Backbone-Ionized Polymers for Quantitatively Guided Extra-Hepatic mRNA Expression via Differential Local Structural Rearrangement

The rational structural design of extra-hepatic mRNA carriers lacks guidance from quantitative structure-activity relationships (QSARs), and the microscopic mechanisms of carrier-mRNA interfacial interactions remain poorly understood. Herein, we report a single-component backbone-ionized polymer platform, establishing QSARs for extra-hepatic mRNA expression and providing an ideal model for elucidating the local structural rearrangement mechanisms. By converting the tertiary amines on poly(β-amino ester) (PβAE) into quaternary ammonium salts (ionization), we developed ionized PβAE (iPβAE) and constructed a platform containing 90 PβAE variants through tuning the ionization proportion and side-chain chemistry. In vivo screening reveals the QSAR, the differential ionization gradient-induced targeting (DIGIT), which summarizes the side-chain-regulated quantitative relationship between the ionization proportion and the organ-selective expression intensity of mRNA. The rearrangement-mediated affinity control and translation (RE-ACT) mechanism elucidates that side chains act as molecular switches, inducing distinct "Electrostatic Pincer" or "Electrostatic-Hydrogen Tethering" modes, thereby precisely controlling the intracellular release and translation efficiency of mRNA. Furthermore, leveraging the lyophilized formulation of iPβAE, antigen mRNA (OVA/MUC1) and p53 mRNA were efficiently delivered to the spleen and lung, respectively. The lifespan of mice was extended by 20 times with vaccine immunotherapy, and chemosensitivity was increased with protein replacement therapy. Collectively, this iPβAE platform establishes a broadly applicable framework for engineering next-generation mRNA carrier systems.