1. INTRODUCTION:

Nanosuspension is submicron colloidal dispersion of drug particles. A pharmaceutical nanosuspension is defined as very finely colloid¹, biphasic², dispersed solid drug particles in an aqueous vehicle, size below 1 μm stabilized by surfactants³ and polymers⁴ prepared by suitable methods for drug delivery⁵applications.

Nanosuspension have revealed their potential to solve the problem⁶ associated with the delivery of poorly water soluble and poorly water and lipid soluble drugs.

It enhances the absorption and bioavailability and help to reduce the dose of conventional oral dosage forms. Drug particle size reduction leads to an increase in the surface area and consequently the rate of dissolution as described by the Noyes whitney⁷ equation. In addition an increase in saturation solubility is postulated by the particle size reduction due to an increased dissolution pressure explained by the ostwald-freundlich equation⁸. Depending upon the production technique applied changes in the crystalline structure of the drug particle may occur. An increase amount of amorphous drug fraction could induce higher saturation solubility. Nanosuspension not only solves the problem of poor solubility and poor bioavailability but also alters the pharmacokinetics of the drug and improves the drug safety and efficacy.⁹

Criteria for selection of drug for nanosuspensions:

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

a) Water insoluble but which are soluble in oil

b) (high logP) or API are insoluble in both water and oils

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

d) API with very large dose.

ADVANTAGES:

a. Most cost effective.

b. Useful for poorly soluble drugs.

c. Physically more stable than liposomes.

d. Provide ease of manufacture and scale up for large scale production.

e. Rapid dissolution and tissue targeting.

f. Reduction in tissue irritation.

g. Higher bioavailability in ocular and inhalational drug delivery

Table1: Advantages of Nano suspensions over conventional formulations.

Route of administration	Disadvantages of conventional formulations	Benefits of Nanosuspensions
Oral	Slow onset of action/ poor absorption	Rapid onset of action/ improved solubility so improved bioavailability
Ocular	Lachrymal wash off/ low bioavailability	Higher bioavailability/ dose consistency
Intravenous	Poor dissolution/ nonspecific action	Rapid dissolution/ tissue targeting
Intramuscular	Low patient compliance due to pain	Reduced tissue irritation
Inhalations	Low bioavailability due to low solubility	Rapid dissolution/ high bioavailability/ dose regulation

Preparation of nanosuspension:

Technically preparations of nanosuspensions are simpler alternative than liposome’s and other conventional colloidal drug carriers but reported to be more cost effective. It is particularly for poorly soluble drugs and to yield a physically more stable product. For manufacturing nanosuspensions there are two converse methods, “Top-down process technology” and “Bottom-up process technology”. The top - down process follows disintegration approach from large particles, micro particles to Nanosized particles.^10,11

Examples are;

a) High pressure homogenization

b) Nanopure

c) Media milling (Nanocrystals).

d) Bottom-up process is an assembly method forms nanoparticles from molecules.

Examples include;

1. Solvent-Antisolvent method

2. Super critical fluid process

3. Emulsification Solvent evaporation technique

4. Lipid emulsion/Micro-emulsion template.

The principle techniques used in recent years for preparing nanosuspensions are:

A. High pressure homogenization:

B. Milling techniques

C. Emulsification-solvent

D. Precipitation

E. Supercritical fluid process

F. Melt emulsification method

G. Lipid emulsion/micro emulsion template:

H. Solvent evaporation:

A. High pressure homogenization:

In the high pressure homogenization method, the suspension of a drug and surfactant is forced under pressure through a nanosized aperture valve of a high pressure homogenizer. The principle of this method is based on cavitation forces of drug particles in the aqueous phase. These forces are sufficiently high to convert the drug micro particles into nanoparticles.

Homogenization in aqueous media (Disso. cubes)

This technology was developed by R.H.Muller using a piston-gap type high pressure homogenizer in 1999. In this method, the suspension containing a drug and surfactant is forced under pressure through a Nanosized aperture valve of a high pressure homogenizer.

Principle:

This method is based on cavitation principle. The dispersion present in 3cm diameter cylinder is suddenly passed through a very narrow gap of 25μm.According to Bernoulli’s law the flow volume of liquid in a closed system per cross section is constant. It leads to increase in dynamic pressure and decrease of static pressure below the boiling point of water at room temperature due to reduction in diameter from 3cm to 25μm.Then water starts boiling at room temperature and forms gas bubbles, which implode when the suspension leaves the gap (called cavitation) and normal air pressure is reached. The particles cavitation forces are sufficiently high to convert the drug micro particles into nanoparticles.

Homogenization in non-aqueous media

i. Nanopure:

Nanopure is suspensions homogenized in water-free media or water mixtures like PEG 400, PEG 1000 etc. The homogenization can be done at room temperature, 0°C and below freezing point (-200°C), hence it is known as “deep freeze” homogenization.

ii. Nanoedge:

Nanoedge technology is the combination of both precipitation and homogenization. The basic principle is same as that of precipitation and homogenization. The major disadvantage of precipitation technique such as crystal growth and long term stability can be overcome by using the Nanoedge technology. Particles of smaller size and better stability in short time can be achieved.

iii. Nanojet:

It is also called as opposite stream technology, uses a chamber where a stream of suspension is divided into two or more parts, which colloid with each other at high pressure, due to the high shear forces produced during the process particle size is reduced.

Figure 1: Schematic Cartoon of the High-Pressure Homogenization Process

Media Milling:

Figure 2: Media milling process

i. Media milling:

Media milling is a further technique used to prepare Nanosuspensions. Nanocrystal is a patent protected technology US 5,145,684 developed by Élan Nanosystems. 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 micro particulate 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.

ii. Dry co-grinding:

Recently, Nanosuspensions can be obtained by dry milling techniques. Dry 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.

C. Emulsification-solvent

Evaporation Technique:

This technique involves preparing a solution of drug followed by its emulsification in another liquid that is a nonsolvent 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.

D. Precipitation

Precipitation technique

(solvent-anti-solvent method):

Precipitation method has been used for long years for the preparation of submicron particles. It is mainly used for the poorly soluble drugs. First drug is dissolved in a suitable solvent.This solution is then mixed with a miscible antisolvent system in the presence of surfactants. Rapid addition of drug solution in to the antisolvent leads to the sudden supersaturation of drug in the mixed solution forms ultrafine drug solids. Precipitation method involves two phases - nuclei formation & crystal growth. When preparing a stable suspension with the minimum particle size, a high nucleation rate and but low growth rate is necessary. Both rate are depend on temperature. In this technique the drug needs to be soluble in at least one solvent which is miscible with nonsolvent.

E. Supercritical fluid process

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. Young et al. prepared cyclosporine nanoparticles in the size range of 400-700 nm using this process. In the PCA method, the drug solution is atomized into a chamber containing compressed CO 2. 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. Nanoparticles of griseofulvin, a drug with poor solubility, using this method.

F. Melt emulsification method

In this method drug is dispersed in the aqueous solution of stabilizer and heated above the melting point of the drug and homogenized to give an emulsion. During this process, the sample holder was enwrapped with a heating tape fitted with temperature controller and the temperature of emulsion was maintained above the melting point of the drug. The emulsion was then cooled down either slowly to room temperature or on an ice‐bath. The main advantage of melt emulsification technique relative to the solvent diffusion method is total avoidance of organic solvents during the production process. Nano suspension of ibuprofen was prepared by this method. Formulating ibuprofen Nano suspension by melt emulsification method show greater dissolution rate than formulating by solvent diffusion method.

G. Lipid emulsion/micro emulsion template

Another way to produce nanosuspensions is to use an emulsion which is formed by the conventional method using a partially water miscible solvent as the dispersed phase. Nanosuspensions are obtained by just diluting the emulsion. Moreover, micro emulsions as templates can produce nanosuspensions. Micro emulsions are thermodynamically stable and isotropically clear dispersions of two immiscible liquids such as oil and water stabilized by an interfacial film of surfactant and co-surfactant. The drug can be either loaded into the internal phase or the pre-formed micro emulsion can be saturated with the drug by intimate mixing. Suitable dilution of the micro emulsion yields the drug nanosuspension. An example of this technique is the griseofulvin nanosuspension which is prepared by the micro emulsion technique using water, butyl lactate, lecithin and the sodium salt of taurodeoxycholate. The advantages of lipid emulsions as templates for nanosuspension formation are that they easy to produce by controlling the emulsion droplet and easy for scale-up. However, the use of organic solvents affects the environment and large amounts of surfactant or stabilizer are required.

H. Solvent evaporation

In the solvent evaporation method, the solutions of polymer are prepared in volatile solvents and emulsions. The emulsion is converted into a nanoparticle suspension on evaporation of the solvent for the polymer, which is allowed to diffuse through the continuous phase of the emulsion. The particle size was influenced by the concentration of polymer, stabilizer and the speed of homogenizer. In the conventional methods, two main strategies are being used for the formation of emulsions, the preparation of single-emulsions, e.g., oil-in-water (o/w) or double-emulsions, e.g., (water-in-oil)-in-water, (w/o)/w.

Figure 3: Schematic representation of the solvent-evaporation technique

These methods require high-speed homogenization or ultra-sonication, followed by evaporation of the solvent, either by continuous magnetic stirring at room temperature or under reduced pressure. By ultracentrifugation the solidified nanoparticles are collected which was washed with distilled water to remove the additives like surfactants, and then it was lyophilized Single-emulsions, e.g., oil-in-water (o/w) or double-emulsions, e.g., (water-in-oil)-in-water, (w/o)/w¹².

Applications:

Nanosuspensions have wide range of applications especially in the case of low solubility and low bioavailability drugs. They are mentioned below.

1. Oral Drug Delivery:

Figure 4: Nanosuspensions for Oral Drug Delivery

Because of the numerous advantages oral route is the most preferable route for many of the drugs (figure 3) especially in the case of orally administering antibiotics such as atovaquone and bupravaquone. By making it in nanosize, its solubility and bioavailability will increase. The oral administration of naproxen nanoparticles leads to an area under the curve (AUC) (0-24 h) of 97.5 mg-h/l compared with naproxen nanosuspension and naproxen tablets 16. In the case of danazole (gonadotropin inhibitor) nanosuspension has absolute bioavailability of 82.3 and the conventional dispersion only 5.2%¹³.

2. Parentral Drug Delivery:

Nanotechnology is also used in the parenteral drug delivery system (figure 4). The advantage of this method is it need only less amount of toxic co-solvent for poorly soluble drugs. This will uplift the therapeutic effect of the drug compared with the conventional oral formulation and targeting the drug to the macrophages. The drug clofazimine is given as iv, the concentration in the liver, spleen and lungs reached a high level i.e.; greater than minimum inhibitory concentration, for most of the mycobacterium avium strains. Tarazepide is formulated as nanosuspension in order to overcome the use of surfactants and cyclodextrinsto improve the bioavailability¹⁴.

Figure 5: Nanosuspension for Parentral Drug Delivery

1. Pulmonary Drug Delivery:

Figure 6: Nanosuspension for Pulmonary Drug Delivery

In pulmonary drug delivery we are using nanopreperations for the drugs which have poor solubility in pulmonary secretions (figure 5). For the lung delivery it is nebulized by mechanical or ultrasonic nebulizer. Uniform distribution of drugis possible and each droplet contain at least single drug particle. Nanosizing improves the diffusion and dissolution of the drug. It enhances the adhesiveness of drug to the mucosal surface and increased residence time at the site of absorption. Nanosuspensions has fast onset of action initially and then controlled release of active moiety occur which is required for most of the pulmonary nanosuspensions has fast onset of action initially and then controlled release of active moiety occur which is required for most of the pulmonary disease¹⁵. E.g.; budesonide

4. Ocular Drug Delivery:

Figure 7: Nanosuspension for Ocular Drug Delivery

Certain drugs have poor solubility in lachrymal fluid. If it is formulated as nanoparticles its saturation solubility and bioavailability will increase. Mainly applied for hydrophobic drugs. It increases the residence time in cul de sac. The best example of nanosuspension is ibuprofen. The anti-inflammatory activity of ibuprofen increased compared with the aqueous preparation.¹⁶

5. Targeted Drug Delivery:

Nanosuspensions also used for targeting their surface properties and changing of the stabilizer can easily alter the in vivo behavior. The drug will be up take by the mononuclear phagocytic system to allow regional specific drug delivery. This can be used for targeting antimycobacterial, fungal drugs to the macrophages. Atovaquone is used as targeting nanosuspension to the brain¹⁷.

6. Mucoadhesion of Nanoparticle:

If the nanosuspension is orally administered it diffuse in to the liquid medium and adhere to the mucosal surface before absorption. it improve the bioavailability and targeting to the parasite persisting the git.eg; bupravaquone against cryptosporidium parvum.¹⁸

CONCLUSION:

Nanosuspension is commercially possible approach to solve the poor solubility as well as poor bioavailability problems of the drugs. Productions techniques such as media milling and high pressure homogenizer are used for large scale production of nanosuspensions. Nanosuspensions can be administered through oral, parenteral, pulmonary, ocular and topical routes. A nanosuspension not only improves the solubility and bioavailability, but also modifies the pharmacokinetics of drug and thus improves drug safety and efficacy.

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ABSTRACT: