Recent advances in strategies for developing tissue-selective mRNA-LNP technology

The approval of mRNA vaccines for coronavirus disease 2019 (COVID-19) has garnered significant attention for mRNA technology worldwide. This milestone has had a profound impact on the industrial transformation of mRNA in China, as evidenced by the increased number of startups and financing events in the past 2-3 years. Regarding mRNA itself, in addition to implementing chemical modifications, such as pseudouridine and N1-methyl pseudouridine, to avoid innate immune responses, efforts have been made to enhance stability and scale up production to meet clinical needs. Also, the successful application of mRNA technology heavily relies on safe and efficient delivery vehicles, particularly lipid nanoparticles (LNP). The initially approved mRNA COVID-19 vaccines, Comirnaty and Spikevax, have benefited from this delivery system. Furthermore, more than 90% of mRNA drug programs in clinical research, including cancer vaccines, infectious disease vaccines, and therapeutics for inherited genetic disorders, also depend on LNP vectors. Notably, mRNA4157, an mRNA-LNP cancer vaccine for melanoma developed by Moderna and Merck, demonstrated positive results in clinical trials and was granted Breakthrough Therapy Designation (BTD) by the Food and Drug Administration (FDA). LNP typically consists of four components: ionizable cationic lipid, phospholipid, cholesterol, and PEGylated lipid. Each component plays a crucial role in the stability, function, efficacy, and safety of LNP. The ionizable cationic lipid, in particular, is considered a key component and researchers invest substantial efforts in designing and screening ionizable lipids. These lipids can interact with mRNA molecules (which are negatively charged) through electrostatic interactions, encapsulating them in a low pH buffer. The charge is then neutralized upon buffer exchange to a physiological pH (pH 7.4). Once internalized by cells, the ionizable lipids can regain their positive charge within endosomes/lysosomes to facilitate mRNA release into the cytoplasm for translation. Phospholipid and cholesterol serve as helper lipids, contributing to the formation and stability of lipid nanoparticles, while PEGylated lipid reduces LNP aggregation to prolong in vivo circulation time and decreases phagocytosis by circulating immune cells. Currently, biomedical applications of mRNALNP technology are more advanced in liver and muscle targeted delivery. Examples include genome editing for genetic diseases in the liver via intravenous injection and the development of infectious vaccines through intramuscular injection. However, targeted delivery to extrahepatic tissues remains a challenge that is in the early stages of development. In order to maximize the application potential of mRNA drugs, researchers have made significant efforts in this direction, resulting in several reported approaches. This review primarily focuses on discussing the current strategies for developing tissue- targeted mRNA-LNP technologies. These strategies include optimizing LNP formulations, screening novel lipid molecules, modifying specific antibodies on LNP, and exploring suitable administration routes. Additionally, potential future directions in this field are briefly introduced. One of them is delivering therapeutic mRNA to specific cell types. To achieve this goal, further efforts are needed, including understanding the mechanism of tissue/cell targeting for better design of chemical structures, applying antibody modifications on the LNP surface, and exploring novel RNA molecules, such as circular RNA, which is believed to have cell-specific expression profiles. Overall, the information conveyed in this article aims to provide readers with a deeper understanding of mRNA-LNP technology and its potential in the design of targeted mRNA-LNP therapeutics.