Mahboob Rehman is a research scientist at Nektar Therapeutics UK Limited, 69 Listerhills Science Park, Campus Road, Bradford, BD7 1HR, UK. Tel. +44 1274 305 540 Fax +44 1274 305 570 mrehman@uk.nektar.com

Mahboob Rehman

The size distribution of pharmaceutical aerosols is important in determining the deposition site of the inhaled drug particles in the respiratory tract. Cascade impaction is the industry's preferred method for particle size analysis, but measurements can be time-consuming and provide no information regarding the dynamics of the aerosol formation. Laser diffraction can accurately provide this information, allowing different drug formulations to be rapidly screened for delivery efficacy. Here, the two techniques have been combined to show that, in dry powder inhaler formulations, drug particles produced using supercritical-fluid crystallization disperse to a smaller particle size than micronized particles, providing more reproducible drug delivery.

Figure 1 Schematic of the supercritical-fluid process.			

Respirable drug delivery is becoming increasingly popular because it provides a non-invasive route with rapid drug uptake, not only for the treatment of respiratory complaints, but also for the systemic delivery of substances that cannot be delivered orally.1 Of the respirable delivery technologies available, dry powder inhalers (DPIs) are rapidly gaining acceptance because they overcome many of the disadvantages associated with devices such as metered dose inhalers (MDIs), particularly with regard to the Montr, Protocol covering propellant use.2	

However, the dispersion of drug particles to a respirable particle size during inhalation from a DPI is a concern because this must occur using the force associated with the patient's inhalation alone. DPI research has, therefore, concentrated on ensuring good particle dispersion, either through inhaler design or by engineering drug particles with specific properties. DPI research3 has shown that crystalline drug particles with smooth surfaces provide improved dispersion during inhalation compared with particles with rough surfaces. This is the conclusion of laser-diffraction-based size distribution studies performed on particles produced using a new pharmaceutical manufacturing process called supercritical-fluid (SCF) crystallization. SCF crystallization produces uniform crystalline particles with smooth surfaces, whereas the industry-standard practice of dry milling or micronizing provides less control of particle diameter and no control of surface morphology. Micronization also produces particles with rough surfaces.	

Drug particle dispersion and size distribution are key parameters in defining the efficiency of respiratory aerosols because they predict the deposition site for the drug within the respiratory tract.4 Laser diffraction was used to rapidly determine particle size distributions in DPI formulations containing micronized and SCF-crystallized particles. Because of their rough surfaces, the micro-nized drug particles will bind more strongly with excipients during storage, thus affecting the particle size of the powder plume emitted by the inhaler. Excipient–drug interactions are minimized in the case of the smooth-surfaced SCF-crystallized particles leading to increased dispersion.		

Figure 2 SEM of salmeterol xinafoate particles produced using the supercritical-fluid process; Figure 3 SEM of micronized salmeterol xinafoate particles.			

An SCF is any material maintained above a critical pressure and temperature that has the properties of both a liquid and a gas. SCF crystallization is a single-stage process that simplifies the particle manufacturing operation and can improve production efficiency. SCF crystallization produces micron-sized particles in a highly pure, crystalline, non-cohesive and solvent-free form. In addition, the physical and surface properties, as well as the crystal forms of the drug particles, can be easily regulated by varying the process parameters.				

SCF crystallization can achieve a yield of greater than 90% and is a self-contained environmentally friendly process that minimizes the risk of contamination and the use of organic solvents. Conventional multistage processes can create a multitude of solid-state manufacturing problems, including batch variation, crystal damage, processing inefficiency and out-of-specification product runs. SCF crystallization, however, gives fine, aerodynamic drug particles of optimal size (1–3 mm) and dispersibility by adjusting bulk powder properties, including geometric particle size and distribution, particle density, morphology, surface roughness and surface energy. Whereas the initial capital cost is high, because there are not many companies in the market that manufacture the specialized components, daily running costs are low because carbon dioxide (CO2) gas is inexpensive.	

One variation of SCF crystallization is solution-enhanced dispersion by SCFs.5 This uses an SCF, such as CO2, as a dispersing antisolvent and as a medium for extracting the solvent. In this process (Figure 1), the drug is dissolved in an organic solvent and the solution is co-introduced with the SCF through the annuli of a two-fluid coaxial nozzle. The two streams are thoroughly mixed in the nozzle to ensure efficient dispersion and mixing. To overcome problems with processing water-soluble compounds, a second solvent, such as ethanol, can be used. This is miscible with both water and supercritical CO2 and is introduced through the annuli of a three-fluid coaxial nozzle.		

Figure 4 Experimental set-up for particle size distribution measurements, showing Malvern's inhalation cell attached to an Andersen cascade impactor.			

This technique is distinguished by three features not available with other SCF techniques:	

Supercritical CO2 and the solvent solution are co-introduced into a jet in which the high velocity, turbulent CO2 flow accelerates both the mixing and particle formation processes. 

The composition of solvent, CO2 and precipitating material in the jet flow is fixed at the mixing point, providing the optimum conditions for uniform and continuous particle formation. 

The temperature and pressure in both the nozzle and vessel are kept constant throughout the process so that stable conditions for the formation of solid forms are established.

Dry particles, formed after solvent extraction by the SCF, are collected in a vessel maintained at a constant temperature and pressure. Unlike other methods using SCFs, this process results in particles with a size range of 0.5–30.0 mm that have a narrow size distribution, controlled solid-state properties and a low residual solvent content.	

Scanning electron micrograph (SEM) images of particles produced by the SCF process show that the particles have a plate-like morphology with defined and smooth edges (Figure 2). By contrast, SEM images of micronized particles of the same drug show irregular, agglomerated particles with less defined edges (Figure 3). Inverse gas chromatography results (Table I) also show that micronized particles have a higher surface energy than particles produced by SCF crystallization. This is possibly related to the surface roughness of the particles whereas the lower surface energy of SCF-produced powder may be because of the smoothness of the particle surfaces. 	

To analyse how this particle morphology affects the dispersion of a DPI formulation, a laser diffraction particle sizer (Spraytec; Malvern Instruments, Malvern, UK) and an Andersen-type cascade impactor (Copley Scientific, Nottingham, UK) were used (Figure 4). A cascade impactor discriminated particles based on their behaviour in airflow, reporting an aerodynamic particle size. The results are believed to correlate well with in vivo performance, hence the acceptance of impaction as the standard method for pharmaceutical aerosol analysis. However, measurements using impactors are time-consuming and provide no information regarding the dynamics of aerosol formation. Multiple actuations of the inhaler under test are also required to collect enough material for analysis.		

Table I Surface energy properties of salmeterol xinafoate particles produced by micronization and by the supercritical-fluid process. 						

Laser diffraction discriminates particles according to their volume. It utilizes the principle that particles of different sizes will scatter the light from a laser beam at an angle that is inversely proportional to their size. This non-invasive technique is rapidly gaining acceptance in the pharmaceutical industry. It allows the dynamics of each actuation of an inhaler to be followed, with much faster result reporting and manipulation compared with cascade impaction.	

In this experiment, the two techniques were combined. An inhalation cell (Malvern Instruments) was used to couple the laser diffraction instrument to the cascade impactor. This cell is specifically designed to allow laser diffraction measurements to be performed under controlled airflow conditions during testing of inhaler systems, enabling measurements in-line with a cascade impactor. This is achieved without causing a bias in the cascade impactor results.6	

Using this set-up, the standard impaction parameters such as the mass median aerodynamic diameter (MMAD), geometric standard deviation and fine particle fraction can be obtained for a series of actuations. The powder delivery dynamics and reproducibility can also be assessed for each actuation using laser diffraction. Data acquisition speeds of up to 2500 Hz (one measurement every 0.4 ms) are possible using laser diffraction, allowing the discrimination of the diffraction patterns produced by different parts of aerosol cloud such as primary drug particles and aggregates, as well as carrier particles and drug particles delayed by adhesion to the inhaler walls. These results can then be compared with the time-averaged aerodynamic size results obtained using the cascade impactor.		

Figure 5 Time-history showing the particle size measured during the actuation of the inhaler containing the micronized salmeterol xinafoate formulation.			

The DPI formulations tested in this study consisted of powders of salmeterol xinafoate produced by micronization and by the solution-enhanced dispersion SCF process. The formulations were prepared with DMV Pharmatose 325M 	

(DMV International, Veghel, The Netherlands) inhalation grade a-lactose monohydrate excipient and tested in a 13 mm3 Clickhaler inhalation device (Innovata Biomed, St Albans, UK). The laser diffraction measurements were done following the passage of the powder aerosol through a USP throat, immediately before the cascade impactor preseparator.	

Figure 5 shows the time-dependence of the particle size measured using laser diffraction for a single actuation of a DPI containing the micronized salmeterol xinafoate formulation. In this case, the powder was released from the device during a period of 400 ms. The results show that the delivered particle size remained relatively constant during the release of the powder, with a mixture of excipient and drug being observed at all times.				

Figure 6 shows the results obtained under the same measurement conditions for a DPI containing drug particles produced using the solution-enhanced dispersion technique. Separation of the fine drug particles and the lactose carrier was observed, suggesting increased deagglomeration of the fine drug particles. Dispersed, fine drug particles initially appeared in the measurement zone (up to 0.07 s). This was as expected because these particles are rapidly entrained in the airflow. During the mid-point of the spray plume (0.07–0.40 s), a mixture of lactose carrier and fine drug particles was observed. Finally, the tail of the spray plume (0.40–0.45 s) contained mainly fine drug particles. It is believed that these particles were delayed by adhesion to the inside of the inhaler.			

Figure 6 Time-history showing the particle size measured during the actuation of the inhaler containing the salmeterol xinafoate formulation produced by supercritical-fluid crystallization.			

The increased dispersion observed with the formulation containing drugs produced by SCF crystallization is believed to be related to the drug particle morphology and lower surface energy. The drug particles produced by SCF crystallization are individual crystals that are smooth-surfaced, thus aiding dispersion. By contrast, micronized drugs are generally rough-surfaced, which causes the lactose and drug particles to bind more strongly during storage, thus affecting the particle size of the powder plume emitted by the inhaler.	

Table I shows inverse gas chromatography data for salmeterol xinafoate samples produced by micronization and by the solution-enhanced dispersion SCF process. The results show that the dispersive component of surface energy (gsd) and the specific component of surface free energy (2Â³GASP) values of SCF-crystallized powder are lower than the micronized sample. Similarly, acid (KA) and base (KD) parameters of solution-enhanced dispersion powder are much lower than micronized material. Therefore, the results have demonstrated considerable changes in the surface properties of the powder prepared by the SCF crystallization process. These changes have possibly contributed towards better dispersion of powder and, as a result, a greater in vitro dose can be delivered to the lower part of lungs compared with micronized material.	

Table II shows in vitro cascade impactor data for salmeterol xinafoate samples produced by micronization and the solution-enhanced dispersion SCF process. These were analysed with a lactose blend (3.8% w/w). The cascade impactor measurements demonstrated that the SCF-processed salmeterol xinafoate has doubled the fine particle fraction compared with micronized salmeterol xinafoate. This correlates well with the increased dispersion observed for the SCF formulation during the laser diffraction measurements. 	

The laser diffraction and cascade impactor particle size measurements described in this article show that particles produced with the solution-enhanced dispersion SCF process will disperse better than micronized particles. This can lead to improved dose uniformity and more reproducible targeting of the correct deposition site for the drug within the respiratory tract.	

Table II Cascade impactor results showing the percentage emitted dose of salmeterol xinafoate particles prepared by micronization and the SCF process.						

Thus, SCF-produced particles are more likely to be deposited in the lower respiratory tract than micronized particles. The laser diffraction results show that the technique is a valuable tool in the rapid prototyping of new formulations in the pharmaceutical industry, with the results showing a good correlation with cascade impactor studies. 	

The cascade impactor provides a useful aerodynamic measure of particle size distribution, which can be utilized to compare different devices and formulations. The deposition pattern of the drug in the respiratory tract is determined by a complex interaction between the inhaler device, the formulation and the patient's inhalation technique.	

The improved performance of the SCF salmeterol xinafoate can be related to the lower surface energies of the particles and the low bulk density of SCF-processed material compared with the micronized particles. Therefore, the drug/carrier adhesion forces are of sufficient magnitude to enable the carrier particles to transfer the drug particles from the inhaler to the lower part of respiratory tract.	

This work demonstrates that products produced by the solution-enhanced dispersion SCF process improve aerosol performance compared with micronized material and give better dose uniformity. The drug particles produced by the SCF process are deposited where they are required in the lung and not in the throat; therefore, less drug is wasted. Considerable savings can be made because of a reduction in overages per device and increased patient compliance because of less frequent dosing.	

1.	J.S. Patten, J. Bukar and S. Nagarajan, "Inhaled Insulin," Adv. Drug Del. Rev. 35, 235–247 (1999).				

2.	United Nations Environment Program (UNEP) - Montr, Protocol on Substances that Deplete the Ozone Layer (1987).	

3.	P.G. Kippax, D.M.J. Higgs and M. Rehman, "Analysis of Dry Powder Aerosols Using Laser Diffraction," paper presented at Drug Delivery to the Lungs XIII  (December 2002, London, UK).	

4.	Task Group on Lung Dynamics, "Deposition and Retention Models for Internal Dosimetry of the Human Respiratory Tract," Health Physics 12, 173–208 (1966).	

5.	P. York and M. Hanna, "Particle Engineering by Supercritical Fluid Technologies for Powder Inhalation Drug Delivery," in proceedings of the Conference on Respiratory Drug Delivery V (Phoenix, Arizona, USA, 1996) pp 231–239.	

6.	C.E. Holmes et al., "Simultaneous Analysis of Respirable Aerosols via Laser Diffraction and Cascade Impaction," paper presented at Drug Delivery to the Lungs XII (December 2001, London, UK).	

Articles

Respirable drug delivery is becoming increasingly popular because it provides a non-invasive route with rapid drug uptake, not only for the treatment of respiratory complaints, but also for the systemic delivery of substances that cannot be delivered orally.