Amplification of Protein Expression by Self-Amplifying mRNA Delivered in Lipid Nanoparticles Containing a β-Aminoester Ionizable Lipid Correlates with Reduced Innate Immune Activation

Self-amplifying mRNA (saRNA) is witnessing increased interest as a platform technology for protein replacement therapy, gene editing, immunotherapy, and vaccination. saRNA can replicate itself inside cells, leading to a higher and more sustained production of the desired protein at a lower dose. Controlling innate immune activation, however, is crucial to suppress unwanted inflammation upon delivery and self-replication of RNA in vivo. In this study, we report on a class of β-aminoester lipids (βAELs) synthesized through the Michael addition of an acrylate to diethanolamine, followed by esterification with fatty acids. These lipids possessed one or two ionizable amines, depending on the use of nonionic or amine-containing acrylates. We utilized βAELs for encapsulating saRNA in lipid nanoparticles (LNPs) and evaluated their transfection efficiency in vitro and in vivo in mice, while comparing them to LNPs containing ALC-0315 as an ionizable lipid reference. Among the tested lipids, OC7, which comprises two unsaturated oleoyl alkyl chains and an ionizable azepanyl motif, emerged as a βAEL with low cytotoxicity and immunogenicity relative to ALC-0315. Interestingly, saRNA delivered via the OC7 LNP exhibited a distinct in vivo transfection profile. Initially, intramuscular injection of OC7 LNP resulted in low protein expression shortly after administration, followed by a gradual increase over a period of up to 7 days. This pattern is indicative of successful self-amplification of saRNA. In contrast, saRNA delivered via ALC-0315 LNP demonstrated high protein translation initially, which gradually declined over time and lacked the amplification seen with OC7 LNP. We observed that, in contrast to saRNA OC7 LNP, saRNA ALC-0315 LNP induced potent innate immune activation by triggering cytoplasmic RIG-I-like receptors (RLRs), likely due to the highly efficient endosomal membrane rupturing properties of ALC-0315 LNP. Consequently, the massive production of type I interferons quickly hindered the amplification of the saRNA. Our findings highlight the critical role of the choice of ionizable lipid for saRNA formulation in LNPs, particularly in shaping the qualitative profile of protein expression. For applications where minimizing inflammation is desired, the use of ionizable lipids, such as the βAEL reported in this study, that elicit a low type I interferon response in saRNA LNP is crucial.