  The stabilization of liquid biological products is a complex task that
depends on the chemical composition of both the active ingredient and any
excipients in solution. Frequently, a large number of unique excipients are
required to stabilize biologics, though it is not well-known how these
excipients interact with one another. To probe these excipient-excipient
interactions, we performed molecular dynamics simulations of arginine -- a
widely used excipient with unique properties -- in solution either alone or
with equimolar lysine or glutamate. We studied the effects of these mixtures on
a hydrophobic polymer model to isolate excipient mechanisms on hydrophobic
interactions, relevant to both protein folding and biomolecular self-assembly.
We observed that arginine is the most effective single excipient in stabilizing
hydrophobic polymer collapse, and its effectiveness can be augmented by lysine
or glutamate addition. We utilized a decomposition of the potential of mean
force to identify that the key source of arginine-lysine and arginine-glutamate
synergy on polymer collapse is a reduction in attractive polymer-excipient
direct interactions. Further, we applied principles from network theory to
characterize the local solvent network that embeds the hydrophobic polymer.
Through this approach, we found that arginine enables a more highly connected
and stable network than in pure water, lysine, or glutamate solutions.
Importantly, these network properties are preserved when lysine or glutamate
are added to arginine solutions. Overall, we highlight the importance of
identifying key molecular consequences of co-excipient selection, aiding in the
establishment of rational formulation design rules.
