Polar vs Non-polar Excipients in BSA Aggregation

In recent years, there has been a rapid expansion in the development of new protein therapeutics in the pharmaceutical industry. Problematically, proteins are conformationally and colloidally marginally stable, and thus it is necessary to formulate them using buffering and stabilizing agents.

In this study, we use experimental and computation techniques to evaluate the aggregation pathway, as well as the stabilizing properties of a range of excipients on a model protein; bovine serum albumin (BSA).

Our results show that polar excipients (L-arginine and trimethylamine N-oxide) decrease BSA stability, whereas non-polar excipients (L-alanine and L-valine) enhances it. To elucidate molecular mechanisms behind these trends, we constructed a multi-step unfolding and aggregation model. Upon thermal stress, the protein system is driven to form a disrupted dimer/trimer species across a steep activation energy barrier. This acts as the primary kinetic bottleneck, as once these intermediate states form sufficiently, large aggregates in tetramers and pentamers form across a significantly lower energy barrier. High temperature molecular dynamics simulations reveal that the unfolding mode of the monomer is characterized by the detachment of its C-terminus domain, creating an extended, aggregation prone structure. Crucially, in the native structure, this flexible region contains the only prominent solvent exposed hydrophobic patch on an otherwise hydrophilic protein surface. Therefore, hydrophobic excipients are predicted to interface with this hotspot, stabilizing the folded structure and suppressing downstream aggregation formation.

Overall, this work demonstrates that understanding the specific protein aggregation mechanisms is key for the rational selection of stabilizers.