Poly(ethylene Glycol) (PEG)–OligoRNA Hybridization to mRNA Enables Fine‐Tuned Polyplex PEGylation for Spleen‐Targeted mRNA Delivery

Organ‐selective targeting of mRNA polyplexes has been rarely explored despite the substantial potential of polymer‐based systems in mRNA delivery. In this study, spleen‐selective delivery of polyplexes is achieved by employing mRNA engineering to coat them with poly(ethylene glycol) (PEG). In this approach, mRNA is hybridized with PEGylated complementary RNA oligonucleotides (PEG–OligoRNAs), followed by the addition of linear poly(ethyleneimine). In this method, it is ensured that nearly all added PEG strands bind to the polyplexes, thereby enabling precise control of PEG amounts on the surface. Following systemic injection into mice, non‐PEGylated polyplexes yield robust protein expression in the lung and spleen. Intriguingly, adding a small number of PEG–OligoRNAs drastically reduces protein expression efficiency in the lung while preserving it in the spleen, realizing spleen targeting of mRNA polyplexes. Furthermore, PEGylated polyplexes demonstrate their potential utility in mRNA vaccination. In mechanistic analyses, non‐PEGylated polyplexes immediately agglomerate in the blood and deposit in the lung. Coating polyplexes with a small amount of short PEG effectively prevents these processes. Notably, even slight changes in PEG amounts and lengths dramatically impact the physicochemical properties and biological functionalities of the polyplexes, emphasizing the benefits of an mRNA engineering‐based approach for fine‐tuning polyplex PEG coating.