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Challenge of Delivering mRNA and the Delivery via LNP 

Lipid nanoparticles have made notable strides in clinical applications, particularly in delivering mRNA. Notably, lipid nanoparticle-mRNA vaccines have been deployed in clinical settings to combat coronavirus disease 2019 (COVID-19), representing a significant milestone in mRNA therapeutics. 

  

The successfully design of the lipid nanoparticles ("LNPs") for mRNA delivery has overcome the physiological barriers - extracellular and intracellular ones (Ref.1,2): 

  • mRNA needs to be protected from nuclease degradation in physiological fluids (Ref.1,2); 

  • The formulation should evade the interception by the MPS and clearance by renal glomerular filtration post systemic administration (Ref.1,2); 

  • Lipid nanoparticle–mRNA systems need to reach target tissues, followed by internalization by target cells (Ref. 1,2); 

  • mRNA molecules must escape endosomes to reach the cytoplasm, where translation occurs (Ref.1,2.) 

  

Lipid nanoparticle-mRNA formulations, produced through rapid mixing techniques, demonstrate a stable nanostructure (Ref. 3,4,5). Within this framework, mRNA molecules can be encapsulated within the core interior via electrostatic interactions with the lipids (Ref. 3,5). This structural attribute shields mRNA molecules from degradation by nucleases and enhances nanoparticle stability in physiological fluids (Ref. 3,6). The incorporation of PEG-lipids additionally reduces recognition by the mononuclear phagocyte system (MPS) and clearance via renal filtration (Ref. 3,7). Furthermore, targeted biodistribution of lipid nanoparticle-mRNA formulations can be enhanced through additional modification and optimization of the nanoparticle structure (Ref. 8,9,10,11,12,13,14,15). For instance, nanoparticles can be coated with antibodies (Ref.12) to facilitate the delivery of mRNA molecules into inflammatory leukocytes and epidermal growth factor receptor (EGFR)-positive tumor cells. This approach holds promise for treating inflammatory bowel disease (Ref.9) and cancer (Ref.13), respectively. Organ selectivity can also be attained by adjusting the ratios of lipid components. For instance, this can involve designing spleen-targeted mRNA vaccines (Ref. 8,9) or lung-targeted genome editing delivery systems (Ref. 11,14). 

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Upon reaching target cells, lipid nanoparticles can be internalized through various mechanisms, including macropinocytosis, clathrin-mediated endocytosis, and caveolae-mediated endocytosis (Ref. 10,3). The specific endocytic pathway employed depends on both the properties of the nanoparticle and the type of cell (Ref.16,17,18). Once internalized, lipid nanoparticles are typically sequestered within endosomal compartments (Ref. 19,20,21), with only a fraction escaping into the cytoplasm (Ref. 19,20,21). This process, known as endosomal escape, is critical for effective mRNA delivery. While the precise mechanism is not fully understood, positively charged lipids may facilitate electrostatic interaction and fusion with negatively charged endosomal membranes, leading to mRNA release into the cytoplasm (Ref. 1,2,14,3,16). Optimization of ionizable lipid pKa values (Ref. 22,23,24,25,26,16,27) and the properties of lipid tails can enhance endosomal escape (Ref. 28,29,11,141). For instance, lipids with branched tails may exhibit superior endosomal escape compared to those with linear tails due to stronger protonation at endosomal pH29. Additionally, adjusting the type and ratio of lipids, such as DSPC and DOPE, can further improve endosomal escape efficiency (Ref. 30,31,32,33,18,34). 

  

  

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