By Mural Quadros1 and Vivek Gupta1,*, 1Department of Pharmaceutical Sciences, College of Pharmacy and Health Sciences, St. John’s University, Queens, NY 11439, USA
*To whom correspondence should be addressed:
Dr. Vivek Gupta, College of Pharmacy and Health Sciences, St. John’s University, 8000 Utopia Parkway, Queens, NY – 11439, USA. Phone: 718-990-3929. Email: guptav@stjohns.edu
Pulmonary Route of Administration
Pulmonary drug delivery i.e., direct delivery to the lungs is considered as a first line approach for treating numerous respiratory diseases and improving systemic absorption of many therapeutics. Pulmonary drug delivery offer various advantages such as large surface area, high density of blood capillary network, escape from first pass metabolism and improved pharmacokinetics (1,2). However, despite these advantages, pulmonary delivery has many limitations such as short residence time, extensive clearance mechanisms, and dose limitations. As the particulate delivery systems become a mainline approach for pulmonary delivery, these limitations can be addressed by modulating the physicochemical properties of the inhaled particles, to achieve desired aerodynamic characteristics. This article briefly discusses various barriers for pulmonary drug delivery, implications of various physiochemical properties on inhalable particles, and a brief overview of landscape of particle engineering from pharmaceutical perspective.
Barriers for Pulmonary Drug Delivery
Particles upon inhalation encounter various lung defense mechanisms such as mucociliary clearance, alveolar macrophages, and enzymatic degradation. The first line of defense is mucociliary clearance which entraps all inhaled particles within a dense mucus layer. This mucus is then pushed toward the pharynx where they are swallowed into the gastrointestinal tract. However, mucociliary clearance depends on numerous physicochemical properties of the particles. For instance, some studies demonstrate that particles greater than 6 µm in size are subjected to more mucus entrapment and clearance than smaller particles (3,4). On the contrary, some studies indicate surface properties such as surface morphology and shape to have greater influence on mucociliary uptake and clearance (5).
Cellular host defenses such as alveolar macrophages adhere and engulf xenobiotic particles by binding to their receptors or by electrostatic interaction. Like mucociliary clearance, macrophage uptake also depends on various particle properties, discussed in detail in later sections. Enzymatic degradation in the lungs poses another barrier that inhibits the efficacy of inhaled particles. Cytochrome (CYP) P450 enzymes such as flavin-containing monooxygenases (FMO),
monoamine oxidase (MAO), aldehyde dehydrogenase, NADPH-CYP450 reductase are predominantly found in the lungs. Hydrolytic enzymes such as carboxylesterases and proteases are also present in the lungs which extensively degrade drugs and other molecules.
Fig. 1: Barriers for Pulmonary Drug Delivery & Modulating Physical Properties for Efficient Lung Deposition
Implications of Particle Physiochemical Properties on Lung Deposition
Size of the Particles
In an aerosol, it is the size, shape, surface charge and density of the individual particles that determine how they travel during inspiration and where they deposit in the airways. A key parameter is the aerodynamic diameter, which can be defined as “the diameter of a sphere of unit density that reaches the same velocity in the air stream as the specific particle that may be non-spherical and of arbitrary density”. The particle size distribution of inhaled particles is thus characterized by a mass median aerodynamic diameter (MMAD), which is an averaged function of aerodynamic diameter. Since MMAD considers the particle size, density, and shape, it may be used to predict the site of particle deposition in the respiratory tract. Studies indicate that particles with MMAD’s greater than 5 µm predominantly deposit in the oropharynx and tracheobronchial region, while particles in the size range of 1-5 µm can achieve deep lung deposition. This is because larger particles deposit by inertial impaction on the curvilinear proximal respiratory regions due to high velocity of respired air. In contrast, medium-sized particles can deposit due to sedimentation under gravity while smaller particles may deposit near alveolar region by diffusional deposition. However, submicron particles tend to remain suspended in the airway and have a high probability of being exhaled out. Therefore, the size of inhaled particles is crucial to predict and fine tune their deposition behavior in the respiratory tract.
Natural respiratory mechanisms can also lead to a change in the size of particles. For instance, the supersaturated warm water vapors in the respiratory tract may condense on the particles to increase their size by up to 4.6 times and thus change their deposition pattern. This phenomenon is termed enhanced condensational growth (ECG). Similarly, excipients too have the tendency to interact with the humidity of the airway, termed as enhanced excipient growth (EEG) (6). Along with deposition, the size of the particles also influences their rate of mucociliary clearance and uptake by alveolar macrophages (7,8). In a study, fasudil encapsulated liposomes of size < 200 nm escaped macrophages when administered via pulmonary route for treating pulmonary arterial hypertension (9). The macrophage uptake and elimination of microparticles can be circumnavigated by grafting PEG on its surface. Studies indicate that PEG grafted on microspheres swells in the moisture present in the respiratory tract thereby imparting stealth like characteristics to escape macrophagic uptake (10).
Density of Particles
The density of inhaled particles can also affect its aerodynamic behavior, with lower density improving particles’ respiratory deposition. For instance, porous insulin loaded PLGA particles had improved in-vivo aerodynamic properties with higher bioavailability and increased residence time of ~96 hours compared to its non-porous counterpart which displayed a residence time of just 4 hours (11). Porous drug delivery carriers have also been observed to have reduced clearance by macrophages. Porous PLGA-PVP particles had reduced phagocytosis by RAW 264.7 macrophages (12). In another similar study, large PEG-PLGA porous particles had reduced uptake by isolated rat alveolar macrophages (13). Although large porous particles escape macrophage clearance and exhibit less aggregation as compared to nanoparticles, they are limited by size dependent penetration and diffusion into the diseased tissues. To overcome this complication, researchers have introduced “Trojan” particles which encompass smaller sized particles within them, thus assisting the delivery of smaller particles to the diseases tissue. One such trojan polymeric microparticle which encapsulated vitamin E in a nano emulsion showed sustained release of vitamin E over an extended period of time (14). Therefore, the density of the inhalable particles can be used to modulate its respiratory deposition and also clearance by macrophages.
Surface Charge
Surface charge is another variable that affects deposition of particles in the respiratory regions. The charged particles deposit in the lungs by way of electrostatic precipitation. Briefly, aerosolized charged particles precipitate when they are in close proximity to oppositely charged respiratory surfaces (15). Studies indicate that charged particles have higher deposition efficiency than that of their uncharged counterpart. For instance, charged radon particles had higher airway deposition than neutral molecules in a simple tracheal model (16). Along with respiratory deposition, the particles’ surface charge also governs their cellular uptake and stability (15). The presence of negatively charged mucin can also be exploited to enhance retention of positively charged particles and thus decrease mucociliary clearance (17). Moreover, studies report that surface charge can also be used to escape macrophage clearance by dysregulating secretion of cytokines responsible for triggering the clearance mechanism (18). Charged particles can also interact with pulmonary surfactants to affect their clearance by macrophages. In one study, differences were observed in the interactions of positively and negatively charged silica particles with both synthetic as well as exogenous pulmonary surfactants and demonstrated a strong columbic interaction between positively charged particles and surfactant to form micron sized aggregates (19). Cationic particles can also bind to the negatively charged sialic acid residues present on the macrophages, thus increasing their clearance (20). The influence of surface groups on polystyrene particles was investigated with its incidence of uptake by macrophages. More positively charged anime functionalized polystyrene particles had enhanced macrophage clearance than carboxylic, sulphate and hydroxyl functionalized particles (21).
Shape of Particles
The shape of the inhalable particle has great influence on its aerodynamic behavior and ultimately its lung deposition. The ratio of particle length to its width is commonly expressed as the elongation ratio or aspect ratio (AR). An AR ratio of greater than 1 indicate elongated or aspherical structure (22). The deposition of asymmetrical particles in the respiratory tract is guided mainly by its diameter compared to its length. Due to their aspherical structure, the deposition of aerosolized particles occurs by an interception mechanism when they come in contact with the small airways. Therefore the shape of the particle can be used to modulate its deposition in the respiratory region (23). Computational fluid dynamics (CFD) is one technique that can be used to predict the deposition trajectory of inhaled particles (24). CFD was used to study the deposition efficiency of high AR fibers. These simulations suggest that particles with a narrow equivalent diameter (dp ) of 6–7 µm have high deposition in the upper and mid-bronchi regions while fibers with dp of 4–6 µm are anticipated to deposit in the deeper lung regions (25). These results suggest that particles with high AR have the ability to penetrate into the distal lung regions (26).
The shape of the particle can also be exploited to escape clearance by alveolar macrophages. After deposition of the particles in the lungs. The cellular uptake occurs via phagocytosis and is dependent on particle cell adhesion, strain energy for membrane deformation and particle concentration at the cellular membrane. Adhesion between particle and cell is the first step in cellular uptake and can be increased by increasing the contact area of the particle. Prolate and oblate ellipsoids have greater attachment efficiency towards RAW 264.7 cells than spherical particles. However, the internalization efficiency was found to be in the order of oblate ellipsoids> spheres> prolate ellipsoids which indicates that particle geometry plays a major role in cellular uptake (27). The next step in phagocytosis involves an energy intensive process for remodeling the cell membrane to form a cup like structure (28). Therefore, particles that require the least energy for cell membrane remodeling are more readily taken up by the cells. In this context, the highest particle uptake was found for particles of spherical nature followed by cubic, rods and disc-shaped particles. The shape related limitation of a spherical particle can be camouflaged by coating its surface to decrease its shape effect (29). The point of first contact (defined as Ω) between the particle and macrophage likely has great influence on internalization time and mechanism of clearance by macrophages. Particles that attach at Ω < 45° are thought to be easily internalized and cleared by the macrophages, while particles that attach at Ω > 45° escape internalization by macrophages (7). Higher macrophage uptake was confirmed in another study for spherical particles compared to rod shaped particles (30).
Researchers have designed stimuli responsive particles which change their shape in the presence of stimuli. For instance, pH responsive hydrogel capsules that transform from a discoid shape to oblate ellipsoids in response to a change in pH had ~60% reduction in macrophage uptake as compared to their spherical counterpart (31). The shape of the particle can also govern its interaction with the pulmonary surfactant. Molecular dynamics predicted that rod-shaped particles had greater penetration ability than barrel-shaped particles.
From Bench to Bedside
The physicochemical properties of inhalable particles including size, density, surface charge, and shape are all tunable variables that can be modulated to attain desirable respiratory deposition and effect. In this context, particle engineering by formulating inhalable particles of a specific shape, size, or density has opened new therapeutic opportunities. Pharmaceutical companies like Liquidia have designed Particle Replication In Nonwetting Templates (PRINT®), a patented process that uses mold templates to manufacture inhalable particles with homogenous properties (32). Comparable bioavailability was observed in a PRINT® manufactured treprostinil dry powder formulation when compared with the standard marketed formulation (Tyvaso®). iSPERSE*TM is another particle engineering technology designed by Pulmatrix, with the ability to manufacture inhalable particles of a small and homogenous size. Moreover, they can also generate aerosols with a high fine particle fraction (FPF) and emitted dose (ED). Clinical studies with iSPERSE*TM manufactured itraconazole particles showed better lung exposure than its oral counterpart to treat allergic bronchopulmonary aspergillosis. PulmoSphere® by Novartis is another particle engineering technology capable of manufacturing porous particles with low density. Aerosphere®, by AstraZeneca Pharmaceuticals LP is an FDA approved product for COPD manufactured using PulmoSphere® technology. Development of these particle engineering technologies offers great control on the inhalable properties, thus controlling their in vivo deposition and aerodynamic behavior.
Conclusion
Pulmonary delivery is a non-invasive and patient-compliant route of drug administration. The physicochemical properties of the inhalable particles largely govern their aerodynamic behavior and thus their efficacy. These properties have a complex relationship with each other, with no one property singly accountable for an inhaled particle’s aerodynamic behavior. Tailoring particle properties such as size, density, surface charge and shape can govern its respiratory deposition as well as its ability to evade alveolar macrophages. Technologies like PRINT, Pulmosphere, Technosphere etc. have paved the way for developing precise and homogenous inhalable particles, which can also be extended towards development of personalized medicines.
Acknowledgements & Conflict of Interest Declaration
This work was supported by research funds provided to VG by College of Pharmacy and Health Sciences (CPHS), St. John’s University. MQ was supported by a teaching assistantship from the College of Pharmacy & Health Sciences. The authors declare no conflict of interest.
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