Structure–Activity Relationship of Ionizable Lipids for siRNA and mRNA Lipid Nanoparticle Design

Lipid nanoparticles (LNPs) represent the most effective vehicle class identified to date for delivering RNA. A stronger understanding of the structure-activity relationships that govern successful mRNA delivery would enable the development of improved LNPs. Herein, ionizable lipids with high mRNA in vitro delivery efficacy among 465 lipids were selected to be evaluated for their in vivo activity and structure-activity relationship. Variations of these ionizable amino hydroxy and amino lipid families were synthesized, and 42 lipids were evaluated to study how chemical alterations of the carbon chain within the core influence LNP potency. To further understand the relationship between chemical structure and in vivo hepatic delivery potency, physicochemical properties including size, PDI, pK_a, and buffering capacity were measured. Our evaluations revealed that both the pK_a and buffering capacity may be valuable in predicting in vivo hepatic delivery based on lipid structures, expanding the range of acceptable LNP pK_a to 6.2-7.4, and showed that the buffering capacity may help predict formulations for successful hepatic delivery of mRNA-LNPs. This study reiterates the importance of the chemical structure of the ionizable amino lipid for LNPs and highlights the intricacies of its relationship with the physical properties of LNPs. We anticipate that understanding the structure-activity relationship of ionizable lipids will be valuable for the continued rational design of ionizable amino lipids for the LNP delivery of small and large RNA cargoes.