The clinical success of many therapeutic agents can be attributed to how these materials escape recognition by the human immune system to reach their desired targets. Upon administration, therapeutic agents adsorb blood serum proteins, resulting in the recognition of the agents by the immune system and their rapid clearance1. An established approach to overcome this challenge is PEGylation — the attachment of polyethylene glycol (PEG) onto the agents to create a stealth coating that improves their bioavailability and pharmacokinetics. For instance, commercial messenger RNA (mRNA) COVID-19 lipid nanoparticle (LNP) vaccines contain PEGylated lipids that are regarded as clinically safe and having low immunogenicity2.

Credit: Yufen Xiao and Daniel Siegwart

Despite its wide acceptance as a therapeutic strategy, PEGylation has some drawbacks. While the antifouling properties of PEG are key to avoid immune recognition, these properties also decrease the interactions of LNPs with cells and tissues, affecting uptake and transfection efficiencies. Moreover, recent studies show that adverse effects can occur upon repeated administration of PEGylated LNP vaccines, and the existence of anti-PEG antibodies may promote the accelerated clearance of LNPs from the body3,4. These findings have prompted researchers to look for polymer alternatives to PEG that may minimize such drawbacks.

In this Focus issue we feature two Articles describing non-PEG-polymer-lipids for mRNA LNP formulations that increase efficacy and reduce immunogenicity. In their Article, Yufen Xiao and colleagues use atom transfer radical polymerization to generate a library of polymer-lipids with distinct polymer structures, charge and molecular weights that could potentially replace the PEGylated lipid 1,2-dimyristoyl-rac-glycero-3-methoxypolyethylene glycol-2000 (DMG-PEG2000) in commercial LNP vaccines and therapies. In vivo screening showed that LNPs containing brush-shaped poly(ethylene glycol) methyl ether methacrylate (PEGMA) lipids (BPLs) displayed high transfection efficiencies and blocked anti-PEG antibody binding. Structure–activity optimization of polymer side chain length, degree of polymerization and the alkyl length of the lipid tails allowed for a comprehensive understanding of how each structural feature affects the potencies of BPL LNP formulations. The two best performing BPL LNP formulations were superior to those made with DMG-PEG2000 in repeated dosing studies and were not recognized by anti-PEG antibodies in mice, achieving better outcomes in protein replacement therapy and genome editing.

In a related Article, Sijin Luozhong and colleagues describe lipids covalently bound to zwitterionic poly(carboxybetaine) (PCB) (PCB-lipids) as possible alternatives for PEGylated lipids used in two commercial mRNA LNP formulations. Reversible addition–fragmentation chain transfer polymerization is used to generate a library of lipids with different molecular weight polymers and two distinct lipid acyl tails. Overall, LNP formulations containing PCB-lipids showed higher mRNA transfection levels compared with those of commercial formulations, with minimal toxicity. Mechanistically, Luozhong and colleagues postulate that the hydrophilicity of the PCB-lipids enhances endosomal membrane fusion. In repeated dosing studies in mice, mRNA LNPs incorporating PCB-lipids maintained transfection efficiency and mitigated the accelerated blood clearance caused by anti-PEG antibodies that affects commercial mRNA LNPs. Furthermore, the PCB LNPs can effectively transfect and deliver gene editing proteins to primary human T cells, supporting their potential for cell engineering and vaccines. As discussed in the accompanying News & Views article by Weiwei Gao and Liangfang Zhang, these studies address the drawbacks with PEGylated LNPs using two different but complementary approaches: modulation of polymer coating architecture to reduce LNP immunogenicity and exploration of LNP charge to improve endosomal escape while promoting immune stealth.

These two studies and others5 describe alternatives to conventional PEGylated lipids used in mRNA therapeutics that might address the pressing issues of their immunogenicity. Nonetheless, doubts remain whether this approach is the best way forward. In a Comment article, Namit Chaudhary and Kathryn Whitehead argue that the field should focus on designing LNPs around the pre-existing immune memory to PEG rather than simply replacing it. Some strategies are proposed: (1) mitigate PEG adverse effects through immune modulation; (2) deepen our understanding of how PEGylated LNPs interact with the human immune system; and (3) develop immunological assays to predict and assess reactogenicity.

This line of thought — calling for more efforts to expand our knowledge of PEG immunogenicity and how it can be modulated to promote better and safe therapeutics — is gaining traction in the field6. Before moving to alternatives that are yet to prove advantageous in clinical testing and are not currently approved by regulatory bodies, focusing on PEGylation to give therapeutic agents their stealth abilities and developing ways to mitigate the adverse effects of PEG may be the best approach to take. In this evolving field, it would be interesting to see if future developments can boost RNA therapeutics to their full potential.