Discovery of Ketal-Ester Ionizable Lipid Nanoparticle with Reduced Hepatotoxicity, Enhanced Spleen Tropism for mRNA Vaccine Delivery.

The safety and efficacy of the lipid nanoparticle (LNP) delivery system are crucial for the successful development of messenger RNA vaccines. We designed and synthesized a series of ketal ester lipids (KELs), featuring a biodegradable ketal moiety in the linker and ester segments in the tail. Through iterative optimization of the head and tail groups of KELs, we tuned the pKa and molecular shapes, and identified (4S)-KEL12 as a safe and efficient ionizable lipid for mRNA delivery. (4S)-KEL12 LNP showed significantly higher delivery efficacy and lower toxicity than the DLin-MC3-DMA LNP. In comparison to SM-102 LNP, (4S)-KEL12 LNP exhibited better spleen tropism, reduced liver tropism, and hepatotoxicity. Additionally, (4S)-KEL12 demonstrated good biodegradability following intramuscular or intravenous injection. Notably, (4S)-KEL12 LNP encapsulated with a therapeutic mRNA cancer vaccine elicited robust cellular immune responses leading to substantial tumor regression along with prolonged survival in tumor-bearing mice. Our results suggest that (4S)-KEL12 LNP holds great promise for mRNA vaccine delivery. The comprehensive analysis of the structure-activity relationship, toxicity, biodegradability, distribution, expression, efficacy, and stereochemistry of these LNPs will greatly contribute to the rational design and discovery of novel lipid-based delivery systems.