr Achleitner, Herberk Pomper, Rainer H.Muller. Development of an intraveneously injectable chemically stable aqueous omeprazole formulation using nanosuspension. Eur. J. Pharm. Biopharm.2004;58:615-619.

36. Benjamin C-Y Lu, Dingan Zang, Wei Sheng. Solubility enhancement in supercritical fluids. Pure and Appl Chem 1990; 62:2277-85.

37. Radtke M. Nanopure: pure drug nanoparticles for the formulation of poorly soluble drugs. New Drugs 2001; 3:62-8.

38. Jan Moschwitzer,Georgr Achleitner, Herberk Pomper, Rainer H.Muller. Development of an intraveneously injectable chemically stable aqueous omeprazole formulation using nanosuspension. Eur. J. Pharm. Biopharm. 2004; 58:615-619.

Received on 22.09.2010 Modified on 12.10.2010

Research J. Pharm. and Tech. 4(4): April 2011; Page 515-520