Ionizable Silyl Lipids with Distinct Membrane Fluidity and mRNA Delivery Profiles

We report the synthesis and evaluation of ionizable silyl-lipids with single or double alkyl chains to modulate RNA encapsulation efficiency (EE), mRNA transfection, particle size, and membrane fluidity in lipid nanoparticles (LNPs). Incorporation of silyl lipid tails with varying tail substituents enables control over LNP properties, with distinct differences in membrane fluidity and transfection efficiency correlating with structure. Single-chain (MA) silyl lipids formed particles with more fluid membranes, while double-chain (DA) silyl lipids (similar to ALC-0315) formed particles with more rigid membranes, as determined using a C-Laurdan fluorescence assay. We show that particles composed of MA silyl lipids can have higher EE and distinct particle morphology (as determined using cryo-TEM) compared to DA lipids. The transfection efficiency was dependent on the particle composition, with MA lipids performing well even without the addition of helper lipids. Overall, this work highlights the potential of single- and double-chain ionizable silyl lipids to rationally modulate membrane fluidity and optimize LNP performance and demonstrates that phenyl-containing cationic silyl lipids can significantly enhance mRNA transfection efficiency relative to controls.