Transferrin-Lipoplexes with Protamine-Condensed DNA for Serum-Resistant Gene Delivery

Recent Advances in Messenger Ribonucleic Acid (mRNA) Vaccines and Their Delivery Systems: A Review

Messenger ribonucleic acid (mRNA) was found as the intermediary that transfers genetic information from DNA to ribosomes for protein synthesis in 1961. The emergency use authorization of the two covid-19 mRNA vaccines, BNT162b2 and mRNA-1273, is a significant achievement in the history of vaccine development. Because they are generated in a cell-free environment using the in vitro transcription (IVT) process, mRNA vaccines are risk-free. Moreover, chemical modifications to the mRNA molecule, such as cap structures and changed nucleosides, have proved critical in overcoming immunogenicity concerns, achieving sustained stability, and achieving effective, accurate protein production in vivo. Several vaccine delivery strategies (including protamine, lipid nanoparticles (LNPs), polymers, nanoemulsions, and cell-based administration) were also optimized to load and transport RNA into the cytosol. LNPs, which are composed of a cationic or a pH-dependent ionizable lipid layer, a polyethylene glycol (PEG) component, phospholipids, and cholesterol, are the most advanced systems for delivering mRNA vaccines. Moreover, modifications of the four components that make up the LNPs showed to increase vaccine effectiveness and reduce side effects. Furthermore, the introduction of biodegradable lipids improved LNP biocompatibility. Furthermore, mRNA-based therapies are expected to be effective treatments for a variety of refractory conditions, including infectious diseases, metabolic genetic diseases, cancer, cardiovascular and cerebrovascular diseases. Therefore, the present review aims to provide the scientific community with up-to-date information on mRNA vaccines and their delivery systems.

Recent Advances in the Molecular Design and Delivery Technology of mRNA for Vaccination Against Infectious Diseases

Vaccines can prevent many millions of illnesses against infectious diseases and save numerous lives every year. However, traditional vaccines such as inactivated viral and live attenuated vaccines cannot adapt to emerging pandemics due to their time-consuming development. With the global outbreak of the COVID-19 epidemic, the virus continues to evolve and mutate, producing mutants with enhanced transmissibility and virulence; the rapid development of vaccines against such emerging global pandemics becomes more and more critical. In recent years, mRNA vaccines have been of significant interest in combating emerging infectious diseases due to their rapid development and large-scale production advantages. However, their development still suffers from many hurdles such as their safety, cellular delivery, uptake, and response to their manufacturing, logistics, and storage. More efforts are still required to optimize the molecular designs of mRNA molecules with increased protein expression and enhanced structural stability. In addition, a variety of delivery systems are also needed to achieve effective delivery of vaccines. In this review, we highlight the advances in mRNA vaccines against various infectious diseases and discuss the molecular design principles and delivery systems of associated mRNA vaccines. The current state of the clinical application of mRNA vaccine pipelines against various infectious diseases and the challenge, safety, and protective effect of associated vaccines are also discussed.

An overview of rational design of mRNA-based therapeutics and vaccines

INTRODUCTION

Messenger RNA (mRNA)-based therapeutics and vaccines have emerged as a disruptive new drug class for various applications, including regenerative medicine, cancer treatment, and prophylactic and therapeutic vaccinations.

AREAS COVERED

This review provides an update about the rational structure-based design of various formats of mRNA-based therapeutics. The authors discuss the recent advances in the mRNA modifications that have been used to enhance stability, promote translation efficiency and regulate immunogenicity for specific applications.

EXPERT OPINION

Extensive research efforts have been made to optimize mRNA constructs and preparation procedures to unleash the full potential of mRNA-based therapeutics and vaccines. Sequence optimization (untranslated region and codon usage), chemical engineering of nucleotides and modified 5'cap, and optimization of in vitro transcription and mRNA purification protocols have overcome the major obstacles (instability, delivery, immunogenicity and safety) hindering the clinical applications of mRNA therapeutics and vaccines. The optimized design parameters should not be applied as default to different biological systems, but rather individually optimized for each mRNA sequence and intended application. Further advancement in the mRNA design and delivery technologies for achieving cell type- and organ site-specificity will broaden the scope and usefulness of this new class of drugs.

Modeling the Kinetics of Lipid-Nanoparticle- Mediated Delivery of Multiple siRNAs to Evaluate the Effect on Competition for Ago2

Drug combinations can improve the control of diseases involving redundant and highly regulated pathways. Validating a multi-target therapy early in drug development remains difficult. Small interfering RNAs (siRNAs) are routinely used to selectively silence a target of interest. Owing to the ease of design and synthesis, siRNAs hold promise for combination therapies. Combining siRNAs against multiple targets remains an attractive approach to interrogating highly regulated pathways. Currently, questions remain regarding how broadly such an approach can be applied, since siRNAs have been shown to compete with one another for binding to Argonaute2 (Ago2), the protein responsible for initiating siRNA-mediated mRNA degradation. Mathematical modeling, coupled with in vitro and in vivo experiments, led us to conclude that endosomal escape kinetics had the highest impact on Ago2 depletion by competing lipid-nanoparticle (LNP)-formulated siRNAs. This, in turn, affected the level of competition observed between them. A future application of this model would be to optimize delivery of desired siRNA combinations in vitro to attenuate competition and maximize the combined therapeutic effect.

Modified mRNA as an alternative to plasmid DNA (pDNA) for transcript replacement and vaccination therapy

Introduction: Current gene therapy involves replacement of defective gene by delivery of healthy genetic material to precede normal function. Virus-mediated gene delivery is the most successful and efficient method for gene therapy, but it has been challenged due to serious safety concerns. Conversely, gene delivery using plasmid DNA (pDNA) is considered safer, but its transfection efficiency is much lower than virus-mediated gene transfer. Recently, mRNA has been suggested as an alternative option to avoid undesired insertion of delivered DNA sequences with higher transfection efficiency and stability.

Area covered: In this review, we summarize the currently available strategies of mRNA modification to increase the therapeutic efficacy; we also highlight the recent improvements of mRNA delivery for in vivo applications of gene therapy.

Expert opinion: The use of mRNA-based gene transfer could indeed be a promising new strategy for gene therapy. Notable advantages include no risk of integration into the genomic DNA, adjustable gene expression and easier modulation of the immune system. By reducing or utilizing the immunogenic properties, mRNA offers a promising tool for gene/or transcript replacement.

Inhibition of Xenograft tumor growth by gold nanoparticle-DNA oligonucleotide conjugates-assisted delivery of BAX mRNA

Use of non-biological agents for mRNA delivery into living systems in order to induce heterologous expression of functional proteins may provide more advantages than the use of DNA and/or biological vectors for delivery. However, the low efficiency of mRNA delivery into live animals, using non-biological systems, has hampered the use of mRNA as a therapeutic molecule. Here, we show that gold nanoparticle-DNA oligonucleotide (AuNP-DNA) conjugates can serve as universal vehicles for more efficient delivery of mRNA into human cells, as well as into xenograft tumors generated in mice. Injections of BAX mRNA loaded on AuNP-DNA conjugates into xenograft tumors resulted in highly efficient mRNA delivery. The delivered mRNA directed the efficient production of biologically functional BAX protein, a pro-apoptotic factor, consequently inhibiting tumor growth. These results demonstrate that mRNA delivery by AuNP-DNA conjugates can serve as a new platform for the development of safe and efficient gene therapy.

Synthesis and transfection efficiency of cationic oligopeptide lipids: role of linker

In the design of new cationic lipids for gene transfection, the chemistry of linkers is widely investigated from the viewpoint of biodegradation and less from their contribution to the biophysical properties. We synthesized two dodecyl lipids with glutamide as the backbone and two lysines to provide the cationic headgroup. Lipid 1 differs from Lipid 2 by the presence of an amide linkage instead of an ester linkage that characterizes Lipid 2. The transfection efficiency of lipoplexes with cholesterol as colipid was found to be very high with Lipid 1 on Chinese Hamster Ovary (CHO) and HepG2 cell lines, whereas Lipid 2 has shown partial transfection efficiency on HepG2 cells. Lipid 1 was found to be stable in the presence of serum when tested in HepG2 and CHO cells albeit with lower activity. Fluorescence-based dye-binding and agarose gel-based assays indicated that Lipid 1 binds to DNA more efficiently than Lipid 2 at charge ratios of >1:1. The uptake of oligonucleotides with Lipid 1 was higher than Lipid 2 as revealed by confocal microscopy. Transmission electron microscopy (TEM) images reveal distinct formation of liposomes and lipoplexes with Lipid 1 but fragmented and unordered structures with Lipid 2. Fusion of Lipids 1 and 2 with anionic vesicles, with composition similar to plasma membrane, suggests that fusion of Lipid 2 was very rapid and unlike a fusion event, whereas the fusion kinetics of Lipid 1 vesicles was more defined. Differential scanning calorimetry (DSC) revealed a high T(m) for Lipid 1 (65.4 °C) while Lipid 2 had a T(m) of 23.5 °C. Surface area-pressure isotherms of Lipid 1 was less compressible compared to Lipid 2. However, microviscosity measured using 1,6-diphenyl-1,3,5-hexatriene (DPH) revealed identical values for vesicles made with either of the lipids. The presence of amide linker apparently resulted in stable vesicle formation, higher melting temperature, and low compressibility, while retaining the membrane fluid properties suggesting that the intermolecular hydrogen bonds of Lipid 1 yielded stable lipoplexes of high transfection efficiency.

Targeted liposomes to deliver DNA to cells expressing 5-HT receptors

Cell targeted delivery of drugs, including nucleic acids, is known to enhance the therapeutic potential of free drugs. We used serotonin (5-HT) as the targeting ligand to deliver plasmid DNA to cells specifically expressing 5-HT receptor. Our liposomal formulation includes the 5-HT conjugated targeting lipid, a cationic lipid and cholesterol. DNA-binding studies indicate that the targeting 5-HT-lipid binds DNA efficiently. The formulation was tested and found to efficiently deliver DNA into CHO cells stably expressing the human serotonin(1A) receptor (CHO-5-HT(1A)R) compared to control CHO cells. Liposomes without the 5-HT moiety were less efficient in both cell lines. Similar enhancement in transfection efficiency was also observed in human neuroblastoma IMR32 and hepatocellular carcinoma (HepG2) cells. Cell uptake studies using CHO-5-HT(1A)R cells by flow cytometry and confocal microscopy clearly indicated that the targeting liposomes through 5-HT moiety may have a direct role in increasing the cellular uptake of DNA-lipid complexes. To our knowledge this is the first report that demonstrates receptor-targeted nucleic acid delivery into cells expressing 5-HT receptor.

A versatile and tunable coating strategy allows control of nanocrystal delivery to cell types in the liver

There are many liver diseases that could be treated with delivery of therapeutics such as DNA, proteins, or small molecules. Nanoparticles are often proposed as delivery vectors for such therapeutics; however, achieving nanoparticle accumulations in the therapeutically relevant hepatocytes is challenging. In order to address this issue, we have synthesized polymer coated, fluorescent iron oxide nanoparticles that bind and deliver DNA, as well as produce contrast for magnetic resonance imaging (MRI), fluorescence imaging, and transmission electron microscopy (TEM). The composition of the coating can be varied in a facile manner to increase the quantity of poly(ethylene glycol) (PEG) from 0% to 5%, 10%, or 25%, with the aim of reducing opsonization but maintaining DNA binding. We investigated the effect of the nanoparticle coating on DNA binding, cell uptake, cell transfection, and opsonization in vitro. Furthermore, we exploited MRI, fluorescence imaging, and TEM to investigate the distribution of the different formulations in the liver of mice. While MRI and fluorescence imaging showed that each formulation was heavily taken up in the liver at 24 h, the 10% PEG formulation was taken up by the therapeutically relevant hepatocytes more extensively than either the 0% PEG or the 5% PEG, indicating its potential for delivery of therapeutics to the liver.

Designed multi-domain protein as a carrier of nucleic acids into cells

Protein-based nucleic acid carriers offer attractive possibilities to enhance in vitro and in vivo gene delivery to combat diseases. A multi-domain fusion protein, namely TAT-NLS-Mu, designated as TNM, has been designed, cloned, heterologously expressed in E. coli and purified to homogeneity by affinity chromatography. The recombinant chimera TNM harbors three epitopes, a cell-penetrating (TAT) domain, a nuclear localization domain comprising of three nuclear localization sequence (NLS) motifs in tandem and a DNA-binding (Mu) domain. Complexes prepared by combining plasmid DNA with TNM (DP) transfect MCF-7, COS, CHO and HepG2 cells. Ternary complexes prepared with DNA, protein and cationic lipid (DPL) resulted in ~5-7 fold enhancement in reporter gene expression over the DP alone. Treatment of cells with chloroquine during transfection, with DP complexes, resulted in remarkable increases in reporter gene expression suggesting the involvement of endosomal compartments in the uptake process. Interestingly, DPL prepared with Lipofectin or 1, 2-Dioleoyl-3-Trimethylammonium-Propane (DOTAP) exhibited enhanced transfection in the presence of serum in MCF-7 and HepG2 cells. Microinjection of DP complexes, with and without NLS sequence, into the cytoplasm and nucleus of smooth muscle cells (SMC) indicated that the presence of NLS sequence in protein carrier significantly enhanced transgene expression. Together the data suggest that modular design of proteins is a promising method to develop gene delivery carriers and also the role of NLS epitopes in mediating nuclear transfer of DNA complexes into various cell types.

Lipid-based nanoparticles for nucleic acid delivery

Lipid-based colloidal particles have been extensively studied as systemic gene delivery carriers. The topic that we would like to emphasize is the formulation/assembly of lipid-based nanoparticles (NP) with diameter under 100 nm for delivering nucleic acid in vivo. NP are different from cationic lipid-nucleic acid complexes (lipoplexes) and are vesicles composed of lipids and encapsulated nucleic acids with a diameter less than 100 nm. The diameter of the NP is an important attribute to enable NP to overcome the various in vivo barriers for systemic gene delivery such as: the blood components, reticuloendothelial system (RES) uptake, tumor access, extracellular matrix components, and intracellular barriers. The major formulation factors that impact the diameter and encapsulation efficiency of DNA-containing NP include the lipid composition, nucleic acid to lipid ratio and formulation method. The particle assembly step is a critical one to make NP suitable for in vivo gene delivery. NP are often prepared using a dialysis method either from an aqueous-detergent or aqueous-organic solvent mixture. The resulting particles have diameters about 100 nm and nucleic acid encapsulation ratios are >80%. Additional components can then be added to the particle after it is formed. This ordered assembly strategy enables one to optimize the particle physico-chemical attributes to devise a biocompatible particle with increased gene transfer efficacy in vivo. The components included in the sequentially assembled NP include: poly(ethylene glycol) (PEG)-shielding to improve the particle pharmacokinetic behavior, a targeting ligand to facilitate the particle-cell recognition and in some case a bioresponsive lipid or pH-triggered polymer to enhance nucleic acid release and intracellular trafficking. A number of groups have observed that a PEG-shielded NP is a robust and modestly effective system for systemic gene or small interfering RNA (siRNA) delivery.