Aminolysis of Highly Branched Poly(β-amino ester)s for Efficient mRNA Delivery

mRNA therapy holds significant promise for preventing and treating diverse diseases. The development of safe and effective cationic polymers for mRNA delivery is essential for advancing its clinical applications. Highly branched poly­(β-amino ester)­s (HPAEs) are considered top nonviral candidates. However, current HPAEs have been synthesized exclusively using conventional “bottom–up” approaches, with optimization efforts primarily focusing on monomer combinations, topological structures, molecular weight, and chemical or physical modifications. Here, we report a “top–down” aminolysis strategy for synthesizing aminolyzed HPAEs (aHPAEs) with superior mRNA delivery efficiency. By intentionally increasing the feed ratio and reaction time, the end-capping amines can simultaneously end-cap the vinyl groups and aminolyze the ester groups of the HPAE base polymers, resulting in the generation of aHPAEs with reduced molecular weight and additional amide and amine groups. The degree of aminolysis significantly impacts both the molecular weight and the type and quantity of terminal groups in aHPAEs. Notably, aHPAE/mRNA polyplexes exhibit higher zeta potential, smaller particle size, and a 2.41-fold enhancement in mRNA transfection efficiency compared to their parent HPAE/mRNA polyplexes. Furthermore, the top-performing aHPAE, 122-2-S3, demonstrates targeted mRNA delivery to the spleen in vivo following systemic administration. The “top–down” aminolysis approach expands the HPAE family and, when integrated with the established “bottom–up” method, offers a pathway for developing more efficient HPAEs for mRNA delivery. This strategy may also be applicable in improving the performance of other cationic polymers.