Investigation of different techniques involved in micronization of particles

2.5.3.1 Spray freeze-drying

One of the conventional methods for the production of uniform micro-particles is spray-drying. However, an important stability issue is associated with the production of spray-dried particles; this is because of the production of thermodynamically active amorphous particles. These particles tend to re-crystallize; which leads to alteration of drug characteristics. Therefore, spray freeze-drying technique is favorable for thermo labile drugs. A spray freeze-drying procedure combines both the atomization step from the spray-drying technique and freezing step involved in the freeze-drying. Typically, the drug solution or suspension is atomized into a spraying chamber filled with a cryogenic liquid 36. Different particles characteristics can be obtained based on of the location of the atomization nozzle. The spraying step can be performed either on the surface or beneath the cryogenic liquid 37. In the process of spray freeze-drying; the surface area available for heat transfer is much larger than the conventional freeze-drying 38. Therefore, spray freeze-dried product can be formulated in the size range less than 5 µm 39,40, in addition to the nano-scale 41-44. The diameter of particles can be manipulated via control of the mass flow rate of the liquid feed 45. A decrease in particle size can be achieved by an increase in mass flow ratio 45,46, while the addition of excipients (e.g., trehalose, ammonium sulfate) may lead to an increase in particle size 47. Further modification of the spray-freezing process has been proposed; instead of spraying the drug solution into the cryogenic medium, the drug solution is atomized and frozen simultaneously by mixing with a liquefied gas or supercritical fluid, such as supercritical CO2 25,41,48.

2.5.3.2 Jet-Milling

Jet-milling has been used as a successful tool for producing very fine particles. The main drawback of jet milling is that the fluidized particles might suffer from a considerable degree of breakage; which resulted from the intense inter-particle collisions. The particle size, shape, morphology could be hardly controlled by jet-milling. In addition, it provides limited control of the surface properties and electrostatic charges 49. The micronized powders produced by jet- milling always demonstrate a broad size distribution. The formed powders are not naturally grown because of the mechanical forces applied for micronization; which leads to breakage of

the crystals at the cleavage plane with the lowest attachment energy 50. The inefficiency of the jet-milling process comes from the reduction in powder crystallinity in addition to the enhanced chemical degradation 51,52. The alteration in the surface properties of drug substance could be related to the creation of thermodynamically-activated surfaces 53,54. Jet-milling also leads to reduction in therapeutic bioavailability, because of the conversion of crystalline surfaces into partially amorphous solids 55. Therefore, the production of disordered structures in the therapeutic substance affects the processing properties of the formulations, such as powder flow and cohesion. Jet-milling produces micronized particles with poor flow properties, due to the increased surface energies 56,57. Because the powders produced by mechanical micronization demonstrated decreased powder dispersibility, the drug delivery from dry powder inhalers may be less effective58. The association between the active sites of the carriers and the micronized drug substance results in reduction of the powder dispersibility. Generally, milling techniques show several drawbacks. However, the main research effort in the pulmonary drug delivery area is focused on the development of dry powder inhaler devices 59

. New techniques for the direct production of micronized particles are desirable. Therefore, micro-crystallization is a technique with high potential for production of particles for pulmonary purposes.

Crystallization as a tool for preparation of inhalable drug

2.6

particles

2.6.1 Micro-crystallization of Proteins using pH Controlled

Method

Production of crystalline protein powders has been found to be more favorable than their amorphous counterparts 60,61. This is because of the higher stability observed for the crystalline materials, which results from organized arrangement of molecules, in addition to the presence of distinguishable crystal lattice 62. Thermodynamically, the lower stability associated with the amorphous state resulted from the lack of crystallinity; which increases the energy content of molecules 62. Due to their high reactivity, amorphous protein particles are rapidly cleared from systemic circulation. Therefore, they are more susceptible to hydrolytic and enzymatic degradation because of their higher reactivity 63. Due to their advantages, crystalline protein

powders are desirable as a fine pharmaceutical ingredient. They possess the advantages of high purity and better handling during processing, storage and delivery. It can also provide the possibility of sustained drug release as a result of controlling the dissolution characteristics 64. However, apart from insulin, limited numbers of crystalline protein are commercially marketed as APIs. This is because of the fact of high degree of oriental freedom resulted from their sheer sizes 63. In addition, crystallization of proteins can lead to particles with wide size distribution. Micronization by milling has been applied for producing microcrystals of proteins. Nevertheless, due to the high energy input, the produced protein microcrystals was characterized by reduced crystallinity, stability and the presence of regions with disordered atoms or molecules 65.

The concept of micro-crystallization has been introduced to overcome milling-induced disorder in the crystalline powders. Microcrystals of α-lactalbumin, a 16 kDa glycoprotein, have been produced through crystallization from acetic acid aqueous solution containing PEG-8000 as a stabilizer. The produced microcrystals showed a controlled diameter between 1 and 2 µm and have a roughly spherical morphology. An enhanced pulmonary delivery was observed for the particles produced by this method 66.