To stabilize a protein formulation to avoid aggregation, we need to know how proteins are aggregated. For proteins to aggregate, the first step is to approach ~3 Å, the second step is desolvation – the removal of everything between the two proteins including water and other molecules in the formulation. Then proteins can go through structural changes over time and eventually form intermolecular β-sheets to be irreversibly aggregated. This serves as a nucleus for the formation of more intermolecular β-sheets and eventually large aggregates.
Excipients are the developer’s ingredients for engineering stable formulations. Different excipients provide different influence and control over different steps described above. The first step is governed by proximity energy and can be hindered via electrostatics, e.g. protein charges, therefore, controlling over pH, ionic strength and dielectric can be very effective.
The second step – desolvation – is governed by protein hydration and exclusive volume. Sugars, some amino acids, can help control protein hydration. Some polymeric chains can be linked to protein surfaces to increase the exclusive volume, therefore stabilizing the protein formulation.