With advantages including flexible design, short development cycles, and strong protein expression capabilities, mRNA technology has been increasingly applied in cutting-edge biomedical fields such as vaccine development, protein replacement therapy, and gene editing. However, due to its limited stability, mRNA is susceptible to degradation by nucleases in the body and cannot directly cross cell membranes. Therefore, achieving safe and efficient intracellular delivery remains a critical challenge limiting the further development and application of mRNA technologies. Lipid nanoparticles (LNPs) are currently one of the most clinically advanced non-viral delivery platforms for mRNA, providing an important technical approach to address mRNA instability and intracellular delivery challenges. In 2018, the FDA approved Onpattro, an LNP-based siRNA therapeutic, marking the clinical advancement of LNP delivery technology. Subsequently, LNP-based mRNA COVID-19 vaccines were approved, further validating the application potential of this delivery platform. With continued advances in research, ionizable lipids, as key functional components of LNPs, have become an important focus for improving mRNA delivery efficiency through structural optimization. Conventional LNPs are mainly composed of ionizable lipids, helper phospholipids, cholesterol, and PEGylated lipids. These components collectively influence the structural stability, in vivo behavior, and delivery performance of LNPs. Among them, ionizable lipids play essential roles in mRNA encapsulation, LNP assembly, and endosomal escape. Their molecular structures also influence cellular uptake and tissue distribution of LNPs. Notably, Onpattro utilizes MC3, Moderna’s mRNA vaccine platform employs SM-102, and the BioNTech/Pfizer mRNA vaccine uses ALC-0315. Despite sharing similar overall LNP compositions, the selection of different ionizable lipids highlights their significant impact on LNP delivery performance. Ionizable Lipids: Key Components Connecting “Encapsulation” and “Intracellular Release” Ionizable lipids typically consist of hydrophilic head groups, linker structures, and hydrophobic tails. Their key feature is the ability to regulate their charge state in response to changes in the surrounding environment. Under acidic conditions during LNP formulation, ionizable lipids become protonated and positively charged, enabling interactions with negatively charged mRNA and promoting nucleic acid encapsulation. Under near-physiological pH conditions (approximately 7.4), ionizable lipids remain largely neutral, which helps reduce nonspecific interactions and improve biocompatibility. After LNPs are taken up by cells and enter acidic endosomes, ionizable lipids undergo protonation again and interact with endosomal membranes, inducing membrane disruption and facilitating mRNA escape into the cytoplasm, thereby initiating protein translation. Therefore, a high-performance ionizable lipid is not simply designed t...
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