Table 3. Mechanisms of Particle Size Reduction

Jet mill

Jet mills are the most common and widely accepted industrial practice for reducing particle size.

Principle: Jet mills operate on the principles of impact and attrition by generating high viscosity collisions between particles that are suspended in nitrogen gas or a compressed air stream. The resulting impacts cause the particles to break down into smaller fragments. Centrifugal forces in the jet mill cause large, heavy particles to separate from smaller and lighter particles. The smaller particles are carried in the fluid stream towards the center of the milling chamber, where they are discharged into a collection unit. Larger particles remain in the milling chamber where they recirculate, causing them to breakdown, creating a uniform particle size distribution. Jet mills are commonly used for particle size reduction because it elimiates the need to use a grinding media, which could  negatively impact the formulation.

(http://www.catalent.com/index.php/offerings/A-Z-Offerings/Catalent-Micron-Technologies/Particle-Size-Reduction-Services).

Advantages: (1) Production of very fine particles possible, (2) relatively little equipment space required, (3) grinding of hard products with a reduced equipment wear, (4) low operating temperature, (5) contamination-free process, and (6) no moving parts and foundation required.

Disadvantages: (1) Small process capacity, (2) high energy requirements, and (3) high maintenance costs (Saravacos G.).

Rotor stator colloid mill

Principle: Colloid mill works on the rotor-stator principle. This method breaks down particles by creating a dispersion of material in liquid. Shearing takes place in a narrow gap between a static cone (the stator) and a rapidly rotating cone (the rotor).

Factors that influence the degree of milling: (1) Homogenization intensity and duration, (2) viscosity of the continuous phase, (3) shear forces within the gap between the spinning rotor, and stationary stator and (4) clearance rate between the rotor and stator are the key factors which impact the particle size reduction (Remington et al., 2006).

Advantages: (1) Wide use of applications including comminution of slurry-fluid materials, (2) pressure less ionization, (3) easy to clean and assembly, (4) simple construction, (5) high capacity with lower maintainence requirements, and (6) low machine noise level.

Disadvantages: (1) No fine grinding, (2) higher energy requirement, and (3) no wide application for solid materials.

Ball Mill (Pebble mill/Tumbling mill)

Principle: Ball mill works on impact and attrition in a fluid energy mill. The Ball mill consists of a hallow cylinder containing balls; mounted on a metallic frame such that it can be rotated along its longitudinal axis. The balls which could be of different diameter occupy 30 - 50% of the mill volume and its size depending on the feed and mill size. The larger balls tend to break down the coarse feed materials and the smaller balls help to form a fine product by reducing void spaces between the balls.

Factors that influence the degree of milling: (1) Residence time of the material in the mill chamber, (2) nature of the balls (type of metal), (3) feed rate and feed level in the vessel, (4) size, density and number of the balls, and (5) rotation speed of the cylinder are some of the key factors that influence the degree of milling (Sud Sushant, 2013; Williams III et al., 2011).

Advantages: (1) Ability to run continuous and easy for a scale up process, (2) simple structure and easy installation, (3) reliable operation and simple maintenance, (4) suitable for toxic material because of completely enclosed process, and (5) milling highly abrasive materials.

Disadvantages: (1) Relatively long and tedious process, (2) difficult to clean and to assemble, (3) high machine noise level when metal used as hollow cylinder, and (4) higher chance of contamination due to the continuous process.

2. Nanosuspension

A nanosuspension is a colloidal dispersion of nano-sized drug particles. They are normaly produced by a suitable method and stabilized by specific stabilizer. In nanosuspension technology, the drug is maintained in the crystalline state with reduced particle size, to enhance the dissolution rate and therefore improve bioavailability. The Nanosuspension formulation approach is most suitable for the compounds that are insoluble in water with higher log P value, higher melting point and high doses. This technology can be used for drugs which are insoluble in both aqueous and organic solvents (Li et al., 2011). In nanosuspension, particle size distribution of solid particles are usually less than 1 micron with an average particle size ranging between 200 nm to 600 nm (Chingunpituk, 2011). Nanosuspension may alter the pharmacokinetics of drug which can improve drug safety and efficacy from enhanced absorbtion. Drug particle size reduction leads to an increase in surface area and consequently in the rate of dissolution as described by the Nernst-Brunner and Levich modification of the Noyes-Whitney equation (Müller et al., 2001).

Additionally, an increase in saturation solubility is postulated by particle size reduction due to an increased dissolution pressure explained by the Ostwald-Freundlich equation.

Advantages of nanosuspension drug delivery:

Ø  Oral administration of nanosuspension provide rapid onset, reduced fed/fasted ratio & improved bioavailability

Ø  Increased dissolution velocity and saturation solubility of the drug

Ø  Improved biological metabolism performance

Ø  Increased resistance to hydrolysis and oxidation

Ø  Increased physical stability to agglomerate formation

Ø  Reduction in injection volumes; essential for intramuscular, subcutaneous, & intraocular/ ophthalmic drug administration

Ø  Increased excipient compatibility

Ø  Ease of manufacture and scale-up

Ø  Long-term physical stability and versatility

Ø  Increased oral absorption

Ø  Improved dose proportionality

Ø  General applicability and simplicity of model to many other drug molecules

Ø  Administration of drugs used for only one particular treatment modality to novel routes

Ø  Possibility of surface-modification of nanosuspension for site specific delivery

Ø  Possibility of  scale up, the prerequisite for the introduction of delivery system to the market

Disadvantages for nanosuspension drug delivery:

Ø  Uniform and efficacious doses cannot be achieved

Ø  Improper dose administration or location

Ø  Special care must be taken during handling in a manufacturing plant and material transportation

Ø  Physical stability, sedimentation and compaction are common drawbacks.

Nanosuspension preparation methods

For manufacturing nanosuspensions, there are two common approaches: ‘Bottom-up’ and the ‘Top-down’ technologies. The ‘Bottom-up’ technologies refers to an assembly method which goes from normal sized molecules to nanosized particles. These include microprecipitation, microemulsion, and melt emulsification method just to name a few. ‘Top Down Technologies’ refer to the disintegration methods from large particles and microparticles to nanoparticles. The later is a more suitable  method since this is used to alter the properties of an existing drug product. Additional ‘Top Down Technologies’ are: 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 (Nanoedge) methods (Lakshmi and Kumar, 2010). Emulsion as templates and microemulsion as templates are also used for preparing nanosuspensions (Banavath et al., 2010). Many of these methods used for nanosuspension preparation are cost effective and technically simple for poorly soluble drugs and may yield a more physically stable product compared to a liposomal encapsulation approach.

List of all major technique for nanosuspension preparations:

Ø  Bottom-up technology (Precipitation)

Ø  Top-down technology

·         Media milling (Nanocrystals)

·         High pressure homogenization in water (Dissocubes)

·         High pressure homogenization in non aqueous media (Nanopure)

·         Combination of precipitation and high-pressure homogenization (Nanoedge)

Ø  Emulsions as templates

Ø  Microemulsions as templates

Ø  Supercritical fluid method

Ø  Melt emulsification method

Ø  Dry co-grinding

Ø  Nanojet technology

2.1.  Bottom-up technology (Precipitation)

Process starts on the molecular level, and goes via molecular association to the formation of a solid particle.

Process: Initially a drug is dissolved in a solvent. Then, the solution is mixed with a miscible antisolvent (normally water) in the presence of surfactants. Rapid addition of a drug solution to the antisolvent generate supersaturation state 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. High nucleation rate but low crystal growth rate is required for manufacturing of stable suspension with the minimum particle size.

Advantages: (1) Economic process equipment and simplistic process,  (2) more suitable for compounds with higher saturation solubility, and (3) ease of scale-up.

Disadvantages: (1) Solvent residues need to be removed, which increase production costs, and (2) preserving the particle character is challenging. In general, this process is follwed up a second consecutive process performed for particle preservation such as spray drying or lyophilization.

Limitations: (1) The solvent needs to be miscible with at least one non-solvent. (2) the drug needs to be soluble in at least one solvent.(Dey, 2012; Patel et al., 2011; Shid, 2013)

2.2  Top-down technology

2.2.1 Media milling (nanocrystals)

This patent-protected technology was developed by Liversidge et al. (1992)(Myerson, 1992). Formerly, this technology was owned by a company named “NanoSystems” but now this patent was transferred to “Elan Drug Delivery”. In this method, drug nanoparticles are obtained using high-shear media milling.  The media mill consists of a milling chamber, milling shaft, and a recirculation chamber. The milling chamber is filled with the milling media, water, drug and stabilizers. The the milling media (pearls) is then rotated at a high shear rate in order to break down the drug particle size. Temperature is critical and has to be closely monitored and controlled throughout the process.

Principle: Large amounts of energy and shear forces are generated as a result of the milling media crashing into the drug particles providing the energy input to break the microparticulate drug into nano-sized particles. The milling medium is composed of glass, zirconium oxide or highly cross-linked polystyrene resin. This process can successfully create micronized and non-micronized drug crystals (Patravale and Kulkarni, 2004). This type of mill is suitable for batch production and continuous operation.  Particle sizes of < 200 nm have been achieved using this process in previous work (Reddy and Anilchowdary, 2012).

Advantages: (1) Drugs with poor solubility in both aqueous and organic media can be formulated into nanosuspensions, (2) batch to batch variations are less and easy to control upon scale-up, (3) simple and convenient technological concept, (4) lower processing costs regarding the milling operation, and (5) a final product with a narrow particle size distribution can be achieved using this method.

Disadvantages: (1) Chances of product contamination from potential erosion of the milling material, (2) duration of the production is longer compared to other techniques, (3) potential microbial growth in water phase during longer milling processes, (4) high amount of homogenization cycles, (5) time and production costs are a major concern (6) separation of milling material from the drug nanoparticles in parenteral sterile products (Buchmann et al., 1996; Pandey et al., 2010).

2.2.2  High pressure homogenization in water

Dissocubes technology was developed by R.H. Muller. The patent rights of Disso Cubes were initially owned by Drug Delivery Services GmbH. Later, the patent was transferred to Skype pharmaceuticals. The suspension of a drug is created to pass through a small orifice resulting in a reduction of the static pressure below the boiling pressure of water, leading to the boiling of water and formation of gas bubbles. When the suspension leaves the gap and normal air pressure is reached again, bubbles implode and the surrounding parts containing the drug particles rush into the center forming colloids with a reduction in particle size. The preferred method is to start with the micronized drug (particle size < 25 µm) for production of nanosuspension to prevent blocking of the homogenization gap. Before subjecting a drug to the homogenization process, presuspension of the micronized drug is recommended in a surfactant solution using high-speed stirrers. During the homogenization process, the drug suspension is passed through the homogenization gap in order to achieve nano-sizing of the drug.

Principle: Particle collisions due to High shear forces and pressure cause the particle size to be reduced. In some cases, viscosity enhancers may be added in order to increase the viscosity of  a nanosuspension. Two key parameters using this method are: Homogenization pressure (higher the homogenization pressure leads lower the particle size) and number of homogenization cycles (can be optimized by particle size and polydispersity index of the drug after each cycle) affecting on the quality of the product.

Advantages: (1) Easy scale up process and less batch to batch variations, (Grau et al., 2000) (2) uniform size distribution in the final product, (3) potenitial for aseptic production of nanosuspension used for parenteral administration, (4) poorly soluble drugs in aqueous and organic media can be formulated, and (5) formulation of diluted or highly concentrated nanosuspensions.

Disadvantages: (1) Must have a micronized drug particles to start, and (2) need a stable suspension using high-speed mixers before subjecting it to homogenization.

2.2.3 High pressure homogenization in non aqueous media (nanopure)

Nanopure is a suspension that is homogenized in water-free media or water mixtures. Patents covering disintegration of polymeric material by high-pressure homogenization mention that higher temperatures of about 80ºC promote disintegration, which are not suitable for thermolabile compounds. In this technology, drug suspensions in the non-aqueous media were homogenized at 0 ºC or below the drugs freezing point; which was named “deep-freeze” homogenization after the low temperature processing (Venkatesh et al., 2011).

2.2.4  Combination of precipitation and high-pressure homogenization (nanoedge)

The basic principles of this technique are the similar to that of precipitation and homogenization. Combination of these two techniques result in smaller size particle with better prodcut stability in a relatively quick successive session. In the precipitation technique, crystal growth and long term stability are major drawbacks. Both problems can be resolved by the Nanoedge technology. In this technique, the precipitated suspension is further homogenized; leading to a reduction in particle size and avoiding crystal growth. Precipitation is performed in water using water-miscible solvents.

2.3  Emulsions as templates

Emulsions are normally used as s drug delivery vehicle but may also be used as templates in order to produce a Nanosuspension. There are two ways of preparing drug nanosuspensions by using the emulsification method.

An organic solvent or mixture loaded with a drug is dispersed in the aqueous phase containing suitable surfactants with high speed homogenization to form an emulsion. The organic phase is then evaporated under reduced pressure so that the drug particles precipitate instantaneously to form a nanosuspension. Particle size of the nanoemulsion as well as drug loading capacity in the emulsion can be control by using the optimum surfactant concentration. In other methods, partially water miscible solvents use as the dispersed phase instead of hazardous solvents (Trotta et al., 2001). The emulsion is manufactured by conventional method and the drug nanosuspension is obtained by just diluting the emulsion. Due to water miscible solvent as disperse phase, dilution of the emulsion with water causes complete diffusion of the internal phase into the external phase, leading to instaneous formation of nanosuspension.

Advantages: (1) No specific required instrumentation needed for manufacturing, (2) particle size can be controlled by modifying the size of emulsion droplets, and (3) ease of manufacturing scale-up processes.

Disadvantages: (1) Drugs poorly soluble in both aqueous and organic media can not be eficently created using this particlar method, (2) require ultrafiltration for purification of the drug nanosuspension, which increases production costs, (3) safety concerns from the use of hazardous solvents, and (4) the large quantities of surfactant/stabilizers required for processing  compared to other production techniques.

2.4  Microemulsions as templates

Microemulsions are thermodynamically stable and is an isotopically clear dispersion of two immiscible liquids, such as oil and water, stabilized by an interfacial film of suitable surfactant and co-surfactant (Eccleston, 1994). The method of preparation uses an organic solvent or solvent mixture added with the drug dispersed in an aqueous phase containing suitable surfactants to form an emulsion. The organic phase is then evaporated under reduced air pressure to make the drug particles precipitate instantaneously forming a nanosuspension. Another method uses partially water-miscible solvents as the dispersed phase instead of hazardous solvents. Oil in water (O/W) microemulsions are preferred for this purpose. The drug can be loaded into the internal phase or pre-formed microemulsions can be saturated with the drug by continuous mixing. Both manufacturing methods are identical as described earlier in Emulsion as template preparation. Microemulsions have a long shelf-life, high drug solubilization and can be scaled up during manufacturing. The manufacturing process of a microemulsion is identical to the emulsion manufacturing. Advantages and disadvantages of this processing method are similar to the described Emulsion as template method.

Industry applications: Nanosuspension formulations of poorly water-soluble and poorly bioavailable antifungal drug such as griseofulvin using microemulsion as templates method; significantly increased the dissolution rate of the drug when compared to the commercial product. However, this technique is still under observation and needs additional testing to determine the pharmacological impact and bioavilibility compared to the finished  products performance in-vivo (Trotta et al., 2003).

2.5   Supercritical fluid method

The various methods attempted for manufacturing are rapid expansion of supercritical solution (RESS) process, supercritical anti-solvent process, and precipitation with compressed anti-solvent (PCA). In rapid expansion of supercritical solution technique, the 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. The supercritical anti-solvent process uses a supercritical fluid in which poorly soluble drug and compatible solvent that is also miscible with the supercritical fluid. In this process, the drug solution is injected into the supercritical fluid and the solvent gets extracted as the drug solution becomes supersaturated.

Example: The drug, griseofulvin, was successfully prepared by Chattopadhyay et al. using this method (Young et al., 2000).

Disadvantages: (1) High amount of hazardous solvent needed for extraction, and (2) particle nucleation overgrowth which may result in the development of amorphous or another undesired polymorphs.

2.6   Melt emulsification method

Melt emulsification method is widely used to prepare solid lipid nanoparticles. In this method, a drug is dispersed in the aqueous solution with a stabilizer. The solution is heated at a temperature higher than the melting point of the drug, then homogenized by a high-speed homogenizer for emulsification. The temperature is set above the drug’s melting during overall process to keep it solubulized. Finally, the emulsion is cooled to precipitate out remaining particles. The particle size of nanosuspension is dependant on various parameters such as: drug concentration, stabilizer concentration, type of the stabilizer, cooling temperature and homogenization speed during processing.

Example: Ibuprofen nanosupension was the first nanosuspension prepared using the this type of melt emulsification method (Kipp et al., 2003; Sutradhar et al., 2013).

Advantages: (1) No organic solvents used during the production process, (2) reduction in processing costs, (3) excellent for drugs that form impurities with other solvents.

2.7  Dry co-grinding

Dry co-grinding is a modern technique used to prepare nanosuspensions. Colloidal particle formation of poorly water soluble drugs such as: Griseofulvin, Glibenclamide and Nifedipine;can be obtained by grinding with Polyvinylpyrrolidone and Sodium dodecylsulfate. Many types of soluble polymers and copolymers have been used in conjunction with this method. Physical properties and dissolution of poorly water-soluble drugs are improved by co-grinding due to changes in the surface polarity and transformation from a crystalline to an amorphous drug form (Banavath et al., 2010; Itoh et al., 2003).

Advantages: (1) Simple process, (2) no organic solvents required during manufacturing process, and (3) short grinding times required.

Disadvantage: (1) Generation of residue from milling media and (2) limitations on formulation components.

2.8   Nanojet technology

This methods is also known as opposite stream technology. This technique uses a chamber where a stream of suspension is divided into two or more parts. Both streams collide with each other at high pressures during the process. These net high shear forces produced during processing results in a particle size reduction.

Example: Atovaquone nanosuspension was prepared by Dearns using the microfluidizer technique.

Disadvantages: (1) High number of passes through the microfluidizer and (2) product obtained contains a relatively larger fraction of microparticles (Dearn, 2003).

3  Sonocrystallization

Recrystallization of poorly soluble materials using liquid solvents and antisolvents is another method that sucessfully reduces particle size and improves the solubility of some drug molecules. In sonocrystallization, particle size reduction on the basis of crystallization achieved by ultrasound method (Patel, 2012).

4  Spray drying

Spray drying is a commonly used method for drying a liquid feed through a hot gas (normally air). Many times, Nitrogen gas is used in special cases in the processing of oxidation sensitive materials or solvents like ethanol require oxygen-free drying. This process of drying is a single step rapid process and eliminates the need for additional processing (Patel, 2012).

Example: Spray drying of the acid dispersed in acacia solutions resulted in as much as a 50% improvement in solubility of poorly water soluble salicylic acid (Kawashima et al., 1975).

5  Newer particle technologies to improve bioavailability

5.1  Solid self-emulsifying drug delivery systems (S-SEDDS)

This technique is suitable to lipophilic drugs and drugs with poor aqueous solubility typically in lipid based formulations. These formulations are typicaly oils, surfactant, dispersions, emulsions, SEDDS, solid lipid nanoparticles and liposomes (Tang et al., 2008) S-SEDDS are formulated by incorporating a liquid,  semisolid self-emulsifying ingredients into powders, or nanoparticles by various solidification techniques. Solidification techniques are spray drying, adsorption to solid carriers, melt granulation and melt extraction techniques. SSEDDS are more desirable than regular liquid SEDDS.

Table 4. Marketed formulation using S-SEDDS drug delivery

Limitations: (1) S-SEDDS such as strong adsorption and physical interaction of the drug with the carriers that causes retarded or incomplete release of the drug from the S-SEDDS.

Disadvantages: (1) Expensive processing costs and (2) limitations to as which materials can be used (Gandhi, 2016).

5.2  Complexation with cyclodextrins

Cyclodextrins are useful solubilizers for both liquid oral and parenteral dosage forms and can increase the solubility of the drug. Cyclodextrin inclusion complexation, which is the formation of host-guest inclusion complexes by weak intermolecular interaction (Loh et al., 2016). There are several methods are designed for the preparation of drug-cyclodextrin complex like freeze drying, spray drying co-precipitation of a cyclodextrin/drug solution, kneading, extrusion and grinding of slurry of drug in a mortar and pestle.

Table 5. Marketed formulation using complexation with cyclodextrins technique

5.3   Polymeric micelles

Polymeric micelles are potential carriers for poorly soluble drugs by solubilizing them in their inner core. In a polymeric micelle, the hydrophobic fragments from the core of the micelle, while hydrophilic fragments form the micelle’s corona. The nonpolar molecules are solubilized within the hydrophobic core while polar molecules will be absorbed on the micelle surface and the substances with intermediate polarity will be distributed along surfactant molecules in intermediate positions. Two core methods for drug loading into the polymeric micelles are used. The first method is the direct dissolution method; which is mostly employed for moderately hydrophobic copolymers. Marketed product from Lupin pharmaceutical using polymeric micelle technique to improve bioavailability of Paclitaxel drug.

The later method is the preparation of drug-loaded micelles by solvent removal which is employed for amphiphilic co-polymers.

5.4   Freeze-dried liposomes

Liposomes are phospholipid vesicles, comprising a phospholipid bilayer surrounding an aqueous compartment and can dissolve lipophilic drugs in their lipid domain.

Table 6. Marketed formulation using freeze-dried liposomes technique

One of the serious limitations with applicability of liposomes as drug delivery systems is associated with its poor stability during storage. The liposomal formulations can thus be stabilized by a freeze drying process in order to obtain dry powders with enhanced stability (Gandhi, 2016) Liposomal incorporation of poorly soluble drugs followed by freeze drying approach can produce powder form of the drug.

5.5  Solid lipid nanoparticles (SLN)

Solid lipid nanoparticles is colloidal drug carrier system similar to nanoemulsions. The main difference from the later is the lipid nature in which the liquid lipid part of emulsion is replaced by a solid lipid at room temperature.

Various methods of SLN preparation have been developed such as cold and hot homogenization, breaking of o/w microemulsion, solvent injection, high shear homogenization, solvent emulsification diffusion and/or ultrasound dispersion. Among all of these methods, the high pressure homogenization method is considered to be the most effective method due to several key advantages such as: uniform particle size distribution, high particle content in the dispersions, avoidance of organic solvents and scale-up feasibility. Marketed product from US WorldMeds, LLC use solid lipid nanoparticles technique to improve bioavailability of Apomorphon drug.

Disadvantages: (1) Low drug loading capacity, and (2) formulation stability are biggest concerns (Gandhi, 2016).

Conclusion

For orally administered drugs, solubility is one of the rate limiting parameters to achieve their desired concentration in systemic circulation for pharmacological response. Drug solubility is the key factor that impacts the formulation and therapeutic efficacy of the drug. These particle size reduction technologies can be divided into two main categories: Conventional methods and the Novel approaches. The conventional size reduction techniques are simple and convenient methods that offer increase the surface area by the reduction of the drug particle size. Novel techniques include the basic particle size reduction principles but overcome many of the obstacles and limitations used with conventional methods.

Various techniques described in this review may be used alone or in combination with other to enhance the solubility of poorly aqueous soluble drugs. Selection of a solubility enhancement technique is important to meet critical formulation attribites such as: good oral bioavailability, reduced dosing frequency, and better patient compliance, with a relatively low production cost. The method for selecting an appropriate solubility enhancement technique is dependant on drug characteristics including: solubility, chemical nature, melting point, absorption site, physical nature, pharmacokinetic behavior, dosage form requirements (tablets or capsules), drug loading, immediate or modified release, and maximum daily dose, regulatory, approved excipients, and analytical quantitation methods. Solubility enhancement of poorly solubility drugs is a challange and many drugs are affected to their bioavailability. Recent advancements in processing technolgoies have now made it possible to increase the solubility of poorly soluble drugs with the various techniques as described in this review article better than before.