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Drug Delivery with Dry Powder Aerosols

Delivery of therapeutic agents in the form of aerosols directly into the lungs is recently gaining attention. Among aerosol generation system, dry powder for inhalation has been an attractive area of research for both local as well systemic administrations of drugs. Effective delivery of dry powder into the lungs is influenced by many factors such as physical properties of dry powders, respiratory function of patients, and devices used for delivery. Among these all factors, the physical properties of dry powder has shown major role in formation of aerosol for optimum delivery into the lungs. The main challenge of any inhalation delivery system is to generate particles within respirable range. In the process of generation of effective particle size for optimum delivery, researchers have recognized the importance of physical properties of dry powder and have utilized various techniques to prepare dry powder for inhalation. Recently, researchers have focused on various aspects of dry powders such as morphology, inhalational airflow rate, delivery device, and carriers but have not mentioned anywhere about the effect of initial particle size. In this work, we have prepared various particle sizes of bovine serum albumin (BSA) microparticles and were converted into dry powder form within respirable range using spray dryer.

Targeting lungs through systemic delivery of drug has always shown low bioavailability with various systemic toxicities, especially in the treatment of lung cancer. Chemotherapy is most effective treatment to target major organs where surgery is not suitable. Unfortunately, chemotherapy can also kill the normal cells during treatment as it is unable to differentiate normal and cancerous cells. The main aim of this work is to prepare paclitaxel loaded BSA microparticles and to investigate its initial particle size in the treatment of lung cancer. The whole work has structured into six chapters.

Chapter one summarized the aim and significance of the whole work. This chapter also included various technical terminologies used to define dry powder for inhalation, various techniques for preparation of dry powder for inhalation and various factors which directly influences the formation of aerosols of dry powder, its delivery and retention in lungs.

In chapter two, the influence of various factors on preparation of PTX loaded BSA microparticles with coacervation process was discussed. The influence of various factors like BSA concentration, pH of solution, rate of addition of coavervating agent, and stirring speed on particles size and poly dispersity index (PDI) were systematically studied using Taguchi Orthogonal experimental design. This also included the development and validation of HPLC method for detection of PTX. The formed PTX loaded microparticles were characterized for yield, size, morphology, and drug entrapment and loading efficiency.

In chapter three, three different sized PTX loaded microparticles were converted into dry powder form using spray dryer. The spray dryer instrumental parameters were optimized by Box Behnken design method. To convert PTX loaded BSA microparticles into dry powder form, various sugar bases like, mannitol, lactose, and trehalose were used. The selection of sugar base as filler during spray drying was done based on the solid behaviors like, bulk densities, size distribution, surface morphology and moisture content. The formed dry powders were characterized for particles size and distribution, surface morphology, and in vitro drug release profile. The compatibility between BSA, PTX, and selected sugar base was studied with DSC, XRD, and IR. The in vitro aerosolization properties such as FPD, FPF, RF, ED, MMAD, and GSD were calculated using NGI. Finally, the stability of dry powder was studied as per ICH guidelines.

In chapter four, in vitro cellular toxicity and uptake study were mentioned. 4T1-Luc cell line was taken for cellular study. Cytotoxicity study shows that Taxol solvent (Cremophore EL) have significant toxic effect on cell. Bare BSA microparticles did not have cytotoxicity even at higher concentration. The IC50 calculated for Taxol solvent, Taxol solution, PTX-BSA-MPs (0.5, 1.0, and 3.0 µm) were 7.213, 2.174, 1.046, 2.164, and 2.416 respectively after 24 h of study. Qualitative cellular uptake study of C6 loaded BSA-MPs (0.5, 1.0, and 3.0 µm) shows time dependent uptake regardless of size of MPs.

In chapter five, selection and optimization of device used for insufflation of dry powder in BALB/C mice was discussed. 20 G cannula plastic tube and 1 mL syringe was customized for delivery of dry powder in the lung of mouse. Otoscope was used to visualize the tracheal opening of mouse for proper intubation of cannula tube in the trachea. Device was optimized for dry powder aerosolization and air pressure applied for blowing of dry powder from the device. The amount of dry powder for each time dosing was fixed to 1 mg. The air applied for blowing of loaded dry powder was characterized by one week study for physiological behavior of test animal such as nasal bleeding and mobility. Further, in vivo toxicity was monitored with loss in relative body weight and histological study of lung tissue. The obtained data showed 1 mL of air is suitable to blow required amount of dry powder form device without any physiological abnormalities.

In chapter six, biodistribution of PTX from Taxol solution via tail vain administration and PTX-BSA-MPs-DPs via intratracheal administration in normal BALB/C mice and in vivo anit-tumor efficacy study were mentioned. From biodistribution study, it was found that only 11% of PTX was available in lungs after 8 h of administration whereas about 76% of PTX was get distributed into liver followed by tail vain administration of Taxol solution. On contrast, higher amount of PTX was detected in the lungs followed by direct administration of PTX-BSA-MPs followed by intratracheal administration. Metastatic lung tumor model in BALB/C mice was developed by tail vain injection and the development of tumor in lung was confirmed by IVIS. After two week development of metastatic lung tumor in animal, medication was started. The anti-tumor efficacy of PTX-BSA-MPs-DPs (0.5, 1.0, and 3.0 µm) was compared with Taxol solution, the obtained results shows significant shrinkage in tumor size and number with PTX-BSA-MPs-DPs. Further for safety evaluation of PTX-BSA-MPs-DPs, health BALB/C mice were taken for study. The toxicity level was characterized for total blood count, loss in relative body weight, variations in biomarkers for kidney and liver and histological changes in major organs.

Hence, the above whole work supports the hypothesis and the obtained results have proved that the initial particle size has great role in systemic absorption and retention when the chemotherapeutic agents delivered in the form of dry powder through tracheal route directly into lung. Furthermore, it can be concluded that the PTX-BSA-MPs-DPs is the most promising delivery system to deliver chemotherapeutic directly in the lung without systemic toxicity.

Historically, it has been noticed that lungs as a filtering organ and responsible for oxygenation of deoxygenated blood in the body [1]. In ancient time pulmonary route has been used to inhale tobacco for mood relaxation. This route is also utilized for therapy, in 1554 BC, Egyptian physicians used pulmonary route to pass vapor of black henbane in breath-compromised patients [2]. Lung can provides a large absorptive surface area with a thin alveo-capillary membrane and a large vascular bed through which the blood flows with every heartbeat.  Due to large absorptive surface area and large vascular bed, pulmonary route have been used to treat various respiratory as well as systemic diseases for centuries [3]. Recently, drug delivery through respiratory tract is a rapidly emerging field in drug delivery field with the advancement in the synthesis of nano and micro drug delivery, pulmonary route has shown a great potential for delivery of local as well as systemic therapy. This is primarily because of the several advantages offered by this route over other route. This route is mainly suitable for those drugs which extensively under go first-pass metabolism when administered through oral route. Other, many more advantages offered by this route i.e, less therapeutic dose required, high bioavailability, rapid onset of action due to large absorptive surface area , high solute permeability, less drug degradation, non-invasive and  easy administration [4] similarly, there are many evidences which shows that the hydrophobic drugs are easily and quickly get absorbed within short time frame followed by inhalation route . However, the absorption mechanism of these hydrophobic molecules are not well described elsewhere but it was hypothesized by many scientist that these molecules get trapped into the airspaces nonspecifically through a combination of tight junction and endocytic vesicles followed by endocytosis [5]. There are many endogenous molecules e.g. albumin, immunoglobulines, and transferrin which are get internalized into the alveolar cell through specific receptor-mediated transport mechanism which are present on the surface of the alveolar epithelial cells [6].

Targeted drug delivery to the respiratory tract has progressed to be one of the most recently investigated approaches for both local and systemic therapy. For local therapy, pulmonary administration offers greater site-specific deposition within the lung; thus lowering the drug dose due to the reduction in first-pass metabolism in comparison to the oral administration [7], as drugs administered through oral route are extensively passed through liver where they interact with efflux transporter P-glycoprotein (P-gp) [8].

Drug delivery to the respiratory tract is classified into two brad categories, i.e, 1) Immediate release, which consist the pure drug suitable for inhalation and, 2) controlled release systems, which includes engineered nano- and micro-particles loaded with drugs for long effect. Drugs loaded into nano-or micro-delivery system provides good stability to drug in biological environment; reduces the incidence of systemic toxicity; and prolong the biological half-life. In case of immediate release dry powders for inhalation get deposited into respiratory tract with the virtue their physic-chemical properties of drugs. However the deposition of engineered nano- and micro-particles is based on the properties of the carriers, because the drugs get loaded inside the core of the carrier.  Therefore, successful delivery of the therapeutic agents deep into lungs depends on pathophysiology and anatomy of lungs, physic-chemical properties of the pure drugs and the carriers through which they can be deliver into the lungs, similarly, the devices through which the dry powder is delivered are also play important role.

  1.          Dry powder inhalation as a promising drug delivery system

Respiratory route is gaining its popularity as a promising route for delivery of both immediate and prolonged release of drugs due to its various advantages over other drug delivery routes e.g. oral and intravenous. Instead of having various advantages e.g. high solute permeability, less drug degradation, non-invasiveness and easy administration over other routes, however, formulations design for successful inhalation required special techniques and have several unique challenges. Drugs formulated for inhalation are directly reached to the epithelium region where they effectively get up-taken [9]. Preparing dry powder for inhalation is an attractive approach because it can bypass many formulation related challenges like solubility and stability [10]. There are many more other advantages like, low chance of microbial growth and suitable for both hydrophobic and hydrophilic drugs [11].  However, for complete deposition of dry powder into lungs needed to be micronized the size < 5µm which is quite small and can easily prone to agglomeration due to electrostatic surface charges and shows poor re-dispersability and flowability [12].

  1.          Clinical significance of inhalational dry powder

Delivery of drugs in the form of dry powder through respiratory route is an attractive non-invasive method. The attractiveness towards this route is increased because of large absorptive area with high vasculature in lungs. Systemic as well as local diseases can be targeted through respiratory route. Local delivery is highly recommended for cystic fibrosis[13],  pulmonary obstructive diseases like asthma[14], chronic obstructive disease[15], and lung cancer[1116]. Drugs delivered to target these diseases get highly benefited through local delivery because high concentration levels of drugs get discharged at target site with minimal or no systemic side effect to produce high bioavailability [17].  Due to high drug concentration availability in lung through local delivery, this route is highly significant for delivery of chemotherapy in the treatment of lung cancer.

Dry powder inhalers have been seen a promising approach in the treatment of lung cancer [1618]. Due to many advantages of dry powders for inhalation, foreseen, that lung cancer can be effectively and successfully treat through respiratory route.

  1.          Pulmonary drug delivery for local and systemic therapy

Drug delivery to the respiratory tract is an interesting alternative to other routes of administration; that generated an increasing consideration over the past decade [19]. Many drugs exhibited an enhanced bioavailability following their pulmonary administration. This can be attributed to: (1) the tremendous surface area of the alveoli (100 m2), (2) a relatively low metabolic activity and (3) an elevated blood flow; which means rapid distribution of drugs throughout the body [20].

Drug delivery to the lungs can combine the advantages of both local and systemic delivery systems. Localized pulmonary administration can be favorable for treatment of various lung disorders, i.e., asthma, chronic obstructive pulmonary diseases, cystic fibrosis, lung tuberculosis, lung cancer and many others. However for systemic therapy, the natural permeability of the lung can be utilized to transfer molecules to the blood stream. Most of the marketed dry powder inhalation therapeutics is for localized treatment of lung disorders. With the approval of Pfizer’s Exubera®, recombinant human insulin for dry powder inhalation, many possible candidates for systemic pulmonary administration are currently under development and being clinically tested.  Nevertheless, the same rationale of improving patient compliance through switching to needle-free delivery encouraged research for inhalation therapy of other active pharmaceutical ingredients (APIs) that are currently administered only via injection. These compounds include morphine and fentanyl as analgesics, di-hydro-ergotamine for migraine, interferon-b for multiple sclerosis, leuprolide acetate for prostate cancer and growth hormone releasing factor to treat pituitary dwarfism.

  1.          Dry powder inhaler devices

Within the pharmaceutical manufacturing, selection of the appropriate inhalation delivery system is a pivotal decision. This is dependent on different aspects such as the clinical objective (acute or chronic treatment) and target patient features (infant, elderly or ambulatory). Different dry powder inhalation devices are available in the market, yet no single inhaler device possesses all the properties of an ideal inhaler [21].

Dry powder inhalers are devices which store the medication as fine particle aggregates, either as a pure drug substance or encapsulated in a nano- or micro-particulate formulation. These inhaler devices have the option of regulating the dose. The dry powdered drug is stored at the bottom of inhaler in the powder reservoir compartment [22]. In some multi-dose inhalers, the drug is separately sealed in individual storage compartments. Figure 1.1 shows photographs for some currently available dry powder inhalers [23]. The patient inspiration comprises the main force that initiate actuation of the inhalation device. As compared to metered dose inhalers, the need for good coordination between the patient’s inspiration and inhaler device actuation is eliminated. The inspiratory airflow rate is a critical factor for delivery of medication. However, some dry powder inhaler devices appear to be relatively independent on the patient’s inspiratory rate [24]. For evaluating all inhalation drug delivery systems, the fractional deposition of drug and its depth of penetration have to be accurately assessed.

  1.          Fundamental aspects of aerosol inhalation from dry powder inhalers
    1.      Patient-related factors

The deposition profile of inhalation dry powders is affected by two major independent factors: (1) patient-related factors; which can be cited as the anatomical and physiological aspects of the respiratory system as well as the inhalation airflow rate, (2) physical properties of dry powders.

  1. Architecture and physiology of pulmonary system

Pulmonary system in human being is divided into three distinct regions, i.e. extrathoracic region (includes- oral-pharyngeal cavity, larynx and entrance of trachea), tracheobronchial region (includes- trachea, bronchi and terminates into bronchioles), and alveolar region (includes-bronchioles, alveolar duct and alveoli) (Figure 1.2) [25]. The anatomical architecture of tracheal system is considered as the most crucial factor in pulmonary drug delivery and it varies based on the age of patients (Figure 1.3). Lungs are responsible for purification and oxygenation of blood, it has been estimated that a normal healthy human inhales about 568.262 mL of air 12-15 times per minute [26]. Approximately, 300 millions of alveoli are present in the lungs which are linked up with more than 280 billion capillaries through which gaseous exchange takes place. The alveolar gas exchange mainly occurs at the interface consisting of alveolar epithelium, endothelium and interstitial cell layers, the gap between interstitial cell layers and capillaries is estimated to be around 0.5µm which facilitates gas exchange and systemic absorption via diffusion mechanism [27]. It is estimated that the total cross-sectional area covered by bronchioles is about 10 m2 and by alveoli is about 100 m2 [28] which provides huge surface area for absorption of inhaled particulate matter. Lungs are highly vascularized organ in human body about 5700 mL/min of blood flow across the lungs which ensure the prompt systemic absorption of the drugs for systemic effect [29]. The alveoli are protected within alveolar fluid and mucus, which are composed of phospholipids and proteins and are responsible for reducing surface tension, facilitate gaseous exchange and provide anti-shock cushion to alveoli. The alveolar tubes are lined up with a thin layer of connective tissue, and are also surrounded with different types of cells such as fibroblasts, nerves, macrophages and lymph vessels; these all architectural makeup highly facilitate delivery of drugs through the pulmonary route [27].

  1. Inhalation mode and airflow rate

The site of particles’ deposition in the respiratory tract is affected by the mode of aerosol inhalation. The mode of inhalation comprises the airflow rate, volume of air inhaled, and the period of breath holding. Deposition by gravitational sedimentation is decreased as the airflow rate decreases. Therefore, the deposition of particles in the respiratory system can be enhanced by forceful expiration prior to inhalation and deep inhalation followed by a period of breath holding. The driving force for deposition in the respiratory airways is the patient’s inspiration effort. The inhalation airflow rate is important to achieve an acceptable disaggregation of particles.

However, the patient’s inhalation rate is difficult to control. In one report, it was mentioned that the dependence of the regional aerosol deposition of inhaled particles on the patient’s sex [30]. In general, the total pulmonary deposition in both male and female patients is similar.  However, female patients demonstrated higher aerosol deposition in the upper respiratory tract and trachea-bronchial region. This effect may be referred to the differences in airway 10 caliber between male and female patients. Similarly, it was found that the effect of inhalation airflow rate on the pulmonary deposition of dry powders [31]. This, in turn, depends on the patient’s disease state, age, sex and height. Generally, the mean peak inspiration airflow rate in healthy human was found to be 300 L/min. However, in asthmatic patients, this value is changed to be as low as 200 L/min [19].  Many recent studies focus on understanding the nature of airflow (laminar or turbulent) created inside the inhaler device.  It has been demonstrated that turbulent airflow is more effective for dispersing the dry powder mixture.

  1. Particle deposition in respiratory airways

The extent to which particles are deposited in the respiratory trajectory depends on patient’s physiological condition (i.e., breathing pattern, health condition and geometry of lungs) as well as physicochemical properties of inhaled particles (i.e.,  size, shape, surface charge, bulk density, hygroscopicity and moisture content) [32]. Deposition of inhaled particles is mostly influenced by particle’s physicochemical properties rather than the patient’s conditions. Size and shape of particles play critical role in their deposition throughout the respiratory trajectory. During inhalation, particles are dragged inside the trajectory by mechanical force along with inhaled air volume and eventually deposited on the surrounding airway surfaces. There are three major mechanisms which cause inhaled particles to deposit on the surrounding airway surfaces: impaction due to inertial forces, sedimentation due to gravity and Brownian diffusion [25]. Furthermore, there are other few mechanisms through which particles deposition takes place such as interception and electrostatic precipitation. These two mechanisms have minor role in particles deposition and are based on particles shape and electrostatic charges.  Particles with elongated fibrous shape are mainly deposited by interception mechanism [33].

Inertial impaction is major mechanism through which particles are deposited throughout the trajectory. During inhalation, aerosolized particles in the air streamLine collide with the wall of the airway due to centrifugal force and are deposited to bronchial region depending on size. Particles greater than 5µm are mostly get deposited in the upper respiratory trajectory whereas, particles with smaller size (1-5µm) are slowly deposited via sedimentation mechanism in the bronchioles region due to gravitational force. The third mechanism is Brownian diffusion through which inhaled particles are deposit in the deeper alveolar region.  Particles < 1µm undergo Brownian diffusion and < 0.5µm get exhaled out during expiration [27].

  1.      Formulation and device related factors
    1. Physicochemical properties of dry powder

Physicochemical properties of aerosolized dry powder directly influence the delivery of dry powder to lungs in the form of aerosol. Dry powders formulated for inhalation are very fine in size and tend to form agglomerates. The physical properties which directly influence the aerosol and release from device are particle size and size distribution, shape and morphology, hygroscopicity and moisture content, and surface electrostatic charge.

Particle size and size distribution are considered to be important parameters in the inhalation technique and have vital role in the formation of aerosol [34]. The size distribution is calculated by span (or polydispersibility index). Larger span value is considered to result in heterogeneity in size distribution which affects deposition of drug in the lung upon inhalation [35]. The optimum size for aerosol formation for inhalation and deep lung deposition is approximately 2-5µm. When the particle size is <2 µm the cohesiveness between particles increases as a result the aerosolization of particles is difficult however, when the size is >5 µm, the cohesiveness between particles decreases. Particles with size <2 µm need more air force in order to reduce the cohesion between particles to form aerosol. The percentage of fine particle fraction (FPF) of inhaled particles depends on the size and size distribution [36].

Particle shape and surface morphology are other important factors that affect particle aerosolization [37]. Particles having irregular shape have low contact area and have low Van der Walls force and have low tendency to aggregate [38]. Mostly dry powder for inhalation has been studied with spherical shape and with smooth surface. Particle morphology and shape are considered to be important factor for optimum aerosol formation and lung deposition [39]. Particles with different shape have different contact force which helps to produce optimum aerosol that further increases the particle deposition rate in the lung [39].

Hygroscopicity is a physical phenomenon of the solid materials. The moisture uptake by solid mass depends on its surrounding environmental condition as well as the nature of the solid materials (i.e., lipophilic or hydrophilic) [40]. Moisture absorption is the property of the hygroscopic materials to absorb water content from the surrounding environment till an equilibrium state is reached. After gaining the moisture from the environment the water content increases affecting the bulk density, surface charge and aerodynamic size of the powder [41].

Development of electrostatic charges on the surface of particles results from a  number of factors which directly influence the aerosolization of the particles [42]. Surface charges on particles depend on the size of particles and their surface properties such as crystal lattice, surface energy and surface area [43]. Larger particles tend to have rough surface and irregular shapes, reduced disorder in crystal lattice and lower moisture uptake compared to smaller particles [44].

  1. Design of device

The characteristics of an ideal inhaler are based around the design and formulation of the device, patient use and the clinical effect, together with concordance and patient preference. The device used for dry powder inhalation, should be resistance free and provide sufficient safety to the patients during inhalation. Recent inventions of the powder inhaler device are aim at improving the inhaler’s dispersion efficiency and reducing the resistance of the device as well as decoupling powder dispersion from the patient’s aspiratory effort in order to deliver accurate and flexible dosages for different patients need.

The drug deposition profile in the respiratory system is significantly affected by the design of dry powder inhaler device. A comparison between the deposition profiles of sodium cromoglycate emitted from two different inhaler devices, namely Inhalator Ingelheim® and Rotahaler®, indicated the dependence of aerosolization performance on the design of dry powder inhaler device 20. Another study pointed to the influence of inhaler design on the amount of particles retained in the gelatin capsule and adhered to the walls of dry powder device.

  1.          Particles engineering techniques

The production of dry powder for inhalation with desired aerodynamic size (1-5µm) is technically challenging [45]. Several techniques are used to produce dry powder for inhalation. The most commonly used technique is milling. To obtained particle size within respirable range, large particles are passed through several types of mills, such as jet-mill, ball-mill or high-pressure homogenizer [4647]. Recently, several sophisticated techniques such as freeze-drying, spray drying, spray freeze drying, and super critical fluid drying are utilized for production of dry powder for inhalation. Among these techniques, ‘spray drying’ and ‘spray freeze drying’ and are the two frequently used techniques [4849]. Besides these techniques, there are also few other methods mentioned in the literatures, such as emulsion solvent diffusion (ESD) method [5051] and crystallization method [52].

  1.      Milling

Milling is a traditional approach to reduce the size from macro to micro. In milling technique, coarse particles are subjected to mechanical impaction or are passed through high pressure air in order to generate fine particles order to generate fine particles. There are many types of  milling techniques utilized in the pharmaceutical industries for the improvement of pharmacological and physical properties of drugs such as, bioavailability, solubility and stability [53]. Amongst many types of milling techniques, the most frequently used techniques are jet milling, ball milling and high pressure homogenizer (Figure 1.4). Although these techniques are easy to handle and are economical, they have several drawbacks including production of particles with irregular shape, rough surface, and high load of electrostatic charge [5455].  Milling process is further categorized into two groups, dry-milling and wet-milling. Among these two groups, dry-milling is well-validated techniques for the production of dry powder for inhalation. Jet-milling process is a typical example of dry-milling process. In jet-milling process, the fed coarse particles undergo high impaction of supplied compressed air/gas whereby the coarse particles break into micro-sized particles and are separated from larger particles by inertial impaction [55]. Jet-milling is also referred as fluid-energy milling [56] because during milling the particles get fluidized in the stream of supplied compressed air. This method is mainly suitable for thermolabile and meltable materials [57].  Ball-milling is both dry and wet milling technique that is frequently used in the pharmaceutical industries for reduction of particles size. It is composed of milling chamber in which different sized solid balls are loaded which are either made of iron, steel, ceramic, tungsten carbide or plastic polyamide. The coarse particles are loaded into the chamber along with milling balls into the chamber with and without a suitable liquid for wet-milling and dry milling respectively; then the chamber is rotated at desirable speed. The balls inside the chamber create impaction on the materials during rotation which result in the production of micronized particles [5859].  The size reduction in ball-milling is based on speed of vessel rotation and number of placed balls inside the chamber. This method has some drawbacks such as, it generates intensive heat which could alter the chemical properties of thermosensitive drugs, and also erodes metal during milling which may contaminate the product [6061]. High pressure homogenizer is an old technique that was first used by Muller in 1994 [62]. This technique is still commonly used in pharmaceutical field to reduce the particle size. This is a wet-milling technique in which salary of coarse particles in suitable liquid is passed through a narrow gap. During this, the coarse particles are reduced to fine particles due to static high pressure on the suspension [6364]. The desired size can be obtained by adjusting the pressure or by prolonging the homogenization cycle, however it is generally preferred to adjust homogenization cycle rather that adjusting pressure as there is the risk of damaging the product, especially thermolabile product, due to excess pressure [65]. Although high pressure poses the risk of damaging the product through the generation of excessive heat, the risk can be minimized by the use of a suitable carrier liquid which could prevent the increase in temperature [66]. Particles obtained by this technique are uniform in size and size distribution in comparison to those obtained by dry-milling techniques [67].  The micronized particles obtained in solution form can be dried to obtain dry powder for inhalation [68]. Products obtained by milling process are generally cohesive in nature due to high number of surface charge and they often exhibit poor flow property in comparison to the parent coarse particles. During milling process, the particles lose crystalinity property and turn into complete amorphous and disordered structures [6970].  During milling high input energy is required which creates a thermodynamically-activated surface on the particles. Thus, disordered structure of the particles influences the flowability property [71].  The milled powders show high cohesion force and are less effectively delivered from an inhaler device although the particle median size is smaller [72].

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