Composition and function of nucleic acid lipid nanoparticles (LNPs) (Reprint)

Gene therapy is a therapeutic approach that introduces exogenous genes into target cells to correct or compensate for diseases caused by defective or abnormal genes. Among the various steps involved, ensuring the efficient delivery of exogenous genes into cells is a critically important aspect. Of the many gene delivery methods available, the approach of formulating suitable lipid nanoparticles (LNPs) to encapsulate nucleic acids—enabling targeted delivery to specific cells and intracellular transport of the genetic material—has gained increasing adoption among scientists. For researchers already familiar with the field, adjusting the ratios of the various components may be relatively straightforward. However, for newcomers just entering the area, individual questions—such as what cationic lipids are or what role PEG‑lipids play—may require considerable time to fully grasp. Therefore, today we offer a brief introductory overview to help early‑stage researchers quickly acquire the foundational concepts.

Lipid nanoparticles (LNPs) are a class of nanoparticles formed using lipids. In the past, researchers typically used LNPs to directly encapsulate small‑molecule drugs. In the field of gene therapy, however, LNPs are now being employed to encapsulate nucleic acids—such as mRNA, siRNA, and pDNA—giving rise to what are known as nucleic acid lipid nanoparticles.

Fig.1 Schematic diagram of nucleic acid lipid nanoparticles

A notable difference between nucleic acids and conventional small‑molecule drugs is that nucleic acids carry a large number of phosphate groups and are therefore negatively charged. To enable better encapsulation within lipid nanoparticles, researchers turned to a special class of lipids—cationic lipids. Cationic lipids typically possess a hydrophilic head group containing an ammonium moiety, which can bind protons under acidic conditions to acquire a positive charge. Through electrostatic adsorption between the positively charged lipid head groups and the negatively charged nucleic acid backbone, the nucleic acid can be encapsulated within the lipid nanoparticle. After encapsulation, the exterior of the particle becomes hydrophobic due to the outward orientation of the hydrophobic tails of the cationic lipids. At this point, PEG‑lipids—lipids modified with a polyethylene glycol (PEG) chain at one end, commonly used in conventional liposome formulations—can be introduced. The hydrophobic tails of the PEG‑lipids associate with those of the cationic lipids, while the hydrophilic PEG‑modified heads face outward, forming the outer shell of the nucleic acid lipid nanoparticle. To further enhance the stability of the nucleic acid lipid nanoparticles, additional components such as cholesterol may be incorporated, which reinforces the hydrophobic interactions between the tails of the PEG‑lipids and the cationic lipids, ultimately yielding the final nucleic acid lipid nanoparticle product.

Fig.2 Schematic diagram of the components and their functions in nucleic acid lipid nanoparticles