Reducing off-target expression of mRNA therapeutics and vaccines in the liver with microRNA binding sites

Lipid nanoparticles (LNPs) are often liver tropic, presenting challenges for LNP-delivered mRNA therapeutics intended for other tissues, as off-target expression in the liver may increase side effects and modulate immune responses. To avoid off-target expression in the liver, miR-122 binding sites have been used by others in viral and non-viral therapeutics. Here, we use a luciferase reporter system to compare different copy numbers and insertion locations of miR-122 binding sequences to restrict liver expression. We inserted one to five miR-122 binding sites into the 5' or 3' untranslated regions (UTRs) of luciferase mRNAs and tested them in LNPs <i>in vitro</i> and <i>in vivo</i> via systemic intravenous and local intramuscular injections in mice. Our results showed no significant differences in de-targeting efficacy between mRNAs harboring one or multiple miR-122 binding sites or between those with 5' or 3' UTR placements. To test the impact of miR-122 binding sites on antibody response to a mRNA vaccine, Ebola virus matrix protein VP40 mRNAs were modified with or without miR-122 binding sites and injected in mice intramuscularly. This work reinforces the utility of miR-122 binding sites while providing a comparison of these sites to aid the future development of LNP-mRNA therapies for non-hepatic tissues.