Macromolecular dimensions of a synthetic polyelectrolyte as a factor in its interactions with protein and cells: desirability for longer chains

Understanding the mechanism of interactions between synthetic polyelectrolytes and ionic matter, which are ubiquitous in living systems, such as proteins and cells, is a fundamental challenge and an important requirement for their clinical development as biomaterials and drug delivery systems. In contrast to small molecules or proteins, in which an active center or epitope largely defines the binding pattern, ionic polymers utilize a plurality of repeat units, which are capable of only weak interactions with the target individually. Although it can be expected that the effects of the chain length and cooperativity play important roles in such interactions, these effects are often overlooked in practical research. Nevertheless, preclinical experience demonstrates the existence of an activity-molar mass relationship in polyelectrolytes. Here, we focus on studying the <i>in vitro</i> interactions of a clinical-grade macromolecule, poly[di(carboxylatophenoxy)phosphazene] (PCPP), for which such a relationship has already been established <i>in vivo</i>. We found that polymers of various molar masses show different <i>in vitro</i> avidities to a model antigenic protein, lysozyme, with longer PCPP chains displaying lower dissociation constants and reduced entropic penalties. Higher-molar-mass polymers result in less compact complex morphologies, in which the protein is more easily accessed by the antibody. The trend of the greater <i>in vitro</i> activation of engineered immune cells with longer polymer chains is also observed. Results suggest that morphological and entropic benefits provided by higher-molar-mass polymers are critical in explaining previously observed <i>in vivo</i> trends, and these aspects should be prioritized in designing next-generation macromolecular immunoadjuvants.