Formulation-Driven Control of mRNA Polyplex Physicochemical Properties Enables Spleen-Targeted Systemic Delivery

Efficient and organ-selective delivery remains a major challenge for messenger RNA (mRNA) therapeutics, motivating extensive material design efforts across diverse delivery systems. Here, we report a simple formulation-based strategy to control the physicochemical properties and in vivo biodistribution of mRNA polyplexes using an amphiphilic polyaspartamide derivative bearing cationic diethylenetriamine (DET) and hydrophobic 2-cyclohexylethyl (CHE) side chains, PAsp(DET/CHE). Systematic modulation of the molar ratio of polyaspartamide amine to mRNA phosphate (N/P) revealed that polyplex size and surface potential strongly influence biodistribution following systemic administration. The physicochemical properties of mRNA polyplexes could also be adjusted by incubation with physiological saline, 150 mM NaCl. This post-assembly modulation induced controlled particle growth without mRNA release, resulting in a marked shift in tropism from lung- to spleen-selective mRNA expression, accompanied by preferential accumulation in antigen-presenting cells. The NaCl-processed polyplexes elicited robust antigen-specific humoral and cellular responses following <i>ovalbumin</i> (<i>OVA</i>) mRNA delivery. These findings demonstrate that fine-tuning the ionic environment provides a facile means to control mRNA polyplex assembly and in vivo distribution, offering a scalable route to organ-selective mRNA delivery systems for vaccination and beyond.