Ionizable Lipid Nanoparticles for mRNA Delivery: Internal Self-Assembled Inverse Mesophase Structure and Endosomal Escape

Abstract

The clinical use of mRNA COVID-19 vaccines developed by Moderna and Pfizer-BioNTech has highlighted the critical role of ionizable lipid nanoparticles (LNPs) in the efficient loading, intracellular delivery, and cytoplasmic release of mRNAs. These LNPs typically comprise an ionizable lipid, a helper lipid, cholesterol, and a PEGylated-lipid, each contributing to the stability, structure, encapsulation efficiency, and nanoparticle–biology interactions of the final mRNA–LNPs both in vitro and in vivo. Notably, the ionizable amino-lipids, ALC-0315 used in the BioNTech/Pfizer vaccine and SM-102 in the Moderna vaccine, possess similar molecular structures, featuring multiple saturated aliphatic chains linked to a tertiary amine group via ester bonds. The acidification-induced ionization behavior of these amino-lipids is essential for enabling endosomal escape and facilitating the intracellular transfection of therapeutic mRNAs. However, despite their widespread clinical use, the physicochemical property–biological interaction and function relationships for LNPs remain poorly understood, particularly regarding how the internal nanostructural evolution during endosomal maturation influences mRNA release, endosomal escape, and gene expression. The rudimentary understanding continues to impede the rational design and optimization of RNA therapeutics.