Biophysical Considerations in the Rational Design and Cellular Targeting of Flexible Polymeric Nanoparticles

Abstract

Physicochemical characteristics of nanoparticles (NPs) can be engineered for tuning their biological function in cellular delivery. How NP mechanical properties impact multivalent ligand-receptor mediated binding to cell surfaces, the avidity of NP adhesion to cells, propensity for internalization, and effects due to crowding remain unknown or unquantified. We report computational analyses of binding mechanisms of three distinct NPs that differ in type and rigidity (core-corona flexible NP, rigid NP, and rigid-tethered NP) but are otherwise similar in size and ligand surface density; moreover, for the case of flexible NP, we tune NP stiffness by varying the internal crosslinking density. We employ biophysical modeling of NP binding to membranes together with thermodynamic analysis powered by free energy calculations and show that efficient cellular targeting and uptake of NP functionalized with targeting ligand molecules can be shaped by factors including NP flexibility and crowding, receptor-ligand binding avidity, state of the membrane cytoskeleton, and curvature inducing proteins. Owing to this multitude of factors, we demonstrate that the binding avidity of a flexible NP depends on engineered changes in NP flexibility governed by significant enthalpy entropy compensations arising from multiple competing terms associated with NP, receptor density, and membrane. Analyses of the individual enthalpic and entropic contributions associated with NP, membrane, and receptor-ligand binding and receptor diffusion collectively illuminate this complex dependence of avidity on crosslinking. These findings provide strong evidence that NP flexibility is an important design parameter for rationally engineering NP targeting and uptake in a crowded cellular adhesion microenvironment.