International Journal of Drug Development and Research

Key words

Poor water soluble drugs, Bioavailability, Drug delivery, Nanosuspension

Introduction

Poorly water-soluble drugs pose a great challenge in drug formulation development1. The low saturated solubility and dissolution velocity lead to poor bioavailability. With the increasing number of newly developed lipophilic drug compounds, many techniques have been proposed, such as solid dispersions, cosolvents, emulsions, liposomes and nanoparticles based on lipidic or polymer carriers.

However, the use of large amounts of excipients or organic solvents is limited in pharmaceutical formulations due to possible toxicity of the compounds. A pharmaceutical nanosuspension is defined as very finely dispersed solid drug particles in an aqueous vehicle for either oral and topical use or parenteral and 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 Nanosuspension is a sub-micron colloidal dispersion of drug particles which are stabilized by surfactants, polymers or a mixture of both2 This formulation has a high drug loading, low incidence of side effects by the excipients, and low cost3 Owing to the increased surface-to- volume ratio of the nanocrystals, an increase in saturated solubility and very fast dissolution rate can be seen ,especially below particle sizes of 1 ?m 4 . Nanosuspension technology can also be used for drugs, which are insoluble in both water and organic solvents. Hydrophobic drugs such as naproxen5, bupravaquone6, nimesulide7, amphotericin B8, omeprazole9, nifedipine10 are formulated as nanosuspension. 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 ostwald 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 particles11.

Various approach to produce nanosuspension

Nanosuspension preparation can be broadly classified into two categories: (i) top-down processes and (ii) bottom-up processes. Top-down processes consist of particle size reduction of large drug particles into smaller particles using various wet milling techniques such as: media milling, microfluidization and high pressure homogenization. No harsh solvents are used in these techniques. However, all media milling processes involve high energy input and are highly inefficient12. Considerable amount of heat is generated in these operations making processing of thermolabile materials difficult. In the bottom-up approach the drug is dissolved in an organic solvent and is then precipitated on addition of an antisolvent in the presence of a stabilizer. Various adaptations of this approach include: (i) solvent–anti-solvent method; (ii) supercritical fluid processes and (iii) emulsion– solvent evaporation13,14 .

Top Down Process Technology

High pressure homogenization (Dissocubes) : High pressure homogenization has been used to prepare nanosuspension of many poorly water soluble drugs. 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 in the aqueous phase. The particles cavitations forces are sufficiently high to convert the drug microparticles into nanoparticles. The concern with this method is the need for small sample particles before loading and the fact that many cycles of homogenization are required. Dissocubes technology is an example of this technology developed by R.H. Müller using a piston-gap-type high pressure homogenizer, which was recently released as a patent owned by SkyePharm15 Other technologies and patents which are based on the homogenization processes are shown in Table 1 16

Media milling (Nanocrystals or Nanosystems)

The method is first developed by liversidge et.al. In this method the nanosusensions are produced using high-shear media mills or pearl mills. Wet milling is an attrition based process in which the drug suspension is subjected using a pearl mill in the presence of milling media.. The grinding media consists of glass, zirconium oxide stabilized with zirconium silicate or highly cross linked polystyrene resin in a spherical form (0.4-3.0 mm diameter). Temperature is less than 40ºC and pressure is as high as 20 psi.17. The high energy and shear forces generated as a result of the impaction of the milling media with the drug provide the energy input to break the microparticulate drug into nano-sized particles. The unimodal distribution profile and mean diameter of <200nm, require a time profile of 30-60 min. The media milling procedure can successfully process micronized and non- micronized drug crystals. Once the formulation and the process are optimized, very short batch to batch variation is observed in the quality. A nanosuspension of Naproxen with a mean particle size of 300-600 nm was prepared using pearl milling technique18.

Nanoedge : This technique is also called opposite stream technology, uses a chamber where a stream of suspension is divided into two or more parts. Both streams are colloid with each other at high pressure. The high shear force produced during the process results in particle size reduction. Dearns had 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 microparticles19

Bottom up Process Technology

a) Solvent – Antisolvent method :. Precipitation has been applied for years to prepare submicron particles within the last decade 20, especially for the poorly soluble drugs. Typically, the drug is firstly dissolved in a solvent. Then this solution is mixed with a miscible antisolvent in the presence of surfactants. Rapid addition of a drug solution to the antisolvent (usually water) leads to sudden supersaturation of drug in the mixed solution, and generation of ultrafine crystalline or amorphous drug solids. This process involves two phases nuclei formation and crystal growth. When preparing a stable suspension with the minimum particle size, a high nucleation rate but low growth rate is necessary. Both rates are dependent on temperature: the optimum temperature for nucleation might lie below that for crystal growth, which permits temperature optimization 21. Precipitation technique is not applicable to drugs which are poorly soluble in aqueous and non aqueous media. In this technique, the drug needs to be soluble in at least one solvent which is miscible with nonsolvent. The major challenge is to avoid crystal growth due to ostwald ripening being caused by different saturation solubilities in the vicinity of the differently sized particles.

b) Supercritical fluid processes: Various methods like rapid expansion of supercritical solution (RESS) process, supercritical antisolvent process, and precipitation with compressed antisolvent (PCA) process are used to produce nanoparticles. In RESS technique, drug solution is expanded through a nozzle into supercritical fluid, resulting in precipitation of the drug as fine particles by loss of solvent power of the supercritical fluid. By using RESS method, Young et al. prepared cyclosporine nanoparticles having diameter of 400 to 700 nm. In the PCA method, the drug solution is atomized into the CO2 compressed chamber. As the removal of solvent occurs, the solution gets supersaturated and finally precipitation occurs. In supercritical antisolvent process, drug solution is injected into the supercritical fluid and the solvent gets extracted as well as the drug solution becomes supersaturated 22.

C) 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.

Lipid Emulsion/Microemulsion Template: Lipid emulsions as templates are applicable for drugs that are soluble in either volatile organic solvents or partially water miscible solvents. In this method the drug will be dissolved in the suitable organic solvent and then emulsified in aqueous phase using suitable surfactants. Then the organic solvent will be slowly evaporated under reduced pressure to form drug particles precipitating in the aqueous phase forming the aqueous suspension of the drug in the required particle size. Then the suspension formed can be diluted suitably to get nanosuspensions .Moreover, microemulsions as templates can produce nanosuspensions23. Microemulsions 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 microemulsion can be saturated with the drug by intimate mixing. Suitable dilution of the microemulsion yields the drug nanosuspension23. An example of this technique is the griseofulvin nanosuspension which is prepared by the microemulsion23. The advantages of lipid emulsions as templates for nanosuspension formation are that they easy to produce by controlling the emulsion droplet and easy for scaleup. However, the use of organic solvents affects the environment and large amounts of surfactant or stabilizer are required.

Other techniques

Dry Co-Grinding: Stable nanosuspensions using dry-grinding of poorly soluble drugs with soluble polymers and copolymers after dispersing in a liquid media has been reported 24. Physicochemical 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 25. The co-grinding technique can reduce particles to the submicron level. Itoh et al 26 reported the colloidal particles formation of many poorly water soluble drugs; griseofulvin, glibenclamide and nifedipine obtained by grinding with polyvinylpyrrolidone (PVP) and sodium dodecylsulfate (SDS).

3. Characterization of Nanosuspensions

Nanosuspensions are characterized in similar ways as those used for conventional suspensions such as appearance, color, odor, assay, related impurities, etc. Apart from the aforementioned parameters, the nanosuspensions should be evaluated for their particle size, zeta potential, crystalline status, dissolution studies and in vivo studies

a) Particle 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 IV 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 embolism.

b) 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 sufficient27.

c) Crystal morphology

To characterize the polymorphic changes due to the impact of high-pressure homogenization in the crystalline structure of the drug, techniques like Scanning electron microscopy(SEM), X-ray diffraction analysis(XRD) in combination with differential scanning calorimetry or differential thermal analysis (DSC) can be utilized. Nanosuspensions can undergo a change in the crystalline structure, which may be to an amorphous form or to other polymorphic forms because of highpressure homogenization. Nakarani M. et al 28 prepared Itraconazole nonosuspension by media milling techniques and characterize surface morphology by scanning electron microscopy which is shown in figure 2a) and 2b), There study reveals that the particles of pure itraconazole were found to be large and especially irregular(figure 2a) However after nanosuspension formulation, particles disappeared and drug became small and uniform(figure2b). The nanocrystals seem to be more rounded, perhaps because the particles were coated with a surfactant layer.

d) 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. Böhm et al. reported an increase in the dissolution pressure as well as dissolution velocity with a reduction in the particle size to the nanometer range29. 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. Muller explained that the energy introduced during the particle size reduction process leads to an increase in the surface tension and an associated increase in the dissolution pressure30.

4. Physical stability of nanosuspensions

The small particle size of nanosuspensions, which is inherent to their success, is also responsible for their physical instability. Nanosuspensions consist of hydrophobic particles dispersed in a hydrophilic medium (usually water). The enormous surface area associated with the small size of these particles results in high interfacial tension, which in turn results in an increase in the free energy of the system. Accordingly, nanosuspensions are essentially thermodynamically unstable systems To decrease their free energy nanoparticles tend to reduce interaction with water via flocculation, aggregation or crystal growth. However, these processes adversely affect the central characteristics of nanosuspensions (i.e., small size and high surface area) and consequently the benefits of the nanosuspension formulations, as discussed above, are lost. Stabilizers are added to reduce the free energy of the system by decreasing interfacial tension, and to prevent nanoparticle aggregation by electrostatic or steric stabilization. Stabilizers can be surfactants, polymers or a mixture of both. Examples of some of the commonly used surfactants include Tween 80, sodium lauryl sulfate and poloxamer 188. Polyvinylpyrrolidone (PVP), hydroxypropyl methyl cellulose (HPMC), hydroxypropyl cellulose (HPC), and polyvinyl alcohol (PVA) are examples of polymeric stabilizers 31.

5. Applications of Nanosuspensions

Nanosuspension have various pharmaceutical and biopharmaceutical application a few of them highlighted here are :.

a) ORAL DRUG DELIVERY: Nanosizing of orally administered antibiotics such as atovaquone and bupravaquone. lead to a dramatic increase in their oral absorption and subsequently bioavailability. The oral administration of naproxen nanoparticles lead to an area under the curve (AUC) (0-24 h) of 97.5 mg-h/l compared with just 44.7 mg-h/l for naprosyn suspensions and 32.7 mg-h/l for anaprox tablets.32 Oral administration of the gonadotrophin inhibitor Danazol as a nanosuspension leads to an absolute bioavailability of 82.3 and the conventional dispersion (Danocrine) only to 5.2%. 33 A nanosuspension of Amphotericin B developed by Kayser et al. showed a significant improvement in its oral absorption in comparison with the conventional commercial formulation34

b) PARENTERAL DRUG DELIVERY: Nanosizing of poorly soluble drug use in parentral formulation have 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. Peters et al 35. prepared clofazimine nanosuspensions for IV use and showed that the drug concentrations in the liver, spleen and lungs reached a comparably higher level, well in excess of the minimum inhibitory concentration for most Mycobacterium avium strains

C) 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.36 The pharmacokinetics of the nebulized nanocrystal budenoside suspension showed comparable AUC, higher Cmax and lower Tmax as that of the pulmicort respules.

D) 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..One example of a nanosuspension intended for ophathamic controlled delivery was developed as a polymeric nanosuspension of Ibuprofen37. 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

E) Drug Targeting: 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 uptaken by the mononuclear phagocytic system to allow regionalspecific delivery. This can be used for targeting antimycobacterial, fungal or leishmanial drugs to the macrophages if the infectious pathogen is persisting intracellularly. Scholer et al. showed an improved drug targeting to the brain in the treatment of toxoplasmic encephalitis in a new murine model infected with Toxoplasma gondii using a nanosuspension formulation of Atovaquone. 38

Conclusion

Nanosuspensions are chiefly seen as vehicles for administering poorly water soluble drugs have been largely solved the dissolution problems to improve drug absorption and bioavailability. 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. Nanosuspension technology can be combined with traditional dosage forms: tablets, capsules, pellets, and can be used for parenteral products. Many drug delivery and pharmaceutical companies are exploiting this technology to reexamine active ingredients that were abandoned from formulation programs because of their poor solubility. To take advantage of nanosuspension drug delivery, simple formation technologies and variety applications, nanosuspensions will continue to be of interest as oral formulations and non-oral administration develop in the future.

Conflict of Interest

NIL

Source of Support

NONE

Tables at a glance

Table 1 Table 2

Figures at a glance

Figure 1 Figure 2

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