Self-replicating RNA viral vectors in vaccine development and gene therapy

Vector engineering, strategies and targets in cancer gene therapy

Understanding the molecular basis of disease and the design of rationally designed molecular therapies has been the holy grail in the management of human cancers. Gene-based therapies are an important avenue for achieving a possible cure. Focused research in the last three decades has provided significant clues to optimize the potential of cancer gene therapy. The development of gene therapies with a high potential to kill the target cells at the lowest effective dose possible, the development of vectors with significant ability to target cancer-associated antigen, the application of adjunct therapies to target dysregulated microRNA, and embracing a hybrid strategy with a combination of gene therapy and low-dose chemotherapy in a disease-specific manner will be pivotal. This article outlines the advances and challenges in the field with emphasis on the biology and scope of vectors used for gene transfer, newer targets identified, and their outcome in preclinical and clinical studies.

Viral Vectors for COVID-19 Vaccine Development

Vaccine development against SARS-CoV-2 has been fierce due to the devastating COVID-19 pandemic and has included all potential approaches for providing the global community with safe and efficient vaccine candidates in the shortest possible timeframe. Viral vectors have played a central role especially using adenovirus-based vectors. Additionally, other viral vectors based on vaccinia viruses, measles viruses, rhabdoviruses, influenza viruses and lentiviruses have been subjected to vaccine development. Self-amplifying RNA virus vectors have been utilized for lipid nanoparticle-based delivery of RNA as COVID-19 vaccines. Several adenovirus-based vaccine candidates have elicited strong immune responses in immunized animals and protection against challenges in mice and primates has been achieved. Moreover, adenovirus-based vaccine candidates have been subjected to phase I to III clinical trials. Recently, the simian adenovirus-based ChAdOx1 vector expressing the SARS-CoV-2 S spike protein was approved for use in humans in the UK.

Nanomedicine-Based Approaches for mRNA Delivery

Messenger RNA (mRNA) has immense potential for developing a wide range of therapies, including immunotherapy and protein replacement. As mRNA presents no risk of integration into the host genome and does not require nuclear entry for transfection, which allows protein production even in nondividing cells, mRNA-based approaches can be envisioned as safe and practical therapeutic strategies. Nevertheless, mRNA presents unfavorable characteristics, such as large size, immunogenicity, limited cellular uptake, and sensitivity to enzymatic degradation, which hinder its use as a therapeutic agent. While mRNA stability and immunogenicity have been ameliorated by direct modifications on the mRNA structure, further improvements in mRNA delivery are still needed for promoting its activity in biological settings. In this regard, nanomedicine has shown the ability for spatiotemporally controlling the function of a myriad of bioactive agents in vivo. Direct engineering of nanomedicine structures for loading, protecting, and releasing mRNA and navigating in biological environments can then be applied for promoting mRNA translation toward the development of effective treatments. Here, we review recent approaches aimed at enhancing mRNA function and its delivery through nanomedicines, with particular emphasis on their applications and eventual clinical translation.

Potential use of noncoding RNAs and innovative therapeutic strategies to target the 5'UTR of SARS-CoV-2

After the increasing number of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections all over the world, researchers and clinicians are struggling to find a vaccine or innovative therapeutic strategies to treat this viral infection. The severe acute respiratory syndrome coronavirus infection that occurred in 2002, Middle East respiratory syndrome (MERS) and other more common infectious diseases such as hepatitis C virus, led to the discovery of many RNA-based drugs. Among them, siRNAs and antisense locked nucleic acids have been demonstrated to have effective antiviral effects both in animal models and humans. Owing to the high genomic homology of SARS-CoV-2 and severe acute respiratory syndrome coronavirus (80–82%) the use of these molecules could be employed successfully also to target this emerging coronavirus. Trying to translate this approach to treat COVID-19, we analyzed the common structural features of viral 5’UTR regions that can be targeted by noncoding RNAs and we also identified miRNAs binding sites suitable for designing RNA-based drugs to be employed successfully against SARS-CoV-2.

Advances in delivery vectors for gene therapy in liver cancer

Hepatocellular carcinoma (HCC) is the third most common cause of cancer death globally, mainly due to lack of effective treatments – a problem that gene therapy is poised to solve. Successful gene therapy requires safe and efficient delivery vectors, and recent advances in both viral and nonviral vectors have made an important impact on HCC gene therapy delivery. This review explores how adenoviral, retroviral and adeno-associated viral vectors have been modified to increase safety and delivery capacity, highlighting studies and clinical trials using these vectors for HCC gene therapy. Nanoparticles, liposomes, exosomes and virosomes are also featured in their roles as HCC gene delivery vectors. Finally, new discoveries in gene editing technology and their impacts on HCC gene therapy are discussed.

RNA-loaded dendritic cells: more than a tour de force in cancer therapeutics

A wide array of therapeutic strategies has been implemented against cancers, yet their clinical benefit is limited. The lack of clinical efficacy of the conventional treatment options might be due to the inept immune competency of the patients. Dendritic cells (DCs) have a vital role in initiating and directing immune responses and have been frequently used as delivery vehicles in clinical research. The recent clinical data suggest the potential use of DCs pulsed with nucleic acid, especially with RNA holds a great potential as an immunotherapeutic measure with compare to other cancer therapeutics. This review mainly deals with the DCs and their role in transfection with RNA in cancer immunotherapy.

Gene Therapy Today and Tomorrow

In the wake of a breakthrough in biotechnology providing realistic application of recombinant expressed proteins as drugs in the 1990s, gene therapy emerged as the potential approach for providing medicines of the future [...].

Viral Vectors in Gene Therapy

Applications of viral vectors have found an encouraging new beginning in gene therapy in recent years. Significant improvements in vector engineering, delivery, and safety have placed viral vector-based therapy at the forefront of modern medicine. Viral vectors have been employed for the treatment of various diseases such as metabolic, cardiovascular, muscular, hematologic, ophthalmologic, and infectious diseases and different types of cancer. Recent development in the area of immunotherapy has provided both preventive and therapeutic approaches. Furthermore, gene silencing generating a reversible effect has become an interesting alternative, and is well-suited for delivery by viral vectors. A number of preclinical studies have demonstrated therapeutic and prophylactic efficacy in animal models and furthermore in clinical trials. Several viral vector-based drugs have also been globally approved.

Latest development on RNA-based drugs and vaccines

Drugs and vaccines based on mRNA and RNA viruses show great potential and direct translation in the cytoplasm eliminates chromosomal integration. Limitations are associated with delivery and stability issues related to RNA degradation. Clinical trials on RNA-based drugs have been conducted in various disease areas. Likewise, RNA-based vaccines for viral infections and various cancers have been subjected to preclinical and clinical studies. RNA delivery and stability improvements include RNA structure modifications, targeting dendritic cells and employing self-amplifying RNA. Single-stranded RNA viruses possess self-amplifying RNA, which can provide extreme RNA replication in the cytoplasm to support RNA-based drug and vaccine development. Although oligonucleotide-based approaches have demonstrated potential, the focus here is on mRNA- and RNA virus-based methods.

Drug development has suffered from inefficiency, side effects and high costs. For this reason novel approaches for drug discovery are of great importance. RNA-based methods provide the advantage of targeting ‘production’ of drugs to diseased cells and vaccines to immune response-stimulating cells. RNA drugs have demonstrated therapeutic efficacy in eye and heart diseases and in various cancers in clinical trials. Likewise, RNA-based vaccines have provided protection against challenges with lethal doses of viruses such as Ebola and cancer cells in animal models.

Replicon RNA Viral Vectors as Vaccines

Single-stranded RNA viruses of both positive and negative polarity have been used as vectors for vaccine development. In this context, alphaviruses, flaviviruses, measles virus and rhabdoviruses have been engineered for expression of surface protein genes and antigens. Administration of replicon RNA vectors has resulted in strong immune responses and generation of neutralizing antibodies in various animal models. Immunization of mice, chicken, pigs and primates with virus-like particles, naked RNA or layered DNA/RNA plasmids has provided protection against challenges with lethal doses of infectious agents and administered tumor cells. Both prophylactic and therapeutic efficacy has been achieved in cancer immunotherapy. Moreover, recombinant particles and replicon RNAs have been encapsulated by liposomes to improve delivery and targeting. Replicon RNA vectors have also been subjected to clinical trials. Overall, immunization with self-replicating RNA viruses provides high transient expression levels of antigens resulting in generation of neutralizing antibody responses and protection against lethal challenges under safe conditions.