Paving the Road for RNA Therapeutics

Abstract

Therapeutic RNA molecules possess high potential for treating medical conditions if they can successfully reach the target cell upon administration. However, unmodified RNA molecules are rapidly degraded and cleared from the circulation. In addition, their large size and negative charge complicates their passing through the cell membrane. The difficulty of RNA therapy, therefore, lies in the efficient intracellular delivery of intact RNA molecules to the tissue of interest without inducing adverse effects. Here, we outline the recent developments in therapeutic RNA delivery and discuss the wide potential in manipulating the function of cells with RNAs. The focus is not only on the variety of delivery strategies but also on the versatile nature of RNA and its wide applicability. This wide applicability is especially interesting when considering the modular nature of nucleic acids. An optimal delivery vehicle, therefore, can facilitate numerous clinical applications of RNA.

Keywords: RNA therapy; cell-specific targeting; drug delivery; nanoparticles.

Figure 1

Overview of Different Mechanisms of Action of Different RNA Therapeutics. (1) Without therapeutic RNA molecules, the translation of a pathogenic protein proceeds without inhibition (shown in the broken line box). (2) ASOs hybridize to the target mRNA, while the (3) siRNA/miRNA mimics utilize the RISC in the RNAi pathway to (4) inhibit translation of target mRNA. (5) Overexpression of a therapeutic protein that counteracts the function of the pathogenic protein can be done by delivering the mRNA of the therapeutic protein. (6) saRNA can be delivered to the cell where it binds to AGO2, is imported to the nucleus, and in turn activates an endogenous gene. (7) A more permanent approach to remove the pathogenic protein is by gene knockout using Cas9 and sgRNA RNPs. Abbreviations: AGO2, argonaute 2; ASO, antisense oligonucleotide; RISC, RNA-induced silencing complex; RNP, ribonucleoprotein; saRNA, small activating RNA.

Figure 2

Examples of Delivery Vehicles for Different RNA Payloads. (A) Antibody conjugated to RNA molecules which can be mediated by using for instance positively charged protamine (shown as plus signs). (B) Conjugate of RNA with a single-chain variable fragment (scFv). (C) RNA–aptamer conjugates. (D) RNA encapsulated in lipid nanoparticles (LNPs). Cationic or ionizable lipids (shown in green) aid in encapsulating the RNA payload through electrostatic interactions. This way, the RNA is encapsulated in inverted micelles. Cholesterol (shown in grey) provides stability to the LNPs. The surface of the LNPs are generally coated with PEG (black lines). Reactive groups such as maleimide (purple triangles) can be linked to the PEG and are used to functionalize the LNPs with targeting moieties (chemical conjugation of targeting moieties). (E) Cationic polymers can encapsulate RNA therapeutics by electrostatic interactions.