Abstract
Mannitol, leucine, and trehalose have been widely used in spray-dried formulations, especially for inhalation formulations. The individual contribution of these excipients on protein physical stability in spray-dried solids was studied here using bovine serum albumin (BSA) as a model protein. The spray-dried solids were characterized with scanning electron microscopy, powder X-ray diffraction, and solid-state Fourier-transform infrared spectroscopy to analyze particle morphology, crystallinity, and secondary structure change, respectively. Advanced solid-state characterizations were conducted with solid-state hydrogen-deuterium exchange (ssHDX) and solid-state nuclear magnetic resonance (ssNMR) to explore protein conformation and molecular interactions in the context of the system physical stability. Trehalose remained amorphous after spray-drying and was miscible with BSA, forming hydrogen bonds to maintain protein conformation, whereby this system showed the least monomer loss in the stability study. As indicated by ssNMR, both crystalline and amorphous forms of mannitol existed in the spray-dried BSA-mannitol solids, which explained its partial stabilizing effect on BSA. Leucine showed the strongest crystallization tendency after spray-drying and did not provide a stabilizing effect due to substantial immiscibility and phase separation with BSA as a result of crystal formation. This work showed novel applications of ssNMR in examining protein conformation and protein-excipient interaction in dry formulations. Overall, our results demonstrate the pivotal role of advanced solid-state characterization techniques in understanding the physical stability of spray-dried protein solids.