The Impact of COVID-19 on RNA Therapeutics: A Surge in Lipid Nanoparticles and Alternative Delivery Systems

Abstract

The COVID-19 pandemic has significantly accelerated progress in RNA-based therapeutics, particularly through the successful development and global rollout of mRNA vaccines. This review delves into the transformative impact of the pandemic on RNA therapeutics, with a strong focus on lipid nanoparticles (LNPs) as a pivotal delivery platform. LNPs have proven to be critical in enhancing the stability, bioavailability, and targeted delivery of mRNA, facilitating the unprecedented success of vaccines like those developed by Pfizer-BioNTech and Moderna. Beyond vaccines, LNP technology is being explored for broader therapeutic applications, including treatments for cancer, rare genetic disorders, and infectious diseases. This review also discusses emerging RNA delivery systems, such as polymeric nanoparticles and viral vectors, which offer alternative strategies to overcome existing challenges related to stability, immune responses, and tissue-specific targeting. Additionally, we examine the pandemic's influence on regulatory processes, including the fast-tracked approvals for RNA therapies, and the surge in research funding that has spurred further innovation in the field. Public acceptance of RNA-based treatments has also grown, laying the groundwork for future developments in personalized medicine. By providing an in-depth analysis of these advancements, this review highlights the long-term impact of COVID-19 on the evolution of RNA therapeutics and the future of precision drug delivery technologies.

Keywords: COVID-19 vaccines; RNA therapeutics; drug delivery systems; lipid nanoparticles; mRNA technology.

Figure 1

Key historical milestones in the development of mRNA-based therapeutics. Reprinted with permission from Ref. [6] under the Creative Commons Attribution 4.0 International License.

Figure 2

TLR and RLR signaling pathways and LNP effects. (A) Toll-like receptors (TLRs) include TLRs 3, 7, 8, and 9, which are located on the endosomal membrane, while other TLRs reside on the cell surface. Upon recognizing foreign antigens, TLRs initiate downstream signaling cascades, typically through the MyD88 adapter protein, except in the case of TLR3. This leads to the activation of transcription factors such as AP-1, NF-κB, CREB, c/EBP, and IRF3, which translocate to the nucleus to induce an innate immune response. The activation of these transcription factors stimulates the production of cytokines, chemokines, and type 1 interferons (IFNs). Endosomal TLRs 3, 7, 8, and 9 recognize RNA, and mRNA encapsulated in lipid nanoparticles (LNPs) can engage the MyD88 signaling pathway. (B) The retinoic acid-inducible gene I-like receptor (RLR) pathway is triggered when cytosolic RNA helicases, such as MDA5 and RIG-I, detect foreign RNA, or when STING proteins on the endoplasmic reticulum are activated. This pathway signals through the mitochondrial antiviral signaling protein (MAVS), leading to the activation of transcription factors NF-κB and IRF3, which induce cytokines and type 1 IFNs. LNPs can activate MDA5, enhancing the immune response by stimulating these pathways. Reprinted with permission from Ref. [36] under the Creative Commons Attribution 4.0 International License.

Figure 3

Lipid nanoparticles (LNPs) enhancing the systemic delivery of mRNA-loaded nanoparticles. (A) Administration of nanoparticles (NPs) alone. (B) Administration of nanoparticles combined with nanoprimers. Reprinted with permission from Ref. [48] under the Creative Commons Attribution 4.0 International License.

Figure 4

Mechanism of endosomal disruption by ionizable lipids and the five main structural types of ionizable lipids. Ionizable lipids, once protonated within the acidic environment of endosomes, form cone-shaped ion pairs with the anionic phospholipids present in the endosomal membrane. This interaction disrupts the lipid bilayer, facilitating the escape of RNA from the endosome into the cytosol. Ionizable lipids used for RNA delivery can be classified into five distinct structural categories: (1) unsaturated lipids, which contain double bonds; (2) multi-tail lipids, which possess more than two hydrocarbon tails; (3) polymeric lipids, which include polymer or dendrimer structures; (4) biodegradable lipids, which contain bonds that can degrade; and (5) branched-tail lipids, characterized by branched hydrocarbon chains. Reprinted with permission from Ref. [53] under the Creative Commons Attribution 4.0 International License.

Figure 5

Proposed mechanism of action for APC-targeted LNP–mRNA. The interaction between targeted mRNA–lipid nanoparticles (LNPs) and antigen-presenting cells (APCs) occurs through specific ligands that engage with receptors on the APC surface. This receptor engagement can initiate the production of interferons (IFNs) and other cytokines or chemokines (1). Following this, the LNP–mRNA complex is internalized through endocytosis (2), allowing the mRNA within the endosome to engage with membrane-bound Toll-like receptors (TLRs) (3). The activation of TLRs triggers signaling pathways that lead to the production of Type I IFNs and pro-inflammatory cytokines (4). Subsequently, the mRNA escapes the endosome and is released into the cytosol, where it can be translated by ribosomes (5). The resulting protein may be secreted outside the host cell (7a) and taken up by other APCs (8a), where it is processed into peptides for presentation on MHC class II molecules (9), facilitating recognition by CD4+ T lymphocytes (10). Alternatively, the translated proteins can be degraded into peptides by the proteasome within the same cell (6). These antigenic peptides are transported into the endoplasmic reticulum and can be loaded onto MHC class I and/or class II molecules through a less common pathway (7b). The complexes formed by MHC–peptide epitopes are then presented on the APC surface, where they can bind to the T cell receptor (TCR) of CD8+ and/or CD4+ T lymphocytes (8b). Reprinted with permission from Ref. [83] under the Creative Commons Attribution 4.0 International License.

Figure 6

mRNA-based therapeutics: versatile tools in disease treatment. The production process of mRNA-based drugs begins with designing and encoding the target peptide or protein into a plasmid DNA construct. This plasmid DNA is then transcribed into mRNA in vitro using bacteriophage polymerases. The resulting mRNA transcripts are purified through methods such as high-performance liquid chromatography (HPLC) or nanoprecipitation to eliminate impurities and reactants. Purified mRNA is then encapsulated in various delivery systems. The interaction between the mRNA and these vehicles occurs through three mechanisms: (a) electrostatic adsorption involving phosphate groups of ribonucleotides, (b) complementary hydrogen bonding with the nucleotide bases, and (c) coordination with phosphate ions. Delivery systems for mRNA include cationic compounds such as cationic lipids, ionizable lipids, and cationic polymers, along with nucleoside-based lipids like DNCA or amphiphilic polymers such as Chol(+)-oligoRNA. Additionally, metal-based compounds can bind to phosphate ions via coordination. After formulation, the effectiveness, pharmacology, and safety of mRNA drugs are assessed in preclinical models, such as vaccinated mice and primates. Finally, the manufacturing process is scaled up for clinical trials. Reprinted with permission from Ref. [31] under the Creative Commons Attribution 4.0 International License.