Spray Freeze Drying

SFD is a relatively new method for biopharmaceutical powder preparation. This method combines the atomization and FD processes to present a potential advantage for fine-powder preparation. The principle is to atomize the protein solution into a cryogenic medium, such as liquid nitrogen, to quench freeze the droplets. The frozen droplets are then dried by lyophilization (Fig. 4). This process involves no heat for drying so that heat denaturation associated with the SD process can be avoided.

3.3.1. Advantages of the SFD Powder-Formation Method

In our view, the assessment of a protein powder process can be performed based on six criteria (see Note 4). Based on these criteria, SFD is considered a highly attractive process because it inherits the overall advantages of lyophilization, and it involves a viable particle formation mechanism. SFD is highly efficient (e.g., >90% yield) because all atomized droplets can be collected and dried in a confined space with a small-surface area. Also, control of particle characteristics can be achieved through atomization, formulation, and drying conditions. Furthermore, SFD is compatible with a wide range of excipients and biopharmaceuticals, whereas some methods (including SD) may be impossible or impractical, e.g., only possible with extremely low yield. In addition, the relatively mild stresses posed by SFD on biopharmaceuticals can be ameliorated by judicious process and formulation approaches. Moreover, SFD can be readily scaled up or down, the latter an important point when conducting screening studies with small quantities of high-valued biopharmaceuticals. Finally, SFD has been sufficiently developed to the point where it is employed in manufacturing a marketed biopharmaceutical (Nutropin Depot™).

3.3.2. Overview of SFD for Biopharmaceuticals Applications

The utility of SFD has been demonstrated in numerous proteins and other biopharmaceuticals (see Note 5). In each case, the process and formulation variables can be optimized based on the physicochemical properties of the protein and its susceptibility to the process stresses as well as the intended application.

The use of SFD began with microencapsulation (5). For this application, spray freeze-dried protein particles are suspended in an organic solvent containing biodegradable poly(lactide-co-glycolide) (PLG). The suspension is spray frozen, and residual solvents are removed, e.g., by solvent exchange into ethanol followed by drying under vacuum. This process, referred to as ProLease®, is suitable for processing fragile drugs like protein, as the process is cryogenic, and there are no potentially destabilizing water-oil interfaces involved (33). This technology is the basis for an FDA-approved, sustained-release form of rhGH: Nutropin Depot.

Fig. 4. SFD experimental set-up presented graphically (A) and in a real system (B).

Spray Drying Spray Freeze Drying

Atomized droplet

Atomized droplet

Air drying droplet shrinks

Freeze drying droplet size unchanged

Dried particle

Porous particle

Fig. 5. Different drying mechanism by SD and SFD, resulting in distinct particle characteristics.

Another potential application for SFD proteins is the preparation of powders suitable for aerosol delivery. SFD is capable of producing highly porous particles suitable for pulmonary delivery (6). Conceptually, the hydrodynamic diameter of a large porous particle would be comparable to that of a physically smaller, high-density particle. Physically larger porous particle would endure less surface energy, thereby suffering less cohesive forces upon inhalation.

Needle-free skin delivery utilizing high-density SFD particles represents a unique drug delivery application. More specifically, a powder-based intraepidermal delivery focusing on epidermal powder immunization for vaccine delivery has been successfully tested preclinically and in human clinical trials.

The high degree of porosity and high-specific surface area that is obtainable by SFD allows for other opportunities as well. Along these lines, yet another application that has been recently touted for SFD is to improve the dissolution of poorly water-soluble drugs (34,35).

3.3.3. Particle Characteristics: SFD vs SD

SFD and SD produce powders of distinct physical properties. In general, the SFD powders have larger median particle size, larger specific surface area, and are more porous (i.e., less dense) than the SD powders. Whereas particle size is primarily dictated by the droplet size in both cases, the size of SD droplets shrinks during dehydration by hot air, and the particle shape is prone to change (see Subheading 3.2.4.). However, in the absence of hot air, atomized droplets during SFD maintained their spherical shape and size upon immediate freezing, and the subsequent drying process did not affect the shape or size (Fig. 5). Instead, the SFD process rendered the particles quite porous. The significant increase (approx 40-fold) in specific surface area for the SFD powder suggests a highly porous structure. Indeed, SEM analysis confirms that

Fig. 6. Particle manipulation by SFD: (A) porous and (B) dense particles.

the SFD particles are spherical and porous (Fig. 6A). Such characteristics are particularly suitable for aerosol applications.

Despite this inherent nature, SFD can dictate particle density using one of the two following approaches: increasing the solid content of the spraying solution or employing a unique formulation composition. For example, a SFD powder formulation prepared from a solution containing hepatitis B surface antigen vaccine, along with a tertiary composition of trehalose:mannitol:dextran (3:3:4) at a solid content of 35%, results in particles with a "shrunken" morphology (Fig. 6B). Indeed, this powder has a much higher particle density, which has been attributed to the flow of freeze concentrates during drying to fill the voids left behind by ice crystals (see Note 6).

3.3.4. Stabilization of Proteins Upon SFD

A series of stress events are associated with SFD, i.e., atomization, freezing, and dehydration. Each stress event can create different stress sources, and each stress source potentially activates various mechanisms that might damage peptides and proteins. The first step in SFD, atomization, provides shear force that can potentially cause protein conformational changes (36) and an air-water interface that possibly promotes adsorption, conformational changes, and aggregation (37,38). The stresses of the next two steps—freezing and drying—are conceptually the same as that posed by conventional lyophilization. For example, freezing can potentially cause damage to proteins via cold denaturation (39), concentration, and pH changes (2,40,41), as well as possible interaction at the ice interface (42,43). In addition, the loss of hydrogen bonding upon removal of water can cause the structural rearrangement of proteins.

Despite such challenges, a mechanistic rational approach can be successfully employed to stabilize proteins upon SFD. To date, some studies on biopharmaceutical formulations, primarily on rhDNase, anti-IgE MAb, and interferon-y, processed by SFD have appeared in the literature. In these studies, all potential stress sources were assessed, including using various excipients (e.g., amorphous sugars, surfactants, and so on), as well as processes like freeze/thaw, different freezing protocols, annealing, and so forth. Overall, in most cases, stresses posed by SFD were benign to the proteins. However, one particular stress appeared to intensify when the freezing rate exceeded a critical point. It was conjectured that a freezing-related stress event (probably due to shear stress associated with separation between ice- and freeze-concentrated phases) was responsible for protein denaturation upon SFD, ultimately leading to aggregation during drying and/or reconstitution. This freezing stress generated at the freezing front (ice-water interface) had an adverse effect on two therapeutic proteins, rhDNase and rhMAb, at a freezing rate beyond a critical point, which was estimated to be in the range of 8.4 x 10-4 and 3.4 x 10-3°C/s. When the ice front advances faster than this critical rate, it might alter protein conformation, leading to aggregation.

Yet, such an effect can be mitigated through stress relaxation by thawing (see Note 7). However, annealing may compromise the powder properties. Thus, a more effective approach might be to decelerate the freezing rate by making larger droplets. For instance, when three particle sizes were prepared (7 and 19 |im by two-fluid atomization and 32 |im by ultrasonic atomization), the aggregate content of anti-IgE MAb was 14.6%, 9.3%, and 0%, respectively. This is an important realization because SFD has not been considered benign to protein denaturation since the use of ultrasonic atomization to prepare powders with many of the proteins (Note 4).

3.4. Conclusions

SD and SFD present two important dehydration methodologies for biopharma-ceutical formulations with unique particle formation attributes. In the authors' opinion, the use of these two methods side-by-side will enable researchers to engineer particles with a wide spectrum of properties. Furthermore, by understanding the stress posed by each process, appropriate formulation strategies can be taken to minimize detrimental effects on biomolecules. More importantly, these two dehydration methods can not only serve as an effective research tool, but they also offer large-scale manufacturing capabilities.

4. Notes

1. The most widely used powder collection equipment is a cyclone separator, in which the particle-laden gas enters a cylindrical or conical chamber tangentially at one or more points and leaves through a central opening at the top. Solid particles, by virtue of their inertia, move toward the wall of the separator from which they are fed into a receiver. Working essentially as a settling chamber, the cyclone uses centrifugal acceleration to replace gravitational acceleration as the separating force. The centrifugal-separating force can be as high as 2500g in very small, high-resistance cyclone units (44). Powder collection by the cyclone is governed by complex fluid dynamic behaviors in the cyclone and receiver (45). The fluid behavior is affected by cyclone design (46). Therefore, the study to improve powder recovery using different cyclone and receiver designs and their configurations yielded useful information in this area (47).

Mass balance indicated that material loss in the bench-top SD system is due mostly to the attachment of sprayed droplets and dry powder to the wall of the apparatus and the cyclone's poor efficiency in collecting fine particles. Particle adhesion to the wall mainly occurs in the drying chamber, as well as in the cyclone, and is affected by the nature of the spray-dried materials and SD conditions. When the cyclone tends to retain a significant amount of the powder, recovering the powder from the cyclone as part of the product may be necessary and is only possible if the protein's stability and particle properties remain unchanged. Many researchers may find cooling down the cyclone helps maintain the protein's stability (unpublished observations). This may be true for very thermally labile proteins. However, despite the high temperature in the cyclone, most proteins investigated thus far are stable (20,48,49). One possible drawback with reducing the cyclone temperature is that the relative humidity of the air in the cyclone will increase and result in higher moisture content of the powder.

The design of the bench-top dryer (Buchi) has limitations in drying air flow, thereby limiting the batch size for SD. This is because of a bag-filter unit located downstream of the system (Fig. 2A) through which a respirator pulls the drying air. This filter unit presents a major resistance to airflow. Unfortunately, the resistance increases during the drying process as fine particles slowly build on the bag to foul the filter. A report (47) described modifications to the dryer to improve airflow. The modifications (Fig. 1C) include removal of the bag-filter unit, relocation of the aspirator, and addition of a vacuum-filter unit. This modified system increased the capacity of drying air, which allowed droplets to be dried at a lower inlet air temperature, whereas the outlet air temperature remained unchanged. This is important to the SD of heat-sensitive proteins. Both the removal of the bag filter and addition of a vacuum-filter resulted in nearly a 100% increase in the airflow rate. This allows nonstop SD of 2-L batch volume (up to 60 g of solid). Design changes, such as using dual cyclones and dual receivers in different configurations and cyclones of different designs, were tested and their effects on powder collection are minor. Also, the effect of using an antistatic-treated cyclone on powder collection was found to be insignificant. This information suggests that the bench-top spray dryer can be a useful production tool for preparing high-valued, low-volume protein products if powder collection efficiency is acceptable. Powder collection is affected more by protein formulation than by system design.

2. By definition, scale-up of a SD process involves increasing the powder output while maintaining product quality comparable to the small-scale process. Several factors influence the rate of product output; these include liquid throughput, solid (protein/excipient) content in the solution/suspension formulation, and product recovery efficiency. Low recovery of aerosol particles upon SD by cyclone collection is a concern owing to the poor deposition characteristics and cohesive nature of the powder as a result of very small particles. Although the general concept favors improved recovery associated with large dryers, the published results are lacking.

When liquid throughput is increased, the process to produce desired powders is limited by two factors: atomization and heating capacity. Of many atomizing mechanisms, air (two-fluid) atomization, based on kinetic energy, should be one of the most effective methods for generating fine sprays. However, its efficiency will decrease as the liquid feed rate increases, because the atomizing air will be acting on the liquid in a less homogeneous fashion. Drying capacity is measured by the rate of water removal from the dryer in the form of vapor. Drying air with more thermal energy can be achieved with higher temperatures and higher flow rates depending on the power source of the system. Larger dryers are normally equipped with larger power supplies to boost heating capacity. However, because the outlet air temperature is the most critical parameter in the SD process (50), heating capacity is limited by how high the inlet air temperature can go at a fixed outlet air temperature without compromising protein stability during the process.

A more direct way to increase powder production is to maximize the protein concentration or total solid content in the starting liquid formulation. The solubility of the active or inactive components limits the solid content of the feed solution. Nevertheless, the stability of the protein at high concentrations should be monitored and studied to ensure good stability prior to SD. The solid concentration can also affect particle characteristics, e.g., particle size, density, and morphology.

The dimensions of the drying chamber determines the scale of the dryer. The taller the chamber, the longer the residence times for the droplets and the larger the particles that can be produced. Under normal conditions, the bench-top spray dryers, such as Buchi-190, can only produce powders of less than 10 |J,m. The next scale-up is the laboratory-scale spray dryer that is commercially available in different sizes. The Yamato DL-41, Mobile Minor by Niro, and Tower-unit by Bowen Engineering represent the commonly used models at an increasing scale, which can produce powders with a particle size of typically 15, 20, and 30-50 |J,m, respectively. Dryers of larger scale are rarely used for therapeutic protein products because of limited production quantity.

3. In the drying chamber, the drying force for dehydration is the difference of partial water vapor pressure (or relative humidity) between the solid surface and the environment, ^droplet - PDA. When there is enough water initially to keep the surface saturated, the surface relative humidity is 100%. This surface vapor pressure decreases, as the subsurface can no longer supply sufficient water for surface saturation due either because of a decease in liquid diffusion or to the reduced moisture level within the protein solid. Therefore, the final moisture level of the protein solid is determined by two factors: the nature of the material and the humidity of the drying conditions. The former is caused by the interactions of water molecules with protein and/or formulation excipient molecules (51). The number and distribution of strong- and weak-binding sites on protein and the excipient molecules are among the intrinsic properties determining these interactions. Therefore, the moisture level of the protein solid can be primarily controlled by the humidity of the environment where the powder is manufactured, processed, and stored, resulting from the equilibration between the powder's residual moisture and the environmental humidity.

Under normal-drying conditions, the powders prepared by FD are drier than those prepared by SD, 1-4% vs 4-10% (52). During FD, the final moisture content is determined by the secondary drying step, virtually a vacuum-drying process performed near room temperature. Therefore, a long-drying time is normally used to ensure that the solid is dried to its equilibrium moisture content, i.e., in equilibration with the environment. However, inside the drying chamber of the spray dryer, the droplets encounter a continuously changing environment where the drying air temperature decreases and the relative humidity (%RH) increases along the chamber from moisture uptake. Thus, the driving force for heat and mass transfer decreases, as does the rate of water removal. If, during the SD process, it is assumed that the powder reaches equilibrium with the surrounding drying air, the final moisture content of the powder will be determined by the %RH of the air inside the collection vessel where the powder resides for the most time. Therefore, to produce drier spray-dried powders, higher inlet (outlet) air temperatures and lower liquid feed rates are required. Indeed, many spray-dried powders with moisture contents of less than 3% have been reported using inlet air temperatures of 140°C or higher (or outlet air temperatures of 90°C or higher) or liquid feed rates of 2 mL/min or lower (53). This represents an undesirable manufacturing condition that decreases the overall production rate and may impose potential adverse effects on protein denaturation (8,52). Therefore, subjecting the powder to a secondary vacuum-drying process might be a better alternative to reduce the moisture content of the spray-dried powder. Nevertheless, if this powder is exposed to a humid environment, it will pick up moisture until its moisture content equilibrates with the %RH of the surrounding environment. Regardless of the drying method, the final moisture of the powder is determined by the environment where the powder was further processed or stored.

Another approach to improve moisture removal is to decrease PDA by dehumidifying the air prior to entering the chamber, as mass transfer is affected by the driving force of Pdroplet - PDA. As the %RH of ambient air was reduced to 5% or lower by a dehumidifier, this additional dehumidification step does not further reduce the moisture content of the powder (52). However, the dehumidified drying air can improve the drying capacity, i.e., removing more water per unit of time.

4. The six criteria in the assessment of the protein powder process are (1) process efficiency/yield; (2) ability to control particle properties, such as size/size distribution, powder flowability, and density; (3) compatibility with a wide range of excipients and biomolecules; (4) mildness of process stress and impact on stability of biopharma-ceuticals; (5) flexibility for both scale-up and scale-down; and (6) suitability of manufacturing, i.e., ability to conduct steps in a straightforward manner aseptically.

5. Over the years, a host of macromolecules have been tested by SFD: rhDNase (6), rhMAb (6), recombinant human interferon-y (3,4), rhGH (5,54), recombinant human vascular endothelial growth factor (55), recombinant human nerve growth factor (56), recombinant human insulin-like growth factor-I (57), recombinant humanized monoclonal antibody (58), BSA (43), trypsinogen (58), lysozyme and lactate dehydrogenase (60), catalase (61), insulin (62), and vaccines, including hepatitis B surface antigen, diphtheria toxoid, and tetanus toxoid (63,64).

6. It remains unclear which factor affects particle shrinkage more—the solid content of the spraying solution, drying conditions, or formulation composition. Because increasing the solid content is limited by each excipient's solubility maximum and by the solution viscosity maximum where atomization ceases, we attempted to increase particle density by inducing particle shrinkage via formulation composition. As previously reported (59,60), formulations containing trehalose alone or the binary mixture of trehalose/mannitol still produce lower density particles even at high levels of solid content (>25%), whereas the mixture of three excipients, trehalose/mannitol/dextran, resulted in significant particle shrinkage. The working hypothesis is that either the freeze-concentrate structure collapses during ice sublimation (or possibly during secondary drying) when the freeze-concen-trate mixture softens and can flow to fill the void left behind by ice crystals. Certainly, it must be related to the maximally frozen freeze-concentrate's glass transition temperature (Tg'), which drops below the drying temperature. Yet, the collapse occurs only at the intraparticle level, because the powder would lose particle characteristics if the collapse took place at the interparticle level. More specifically, the composition of trehalose: mannitol:dextran at the 3:3:4 weight ratio was particularly effective in plasticizing the mixture and promoting particle shrinkage, thereby resulting in a higher tap density.

7. To test this hypothesized mechanism, a series of experiments were performed to anneal the spray-frozen powder of anti-IgE MAb:trehalose (60:40) at temperatures (-5, -10, and -15°C) higher than the primary drying temperature (-25°C) for 1 h before the lyophiliza-tion cycle began. As the annealing temperature increased, the surface area and aggregate content was reduced: 74.6 m2/g (nonannealing), 15.4 m2/g (-15°C), 11.3 m2/g (-10°C), and 8.3 m2/g (-5°C). Annealing temperatures near the melting point caused large ice crystals to grow at the expense of more energetically unfavorable small crystals (a migratory recrystallization process), thereby reducing the surface area of the particle. Indeed, such annealing appeared to relax the stress as the protein's aggregate content decreased as well: 15.4% (nonannealing), 5.6% (15°C), 4.2% (-10°C), and 2.1% (-5°C).


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