By Cole Tower
Many protein drugs must be formulated in the solid state, typically via lyophilization with a disaccharide, to maintain adequate stability during storage. Two theories exist to explain why a disaccharide stabilizes the protein: the vitrification theory and the water-replacement theory. For either of these theories to work, intimate mixing of the protein and the disaccharide is necessary. Properties of the protein, such as hydrophobicity, may result in separation of the phases into sugar-rich and/or protein-rich domains in the lyophilate, which may result in physical instability of the protein. Solid-state nuclear magnetic resonance spectroscopy (SSNMR) can be used to investigate the degree of mixing of the protein with the sugar. Matrix mobility has also been linked to physical instability and can also be measured using SSNMR.
Human serum albumin (HSA) is known to be physically unstable upon storage, and can be lyophilized with sucrose, trehalose, and/or mannitol to improve stability. Formulations were made with varying protein concentration and stabilizer and were put under accelerated stability conditions for up to 36 weeks. In all cases, formulations with higher sugar concentrations show longer 1H T1 relaxation times, which is correlated with sample mobility, which is correlated with physical stability. Too high sugar concentration can cause the formulation to become inhomogeneous. At high mannitol concentrations, mannitol crystalized and excluded itself from the matrix.