Targeting materials and strategies for RNA delivery

Abstract

RNA-based therapeutics have shown great promise in various medical applications, including cancers, infectious diseases, and metabolic diseases. The recent success of mRNA vaccines for combating the COVID-19 pandemic has highlighted the medical value of RNA drugs. However, one of the major challenges in realizing the full potential of RNA drugs is to deliver RNA into specific organs and tissues in a targeted manner, which is crucial for achieving therapeutic efficacy, reducing side effects, and enhancing overall treatment efficacy. Numerous attempts have been made to pursue targeting, nonetheless, the lack of clear guideline and commonality elucidation has hindered the clinical translation of RNA drugs. In this review, we outline the mechanisms of action for targeted RNA delivery systems and summarize four key factors that influence the targeting delivery of RNA drugs. These factors include the category of vector materials, chemical structures of vectors, administration routes, and physicochemical properties of RNA vectors, and they all notably contribute to specific organ/tissue tropism. Furthermore, we provide an overview of the main RNA-based drugs that are currently in clinical trials, highlighting their design strategies and tissue tropism applications. This review will aid to understand the principles and mechanisms of targeted delivery systems, accelerating the development of future RNA drugs for different diseases.

Keywords: RNA-based therapeutics; Specific organ/tissue tropism; Targeting materials; Targeting strategies; mRNA delivery.

Figure 1

Materials and strategies for targeted RNA delivery. The vector category, vector chemical structure, administration route, and physicochemical property all affect the targeting tropism of nanoparticles.

Figure 2

Representative chemical structures of RNA vectors that mediate targeted delivery. RNA vectors for liver, lung, spleen, skin, and brain-targeting are shown.

Figure 3

Lipids derived from ring-opening reaction or addition reaction of amines, acrylates, and epoxides for liver-targeting. (A) Synthesis route of cationic lipid-modified aminoglycosides according to ring-opening reaction and structures of aminoglycosides. (B) Luciferase expression in vivo of C57BL/6 mice with the delivery of CLA-based LNPs at 6h after intravenous injection. (C) Human erythropoietin expression after 6 h and 24 h with GT-EP10 LNP and MC3 LNP delivery. Adapted with permission from , copyright 2020, Wiley-VCH. (D) Synthesis routes of amino acid derivatives by addition reactions. (E) Expression level of Pten, in different organs and subtypes of liver cells. Adapted with permission from , copyright 2014, National Academy of Sciences.

Figure 4

O-series LNPs for liver-targeting. (A) Synthesis of O-series lipidoids. (B) Whole body luminescence intensity of O-series LNPs compared to MC3 LNP in Balb/c mice at 6h after intravenous injection. (C) Schematic illustration of LNP-mediated gene editing in hepatocytes and reduction of Angptl3 protein resulting disinhibition of lipoprotein lipase. (D) Comparison of Angptl3 gene editing efficiency with the delivery of 306-O12B LNP and DLin-MC3-DMA LNP. Adapted with permission from , copyright 2021, National Academy of Sciences.

Figure 5

Novel LLNs for liver-targeting. (A) Chemical structures of FTT derivatives with TT3 as core. (B) mRNA delivery efficiency of FTT LLNs was represented by the fold of increase of luminescence intensity in vivo and FTT5 showed the highest delivery efficiency. (C) Expression level of hFVIII protein and hFVIII activity in wild-type mice and HA mice after intravenous injection of FTT5-hFVIII mRNA LLNs. And histopathological images of HA mice with injection of FTT5-hFVIII mRNA LLNs and untreated HA mice were shown. (D) FTT5 LLNs with branched ester side chains (left) were less likely to degrade than FTT9 LLNs with linear chains (right) in liver. Adapted with permission from , copyright 2020, American Association for the Advancement of Science. (E) The synthesized routes of amino-ester-derived LLNs. Adapted with permission from , copyright 2017, American Chemical Society.

Figure 6

SORT molecules allowed LNPs to achieve targeted delivery of mRNA for different organs. (A) Addition of different SORT molecules mediated the tissue-specific targeting delivery of LNPs. Adapted with permission from , copyright 2020, Springer Nature. (B) Increase of percentage of different SORT molecules altered the bio-distribution of fluorescence of Cy5-mRNA. (C) The addition of different percentages of SORT molecules affected the pK_a of LNPs determined by TNS assay and plasma proteins absorbed in surface of LNPs visualized by SDS-PAGE. (D) The bioluminescence of functional proteins showed that the targeting of SORT LNPs in lung and spleen was ApoE-independent. Adapted with permission from , copyright 2021, National Academy of Sciences.

Figure 7

Imidazole-based lipidoids for spleen-targeting and N-series LNPs for lung-targeting. (A) Synthesis of imidazole-based lipidoids. (B) Screening LNPs according to bioluminescence images of whole body and each organ with IVIS. (C) Detection of tdTomato expression in spleen by confocal microscopy and T cells were marked by CDε antibody. Adapted with permission from , copyright 2020, Wiley-VCH. (D) Synthesis and screening of N-series LNPs by whole body bioluminescence images with IVIS imaging system. (E) Changing the head structure of N-series LNPs could target different pulmonary cell types represented by immunofluorescence images of lung tissue and quantification of tdTomato⁺ cell percentages in different pulmonary cell types of 306-N16B and 113-N16B. (F) Tsc2 mRNA-loaded LNP could exert therapeutic effect of inhibiting growth of tumor in lung. Adapted with permission from , copyright 2022, National Academy of Sciences.

Figure 8

OF-XX series lipids for spleen-targeting. (A) Synthesis route of OF-XX lipids by ring opening reaction between alkenyl epoxides and polyamine core. (B) EPO expression following mRNA delivery with OF-XX LNPs and cKK-E12 LNP in vivo. Adapted with permission from , copyright 2016, Wiley-VCH. (C) Synthesis route of OF-Deg-Lin. (D) Quantification of cell populations labeled by Cy5 mRNA delivered with OF-Deg-Lin showed that B lymphocytes were the main targeted cell population. (E) Luciferase expression showed that OF-Deg-Lin LNPs could deliver FLuc mRNA to the spleen of mice. Adapted with permission from , copyright 2017, Wiley-VCH.

Figure 9

A series of novel iPhos for mRNA targeted delivery. (A) Synthesis routes of iPhos lipids by conjugating amines and alkylated dioxaphospholane oxide molecules. (B) Schematic illustration of hexagonal transition of biofilm phospholipids with the addition of iPhos lipids, which contained one zwitterionic head and three hydrophobic alkyl tails, in acidic environment. (C) Bioluminescence images and quantification of luciferase expression showed that the delivery efficiency of mRNA by iPhos 9A1P9 were superior to that of commonly used phospholipids, DOPE and DSPC. (D) Cas9 mRNA delivery and gene editing were achieved in liver and lung with 9A1P9-5A2-SC8 iPLNPs and 9A1P9-DDAB iPLNPs respectively. (E) iPhos with alkyl group length of 9 to 12 carbons resulted in highest mRNA expression in liver, 13 to 16 carbons resulted in highest mRNA expression in spleen. Adapted with permission from , copyright 2021, Springer Nature.

Figure 10

Different administration routes for RNA drugs, including systemic administration and local administration.