Impact of Sugar Molecular Weight on the Miscibility and Stability of Lyophilized and Spray-Dried Protein Formulations

Poor stability of biological products such as proteins is a major challenge facing the biopharmaceutical industry. Poor stability is usually mitigated by formulating these products in the solid state, employing sugars as stabilizers. Several studies have pointed out the superior stabilizing ability of disaccharides, including sucrose and trehalose, as compared to polysaccharides such as dextrans. The aim of this study was to investigate the impact of excipient molecular weight on miscibility with a model protein, bovine serum albumin (BSA). Aqueous solutions containing a binary combination of a sugar-based stabilizer and BSA were dried using different methods (air drying to form films, spray drying, and lyophilization). The stabilizers tested varied in molecular weight and were dextran 6, 70, or 2000 kDa, hydroxypropyl methyl cellulose (HPMC), and trehalose. Miscibility was evaluated using a variety of techniques including confocal fluorescence microcopy, infrared and Raman microscopy, and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. The stability of BSA in dried mixtures subjected to accelerated storage conditions was also measured. BSA was more stable in the presence of dextran 2000 kDa compared to dextran 70 and 6 kDa, while stability was highest in trehalose and lowest in HPMC. From ssNMR spectroscopy, BSA-Dex 2000 kDa and BSA-trehalose were miscible over 20 and 5 nm length scales, BSA-Dex 6 kDa was miscible over a 20 nm length scale and phase-separated over a 5 nm length scale, while BSA-Dex 70 kDa and BSA-HPMC were phase-separated over both length scales. It was postulated that for dextran, the size of the polysaccharide relative to the size of the protein determined the extent of the system miscibility and stability. A smaller or similar polysaccharide size compared to that of the protein, as in the case of BSA-Dex 6 kDa and BSA-Dex 70 kDa, leads to depletion-induced phase separation. A much larger polysaccharide size compared to that of the protein allows the protein molecules to be trapped within a polysaccharide mesh, resulting in a miscible system. This study suggests that the impact of the relative size of the stabilizer and protein on miscibility is more complex than previously considered.