LNP Composition
Lipid nanoparticles (LNP) are composed of typically four different types of lipids that have been identified and developed for the specific function of encapsulating, protecting, and delivering the drug cargo to the cell. Though LNP composition can vary widly, they typically consist of:
Ionizable cationic lipid
Helper Phospholipid
Cholesterol
Polyethylene Glycol (PEG) Lipid
LNPs are generally formed by mixing lipids in ethanol with mRNA in an aqueous buffer solution. First, ethanol facilitates the initial solubilization of lipids–an important step to ensure proper mixing and LNP formation. When mixed with the aqueous phase, the lipids exhibit their hydrophobic nature and self-assemble into supramolecular particles due to the hydrophobic effect. This process mitigates unfavorable polar-nonpolar interactions and helps encapsulate mRNA into the nanoparticle core.
It can be said that the defining feature of any LNPs are their ionizable lipids. Some prominent examples include ALC-0315, SM-102, and DLin-MC3-DMA which have all been used in FDA-approved LNP formulations. There are also many new novel ionizable lipids that have been synthesized and researched, many of which improve mRNA encapsulation efficiency, cellular uptake, and drug release.
Their overall positive charge in acidic formulation buffer promotes attractive electrostatic interactions with the negatively-charged phosphate backbone of nucleic acids, encouraging nucleic acid incorporation into the developing LNP. Ionizable lipids have a neutral charge at physiological pH of ~7.4 which promotes the endosomal uptake of LNPs. Their eventual re-protonation aids in the release of the encapsulated cargo in cells.
Helper, zwitterionic phospholipids (e.g. DSPC, DOPC, DOPE, and more) help to both stabilize the lipid shell and disrupt the endosomal membrane of the cell, when delivering the mRNA. Cholesterol helps with LNP stability and to control the fluidity of the membrane through differing interactions with unsaturated lipids (decrease fluidity) and saturated lipids (increase fluidity). The addition of cholesterol to the lipid bilayer also helps to prevent the contents from escaping or leaking out. Lastly, PEG lipids, in addition to phospholipids and cholesterol, can help with the stability of the final product for long-term storage and increase LNP circulation time within the bloodstream.
Together, the ratios and types of these four lipid components can be finely tuned to meet specific requirements, such as the target cell/tissue/organ and for the type of drug. Not limited to just four components, researchers have played around with the number of components, different lipid chemistries, and even conjugating different moieties onto the LNP surface for better targeting and pharmacokinetics. Some notable examples include the selective organ targeting (SORT) LNPs that include a fifth SORT lipid component to enable tissue targeting, peptide-conjugated LNPs to deliver mRNA through ocular barriers, and novel ionizable lipid structure and chemistries to improve RNA release.
Fig. 1Â - Illustration of the components typically seen in LNP formulations.