ABSTRACT Recent clinical success of mRNA vaccines for COVID-19 has sparked enormous interest in mRNA therapy for a wide range of biomedical applications including protein replacement therapy. However, one unique challenge associated with mRNA therapy is dealing with the transient efficacy due to its relatively short half-life. Current nanoparticles including FDA-approved lipid nanoparticles (LNPs) could significantly improve mRNA translation efficiency, but the duration of in vivo protein expression by these mRNA NPs is generally short (limited to a few days), thus requiring frequent re-dosing. The main objective of this project is to advance a new transformative LNP technology enabling long-acting mRNA replacement therapy of genetic disorders associated with loss of function of a particular protein. In our recent studies, we developed a new generation of LNPs and performed the head-to-head comparison in vitro and in vivo to the benchmark LNP formulations composed of FDA-approved ionizable lipids. We observed a dramatic increase of the duration of model protein expression in vitro and in vivo by our new mRNA LNPs. Preliminary safety studies showed that our mRNA LNPs were well tolerated without observable adverse events in vivo. With the proof-of-concept demonstration of our long-acting mRNA LNPs, this project aims to i) further optimize the mRNA LNP technology for longer-term, high level protein expression, and ii) rigorously validate this transformative mRNA delivery platform using hemophilia A as a model disease. We expect that with successful validation in normal and hemophilia A mice, this long-acting mRNA LNP platform could be readily moved into clinical testing for hemophilia and expanded to other genetic diseases that require restoration of normal protein functions.