Degradation of lactoferrin caused by droplet atomization process via two-fluid nozzle: The detrimental effect of air–water interfaces

Abstract

ABSTRACTBiological macromolecules, especially therapeutic proteins, are delicate and highly sensitive to degradation from stresses encountered during the manufacture of dosage forms. Thin-film freeze-drying (TFFD) and spray freeze-drying (SFD) are two processes used to convert liquid forms of protein into dry powders. In the production of inhalable dry powders that contain proteins, these potential stressors fall into three categories based on their occurrence during the primary steps of the process: (1) droplet formation (e.g., the mechanism of droplet formation, including spray atomization), (2) freezing, and (3) frozen water removal (e.g., sublimation). This study compares the droplet formation mechanism used in TFFD and SFD by investigating the effects of spraying on the stability of proteins, using lactoferrin as a model. This study considers various perspectives on the degradation (e.g., conformation) of lactoferrin after subjecting the protein solution to the atomization process using a pneumatic two-fluid nozzle (employed in SFD) or a low-shear drop application through the nozzle. The surface activity of lactoferrin was examined to explore the interfacial adsorption tendency, diffusion, and denaturation process. Subsequently, this study also investigates the secondary and tertiary structure of lactoferrin, the quantification of monomers, oligomers, and ultimately, aggregates. The spraying process affected the tertiary structure more negatively than the tightly woven secondary structure, resulting in a 1.5 nm red shift in peak position corresponding to the Tryptophan (Trp) residues. This conformational change can either (a) be reversed at low concentrations via relaxation or (b) proceed to form irreversible aggregates at higher concentrations. Interestingly, when the sample was allowed to progress into micron-sized aggregates, such a dramatic change was not detected using methods such as size-exclusion chromatography, polyacrylamide gel electrophoresis, and dynamic light scattering at 173°. A more complete understanding of the heterogeneous protein sample was achieved only through a combination of 173° and 13° backward and forward scattering, a combination of derived count rate measurements, and micro-flow imaging (MFI). Finally, compared to the low-shear dripping used in the TFFD process, lactoferrin underwent a relatively fast conformational change upon exposure to the high air-water interface of the two-fluid atomization nozzle used in the SFD process as compared to the low shear dripping used in the TFFD process. The interfacial induced denaturation that occurred during spraying was governed primarily by the size of the atomized droplets, regardless of the duration of exposure to air.