Randomized PEG-Based Polycarbonate Micelles with PEG-like Stealth Properties and Biodistribution

The increase in anti-PEG antibodies (APAs) as well as PEG-related adverse effects have questioned the benefits of PEGylated nanotherapeutics. Growing awareness of the safety and efficacy of PEGylation has prompted research into countermeasures, including alternative polymers. Among these, randomized PEG (rPEG) was established as a fundamentally new approach that preserves the polyether structure while preventing antibody recognition through isomerization. Here, rPEG was incorporated into polymeric micelles (PMs) for immunodrug delivery to assess its suitability as a PEG replacement. Amphiphilic rPEG polycarbonate block copolymers were synthesized via chain extension using rPEG macroinitiators with primary hydroxy end groups. The respective PMs self-assembled via solvent-exchange evaporation, encapsulating the hydrophobic TLR7/8 agonist CL075 as the immunodrug. rPEG-based PMs demonstrated size distributions and encapsulation efficiencies similar to those of PEG. In vitro assays confirmed that rPEG-PMs had comparable cellular uptake, no cellular toxicity, and similar drug-delivery efficiencies to PEG-PMs. Notably, competitive ELISAs revealed markedly decreased antigenicity of rPEG-PMs towards backbone- and end group-specific APAs. In a fiveday biodistribution study, rPEG-PMs showed whole-body and organ distribution patterns comparable to mPEG-PMs, as would be expected for a stealth polymer. Strikingly, despite the immunostimulatory payload, serum ELISA after a single dose detected anti-rPEG IgM in one of five mice, compared with anti-PEG IgM in three of five mice receiving mPEG-PMs, suggesting lower immunogenic potential for rPEG-PMs, though this remains to be confirmed in repeat-dose studies. Overall, these results position rPEG as a highly promising alternative to PEG for polycarbonate-based PMs and next-generation nanotherapeutics.