Alignment of Magnetite-Loaded High Aspect Ratio Aerosol Drug Particles with Magnetic Fields

Abstract

The present note demonstrates magnetic field alignment of high aspect ratio pharmaceutical aerosol particles loaded with magnetite nanoparticles. This alignment is used to reduce the penetration of the particles through polycarbonate membrane filters when a magnetic field is generated parallel to the face of the membrane. Possible application of magnetic field alignment to noninvasively target respiratory tract deposition during aerosol drug delivery is discussed.

While inhaled pharmaceutical aerosols have long been prescribed to treat asthma and chronic obstructive pulmonary disease (COPD), more recently the inhalation route has been explored for delivery of antibiotics to treat lung infections and chemotherapeutic agents to treat lung cancers. In the latter case especially, the onset of disease is often limited to specific locations within the lung. Clearly, the ability to target drug delivery to the site of disease would be of great clinical benefit, allowing for relatively large doses to be delivered to the targeted site, while reducing side effects associated with delivery to non-targeted regions of the lung.

For the inhalation route, the dose of drug delivered to the lung is primarily determined by deposition of aerosol particles onto airway walls, after which drug absorption and translocation due to clearance mechanisms play secondary, though important, roles. Control of particle size and density is known to allow a certain amount of broad targeting of aerosol deposition between, for example, central versus peripheral regions of the lung. Broadly targeting deposition in such a manner to airways either more or less distal to the trachea is now a widely used approach in optimizing aerosol delivery systems. However, the ability to noninvasively target deposition to one lung but not the other, or to one lobe of a lung but not other lobes, remains elusive. One can imagine this second type of targeting, which can be thought of as localized targeting, being particularly useful in cases where disease is limited to a localized region within the lungs.

Some time ago, Chan and Gonda (1989) proposed that elongated, high aspect ratio drug particles might be particularly well suited for targeting the peripheral regions of the lung, and this concept has since been explored by a number of other researchers (Johnson and Martonen 1994; Zeng et al. 2000; Crowder et al. 2002). Briefly, above an aspect ratio of about 5, the aerodynamic diameter of a randomly oriented, elongated particle is smaller than its mass-equivalent spherical diameter, meaning that the elongated particle is less likely to deposit by inertial impaction onto the walls of the upper airways than is a compact particle of the same mass. However, as the elongated particle penetrates into the peripheral regions of the lung, where airway diameters are much smaller, the possibility that a tip of the particle will intercept an airway wall enhances the likelihood of deposition over that expected for the same mass-equivalent compact particle.

The combined conclusion that elongated particles are especially adept at avoiding deposition in the upper airways, but are expected to exhibit enhanced deposition in the peripheral airways, makes them candidates for broad targeting of the peripheral airways. That said, the increased deposition in peripheral airways that can be expected due to interception is limited by the tendency of elongated particles to align with their longer axes parallel to airway walls due to shear in the entraining airflow. We have recently been exploring the possibility that deposition by interception might be increased if the orientation of elongated particles inhaled into the lung could be controlled by some noninvasive means. Because peripheral airways in the lung are essentially oriented randomly, aligning particles in any one direction would yield randomly oriented particles with respect to airways, and increase interception over that expected for particles aligned parallel to airways. Such control would not only be useful in enhancing broad targeting to the peripheral airways, but, if orientation was adjusted to promote interception in only one location of the lung, the technique could allow for localized targeting to that particular location. This type of localized targeting would require small particle diameters in order to limit deposition due to impaction and sedimentation in both targeted and non-targeted regions. Indeed, targeting by orientation is likely to be most effective in the bronchioles and first few generations of the alveolar region, where airway diameters are small, but deposition due to sedimentation does not dominate that due to interception, as likely occurs in the last few generations of the alveolar region.

As a first step towards the goal of achieving localized targeting of inhaled aerosols, we investigated the possibility that high aspect ratio aerosol drug particles loaded with superparamagnetic magnetite would align with their long axes parallel to an applied magnetic field. In general, alignment of an elongated, paramagnetic, or ferromagnetic particle occurs due to competing tendencies for the particle's internal magnetization vector to orient parallel to the externally applied field and to the particle's long axis (Shine and Armstrong 1987). While almost all substances used as drugs are expected to have very low magnetic susceptibilities, and as such exhibit little or no response to all but extremely high magnetic fields, it is possible that low susceptibility particles can be made magnetically responsive by loading them with a material of high susceptibility, such as magnetite. For example, though for different application, Kim et al. (2005) have recently reported aligning magnetite loaded, plate-like particles with a 20 mT magnetic field. In a similar vein, Zauscher and Humphrey (1997) have previously demonstrated that alignment of wood fibers with magnetic fields on the order of 100 mT was feasible if the fibers were first loaded with smaller, compact nickel particles.

For the study described here, high aspect ratio particles of cromoglycic acid (CA) were prepared by crystallization according to the method of Chan and Gonda (1989). Dried CA powders were dispersed in deionized water to yield suspensions containing 2 mg CA/ml water. Superparamagnetic magnetite particles were prepared by precipitation following the method of Cheng et al. (2005). Transmission electron micrographs confirmed that the magnetite particles were reasonably monodisperse in diameter, with a mean diameter of about 10 nm, as was also reported by Cheng et al. (2005). Magnetite was added to the CA suspensions by one of two methods: in the first, dilute suspensions of magnetite in deionized water were sonicated for 30 minutes to deaggregate the magnetite particles as much as possible, then these suspensions were used to disperse the CA powders; in the second method, the magnetite was added in the last stage prior to crystallization of CA, and then the combined CA/magnetite powders were dispersed in deionized water. In either case, the concentration of magnetite in suspension ranged from 10 to 20% by weight over several preparations.

Formulations of CA, CA with magnetite added post-crystallization, and CA with magnetite added pre-crystallization, were nebulized using conventional jet nebulizers (Up-Draft II; Hudson Respiratory Care, Inc., Temecula, CA) driven by compressor (PulmoAide 5650C; DeVilbiss Canada, Barrie, ON), and drawn along with ambient, drying air at 10 l/min into a large volume delivery chamber. Evaporation of the nebulized droplets left behind an aerosol of CA particles, and, for the two formulations containing magnetite, allowed any free magnetite particles not attached to CA particles in suspension to adhere to them upon drying of the droplet. Although a small fraction of the magnetite in suspension may be nebulized in extremely small droplets that do not contain CA particles, and thus is not associated with the CA particles, analysis of CA/magnetite aerosols by scanning and transmission electron microscopy revealed very little evidence of free magnetite.

The aerosols were sampled from the delivery chamber onto 0.2 μ m pore size polycarbonate membrane filters (Isopore GTTP; Millipore, Billerica, MA) for analysis of particle sizes by scanning electron microscopy (SEM). Figure 1 shows particles of CA collected in such a manner. Figure 2 displays volume-weighted distributions in diameter and length for CA and CA/magnetite aerosols, determined for either aerosol from nine SEM images, each containing between 200 and 300 particles. Particle sizing rules followed those established by Platek et al. (1992) for asbestos fibers. The presented data is for the CA/magnetite formulation with magnetite added prior to crystallization of CA; however, there were no apparent size differences between the two CA/magnetite formulations. Particle lengths and diameters both tended to be larger for CA/magnetite than for CA alone, possibly due to increased aggregation of CA crystals in suspension in the presence of colloidal magnetite.

FIG. 1 Cromoglycic acid aerosol after nebulization and drying, sampled from a large volume delivery chamber onto a 0.2 μ m pore size polycarbonate membrane at 0.21 l/min.

FIG. 2 Normalized cumulative volume distributions in diameter and length for cromoglycic acid (CA) and CA/magnetite (CA/mag) aerosols determined from analysis of scanning electron micrographs.

As a test of particle alignment with an external magnetic field, and of the effect of alignment on interception, aerosols produced from each of the three formulations were drawn from the delivery chamber through polycarbonate track-etched membrane filters with pore size of 5 μ m and diameter of 47 mm (Isopore TMTP; Millipore, Billerica, MA). The membrane filters were sealed tightly within an in-house filter casing, and the flow rate through the membrane was monitored to within 0.21 ± 0.02 l/min with a low flow rate rotameter (FL-2010; OMEGA Canada, Laval, QC). A bacterial air filter (Respirgard; Vital Signs, Inc., Totowa, NJ) was placed downstream from the membrane to capture the aerosol that penetrated the membrane. The masses of CA collected on the membrane and the bacterial filter were measured by washing with 0.01 N sodium hydroxide, to convert the CA to its sodium salt, and subsequent assay by UV spectrophotometry (8452A; Hewlett-Packard, Palo Alto, CA) at a wavelength of 326 nm. On average, the total mass recovered from both filters (i.e., the challenge mass) was 72 ± 16 μ g (mean ± one standard deviation, n = 12).

Penetration efficiency, calculated as the mass of CA recovered from the bacterial filter as a percentage of the total mass recovered from both filters, was determined for each formulation, with and without the presence of a magnetic field across the membrane filter. The magnetic field was produced parallel to the face of the membrane by placing neodymium permanent magnets (2″ × 2″ × 0.5″ N42; Indigo Instruments, Waterloo, ON) on either side of the filter casing. The flux density at the center of the membrane was measured using a gauss meter (F.W. Bell 5180; Sypris Test and Measurement, Orlando, FL) to be 90 mT.

Table 1 displays the penetration efficiency for each formulation, with and without the magnetic field across the face of the membrane filter. As expected, for the formulation of CA alone, there was no difference in the penetration of aerosol particles through the membrane with and without the magnetic field. In contrast, for both the formulations containing magnetite, penetration efficiency decreased significantly (one tailed student's t-test, p < 0.05) when the magnetic field was produced across the face of the membrane. Neither of the two methods for formulating CA and magnetite proved clearly superior to the other; however, from a handling perspective, adding the magnetite prior to crystallization of CA creates as an end product a powder that needs only to be dispersed in water prior to nebulization. Figure 3 shows scanning electron micrographs of CA/magnetite particles captured on membranes with and without an external magnetic field. Particle alignment with the magnetic field is clearly visible in Figure 3b.

FIG. 3 Cromoglycic acid particles loaded with magnetite (added after crystallization of cromoglycic acid) captured in the pores of a 5 μ m pore size polycarbonate membrane (a) without a magnetic field across the face of the membrane and (b) with a 90 mT magnetic field. As they were captured, individual aerosol particles formed clusters at the site of each pore.

TABLE 1 Penetration efficiencies through 5 μ m pore size polycarbonate membrane filters with and without external magnetic fields

	Penetration Efficiency (%)
Formulation	No field	With field
Cromoglycic Acid (CA)	12.9 ± 5.8	13.0 ± 3.6
CA with magnetite	19.6 ± 5.8	5.5 ± 0.1
added post-crystallization
CA with magnetite	12.7 ± 1.2	4.5 ± 0.6
added pre-crystallization
All entries expressed as mean ± one standard deviation, n = 2.

In summary, high aspect ratio aerosol particles of CA loaded with magnetite were aligned with external magnetic fields, and this alignment reduced aerosol penetration through polycarbonate membrane filters. We believe that magnetically responsive, high aspect ratio aerosols show promise for targeting inhalation drug delivery to specific locations in the lung. Unlike drug targeting techniques that rely on the magnetic force exerted on a particle, targeting by magnetic alignment does not require magnetic field gradients, and can be achieved with smaller amounts of magnetic material compared to the active drug. Our future work will examine the effects of particle alignment on penetration through geometries representative of the small, bifurcating airways found in the lung.

Acknowledgments

The authors acknowledge funding from the Natural Sciences and Engineering Research Council of Canada, the Alberta Ingenuity Fund, and the Killam Fund. The authors also thank Dr. Ming Chen for the electron microscope imaging used in this study.