Hydrogels for RNA delivery

Abstract

RNA-based therapeutics have shown tremendous promise in disease intervention at the genetic level, and some have been approved for clinical use, including the recent COVID-19 messenger RNA vaccines. The clinical success of RNA therapy is largely dependent on the use of chemical modification, ligand conjugation or non-viral nanoparticles to improve RNA stability and facilitate intracellular delivery. Unlike molecular-level or nanoscale approaches, macroscopic hydrogels are soft, water-swollen three-dimensional structures that possess remarkable features such as biodegradability, tunable physiochemical properties and injectability, and recently they have attracted enormous attention for use in RNA therapy. Specifically, hydrogels can be engineered to exert precise spatiotemporal control over the release of RNA therapeutics, potentially minimizing systemic toxicity and enhancing in vivo efficacy. This Review provides a comprehensive overview of hydrogel loading of RNAs and hydrogel design for controlled release, highlights their biomedical applications and offers our perspectives on the opportunities and challenges in this exciting field of RNA delivery.

Fig. 1 |. Timeline of recent preclinical studies of hydrogel-based RNA delivery.

Coloured boxes indicate the type of biomedical application: cancer therapy (orange), bone regeneration (blue), immunomodulation (yellow), cardiac repair (red) and angiogenesis (grey). ACpG-STAT3, cytosine-phosphorothioate-guanine-signal transducer and activator of transcription 3; DextranVS, dextran vinylsulfone; GelMA, gelatin methacryloyl; HP-HA-PEG, a thiol-modified analogue of heparin-thiol-modified hyaluronan-poly(ethylene glycol) diacrylate; hyd, hydrogel; IL, interleukin; MPEG, methoxypolyethylene glycol; mTOR, mammalian target of rapamycin; PAA, polyacrylamide; PCL, poly(ε-caprolactone); PE, polyethylene; PEG4SH, tetra-thiolated polyethyleneglycol; PEI-DA, deoxycholic acid-modified polyethylenimine polymeric conjugates; PLA-DX-PEG, poly-d,l-lactic acid-p-dioxanone-polyethylene glycol block copolymer; PLK, serine/threonine-protein kinase; Rb1/Meis2, retinoblastoma1/meis homeobox 2; RGM, RNA gene for miRNAs; SPARC, secreted protein acidic and rich in cysteine^–.

Fig. 2 |. Advantages of hydrogels as a platform for RNA delivery.

Hydrogels provide a unique strategy for local administration of RNA, overcoming some of the difficulties associated with systemic RNA delivery. They enable a localized, controlled and sustained delivery of high levels of payloads, while maintaining RNA biological activity. Off-target effects and the need for multiple payload administrations in systemic delivery may thus be avoided.

Fig. 3 |. Functional hydrogels for RNA loading and delivery.

a, RNA is loaded into hydrogel either with no manipulation (naked RNA) or by means of nanocarriers. b, RNA-loaded hydrogels can be used as implantable scaffolds or as injectable gels for local RNA delivery. The fine-tunable physical, biochemical and biological features of hydrogels allow the sustained and/or controllable release of RNA. Upon cellular entry (for example, via naked RNA or RNA-loaded nanocarriers), RNA reaches the proper subcellular compartment to initiate protein production/inhibition.

Fig. 4 |. Strategies for loading naked RNA into a hydrogel network.

RNA therapeutics can interact with hydrogel networks through ionic bonds between negatively charged RNA parts and positively charged hydrogel network parts; hydrogen bonds produced when positively charged hydrogen atoms come within a certain radius of an electronegative acceptor atom; covalent bonds that chemically link the RNA to polymer hydrogel chains; hydrophobic interactions that use modified RNA; and non-specific interactions.

Fig. 5 |. Hydrogel functional properties for controlled RNA delivery.

a, The ultimate release profile of the encapsulated naked RNA and/or RNA nanocarriers is determined by the hydrogel’s physical features and RNA–hydrogel interactions. b, Upon local administration, the release of the encapsulated RNA can be triggered by external or internal stimuli. c, Illustrative release profiles of encapsulated naked RNA and/or RNA nanocarriers. d, Illustrative biodistribution profiles of RNA therapeutics administered in the naked form or in combination with a hydrogel system. e, Illustrative local accumulation profiles of payload at the implantation site in the naked form or in combination with a hydrogel system.

Fig. 6 |. Biomedical applications of hydrogel-based RNA delivery.

The conjunction of naked RNA or RNA nanocarriers with multifunctional hydrogels can find multiple biomedical applications, such as cancer therapy, wound healing, bone regenearation and cardiac repair.