Messenger RNAs (mRNAs) are biological molecules that transfer information encoded by genes from the nucleus to the cytoplasm for protein synthesis by ribosomes. mRNA sequences can be designed to code for specific proteins; the best-known example of this are mRNA vaccines for COVID-19. mRNA molecules are large and chemically unstable, so a vector must be used to deliver mRNA to cells. One of the most advanced technologies for mRNA delivery are lipid nanoparticles (LNPs), which are composed of ionizable lipids, cholesterol, helper lipids, and polyethylene glycol.
A team of researchers led by Assistant Professor Yusuke Sato and Professor Hideyoshi Harashima of Hokkaido University’s Faculty of Pharmaceutical Sciences and Kazuki Hashiba of Nitto Denko Corporation have developed a new branched ionizable lipid that, when ‘It is included in NLPs, greatly increases the efficiency of mRNA delivery. Their results were published in the journal Little Science.
Previous work has shown that ionizable lipids with branched tails increase the efficiency of mRNA delivery by LNPs. However, two major issues have prevented a systematic analysis of the branching effect of ionizable lipids. First, tail branching leads to a huge diversity of chemicals; second, the number of commercially available branched ionizable lipids is limited. To overcome these obstacles, the researchers generated a systematic lipid library of branched ionizable lipids and limited this library to a specific subset of branched lipids that could be described with just two parameters: total carbon number and symmetry. They then tested the 32 lipids in this library for their effect on the stability of LNPs containing mRNA (LNP-RNA).
The team found that RNA-LNPs that contained highly symmetric branched lipids exhibited greater microviscosity, and that higher microviscosity was positively correlated with increased stability of RNA-LNPs in storage. Highly symmetric branched lipids in RNA-LNPs are also positively correlated with protein expression in liver and spleen in mice. They determined that branch chain length affects organ selectivity.
The most stable storage and efficient delivery of mRNA was achieved by the branched lipid CL4F 8-6. The authors demonstrated that this particular lipid could be used in LNPs designed for gene editing, achieving a 77% deletion of the target gene in mice with a single dose of LNP.
This study revealed that branched lipids with a high level of symmetry contribute to optimal LNP properties for efficient intracellular delivery and stable formulations. Future work will focus on the development of expanded lipid libraries to understand the properties of other branched lipids and may lead to the design of novel lipids.
Hashiba, K. et al. (2022) Branching of ionizable lipids can improve mRNA stability, fusogenicity, and functional delivery. Little Science. doi.org/10.1002/smsc.202200071.