Binding of excipients is a poor predictor for aggregation kinetics of biopharmaceutical proteins

One of the major challenges in formulation development of biopharmaceuticals is improving long-term storage stability, which is often achieved by addition of excipients to the final formulation. Finding the optimal excipient for a given protein is usually done using a trial-and-error approach, due to the lack of general understanding of how excipients work for a particular protein. Previously, preferential interactions (binding or exclusion) of excipients with proteins were postulated as a mechanism explaining diversity in the stabilisation effects. Weak preferential binding is however difficult to quantify experimentally, and the question remains whether the formulation process should seek excipients which preferentially bind with proteins, or not. Here, we apply solution NMR spectroscopy to comprehensively evaluate protein-excipient interactions between therapeutically relevant proteins and commonly used excipients. Additionally, we evaluate the effect of excipients on thermal and colloidal protein stability, on aggregation kinetics and protein storage stability at elevated temperatures. We show that there is a weak negative correlation between the strength of protein-excipient interactions and effect on enhancing protein thermal stability. We found that the overall protein-excipient binding per se can be a poor criterion for choosing excipients enhancing formulation stability. Experiments on a diverse set of excipients and test proteins reveal that while excipients affect all of the different aspects of protein stability, the effects are very much protein specific, and care must be taken to avoid apparent generalisations if a smaller dataset is being used.