Oncolytic Reovirus and Immune Checkpoint Inhibition as a Novel Immunotherapeutic Strategy for Breast Cancer

Intratumoral delivery of immunotherapy to treat breast cancer: current development in clinical and preclinical studies

Breast cancer poses one of the largest threats to women's health. Treatment continues to improve for all the subtypes of breast cancer, but some subtypes, such as triple negative breast cancer, still present a significant treatment challenge. Additionally, metastasis and local recurrence are two prevalent problems in breast cancer treatment. A newer type of therapy, immunotherapy, may offer alternatives to traditional treatments for difficult-to-treat subtypes. Immunotherapy engages the host's immune system to eradicate disease, with the potential to induce long-lasting, durable responses. However, systemic immunotherapy is only approved in a limited number of indications, and it benefits only a minority of patients. Furthermore, immune related toxicities following systemic administration of potent immunomodulators limit dosing and, consequently, efficacy. To address these safety considerations and improve treatment efficacy, interest in local delivery at the site of the tumor has increased. Numerous intratumorally delivered immunotherapeutics have been and are being explored clinically and preclinically, including monoclonal antibodies, cellular therapies, viruses, nucleic acids, cytokines, innate immune agonists, and bacteria. This review summarizes the current and past intratumoral immunotherapy clinical landscape in breast cancer as well as current progress that has been made in preclinical studies, with a focus on delivery parameters and considerations.

Oncolytic virus-based combination therapy in breast cancer

Breast cancer continues to pose significant challenges in the field of oncology, necessitating innovative treatment approaches. Among these, oncolytic viruses have emerged as a promising frontier in the battle against various types of cancer, including breast cancer. These viruses, often genetically modified, have the unique ability to selectively infect and destroy cancer cells while leaving healthy cells unharmed. Their efficacy in tumor eradication is not only owing to direct cell lysis but also relies on their capacity to activate the immune system, thereby eliciting a potent and sustained antitumor response. While oncolytic viruses represent a significant advancement in cancer treatment, the complexity and adaptability inherent to cancer require a diverse array of therapies. The concept of combining oncolytic viruses with other treatment modalities, such as chemotherapy, immunotherapy, and targeted therapies, has received significant attention. This synergistic approach capitalizes on the strengths of each therapy, thus creating a comprehensive strategy to tackle the heterogeneous and evolving nature of breast cancer. The purpose of this review is to provide an in-depth discussion of preclinical and clinical viro-based combination therapy in the context of breast cancer.

New hopes for the breast cancer treatment: perspectives on the oncolytic virus therapy

Oncolytic virus (OV) therapy has emerged as a promising frontier in cancer treatment, especially for solid tumours. While immunotherapies like immune checkpoint inhibitors and CAR-T cells have demonstrated impressive results, their limitations in inducing complete tumour regression have spurred researchers to explore new approaches targeting tumours resistant to current immunotherapies. OVs, both natural and genetically engineered, selectively replicate within cancer cells, inducing their lysis while sparing normal tissues. Recent advancements in clinical research and genetic engineering have enabled the development of targeted viruses that modify the tumour microenvironment, triggering anti-tumour immune responses and exhibiting synergistic effects with other cancer therapies. Several OVs have been studied for breast cancer treatment, including adenovirus, protoparvovirus, vaccinia virus, reovirus, and herpes simplex virus type I (HSV-1). These viruses have been modified or engineered to enhance their tumour-selective replication, reduce toxicity, and improve oncolytic properties.Newer generations of OVs, such as Oncoviron and Delta-24-RGD adenovirus, exhibit heightened replication selectivity and enhanced anticancer effects, particularly in breast cancer models. Clinical trials have explored the efficacy and safety of various OVs in treating different cancers, including melanoma, nasopharyngeal carcinoma, head and neck cancer, and gynecologic malignancies. Notably, Talimogene laherparepvec (T-VEC) and Oncorine have. been approved for advanced melanoma and nasopharyngeal carcinoma, respectively. However, adverse effects have been reported in some cases, including flu-like symptoms and rare instances of severe complications such as fistula formation. Although no OV has been approved specifically for breast cancer treatment, ongoing preclinical clinical trials focus on four groups of viruses. While mild adverse effects like low-grade fever and nausea have been observed, the effectiveness of OV monotherapy in breast cancer remains insufficient. Combination strategies integrating OVs with chemotherapy, radiotherapy, or immunotherapy, show promise in improving therapeutic outcomes. Oncolytic virus therapy holds substantial potential in breast cancer treatment, demonstrating safety in trials. Multi-approach strategies combining OVs with conventional therapies exhibit more promising therapeutic effects than monotherapy, signalling a hopeful future for OV-based breast cancer treatments.

Neutralizing Antibodies Impair the Oncolytic Efficacy of Reovirus but Permit Effective Combination with T cell-Based Immunotherapies

Reovirus type 3 Dearing (Reo), manufactured for clinical application as pelareorep, is an attractive anticancer agent under evaluation in multiple phase 2 clinical trials for the treatment of solid tumors. It elicits its anticancer efficacy by inducing both oncolysis and intratumoral T-cell influx. Because most people have been preexposed to Reo, neutralizing antibodies (NAb) are prevalent in patients with cancer and might present a barrier to effective Reo therapy. Here, we tested serum of patients with cancer and healthy controls (n = 100) and confirmed that Reo NAbs are present in >80% of individuals. To investigate the effect of NAbs on both the oncolytic and the immunostimulatory efficacy of Reo, we established an experimental mouse model with Reo preexposure. The presence of preexposure-induced NAbs reduced Reo tumor infection and prevented Reo-mediated control of tumor growth after intratumoral Reo administration. In B cell-deficient mice, the lack of NAbs provided enhanced tumor growth control after Reo monotherapy, indicating that NAbs limit the oncolytic capacity of Reo. In immunocompetent mice, intratumoral T-cell influx was not affected by the presence of preexposure-induced NAbs and consequently, combinatorial immunotherapy strategies comprising Reo and T-cell engagers or checkpoint inhibitors remained effective in these settings, also after a clinically applied regimen of multiple intravenous pelareorep administrations. Altogether, our data indicate that NAbs hamper the oncolytic efficacy of Reo, but not its immunotherapeutic capacity. Given the high prevalence of seropositivity for Reo in patients with cancer, our data strongly advocate for the application of Reo as part of T cell-based immunotherapeutic strategies.

Hypoxia effects on oncolytic virotherapy in Cancer: Friend or Foe?

Researchers have tried to find novel strategies for cancer treatment in the past decades. Among the utilized methods, administering oncolytic viruses (OVs) alone or combined with other anticancer therapeutic approaches has had promising outcomes, especially in solid tumors. Infecting the tumor cells by these viruses can lead to direct lysis or induction of immune responses. However, the immunosuppressive tumor microenvironment (TME) is considered a significant challenge for oncolytic virotherapy in treating cancer. Based on OV type, hypoxic conditions in the TME can accelerate or repress virus replication. Therefore, genetic manipulation of OVs or other molecular modifications to reduce hypoxia can induce antitumor responses. Moreover, using OVs with tumor lysis capability in the hypoxic TME may be an attractive strategy to overcome the limitations of the therapy. This review summarizes the latest information available in the field of cancer virotherapy and discusses the dual effect of hypoxia on different types of OVs to optimize available related therapeutic methods.

Current perspectives on Vaxinia virus: an immuno-oncolytic vector in cancer therapy

Viruses are being researched as cutting-edge therapeutic agents in cancer due to their selective oncolytic action against malignancies. Immuno-oncolytic viruses are a potential category of anticancer treatments because they have natural features that allow viruses to efficiently infect, replicate, and destroy cancer cells. Oncolytic viruses may be genetically modified; engineers can use them as a platform to develop additional therapy modalities that overcome the limitations of current treatment approaches. In recent years, researchers have made great strides in the understanding relationship between cancer and the immune system. An increasing corpus of research is functioning on the immunomodulatory functions of oncolytic virus (OVs). Several clinical studies are currently underway to determine the efficacy of these immuno-oncolytic viruses. These studies are exploring the design of these platforms to elicit the desired immune response and to supplement the available immunotherapeutic modalities to render immune-resistant malignancies amenable to treatment. This review will discuss current research and clinical developments on Vaxinia immuno-oncolytic virus.

Viral Vectors in Gene Therapy: Where Do We Stand in 2023?

Viral vectors have been used for a broad spectrum of gene therapy for both acute and chronic diseases. In the context of cancer gene therapy, viral vectors expressing anti-tumor, toxic, suicide and immunostimulatory genes, such as cytokines and chemokines, have been applied. Oncolytic viruses, which specifically replicate in and kill tumor cells, have provided tumor eradication, and even cure of cancers in animal models. In a broader meaning, vaccine development against infectious diseases and various cancers has been considered as a type of gene therapy. Especially in the case of COVID-19 vaccines, adenovirus-based vaccines such as ChAdOx1 nCoV-19 and Ad26.COV2.S have demonstrated excellent safety and vaccine efficacy in clinical trials, leading to Emergency Use Authorization in many countries. Viral vectors have shown great promise in the treatment of chronic diseases such as severe combined immunodeficiency (SCID), muscular dystrophy, hemophilia, β-thalassemia, and sickle cell disease (SCD). Proof-of-concept has been established in preclinical studies in various animal models. Clinical gene therapy trials have confirmed good safety, tolerability, and therapeutic efficacy. Viral-based drugs have been approved for cancer, hematological, metabolic, neurological, and ophthalmological diseases as well as for vaccines. For example, the adenovirus-based drug Gendicine® for non-small-cell lung cancer, the reovirus-based drug Reolysin® for ovarian cancer, the oncolytic HSV T-VEC for melanoma, lentivirus-based treatment of ADA-SCID disease, and the rhabdovirus-based vaccine Ervebo against Ebola virus disease have been approved for human use.

Combination therapy with oncolytic viruses and immune checkpoint inhibitors in head and neck squamous cell carcinomas: an approach of complementary advantages

Squamous cell carcinomas are the most common head and neck malignancies. Significant progress has been made in standard therapeutic methods combining surgery, radiation, and chemotherapy. Nevertheless, the 5-year survival rate remains at 40-50%. Immune checkpoint inhibitors (ICIs) are a new strategy for treating head and neck squamous cell carcinomas (HNSCCs). Still, the overall response and effective rates are poor, as HNSCCs are 'cold' tumors with an immunosuppressive tumor microenvironment (TME), limiting ICI's beneficial effects. In this case, transforming the tumor suppression microenvironment before using ICIs could be helpful. Oncolytic viruses (OVs) can transform cold tumors into hot tumors, improving the situation. Talimogene laherparepvec (T-VEC), oncolytic immunotherapy authorized for advanced melanoma, also showed good safety and antitumor activity in treating head and neck cancer and pancreatic cancer. In combination with pembrolizumab, T-Vec may have more anticancer efficacy than either drug alone. Therefore, understanding the mechanisms underpinning OVs and their potential synergism with ICIs could benefit patients with HNSCC.

Gene Therapy Cargoes Based on Viral Vector Delivery

Viral vectors have been proven useful in a broad spectrum of gene therapy applications due to their possibility to accommodate foreign genetic material for both local and systemic delivery. The wide range of viral vectors has enabled gene therapy applications for both acute and chronic diseases. Cancer gene therapy has been addressed by the delivery of viral vectors expressing anti-tumor, toxic, and suicide genes for the destruction of tumors. Delivery of immunostimulatory genes such as cytokines and chemokines has also been applied for cancer therapy. Moreover, oncolytic viruses specifically replicating in and killing tumor cells have been used as such for tumor eradication or in combination with tumor killing or immunostimulatory genes. In a broad meaning, vaccines against infectious diseases and various cancers can be considered gene therapy, which has been highly successful, not the least for the development of effective COVID-19 vaccines. Viral vector-based gene therapy has also demonstrated encouraging and promising results for chronic diseases such as severe combined immunodeficiency (SCID), muscular dystrophy, and hemophilia. Preclinical gene therapy studies in animal models have demonstrated proof-of-concept for a wide range of disease indications. Clinical evaluation of drugs and vaccines in humans has showed high safety levels, good tolerance, and therapeutic efficacy. Several gene therapy drugs such as the adenovirus-based drug Gendicine® for non-small-cell lung cancer, the reovirus-based drug Reolysin® for ovarian cancer, lentivirus-based treatment of SCID-X1 disease, and the rhabdovirus-based vaccine Ervebo against Ebola virus disease, and adenovirus-based vaccines against COVID-19 have been developed.

Optimal timing of PD-1 blockade in combination with oncolytic virus therapy

Anti-PD-1 and oncolytic viruses (OVs) have non-overlapping anti-tumor mechanisms, since each agent works at different steps of the cancer-immunity cycle. Evidence suggests that OVs improve therapeutic responses to anti-PD-1 therapy by reversing immunosuppressive factors, increasing the number and diversity of infiltrating lymphocytes, and promoting PD-L1 expression in both injected and non-injected tumors. Many studies in preclinical models suggest that the timing of anti-PD-1 administration influences the therapeutic success of the combination therapy (anti-PD-1 + OV). Therefore, determining the appropriate sequencing of agents is of critical importance to designing a rationale OV-based combinational clinical trial. Currently, the combination of anti-PD-1 and OVs are being delivered using various schedules, and we have classified the timing of administration of anti-PD-1 and OVs into five categories: (i) anti-PD-1 lead-in → OV; (ii) concurrent administration; (iii) OV lead-in → anti-PD-1; (iv) concurrent therapy lead-in → anti-PD-1; and (v) OV lead-in → concurrent therapy. Based on the reported preclinical and clinical literature, the most promising treatment strategy to date is hypothesized to be OV lead-in → concurrent therapy. In the OV lead-in → concurrent therapy approach, initial OV treatment results in T cell priming and infiltration into tumors and an immunologically hot tumor microenvironment (TME), which can be counterbalanced by engagement of PD-L1 to PD-1 receptor on immune cells, leading to T cell exhaustion. Therefore, after initial OV therapy, concurrent use of both OV and anti-PD-1 is critical through which OV maintains T cell priming and an immunologically hot TME, whereas PD-1 blockade helps to overcome PD-L1/PD-1-mediated T cell exhaustion. It is important to note that the hypothetical conclusion drawn in this review is based on thorough literature review on current understanding of OV + anti-PD-1 combination therapies and rhythm of treatment-induced cancer-immunity cycle. A variety of confounding factors such as tumor types, OV types, presence or absence of cytokine transgenes carried by an OV, timing of treatment initiation, varying dosages and treatment frequencies/duration of OV and anti-PD-1, etc. may affect the validity of our conclusion that will need to be further examined by future research (such as side-by-side comparative studies using all five treatment schedules in a given tumor model).

GOBLET: a phase I/II study of pelareorep and atezolizumab +/- chemo in advanced or metastatic gastrointestinal cancers

Most gastrointestinal (GI) cancers have microsatellite-stable (MSS) tumors, which have an immunologically 'cold' phenotype with fewer genetic mutations, reduced immune cell infiltration and downregulated immune checkpoint proteins. These attributes make MSS tumors resistant to conventional immunotherapy including checkpoint blockade therapy. Pelareorep is a naturally occurring, nongenetically modified reovirus. Upon intravenous administration, pelareorep selectively kills tumor cells and promotes several immunologic changes that prime tumors to respond to checkpoint blockade therapy. Given its demonstrated synergy with checkpoint blockade, as well as its encouraging efficacy in prior GI cancer studies, pelareorep plus atezolizumab will be evaluated in the GOBLET study in multiple GI cancer indications.

Preinduced reovirus-specific T-cell immunity enhances the anticancer efficacy of reovirus therapy

BACKGROUND

Many solid tumors do not respond to immunotherapy due to their immunologically cold tumor microenvironment (TME). We and others found that oncolytic viruses (OVs), including reovirus type 3 Dearing, can enhance the efficacy of immunotherapy by recruiting CD8⁺ T cells to the TME. A significant part of the incoming CD8⁺ T cells is directed toward reovirus itself, which may be detrimental to the efficacy of OVs. However, here we aim to exploit these incoming virus-specific T cells as anticancer effector cells.

METHODS

We performed an in-depth characterization of the reovirus-induced T-cell response in immune-competent mice bearing pancreatic KPC3 tumors. The immunodominant CD8⁺ T-cell epitope of reovirus was identified using epitope prediction algorithms and peptide arrays, and the quantity and quality of reovirus-specific T cells after reovirus administration were assessed using high-dimensional flow cytometry. A synthetic long peptide (SLP)-based vaccination strategy was designed to enhance the intratumoral frequency of reovirus-specific CD8⁺ T cells.

RESULTS

Reovirus administration did not induce tumor-specific T cells but rather induced high frequencies of reovirus-specific CD8⁺ T cells directed to the immunodominant epitope. Priming of reovirus-specific T cells required a low-frequent population of cross-presenting dendritic cells which was absent in Batf3^-/- mice. While intratumoral and intravenous reovirus administration induced equal systemic frequencies of reovirus-specific T cells, reovirus-specific T cells were highly enriched in the TME exclusively after intratumoral administration. Here, they displayed characteristics of potent effector cells with high expression of KLRG1, suggesting they may be responsive against local reovirus-infected cells. To exploit these reovirus-specific T cells as anticancer effector cells, we designed an SLP-based vaccination strategy to induce a strong T-cell response before virotherapy. These high frequencies of circulating reovirus-specific T cells were reactivated on intratumoral reovirus administration and significantly delayed tumor growth.

CONCLUSIONS

These findings provide proof of concept that OV-specific T cells, despite not being tumor-specific, can be exploited as potent effector cells for anticancer treatment when primed before virotherapy. This is an attractive strategy for low-immunogenic tumors lacking tumor-specific T cells.

Immune-Based Therapy in Triple-Negative Breast Cancer: From Molecular Biology to Clinical Practice

Triple-negative breast cancer (TNBC) has been considered for many years an orphan disease in terms of therapeutic options, with conventional chemotherapy (CT) still representing the mainstay of treatment in the majority of patients. Although breast cancer (BC) has been historically considered a "cold tumor", exciting progress in the genomic field leading to the characterization of the molecular portrait and the immune profile of TNBC has opened the door to novel therapeutic strategies, including Immune Checkpoint Inhibitors (ICIs), Poly ADP-Ribose Polymerase (PARP) inhibitors and Antibody Drug Conjugates (ADCs). In particular, compared to standard CT, the immune-based approach has been demonstrated to improve progression-free survival (PFS) and overall survival (OS) in metastatic PD-L1-positive TNBC and the pathological complete response rate in the early setting, regardless of PD-L1 expression. To date, PD-L1 has been widely used as a predictor of the response to ICIs; however, many patients do not benefit from the addition of immunotherapy. Therefore, PD-L1 is not a reliable predictive biomarker of the response, and its accuracy remains controversial due to the lack of a consensus about the assay, the antibody, and the scoring system to adopt, as well as the spatial and temporal heterogeneity of the PD-L1 status. In the precision medicine era, there is an urgent need to identify more sensitive biomarkers in the BC immune oncology field other than just PD-L1 expression. Through the characterization of the tumor microenvironment (TME), the analysis of peripheral blood and the evaluation of immune gene signatures, novel potential biomarkers have been explored, such as the Tumor Mutational Burden (TMB), Microsatellite Instability/Mismatch Repair Deficiency (MSI/dMMR) status, genomic and epigenomic alterations and tumor-infiltrating lymphocytes (TILs). This review aims to summarize the recent knowledge on BC immunograms and on the biomarkers proposed to support ICI-based therapy in TNBC, as well as to provide an overview of the potential strategies to enhance the immune response in order to overcome the mechanisms of resistance.

Kickstarting Immunity in Cold Tumours: Localised Tumour Therapy Combinations With Immune Checkpoint Blockade

Cancer patients with low or absent pre-existing anti-tumour immunity ("cold" tumours) respond poorly to treatment with immune checkpoint inhibitors (ICPI). In order to render these patients susceptible to ICPI, initiation of de novo tumour-targeted immune responses is required. This involves triggering of inflammatory signalling, innate immune activation including recruitment and stimulation of dendritic cells (DCs), and ultimately priming of tumour-specific T cells. The ability of tumour localised therapies to trigger these pathways and act as in situ tumour vaccines is being increasingly explored, with the aspiration of developing combination strategies with ICPI that could generate long-lasting responses. In this effort, it is crucial to consider how therapy-induced changes in the tumour microenvironment (TME) act both as immune stimulants but also, in some cases, exacerbate immune resistance mechanisms. Increasingly refined immune monitoring in pre-clinical studies and analysis of on-treatment biopsies from clinical trials have provided insight into therapy-induced biomarkers of response, as well as actionable targets for optimal synergy between localised therapies and ICB. Here, we review studies on the immunomodulatory effects of novel and experimental localised therapies, as well as the re-evaluation of established therapies, such as radiotherapy, as immune adjuvants with a focus on ICPI combinations.

Antibody-drug conjugates, immune-checkpoint inhibitors, and their combination in breast cancer therapeutics

INTRODUCTION

Advanced breast cancer (aBC) remains incurable and the quest for more effective systemic anticancer agents continues. Promising results have led to the FDA approval of three antibody-drug conjugates (ADCs) and two immune checkpoint inhibitors (ICIs) to date for patients with aBC.

AREAS COVERED

With the anticipated emergence of newer ADCs and ICIs for patients with several subtypes of breast cancer, and given their potential synergy, their use in combination is of clinical interest. In this article, we review the use of ADCs and ICIs in patients with breast cancer, assess the scientific rationale for their combination, and provide an overview of ongoing trials and some early efficacy and safety results of such dual therapy.

EXPERT OPINION

Improvement in the medicinal chemistry of next-generation ADCs, their rational combination with ICIs and other agents, and the development of multiparametric immune biomarkers could help to significantly improve the outlook for patients with refractory aBC.

Cytotoxic T lymphocyte-associated protein 4 antibody aggrandizes antitumor immune response of oncolytic virus M1 via targeting regulatory T cells

Oncolytic virotherapies are perceived as remarkable immunotherapies coming into view and represent highly promising cancer treatments, yet to figure out its specific immune responses and underlying barriers remains critical. Albeit recent studies have demonstrated that oncolytic viruses (OVs) could fine tune tumor microenvironment (TME) to elicit tumor suppression mainly due to effective T-cell responses, the interaction between suppressive T cells and OVs is barely undetermined. Herein, we found that regulatory T cells (Treg cells) were increased in the TME following systemic administration of oncolytic virus M1 along with the higher expression of relative cytokines and chemokines in both mouse RM-1 prostatic carcinoma model and mouse B16F10 melanoma model. Besides, Treg cells expressed high levels of CD25 post-M1 treatment, and its suppressive effect on CD8⁺ T cells was also elevated. Depletion of Treg cells in M1-treated groups significantly reinforced antitumor effect of M1. Specific targeting of Treg cells using cytotoxic T lymphocyte-associated protein 4 (CTLA-4) antibody (Ab) in combination with M1 treatment elicited a more profound tumor suppression and longer overall survival time than M1 alone in both tumor models. Moreover, CTLA-4 Ab further aggrandized antitumor immune response elicited by M1, including increased infiltration of CD45⁺ immune cells and CD8⁺ or CD4⁺ T lymphocytes, decreased ratio of Treg cells to CD4⁺ T lymphocytes, the intensified lymphocytotoxicity and elevated secretion of cytotoxic cytokines like interferon-γ, granzyme B and perforin. Therefore, our findings constituted a suggestive evidence that targeting Treg cells in M1-based oncolytic virotherapy may achieve a highly response in clinical cancer research.

Oncolytic Virotherapy Treatment of Breast Cancer: Barriers and Recent Advances

Oncolytic virotherapy (OV) is an emerging class of immunotherapeutic drugs. Their mechanism of action is two-fold: direct cell lysis and unmasking of the cancer through immunogenic cell death, which allows the immune system to recognize and eradicate tumours. Breast cancer is the most common cancer in women and is challenging to treat with immunotherapy modalities because it is classically an immunogenically "cold" tumour type. This provides an attractive niche for OV, given viruses have been shown to turn "cold" tumours "hot," thereby opening a plethora of treatment opportunities. There has been a number of pre-clinical attempts to explore the use of OV in breast cancer; however, these have not led to any meaningful clinical trials. This review considers both the potential and the barriers to OV in breast cancer, namely, the limitations of monotherapy and the scope for combination therapy, improving viral delivery and challenges specific to the breast cancer population (e.g., tumour subtype, menopausal status, age).

Mesenchymal stem cells loaded with oncolytic reovirus enhances antitumor activity in mice models of colorectal cancer

Oncolytic viruses (OVs) are promising alternative biological agents for treating cancer. However, triggered immune responses against viruses and their delivery to tumor sites are their primary limitations in cancer therapy. To address these challenges, mesenchymal stem cells (MSCs) can serve as permissive tools for OVs loading and delivery to tumor sites. Here, we evaluated the in vitro and in vivo antitumor capability of adipose-derived mesenchymal stem cells (AD-MSCs) as a new vehicle for Dearing strain of reovirus (ReoT3D) loading. We first isolated and confirmed the purity of MSCs, and the optimized dose of ReoT3D for MSCs loading was computed by a standard assay. Next, we used murine CT26 cell line to establish the colorectal cancer model in BALB/c mice and demonstrated the antitumor effects of MSCs loaded with reovirus. Our results demonstrated that multiplicity of infection (MOI) 1 pfu/cells of reovirus was the safe dose for loading into purified MSCs. Moreover, our anticancer experiments exhibited that treatment with MSCs loaded with ReoT3D was more effective than ReoT3D and MSCs alone. Higher anticancer impact of MSCs loaded with OV was associated with induction of apoptosis, cell cycle arrests, P53 expression in tumor sections, and reduced tumor growth and size. The present results suggest that MSCs as a permissive shuttle for oncolytic virus (OV) delivery increased the anticancer activity of ReoT3D in mice models of colorectal cancer and these findings should be supported by more preclinical and clinical studies.

Oncolytic Reovirus-Mediated Recruitment of Early Innate Immune Responses Reverses Immunotherapy Resistance in Prostate Tumors

Prostate cancers are considered "cold" tumors characterized by minimal T cell infiltrates, absence of a type I interferon (IFN) signature, and the presence of immunosuppressive cells. This non-inflamed phenotype is likely responsible for the lack of sensitivity of prostate cancer patients to immune checkpoint blockade (ICB) therapy. Oncolytic virus therapy can potentially overcome this resistance to immunotherapy in prostate cancers by transforming cold tumors into "hot," immune cell-infiltrated tumors. We investigated whether the combination of intratumoral oncolytic reovirus, followed by targeted blockade of Programmed cell death protein 1 (PD-1) checkpoint inhibition and/or the immunomodulatory CD73/Adenosine system can enhance anti-tumor immunity. Treatment of subcutaneous TRAMP-C2 prostate tumors with combined intratumoral reovirus and anti-PD-1 or anti-CD73 antibody significantly enhanced survival of mice compared with reovirus or either antibody therapy alone. Only combination therapy led to rejection of pre-established tumors and protection from tumor re-challenge. This therapeutic effect was dependent on CD4+ T cells and natural killer (NK) cells. NanoString immune profiling of tumors confirmed that reovirus increased tumor immune cell infiltration and revealed an upregulation of the immune-regulatory receptor, B- and T-lymphocyte attenuator (BTLA). This expression of BTLA on innate antigen-presenting cells (APCs) and its ligand, Herpesvirus entry mediator (HVEM), on T cells from reovirus-infected tumors was in keeping with a role for the HVEM-BTLA pathway in promoting the potent anti-tumor memory response observed.

Natural killer T cell immunotherapy combined with oncolytic vesicular stomatitis virus or reovirus treatments differentially increases survival in mouse models of ovarian and breast cancer metastasis

BACKGROUND

Oncolytic viruses reduce tumor burden in animal models and have generated promising results in clinical trials. However, it is likely that oncolytic viruses will be more effective when used in combination with other therapies. Current therapeutic approaches, including chemotherapeutics, come with dose-limiting toxicities. Another option is to combine oncolytic viruses with immunotherapeutic approaches.

METHODS

Using experimental models of metastatic 4T1 breast cancer and ID8 ovarian peritoneal carcinomatosis, we examined natural killer T (NKT) cell-based immunotherapy in combination with recombinant oncolytic vesicular stomatitis virus (VSV) or reovirus. 4T1 mammary carcinoma cells or ID8 ovarian cancer cells were injected into syngeneic mice. Tumor-bearing mice were treated with VSV or reovirus followed by activation of NKT cells via the intravenous administration of autologous dendritic cells loaded with the glycolipid antigen α-galactosylceramide. The effects of VSV and reovirus on immunogenic cell death (ICD), cell viability and immunogenicity were tested in vitro.

RESULTS

VSV or reovirus treatments followed by NKT cell activation mediated greater survival in the ID8 model than individual therapies. The regimen was less effective when the treatment order was reversed, delivering virus treatments after NKT cell activation. In the 4T1 model, VSV combined with NKT cell activation increased overall survival and decreased metastatic burden better than individual treatments. In contrast, reovirus was not effective on its own or in combination with NKT cell activation. In vitro, VSV killed a panel of tumor lines better than reovirus. VSV infection also elicited greater increases in mRNA transcripts for proinflammatory cytokines, chemokines, and antigen presentation machinery compared with reovirus. Oncolytic VSV also induced the key hallmarks of ICD (calreticulin mobilization, plus release of ATP and HMGB1), while reovirus only mobilized calreticulin.

CONCLUSION

Taken together, these results demonstrate that oncolytic VSV and NKT cell immunotherapy can be effectively combined to decrease tumor burden in models of metastatic breast and ovarian cancers. Oncolytic VSV and reovirus induced differential responses in our models which may relate to differences in virus activity or tumor susceptibility.

Intravenous injection of the oncolytic virus M1 awakens antitumor T cells and overcomes resistance to checkpoint blockade

Reversing the highly immunosuppressive tumor microenvironment (TME) is essential to achieve long-term efficacy with cancer immunotherapy. Despite the impressive clinical response to checkpoint blockade in multiple types of cancer, only a minority of patients benefit from this approach. Here, we report that the oncolytic virus M1 induces immunogenic tumor cell death and subsequently restores the ability of dendritic cells to prime antitumor T cells. Intravenous injection of M1 disrupts immune tolerance in the privileged TME, reprogramming immune-silent (cold) tumors into immune-inflamed (hot) tumors. M1 elicits potent CD8⁺ T cell-dependent therapeutic effects and establishes long-term antitumor immune memory in poorly immunogenic tumor models. Pretreatment with M1 sensitizes refractory tumors to subsequent checkpoint blockade by boosting T-cell recruitment and upregulating the expression of PD-L1. These findings reveal the antitumor immunological mechanism of the M1 virus and indicated that oncolytic viruses are ideal cotreatments for checkpoint blockade immunotherapy.

Oncolytic virotherapy: new weapon for breast cancer treatment

The recent introduction of viruses as a weapon against cancer can be regarded as one of the most intriguing approaches in the context of precision medicine. The role of immune checkpoint inhibitors has been extensively studied in early and advanced cancer stages, with extraordinary results. Although there is a good tolerability profile, especially when compared with conventional chemotherapy, severe immune-related adverse events have emerged as a potential limitation. Moreover, there are still treatment-resistant cases and thus further treatment options need to be implemented. Several in vitro and in vivo studies have been conducted and are ongoing to develop oncolytic viruses (OVs) as a tool to modulate the immune system response. OVs are attenuated viruses that can kill cancer cells after having infected them, producing microenvironment remodelling and antitumour immune response. The potential of oncolytic virotherapy is to contrast the absence of T cell infiltrates, converting ‘cold’ tumours into ‘hot’ ones, thus improving the performance of the immune system. Breast cancer, the second most common cause of cancer-related deaths among women, is considered a ‘cold’ tumour. In this context, oncolytic virotherapy might well be considered as a promising strategy. This review summarises the current status, clinical applications and future development of OVs, focusing on breast cancer treatment.

Viroimmunotherapy for breast cancer: promises, problems and future directions

Virotherapy, a strategy to use live viruses as therapeutics, is a relatively novel field in the treatment of cancer. With the advancements in molecular biology and virology, there has been a huge increase in research on cancer virotherapy. For the treatment of cancer, viruses could be used either as vectors in gene therapy or as oncolytic agents. A variety of viruses have been studied for their potential usage in gene therapy or oncolytic therapy. In this review, we discuss virotherapy with a special focus on breast cancer. Breast cancer is the most common cancer and the leading cause of cancer-related deaths in women worldwide. Current treatments are insufficient to cure metastatic breast cancer and are often associated with severe side effects that further deteriorates patients' quality of life. Therefore, novel therapeutic approaches such as virotherapy need to be developed for the treatment of breast cancer. Here we summarize the current treatments for breast cancer and the potential use of virotherapy in the treatment of the disease. Furthermore, we discuss the use of oncolytic viruses as immunotherapeutics and the rational combination of oncolytic viruses with other therapeutics for optimal treatment of breast cancer. Finally, we outline the progress made in virotherapy for breast cancer and the shortcomings that need to be addressed for this novel therapy to move to the clinic for better treatment of breast cancer.

Past, Present and Future of Oncolytic Reovirus

Oncolytic virotherapy (OVT) has received significant attention in recent years, especially since the approval of talimogene Laherparepvec (T-VEC) in 2015 by the Food and Drug administration (FDA). Mechanistic studies of oncolytic viruses (OVs) have revealed that most, if not all, OVs induce direct oncolysis and stimulate innate and adaptive anti-tumour immunity. With the advancement of tumour modelling, allowing characterisation of the effects of tumour microenvironment (TME) components and identification of the cellular mechanisms required for cell death (both direct oncolysis and anti-tumour immune responses), it is clear that a "one size fits all" approach is not applicable to all OVs, or indeed the same OV across different tumour types and disease locations. This article will provide an unbiased review of oncolytic reovirus (clinically formulated as pelareorep), including the molecular and cellular requirements for reovirus oncolysis and anti-tumour immunity, reports of pre-clinical efficacy and its overall clinical trajectory. Moreover, as it is now abundantly clear that the true potential of all OVs, including reovirus, will only be reached upon the development of synergistic combination strategies, reovirus combination therapeutics will be discussed, including the limitations and challenges that remain to harness the full potential of this promising therapeutic agent.

Combination therapy with oncolytic viruses and immune checkpoint inhibitors

Introduction: Immune checkpoint inhibitors (ICI) have dramatically improved the outcome for cancer patients across multiple tumor types. However the response rates to ICI monotherapy remain relatively low, in part due to some tumors cultivating an inherently 'cold' immune microenvironment. Oncolytic viruses (OV) have the capability to promote a 'hotter' immune microenvironment which can improve the efficacy of ICI.Areas covered: In this article we conducted a literature search through Pubmed/Medline to identify relevant articles in both the pre-clinical and clinical settings for combining OVs with ICIs and discuss the impact of this approach on treatment as well as changes within the tumor microenvironment. We also explore the future directions of this novel combination strategy.Expert opinion: The imminent results of the Phase 3 study combining pembrolizumab with or without T-Vec injection are eagerly awaited. OV/ICI combinations remain one of the most promising avenues to explore in the success of cancer immunotherapy.

In Vitro Anti-cancer Activity of Adipose-Derived Mesenchymal Stem Cells Increased after Infection with Oncolytic Reovirus

Purpose: Reovirus type 3 Dearing (ReoT3D), a wild type oncolytic virus (OV) from the Reoviridae family, kills KRAS mutant cancer cells. However, the use of OVs has faced with some limitations such as immune responses, and delivery of OVs to the tumor sites in systemic therapy. To solve this, and also to increase the anti-cancer effects of these OVs, mesenchymal stem cells (MSCs) might be used as an effective vehicle for OVs delivery. In this study, we examined the anti-cancer effects of human adipose derived-MSCs (AD-MSCs) as a vehicle of ReoT3D against human glioblastoma cells.

Methods: Here, AD-MSCs were characterized and toxicity of ReoT3D on them was determined by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay. Then, capability of AD-MSCs for virus production was assessed by real-time polymerase chain reaction (PCR), and different in vitro anti-cancer experiments were applied for our anti-cancer purposes.

Results: Our results from toxicity assay revealed that the isolated and provoked AD-MSCs were resistant to nontoxic concentration multiplicity of infection (MOI) >1 pfu/cells of ReoT3D. In addition, the results indicated that AD-MSCs were susceptible for virus life cycle complementation and were capable for production of virus progenies. Furthermore, our results showed that AD-MSCs had oncolysis effects and increased the anti-cancer effects of ReoT3D.

Conclusion: AD-MSCs as a susceptible host for oncolytic reovirus could increase the anti-cancer activity of this OV against glioblastoma multiforme (GBM) cell line.

Immunotherapeutic Potential of TGF-β Inhibition and Oncolytic Viruses

In cancer immunotherapy, a patient's own immune system is harnessed against cancer. Immune checkpoint inhibitors release the brakes on tumor-reactive T cells and, therefore, are particularly effective in treating certain immune-infiltrated solid tumors. By contrast, solid tumors with immune-silent profiles show limited efficacy of checkpoint blockers due to several barriers. Recent discoveries highlight transforming growth factor-β (TGF-β)-induced immune exclusion and a lack of immunogenicity as examples of these barriers. In this review, we summarize preclinical and clinical evidence that illustrates how the inhibition of TGF-β signaling and the use of oncolytic viruses (OVs) can increase the efficacy of immunotherapy, and discuss the promise and challenges of combining these approaches with immune checkpoint blockade.

Combination Therapy with Reovirus and ATM Inhibitor Enhances Cell Death and Virus Replication in Canine Melanoma

Oncolytic virotherapy using reovirus is a promising new anti-cancer treatment with potential for use in humans and dogs. Because reovirus monotherapy shows limited efficacy in human and canine cancer patients, the clinical development of a combination therapy is necessary. To identify candidate components of such a combination, we screened a 285-compound drug library for those that enhanced reovirus cytotoxicity in a canine melanoma cell line. Here, we show that exposure to an inhibitor of the ataxia telangiectasia mutated protein (ATM) enhances the oncolytic potential of reovirus in five of six tested canine melanoma cell lines. Specifically, the ATM inhibitor potentiated reovirus replication in cancer cells along with promoting the lysosomal activity, resulting in an increased proportion of caspase-dependent apoptosis and cell cycle arrest at G2/M compared to those observed with reovirus alone. Overall, our study suggests that the combination of reovirus and the ATM inhibitor may be an attractive option in cancer therapy.

Determinants of combination GM-CSF immunotherapy and oncolytic virotherapy success identified through in silico treatment personalization

Oncolytic virotherapies, including the modified herpes simplex virus talimogene laherparepvec (T-VEC), have shown great promise as potent instigators of anti-tumour immune effects. The OPTiM trial, in particular, demonstrated the superior anti-cancer effects of T-VEC as compared to systemic immunotherapy treatment using exogenous administration of granulocyte-macrophage colony-stimulating factor (GM-CSF). Theoretically, a combined approach leveraging exogenous cytokine immunotherapy and oncolytic virotherapy would elicit an even greater immune response and improve patient outcomes. However, regimen scheduling of combination immunostimulation and T-VEC therapy has yet to be established. Here, we calibrate a computational biology model of sensitive and resistant tumour cells and immune interactions for implementation into an in silico clinical trial to test and individualize combination immuno- and virotherapy. By personalizing and optimizing combination oncolytic virotherapy and immunostimulatory therapy, we show improved simulated patient outcomes for individuals with late-stage melanoma. More crucially, through evaluation of individualized regimens, we identified determinants of combination GM-CSF and T-VEC therapy that can be translated into clinically-actionable dosing strategies without further personalization. Our results serve as a proof-of-concept for interdisciplinary approaches to determining combination therapy, and suggest promising avenues of investigation towards tailored combination immunotherapy/oncolytic virotherapy.

The advent of biological therapies for anti-cancer treatment has had a significant impact on patient outcomes. Targeted xenobiotics, including oncolytic viruses, in combination with existing, more general, immunotherapies like exogenous cytokines show great promise for continuing to improve cancer care. However, determining optimal combination regimens can be difficult, given that testing proposed schedules would require large cohorts of patients enrolled in clinical trials. Fortunately, computational biology can help to address treatment scheduling while simultaneously helping to unravel the mechanisms driving therapeutic responses. In this work, we integrate a mathematical model of GM-CSF and talimogene laherparepvec (T-VEC) oncolytic virotherapy into a virtual clinical trial to optimize their administration in combination. Using this platform, we inferred a clinically-actionable combination schedule for patients with late-stage melanoma that significantly improved virtual patient outcome when compared to GM-CSF and T-VEC monotherapies, and a standard combination strategy. Our results outline a rational approach to therapy optimization with meaningful consequences for how we effectively design and implement clinical trials to maximize their success, and how we treat melanoma with combined immuno- and virotherapy.

Pembrolizumab in Combination with the Oncolytic Virus Pelareorep and Chemotherapy in Patients with Advanced Pancreatic Adenocarcinoma: A Phase Ib Study

PURPOSE

Pelareorep is an intravenously delivered oncolytic reovirus that can induce a T-cell-inflamed phenotype in pancreatic ductal adenocarcinoma (PDAC). Tumor tissues from patients treated with pelareorep have shown reovirus replication, T-cell infiltration, and upregulation of PD-L1. We hypothesized that pelareorep in combination with pembrolizumab and chemotherapy in patients with PDAC would be safe and effective.

PATIENTS AND METHODS

A phase Ib single-arm study enrolled patients with PDAC who progressed after first-line treatment. Patients received pelareorep, pembrolizumab, and either 5-fluorouracil, gemcitabine, or irinotecan until disease progression or unacceptable toxicity. Study objectives included safety and dose-limiting toxicities, tumor response, evaluation for reovirus replication, and immune analysis in peripheral blood and tumor biopsies.

RESULTS

Eleven patients were enrolled. Disease control was achieved in three of the 10 efficacy-evaluable patients. One patient achieved partial response for 17.4 months. Two additional patients achieved stable disease, lasting 9 and 4 months, respectively. Treatment was well tolerated, with mostly grade 1 or 2 treatment-related adverse events, including flu-like symptoms. Viral replication was observed in on-treatment tumor biopsies. T-cell receptor sequencing from peripheral blood revealed the creation of new T-cell clones during treatment. High peripheral clonality and changes in the expression of immune genes were observed in patients with clinical benefit.

CONCLUSIONS

Pelareorep and pembrolizumab added to chemotherapy did not add significant toxicity and showed encouraging efficacy. Further evaluation of pelareorep and anti-PD-1 therapy is ongoing in follow-up studies. This research highlights the potential utility of several pretreatment and on-treatment biomarkers for pelareorep therapy warranting further investigation.

The oncolytic efficacy and safety of avian reovirus and its dynamic distribution in infected mice

Primary liver cancer is a major public health challenge that ranks as the third most common cause of cancer worldwide despite therapeutic improvement. Reovirus has been emerging as a potential anti-cancer agent and is undergoing multiple clinical trials, and it is reported that reovirus can preferentially cause the cell death of a variety of cancers in a manner of apoptosis. As few studies have reported the efficacy of oncolytic activity and safety profile of avian reovirus, in our study, LDH assay, MTT assay, DAPI staining, and flow cytometry assay were performed to demonstrate the oncolytic effects of avian reovirus against the HepG2 cells, and quantitative real-time PCR (qRT-PCR) and animal experiments were conducted to investigate the dynamic distribution of avian reovirus in infected mice and then illustrate the safety and tissue tropism of avian reovirus. LDH assay, DAPI staining, and flow cytometry assay confirmed the efficacy of the oncotherapeutic effects of avian reovirus, and MTT assay has indicated that avian reovirus suppressed the proliferation of HepG2 cells and decreased their viability significantly. qRT-PCR revealed the dynamic distribution of avian reovirus in infected mice that avian reovirus might replicate better and have more powerful oncolytic activity in liver, kidney, and spleen tissues. Furthermore, histopathological examination clearly supported that avian reovirus appeared non-pathogenic to the normal host, so our study may provide the new insights and rationale for the new strategy of removing liver cancer.

Impact statement

We demonstrated the efficacy of oncolytic activity of avian reovirus (ARV) by LDH assay, MTT assay, DAPI staining, and flow cytometry assay, and also investigated the dynamic distribution of ARV in infected mice and then illustrated the safety and tissue tropism of ARV by quantitative real-time PCR (qRT-PCR) and animal experiments. Collectively, our study may provide the new insights and rationale for the new strategy of removing liver cancer.

Oncolytic Viruses and Immune Checkpoint Inhibition: The Best of Both Worlds

Cancer immunotherapy and the emergence of immune checkpoint inhibitors have markedly changed the treatment paradigm for many cancers. They function to disrupt cancer cell evasion of the immune response and activate sustained anti-tumor immunity. Oncolytic viruses have also emerged as an additional therapeutic agent for cancer treatment. These viruses are designed to target and kill tumor cells while leaving the normal cells unharmed. As part of this process, oncolytic virus infection stimulates anti-cancer immune responses that augment the efficacy of checkpoint inhibition. These viruses have the capability of transforming a “cold” tumor microenvironment with few immune effector cells into a “hot” environment with increased immune cell and cytokine infiltration. For this reason, there are multiple ongoing clinical trials that combine oncolytic virotherapy and immune checkpoint inhibitors. This review will detail the key oncolytic viruses in preclinical and clinical studies and highlight the results of their testing with checkpoint inhibitors.

The 13th International Double-Stranded RNA Virus Symposium, Houffalize, Belgium, 24 to 28 September 2018

The triennial International Double-Stranded RNA Virus Symposium, this year organized by J. Matthijnssens, J. S. L. Parker, P. Danthi, and P. Van Damme in Belgium, gathered over 200 scientists to discuss novel observations and hypotheses in the field. The keynote lecture on functional interactions of bacteria and viruses in the gut microbiome was presented by Julie Pfeiffer. Workshops were held on viral diversity, molecular epidemiology, molecular virology, immunity and pathogenesis, virus structure, the viral use and abuse of cellular pathways, and applied double-stranded RNA (dsRNA) virology. The establishment of a plasmid only-based reverse genetics system for rotaviruses by several Japanese research groups in 2017 has now been reproduced by various other research groups and was discussed in detail. The visualization of dsRNA virus replication steps in living cells received much attention. Mechanisms of the cellular innate immune response to virus infection and of viral pathogenesis were explored. Knowledge of the gut microbiome's influence on specific immune responses has increased rapidly, also due to the availability of relevant animal models of virus infection. The method of cryo-electron microscopic (cryo-EM) tomography has elucidated various asymmetric structures in viral particles. The use of orthoreoviruses for oncolytic virotherapy was critically assessed. The application of llama-derived single chain nanobodies for passive immunotherapy was considered attractive. In a satellite symposium the introduction, impact and further developments of rotavirus vaccines were reviewed. The Jean Cohen Lecturer of this meeting was Harry B. Greenberg, who presented aspects of his research on rotaviruses over a period of more than 40 years. He was also interviewed at the meeting by Vincent Racaniello for the 513th session of This Week in Virology.

Oncolytic Viruses for Canine Cancer Treatment

Oncolytic virotherapy has been investigated for several decades and is emerging as a plausible biological therapy with several ongoing clinical trials and two viruses are now approved for cancer treatment in humans. The direct cytotoxicity and immune-stimulatory effects make oncolytic viruses an interesting strategy for cancer treatment. In this review, we summarize the results of in vitro and in vivo published studies of oncolytic viruses in different phases of evaluation in dogs, using PubMed and Google scholar as search platforms, without time restrictions (to date). Natural and genetically modified oncolytic viruses were evaluated with some encouraging results. The most studied viruses to date are the reovirus, myxoma virus, and vaccinia, tested mostly in solid tumors such as osteosarcomas, mammary gland tumors, soft tissue sarcomas, and mastocytomas. Although the results are promising, there are issues that need addressing such as ensuring tumor specificity, developing optimal dosing, circumventing preexisting antibodies from previous exposure or the development of antibodies during treatment, and assuring a reasonable safety profile, all of which are required in order to make this approach a successful therapy in dogs.

Going (Reo)Viral: Factors Promoting Successful Reoviral Oncolytic Infection

Oncolytic viruses show intriguing potential as cancer therapeutic agents. These viruses are capable of selectively targeting and killing cancerous cells while leaving healthy cells largely unaffected. The use of oncolytic viruses for cancer treatments in selected circumstances has recently been approved by the Food and Drug Administration (FDA) of the US and work is progressing on engineering viral vectors for enhanced selectivity, efficacy and safety. However, a better fundamental understanding of tumour and viral biology is essential for the continued advancement of the oncolytic field. This knowledge will not only help to engineer more potent and effective viruses but may also contribute to the identification of biomarkers that can determine which patients will benefit most from this treatment. A mechanistic understanding of the overlapping activity of viral and standard chemotherapeutics will enable the development of better combinational approaches to improve patient outcomes. In this review, we will examine each of the factors that contribute to productive viral infections in cancerous cells versus healthy cells. Special attention will be paid to reovirus as it is a well-studied virus and the only wild-type virus to have received orphan drug designation by the FDA. Although considerable insight into reoviral biology exists, there remain numerous deficiencies in our understanding of the factors regulating its successful oncolytic infection. Here we will discuss what is known to regulate infection as well as speculate about potential new mechanisms that may enhance successful replication. A joint appreciation of both tumour and viral biology will drive innovation for the next generation of reoviral mediated oncolytic therapy.

Oncolytic Virotherapy for Breast Cancer Treatment

Breast cancer continues to be a leading cause of mortality among women. While at an early stage, localized breast cancer is easily treated; however, advanced stages of disease continue to carry a high mortality rate. The discrepancy in treatment success highlights that current treatments are insufficient to treat advanced-stage breast cancer. As new and improved treatments have been sought, one therapeutic approach has gained considerable attention. Oncolytic viruses are uniquely capable of targeting cancer cells through intrinsic or engineered means. They come in many forms, mainly from four major virus groups as defined by the Baltimore classification system. These vectors can target and kill cancer cells, and even stimulate immunotherapeutic effects in patients. This review discusses not only individual oncolytic viruses pursued in the context of breast cancer treatment but also the emergence of combination therapies with current or new therapies, which has become a particularly promising strategy for treatment of breast cancer. Overall, oncolytic virotherapy is a promising strategy for increased treatment efficacy for advanced breast cancer and consequently provides a unique platform for personalized treatments in patients.