Rational design of lipid nanoparticles: overcoming physiological barriers for selective intracellular mRNA delivery

Abstract

This review introduces the typical delivery process of messenger RNA (mRNA) nanomedicines and concludes that the delivery involves a at least four-step SCER cascade and that high efficiency at every step is critical to guarantee high overall therapeutic outcomes. This SCER cascade process includes selective organ-targeting delivery, cellular uptake, endosomal escape, and cytosolic mRNA release. Lipid nanoparticles (LNPs) have emerged as a state-of-the-art vehicle for in vivo mRNA delivery. The review emphasizes the importance of LNPs in achieving selective, efficient, and safe mRNA delivery. The discussion then extends to the technical and clinical considerations of LNPs, detailing the roles of individual components in the SCER cascade process, especially ionizable lipids and helper phospholipids. The review aims to provide an updated overview of LNP-based mRNA delivery, outlining recent innovations and addressing challenges while exploring future developments for clinical translation over the next decade.

Keywords: Cellular uptake; Cytosolic mRNA release; Endosomal escape; Lipid nanoparticle; Targeted mRNA delivery.

Figure 1.

Overcoming physiological barriers for selective intracellular mRNA delivery using LNPs. This is a four-step process including selective organ-targeting delivery, cellular uptake, endosomal escape, and cytosolic mRNA release. E_S, efficiency of selective organ-targeting delivery; E_C, efficiency of cellular uptake; E_E, efficiency of endosomal escape; and E_R, efficiency of mRNA release.

Figure 2.

(a) SORT LNPs for tissue-specific mRNA delivery by adding a supplemental SORT molecule of cationic, anionic or ionizable lipids. (b) Imidazole-based synthetic lipids for in vivo mRNA delivery into primary T lymphocytes. (c) Engineering LNPs for enhanced intracellular delivery of mRNA through inhalation. (d) Adjuvant LNPs augment the immunogenicity of SARS-CoV-2 mRNA vaccines. (e) Phosphatidylserine LNPs promote systemic mRNA delivery to secondary lymphoid organs. (f) Rational design of bisphosphonate lipids for mRNA delivery to the bone microenvironment.

Figure 3.

(a) Ionizable lipids interact with anionic endosomal phospholipids, giving rise to cone-shaped ion pairs that are incompatible with a bilayer structure. These cationic-anionic lipid pairs promote the transition from a bilayer structure to the inverted hexagonal H_II phase, which facilitates membrane fusion/disruption, enabling endosomal escape. (b) mRNA-delivering ionizable lipids can be categorized into unsaturated ionizable lipids, multi-tail ionizable lipids, branched-tail ionizable lipids, and biodegradable ionizable lipids. (c) ³¹P NMR spectra of a mixture of endosomal mimic and iPhos. The iPhos lipid mixing that induces membrane hexagonal H_II transformation. (d) Helper phospholipids in LNPs affect mRNA expression in vivo.

Figure 4.

(a) LNP–endosome membrane fusion involves a topological transformation of activation elastic energy. (b) LNPs of different nanostructures (L, H_II, and Q_II) have distinct spontaneous curvature C₀, total curvature J, and Gaussian curvature K properties. (c) The escape of LNPs from endosomal entrapment will be enhanced by lowering the energetic barrier for fusion and fusion pore formation with the endosomal membrane for which Q_II LNPs should have preferential properties. (d and e) Live cell imaging of HeLa cells treated with lipoplexes (d) and cuboplexes (e) labeled with 1% self-quenching dye DiD. Recovery of DiD signal (red) implies LNP–endosomal fusion. Scale bar, 10 μm. (f) Nanomechanical action opens endo-lysosomes. (g) Molecular dimensions of light-driven lipid. (h) Fluorescence images of the cytosolic transport capabilities of light-driven LNPs in HeLa cells. Scale bar, 20 μm.