POx-Lipids as an Alternative to PEG-Lipids? Multimethod Assessment of Chemistry and Structure

The characterization of polymer-lipid conjugates is essential for their successful application in pharmaceutical formulations, particularly in lipid nanoparticles (LNPs). While poly(ethylene glycol) (PEG) lipids are widely used in LNPs, growing concerns over PEG immunogenicity have prompted the search for alternatives. Here, we investigate a series of poly(2-ethyl-2-oxazoline) (PEtOx) lipids as promising PEG substitutes featuring biocompatibility, tunable hydrophilicity, and stealth-like properties. We apply a comprehensive, multimethod approach utilizing liquid chromatography (LC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) to assess end group homogeneity, sample purity, and information about overall hydrophobicity/hydrophilicity of PEtOx-lipids compared to benchmark PEG-lipids used in COVID vaccines. We also demonstrate that under conditions of high sample purity only, hydrodynamic techniques such as analytical ultracentrifugation (AUC) become robust tools for analyzing conformational properties, aggregation behavior, and hydration of the polymers in aqueous environments. Though PEtOx-lipid analogues form micelles with lower aggregation numbers than PEG-lipids, they feature the same hydrodynamic size and values of hydration. The large levels of hydration of PEtOx-lipid micelles cannot be explained by hydrogen bonded water. Water present in the micelles constitutes a major part of their overall hydrodynamic volume and is located between the extended chain conformation of assembled polymers. Our study, with a unique multimethod approach, shows the importance of sample purity in high-end hydrodynamic analysis and paves the way for a quantitative replacement of PEG-based lipid conjugates by PEtOx-based lipid conjugates through a distinct set of properties to be considered for a particular application.