There are several critical attributes that need to be addressed by judicious selection of formulation components and processing variables. These key properties also need to be retained over the inhaled product’s shelf-life [2]:
Biologic Stability – Biologic molecules are thermolabile and susceptible to mechanical stress compared to small molecules. Hence proper choice of excipients and processing technology may be essential to ensure that the biologic retains its stability. For example, with lyophilization, buffering agents and a cryoprotectant are essential to avoid degradation during the freezing step, while for spray drying an adequate surface-active compound may be required to prevent shear-stress induced degradation of the biologic molecule at the gas-liquid interface. For both lyophilization and spray drying [3], it may also involve choosing the right matrix whereby the biologic and excipients are locked into an amorphous form or utilizing excipients that form hydrogen bonds with the biologic to stabilize it during the drying process and also during storage.
Particle size – The particle size distribution is a key attribute as it directly influences the proportion of the particles and thus the dose of the biologic that may reach the lung. When discussing particle size for inhalation products, one refers to the aerodynamic particle size, Da, rather than the physical (geometric) particle size, Dp, as it’s the aerodynamic behavior that determines whether a particle of a given size deposits in the upper airways (i.e., the mouth and throat), in the lung, or is exhaled. The aerodynamic particle size is directly proportional to the geometric particle diameter but is also dependent on the particle density and a dynamic shape factor (i.e., morphology). For example, during inspiration, a low-density particle will behave as if it were smaller in size than its physical geometry and so will have less inertial momentum to impact in the upper respiratory tract.
Morphology – The shape of a particle can directly impact the aerodynamic particle size as it influences the particle packing, particle density, the flow around a particle, the cohesive nature of the particles, the specific surface area, and flowability. In many inhalation applications low-density particles, due to their larger physical size, are less cohesive (thereby easier to handle and fill), have improved aerosol performance and have reduced clearance by macrophages [3]. Low-density particles can be produced by formulation with pore-forming agents (i.e., volatile excipients) that are removed during drying and leave behind pores or voids in the particle (e.g. Pulmospheres™ [4], Figure 1) or by choice of process technology (e.g., Thin Film Freeze Drying [5] or Spray Freeze Drying [6] or Spray drying [7]). However, the lower density can be a drawback for therapies requiring a high lung dose as the larger volume of powder may necessitate the use of several blisters or capsules to achieve an effective dose. This can have a negative impact on treatment compliance.
Figure 1 Low density particle preparation for inhalable drug product a) pulmosphere b) thin film freezing and c) spray freeze drying
Dispersibility – Due to the small geometric particle size, inhalation powders tend to be cohesive which reduces their aerosol performance. This has historically been addressed by using carrier particles that are much larger in size (80-120 µm). Further, the dispersibility can be improved using hydrophobic amino acids (e.g. leucine), which may also reduce water uptake, and improvements of 15-20% in the aerosol fine particle fraction have been observed. Powder dispersibility can also be improved by adding lubricants, similar to those used in tablet formulations, and the use of magnesium and especially sodium stearate have been cited [3]. A relatively new technology, atomic layer deposition, has been used to deposit a nano meter sized coating of a metal oxide on budesonide particles resulting in a 2-fold improvement in the aerosol fine particle fraction [8].
Water uptake - Many formulations are hygroscopic and it’s crucial to address this factor as water uptake can change the particle size, aggregation potential, solid form (crystallization of amorphous components), and morphology thereby resulting in decreased drug product performance. As mentioned above, one approach to reduce water uptake is to include hydrophobic amino acids (e.g. leucine and methionine) in the formulation to form a shell around the hygroscopic inner core particle [2,9]. As an excipient, leucine has been the most investigated due to its dual function having both surface-active and hydrophobic characteristics. It slows down water uptake by forming a crystalline shell around the core particle and it further improves the dispersibility of the powders resulting in increased aerosol performance. The amount of hydrophobic amino acid excipient needed to achieve the desired effect is dependent on the properties of the specific biologic, but generally is no more than approximately 30-40 % of the composition.
Performance in the Dry Powder Inhaler (DPI) – The performance of the powder is inextricably linked to the specific DPI that is being used to deliver the powder. The emitted dose (proportion of the powder that leaves the device) and the fine particle fraction (proportion of the powder that is in a respirable size typically between around 1 and 5 µm) are used to characterize the dose of the biologic that can deposit in the lung during inspiration. The shear force generated by the DPI during inspiration should be adequate to overcome the cohesive forces of the particles for effective fluidization and de-agglomeration of the discrete powder particles from the device. If incomplete deagglomeration of the primary particles occurs, then the aerodynamic diameter of the agglomerated particles will be greater than the individual particles alone and may lead to increased upper airway deposition and a lowering of the lung dose.
As mentioned above, excipients need to be included in the formulation to achieve adequate stability of the biologic as well as the powder particles and to ensure effective aerosol performance. The most common excipients used to stabilize biologics are disaccharides, polysaccharides, and polyols like trehalose, mannitol, dextran, sucrose, maltodextrin [Table 1]. All of these have been shown to improve the stability [10-13] or the aerosol performance of biologic powders for inhalation [10]. The stabilization is hypothesized to occur according to one or both of the proposed mechanisms:
Vitrification/Amorphous Glass – The excipient forms an amorphous glass phase in which the biologic is immobilized whereby the rate of the degradation processes is reduced. The excipient preferably should have a high glass transition temperature (Table 1).
Water Replacement Theory – The excipient forms hydrogen bonds with the biologic replacing interactions of polar amino acids in the biologic with water, which stabilizes the biologic during drying. The sugar excipients mentioned above contain hydroxyl groups that effectively interact with the phosphate groups. Figure 2 demonstrates the stabilization of a live attenuated bacterial vaccine (Listeria monocytogenes) by spray drying and using sugar excipients. The replacement of water molecules with sugar also helps in maintaining the three-dimensional structure of biologics by providing sites for hydrogen bonding (14).
Figure 2 - Spray drying of live attenuated Listeria monocytogenes with sugars provides structural stability to the bacterial membrane. The phosphate group in the lipid bilayer (or peptidoglycan layer) forms multiple hydrogen bonds with the hydroxyl groups on the sugar molecules that stabilize the cell membrane of the bacteria during the spray drying process.
These stabilization mechanisms also apply to amorphous solid dispersion formulations when optimizing the choice of polymer to stabilize an amorphous small molecule. One of the most important parameters during inhalation formulation development is the ratio between the drug substance and the excipient. The amount of excipient should be large enough to saturate the available hydrogen bonding sites (if stabilization in that way is possible) or if stabilization is by vitrification, then the glass transition should be high enough to enable storage at reasonable temperature (preferably room temperature) and humidity conditions. Packaging measures (e.g., the addition of desiccants to limit moisture uptake, the use of reactive oxygen scavengers to quench reactive oxygen species, and an inert headspace in the vaccine vial) can also be implemented to reduce the impact of storage conditions on the stability of the biologic [10,14].
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