Supervised by: Yuhui Zhou, BA (Hons). Yuhui is a 5th year medical student at the University of Cambridge. She gained a First class degree in her intercalated year studying Pathology. She has an interest in Cancer & Immunology and has been awarded a Wellcome Trust Biomedical Vacation Scholarship to study host responses to infection.
Abstract
In this informative review article, we discuss the recent advances that have allowed oncolytic viruses to become an emerging new method of immunotherapy, particularly as an adjunct treatment. Oncolytic viruses have demonstrated substantial potential in the field of cancer treatment as they have fewer side effects when compared to other cancer therapies. However, global recognition of the potential benefits of oncolytic viruses was achieved in the 1990s, as improvements in technology made genetic modification of these viruses possible. Oncolytic viruses are particularly effective when combined with other forms of cancer treatments including chemotherapy and radiotherapy. Due to the rapidly evolving data, the implications of the use of oncolytic viruses are immense.
Meanwhile, despite the promising research surrounding oncolytic viruses, more evidence is required to provide definitive proof about the safety and efficiency of the use of oncolytic viruses in cancer therapy. Literary reviews, clinical trial data, and systematic reviews were critically analyzed in order to ensure that conclusions made were based upon reliable and recent data. This informative review aims to leave readers with an understanding of the existing benefits and limitations of oncolytic viruses as a cancer treatment, as well as the ways in which oncolytic viruses can be genetically altered to increase their efficiency.
This literature review examines the mechanisms of oncolytic viruses, current research and experiments, as well as promising new experiments and future prospects.
Introduction
This study focuses on the emerging use of oncolytic viruses and the future prospects for the use of the virus within cancer therapeutics. This review will also discuss how oncolytic viruses can be used as an adjunct to immunotherapy.
Oncolytic viruses are a newly introduced virotherapy showing great potential for their therapeutic efficacy, safety, and reduced side effects compared to chemotherapy and radiotherapy. Oncolytic viruses are genetically engineered or naturally occurring viruses that selectively replicate in – and kill – cancer cells without harming healthy tissue (Fukuhara, et al.). It has been observed that cancer patients went into remission when they were infected with certain viruses and this has emerged as a new field of cancer therapeutics (Mondal, et al.). Additionally, for more stable and long-lasting results, oncolytic viruses are often combined with other cancer therapies. However, this study will highlight how oncolytic viruses are used with other methods of cancer therapies as an adjunct.
Immunotherapy is a form of cancer treatment that utilizes the immune system to fight cancer by stimulating the natural defenses of the immune system or creating substances that act like immune system components. This improves the immune system’s effectiveness in finding and attacking cancer cells. Oncolytic viruses are a form of immunotherapy in which viruses are modified to infect and kill certain tumor cells (American Cancer Society).
The adenovirus is one of the most frequently employed oncolytic viruses due to its effectiveness in cell lysis and immune response stimulation (Zhao, et al.). The adenovirus is also advantageous due to the low pathogenic risk, high genome stability, wide range of tissue tropism, and relatively large DNA loading capacity (Huang, et al.).
This study will detail the mechanisms of oncolytic viruses, as well as the existing and promising new clinical trials surrounding oncolytic viruses. As stated above, this study will focus on how oncolytic viruses are used in combination with immunotherapy.
Cancer is among the leading causes of death worldwide. In 2018, 18.1 million new cases were diagnosed, with 9.5 million deaths worldwide (National Cancer Institute). There has been significant progress in the prevention and diagnosis of cancer, but the incidence and mortality are still increasing. Therefore, the implications of the newly introduced oncolytic viruses demonstrate promising potential as cancer therapeutics which can significantly improve patient outcomes.
The future research concerning oncolytic viruses aims to develop different viruses which are able to target specific cancers, and improve the efficacy and safety of oncolytic viruses.
Mechanism of Oncolytic Viruses
Cancer immunotherapy is proven effective when an increase in the frequency and function of tumor-infiltrating immune cells are observed, resulting in improved therapeutic efficacy (Ylösmäki and Cerullo). Difficulties arise in the development of oncolytic viruses as some cancer cells and non-transformed supporting cells are resistant to certain oncolytic viruses (Mondal, et al.). Therefore, a single type of virotherapy is not effective in all types of cancers due to the complexity of cancer cells and the diversity of cancer tissues (Mondal, et al.). Oncolytic viruses have two main objectives: the direct induction of tumor cell lysis and the indirect stimulation of the immune system (Terrível, et al.).
The method by which viruses enter into cells begins with the binding of the proteins found on a virus’s surface to a particular receptor on the surface of a host cell (cancer cell). The virus then penetrates the cell, and goes through the process of viral uncoating, allowing the viral genome to replicate. The final stage occurs when the viral progeny is released from the cell through cell lysis or exocytosis to infect adjacent cells. Cell lysis is the main objective in oncolytic virotherapy as the constant repetition of this process kills tumor cells, leading to progessive tumor mass destruction. In addition to the release of viral particles cell lysis also releases tumor-derived antigens (TDAs) and damage-associated molecular patterns (DAMPs), in the process of immunogenic cell death (ICD). The released molecules induce an immune response further serving to combat the tumor growth and promote tumor removal as immune system components are led to the tumor environment (Terrível, et al.). The infection of cancer cells by oncolytic viruses induces the release of toll-like receptor (TLR) ligands including the pathogen-associated molecular patterns (PAMPs) and DAMPs which play a critical role in the activation of antigen-presenting cells (APCs), natural killer (NK) cells, and T cells. The combination of cytokines and TLR ligands nullifies the tumor-induced immune suppression. The anti-tumor immune response will act against the primary tumor and existing secondary malignant growths. Memory T cells may also be produced, allowing for a later tumor protection in the event of a relapse (Terrível, et al.).
Human adenovirus is the most potent virus being used in oncolytic virotherapy (Mondal, et al.). There are several strains of DNA viruses from the Adenoviridae family, but serotype 5 is the most commonly used in oncolytic therapy (Terrível, et al.). Ad5-yCD/mutTKSR39rep-hIL12 is an adenovirus serotype with an insertion of yCD, TKSR39, and human interleukin (IL)-12 which is currently being tested in prostate and pancreatic cancer (Terrível, et al.). yCD and TKSR39 are suicide genes which allow the conversion of prodrugs 5-flurocytosine (5-FC) and valganciclovir in their toxic forms within infected cells (Terrível, et al.). The IL-12 insertion contributes to the activation of innate and adaptive immunity, resulting in an enhanced immune response against tumor cells (Terrível, et al.). Jason Chesney, M.D., Ph.D., director of the University of Louisville’s James Graham Brown Cancer Center described the action of oncolytic viruses as serving to “[alert] the immune system that something’s wrong”.
Current Research and Experiments
In recent years, oncolytic viruses have been taking the front stage in biological therapy for cancer. Many systematic reviews on the clinical application of oncolytic viruses have shown the potential of oncolytic viruses working as an immunotherapeutic tool. This is currently being extensively explored in cancer therapeutics.
Since the use of viruses in cancer therapy was recognized over a century ago, oncolytic viruses are a time-tested method of cancer treatment. However, several notable shifts in the study of oncolytic viruses have recently occured. Matthias Gromeier, M.D., of the Duke Cancer Institute, noted that the science on oncolytic viruses only began to move forward in the 1990s with advances in genetic engineering technology, making modern oncolytic viruses a fairly new field of study for researchers (National Cancer Institute). Another shift occurred around 2005, when researchers discovered the true value of oncolytic viruses as a form of immunotherapy for cancer treatment (National Cancer Institute).
Apart from this, oncolytic viruses have shown many benefits in terms of their efficacy, safety, and limited side effects. Additionally, some reports have stated that oncolytic viruses are limited in their ability to replicate outside of cancer cells – they selectively replicate within cancer cells above all other cell types – making them rather simple to administer (Fukuhara, et al.). These advantages made biological therapy a preferred treatment option for many patients. Additionally, recent clinical trials have provided supporting evidence for the versatility of viruses in cancer treatments, by showing how they can be used to complement other cancer therapies, while gaining optimal patient benefits (Hemminki, et al.).
Though oncolytic viruses have initially shown these benefits, more evidence and research is required to provide definitive proof thereof. Therefore, now that clinical trials have improved the safety and tolerability of oncolytic viruses, many are moving on to increase efficacy in a wide array of approaches, including adding different immunomodulatory transgenes to the viruses. However, improving the efficacy of oncolytic viruses is challenging due to the limited understanding of increasing specificity of cancer cells and human immune responses against antigens produced from the cancer cells.
Recent clinical trials have shown that a single type of oncolytic virus is oftentimes not effective in destroying all cancer cells and avoiding metastasis (Mondal, et al.). Moreover, adjunct properties of oncolytic viruses were shown through the higher effectiveness when used alongside other cancer treatments (Fukuhara, et al.). However, as some oncolytic viruses have also shown their function as an “alerting tool to the immune system”, some researchers are considering oncolytic viruses to be a form of immunotherapy. Meanwhile, as not all oncolytic viruses act in an immunotherapeutic manner, researchers are currently studying how different oncolytic viruses may be applied against cancer tissues for a multi-layered approach (National Cancer Institute).
Apart from the adenovirus, other viruses, such as genetically engineered HSV-based oncolytic virus, talimogene laherparepvec (Imlygic®), also called T-VEC, and vaccine strains of the measles virus have also shown impressive results in pre-clinical and clinical trials (Mondal, et al.). Clinical trials have shown that T-VEC can be engineered to produce a protein that stimulates the production of immune cells in the body and reduces the risk of causing herpes as a side-effect when it is injected into the tumors (National Cancer Institute).
Hence, this genetically modified herpes virus is, to date, the only oncolytic virus that has been approved by the U.S. Food and Drug Administration (FDA) for use in biological cancer therapy for the treatment of patients with melanoma lesions in the skin and lymph nodes (Wolters Kluwer Health). This has contributed to major advancements in the application of viruses within cancer treatment. Meanwhile, clinical trials have also shown room for improvement for their uses, and have suggested that checkpoint inhibitors, and other immunomodulatory drugs could make ideal companions to oncolytic viruses (Terrível, et al.). There are also a number of other viruses being evaluated as potential treatments for cancer in clinical trials.
An NCI-sponsored conference on using microbes as cancer therapies in 2017 also introduced that as of today, not only are several dozen viruses being studied, but a few strains of bacteria are also being studied as potential cancer treatments.
Promising New Experiments and Future Projections
Adenovirus-based oncolytic viruses have shown great success in recent clinical trials (Hemminki, et al.). However, clinical trials have expressed shortcomings due to the high levels of neutralizing antibodies in the vector itself, which has been shown to impair the therapeutic efficacy in patients. To combat this issue research suggests that chimpanzee adenovirus-based oncolytic viruses have the potential to be applied clinically due to their ability to overcome the problem of preexisting immunity (Hemminki, et al.). Other potential strategies that are currently being studied in an attempt to overcome the issue of preexisting immunity include: using less seroprevalence adenovirus serotypes, candid pseudotyping, genetic masking, and chemical shielding (Mondal, et al.).
Studies have supported that the potency of oncolytic viruses can be increased by the expression of cytotoxicity enhancing proteins (Hemminki, et al.) (Fukuhara, et al.) (Singh et al.). Adenovirus death protein (ADP) is a glycoprotein which can be applied in the late stages of infection to efficiently lyse and release viruses from cells. Data suggests that in viruses where ADP is overexpressed the virus is able to spread more rapidly and efficiently through tumors (Terrível, et al.). Two types of adenoviruses being extensively studied include DNX-2401 and VCN-01.
DNX-2401 is a promising adenovirus-based oncolytic virus, which consists of a deletion of 24 bps in the E1A region and the engineering of the RGD motif into the HI-loop of the fiber knob. DNX-2401 has been shown to be more potent and safe as the RGD-4C motif enhances replication and infectivity of the adenovirus while reducing the sequestration of the adenovirus by CAR-expressing normal cells. Success in a clinical trial treatment regimen demonstrated the benefits of DNX-2401 as a potential therapeutic agent in combination therapy for gliomas (Mondal, et al.).
VCN-01 is an RB pathway selective- and hyaluronidase-armed oncolytic adenovirus being currently applied in several clinical trials. VCN-01 has demonstrated cytotoxic effects on glioma cells in vitro and in vivo. While much research is still required as the Adenoviridae family is extremely vast and complex, the promising potential of the adenovirus has been demonstrated allowing further investigations (Mondal, et al.).
Methodology
Please see out methodology table here: https://docs.google.com/spreadsheets/d/18KAhUZQKuYj3iY2Qu0ePMF0WW3PQMI7E9Nf0g5jOTpY/edit?usp=sharing
Discussion
In order to form reasonable conclusions about the practicality of oncolytic viruses, several clinical trials, review articles, and web pages were critically analyzed. While the conclusions made were mostly based upon recent data, one must acknowledge that scientific discoveries and consequently scientific data are always evolving. One must also recognize that limitations to data are inevitable in the forms of flaws in methodology, and access to data.
This literature review only evaluated papers written in English ,demonstrating an evident limitation as research concerning oncolytic viruses is ongoing globally. It is impossible to have all of the current information on oncolytic viruses especially as the scale of research continues to expand at a rapid pace. Therefore, while well-founded, the conclusions made were based on an interpretation of available scientific literature rather than first-hand data.
The articles examined rarely expressed potential conflicts of interest in the form of funding for clinical trials. Paid journals also posed a challenge as data was not freely available, especially for raw data as many clinical trials are in early stages. The use of oncolytic viruses is not widely utilized, causing one to question the validity of early stage clinical trial data as subject sizes and other constraints were not advertised. However, the data currently available would indicate further support for the conclusions made rather than the opposite.
Despite the astounding advantages of oncolytic viruses and the continuous study over years, oncolytic viruses are not extensively used in cancer therapeutics, especially when compared to chemo and radio therapies that cause significant side effects.
The biggest issue with oncolytic viruses is the preexisting uncertainty of its application. Oncolytic virotherapy is a “new field” of cancer treatment, and there is a lack of evidence that supports the idea that oncolytic viruses are a definitive tool to treat cancer. This also relates to the points previously mentioned in this literature review, including the need to work on the function and efficacy of the oncolytic viruses, as well as their need to be personalized for individual patients, regarding the different types of cancer cells in different locations of the body.
Additionally, the examined articles demonstrated many cases where ‘manmade-mutations’ showed successful results in treating tumor cells. In 1991, Martuza demonstrated genetically engineered HSV-1 with a mutation in the TK gene replicated selectively in cancer cells successfully treating experimental brain tumors (Martuza, et al.). As the clinical trials focused on intentional mutations for clinical purposes, it lacked information about ‘natural mutations’. Therefore, it raised the question of whether mutations of pre-genetically-modified oncolytic viruses may infect other cells, potentially becoming a significant side effect of their use. However, this is a subject to be further studied in the future.
Cost and accessibility is also another problem with the use of oncolytic viruses. Due to the need to be personalized, it is difficult for specialists to research specific types of oncolytic viruses and their combinations. This naturally leads to growing pressure for specialists as demand continues to rise. Therefore, the cost for private treatments are exceptionally high, leading to the problem that only those with a high income are able to afford access to these private treatments.
New advances to technology and a greater understanding of genetics have allowed science to manipulate viruses to target cancer cells. The recent interest in oncolytic viruses is well founded as they are able to target cancer cells while preventing the destruction of healthy cells. The recorded success of oncolytic viruses such as the adenovirus provides aspirations to treat different forms of cancers which require the specialization and genetic modification of other emerging viruses. However, the promising data does not discount the necessity for future analysis, as evident weaknesses in efficacy have been identified. As further research is done, shortcomings can be overcome with the use of gene manipulation and different viruses.
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