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Review
. 2022 May;23(5):265-280.
doi: 10.1038/s41576-021-00439-4. Epub 2022 Jan 4.

Drug delivery systems for RNA therapeutics

Kalina Paunovska  1 , David Loughrey  1 , James E Dahlman  2
Affiliations
Review

Drug delivery systems for RNA therapeutics

Kalina Paunovska et al. Nat Rev Genet. 2022 May.

Abstract

RNA-based gene therapy requires therapeutic RNA to function inside target cells without eliciting unwanted immune responses. RNA can be ferried into cells using non-viral drug delivery systems, which circumvent the limitations of viral delivery vectors. Here, we review the growing number of RNA therapeutic classes, their molecular mechanisms of action, and the design considerations for their respective delivery platforms. We describe polymer-based, lipid-based, and conjugate-based drug delivery systems, differentiating between those that passively and those that actively target specific cell types. Finally, we describe the path from preclinical drug delivery research to clinical approval, highlighting opportunities to improve the efficiency with which new drug delivery systems are discovered.

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Conflict of interest statement

J.E.D. is a consultant for GV. The other authors declare no competing interests.

Figures

Fig. 1
Fig. 1. The expanding universe of therapeutic RNA payloads.
a | One class of RNA therapeutics requires delivery of small RNA molecules. Small interfering RNAs (siRNAs) can reduce gene expression via RNA-induced silencing complex (RISC)-mediated mRNA degradation, antisense oligonucleotides (ASOs) can alter isoforms by binding to splice sites, and adenosine deaminase acting on RNA ASOs (ADAR-oligonucleotides) can edit RNA. In all three cases, these small RNAs can be designed with site-specific chemical modifications using solid-phase synthesis and can be delivered using nanoparticles or conjugate delivery systems. In this figure, the blue molecule represents the small therapeutic RNA being ferried into the cell. b | A second class of RNA therapeutics requires delivery of large RNA molecules. In vitro transcribed mRNA consists of a 5′ cap, 5′ and 3′ untranslated regions (UTRs), an open reading frame encoding antigen(s), and a 3′ poly(A) tail. c | mRNA payloads can encode nucleases that mediate DNA or RNA editing. mRNA can also be used to replace dysfunctional protein or encode antigens that confer longer-term immunity to a pathogen, such as SARS-CoV-2. mRNAs are transcribed in vitro and thus cannot currently be made with site-specific chemical modifications. In this figure, the blue molecule represents the protein encoded by the mRNA. Cas, CRISPR-associated protein; crRNA, CRISPR RNA; dCas9, dead Cas9; GFP, green fluorescent protein; pegRNA, prime editing guide RNA; sgRNA; single-guide RNA.
Fig. 2
Fig. 2. FDA-approved lipid-based structures contain some variation of the four basic components: cholesterol, a helper lipid, a PEG-lipid, and a cationic or ionizable lipid.
a | Lipid-based structures can include micelles, which consist of a lipid monolayer, or liposomes, which consist of a bilayer. Lipid nanoparticles are composed of multiple lipid layers as well as microdomains of lipid and nucleic acid. b,c | In addition to the RNA payload, LNPs often consist of cholesterol, a helper lipid, a PEG-lipid (all shown in part b), and a cationic or ionizable lipid (part c). Despite the variety of chemical structures and lipid lengths, all the lipids in part c contain amine groups, which become positively charged at lower pH; this charge binds the anionic backbone of the RNA, providing a driving force for the formation of a stable LNP. d | The molar ratios of the four components making up the FDA-approved Acuitas/BioNTech/Pfizer COVID vaccine and patisiran, which delivers siRNA to the liver.
Fig. 3
Fig. 3. RNA can be delivered using nanoparticles formulated with polymers or dendrimers.
a | Polymeric nanoparticles and polymers based on poly(ethylenimine) (PEI), poly(l-lysine) (PLL), and poly(beta-amino-ester) (PBAE) use cationic amine groups to complex the anionic phosphodiester backbone of RNA. Polymers based on poly(lactic-co-glycolic acid) (PLGA) are typically engineered to contain separate cationic groups. b | Dendrimers are polymeric structures with a defined number of molecules emanating from a core. PAMAM, poly(amidoamine).
Fig. 4
Fig. 4. Drug delivery vehicles can use two mechanisms of action to reach their target cell type.
a–d | Delivery vehicles can reach desired cells using passive or endogenous targeting (part a), which leads to adsorption of serum biomolecules onto the outside of the lipid nanoparticle (LNP) in the bloodstream. For example, the serum lipoprotein ApoE binds to LNPs, leading to delivery via low-density lipoprotein receptor (LDLR) expressed on hepatocytes. Active targeting employs a ligand directly conjugated to a nucleic acid (part b), an antibody directly conjugated to a nucleic acid (part c) or a ligand or antibody conjugated to a nanoparticle to target a receptor expressed on the cell (part d). ASGPR, asialoglycoprotein receptor; GalNAc, N-acetylgalactosamine. ApoE, apolipoprotein E.
Fig. 5
Fig. 5. The preclinical nanoparticle discovery pipeline.
a | Thousands of nanoparticles are often synthesized and tested in vitro before a few nanoparticles are evaluated in mice. However, in vitro delivery can be a poor predictor of in vivo delivery. A smaller number of nanoparticles are then often tested in rats or non-human primates (NHPs). Given that NHPs are the best preclinical models for human delivery, one key opportunity to improve the efficiency of this pipeline is to understand which small-animal model is most predictive of NHP efficacy and tolerability; this species-to-species drug delivery relationship is understudied. b | Tissue weight as a percentage of total animal body weight (bw) in female animals. Relative organ size varies across species, which may alter nanoparticle on-target and off-target delivery.
Fig. 6
Fig. 6. The hallmarks of a clinically relevant delivery system.
These six traits, which can be studied early in the development of the drug delivery vehicle, can help to increase the odds that a drug delivery system is approved by the FDA. NHP, non-human primate.
See this image and copyright information in PMC

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