Bugs and drugs: oncolytic virotherapy in combination with chemotherapy

Oncolytic vaccinia virus and cancer immunotherapy

Oncolytic virotherapy (OVT) is a promising form of cancer treatment that uses genetically engineered viruses to replicate within cancer cells and trigger anti-tumor immune response. In addition to killing cancer cells, oncolytic viruses can also remodel the tumor microenvironment and stimulate a long-term anti-tumor immune response. Despite achieving positive results in cellular and organismal studies, there are currently only a few approved oncolytic viruses for clinical use. Vaccinia virus (VACV) has emerged as a potential candidate due to its ability to infect a wide range of cancer cells. This review discusses the mechanisms, benefits, and clinical trials of oncolytic VACVs. The safety and efficacy of different viral backbones are explored, as well as the effects of oncolytic VACVs on the tumor microenvironment. The potential combination of oncolytic VACVs with immunotherapy or traditional therapies is also highlighted. The review concludes by addressing prospects and challenges in the field of oncolytic VACVs, with the aim of promoting further research and application in cancer therapy.

Clinical Activity of Olvimulogene Nanivacirepvec-Primed Immunochemotherapy in Heavily Pretreated Patients With Platinum-Resistant or Platinum-Refractory Ovarian Cancer: The Nonrandomized Phase 2 VIRO-15 Clinical Trial

Importance

Patients with platinum-resistant or platinum-refractory ovarian cancer (PRROC) have limited therapeutic options, representing a considerable unmet medical need.

Objective

To assess antitumor activity and safety of intraperitoneal (IP) olvimulogene nanivacirepvec (Olvi-Vec) virotherapy and platinum-based chemotherapy with or without bevacizumab in patients with PRROC.

Design, Setting, and Participants

This open-label, nonrandomized multisite phase 2 VIRO-15 clinical trial enrolled patients with PRROC with disease progression following their last prior line of therapy from September 2016 to September 2019. Data cutoff was on March 31, 2022, and data were analyzed between April 2022 and September 2022.

Interventions

Olvi-Vec was administered via a temporary IP dialysis catheter as 2 consecutive daily doses (3 × 109 pfu/d) followed by platinum-doublet chemotherapy with or without bevacizumab.

Main Outcomes and Measures

Primary outcomes were objective response rate (ORR) via Response Evaluation Criteria in Solid Tumors, version 1.1 (RECIST 1.1) and cancer antigen 125 (CA-125) assay, and progression-free survival (PFS). Secondary outcomes included duration of response (DOR), disease control rate (DCR), safety, and overall survival (OS).

Results

Twenty-seven heavily pretreated patients with platinum-resistant (n = 14) or platinum-refractory (n = 13) ovarian cancer were enrolled. The median (range) age was 62 (35-78) years. The median (range) prior lines of therapy were 4 (2-9). All patients completed both Olvi-Vec infusions and chemotherapy. Median follow-up duration was 47.0 months (95% CI, 35.9 months to NA). Overall, ORR by RECIST 1.1 was 54% (95% CI, 33%-74%), with a DOR of 7.6 months (95% CI, 3.7-9.6 months). The DCR was 88% (21/24). The ORR by CA-125 was 85% (95% CI, 65%-96%). Median PFS by RECIST 1.1 was 11.0 months (95% CI, 6.7-13.0 months), and the PFS 6-month rate was 77%. Median PFS was 10.0 months (95% CI, 6.4-NA months) in the platinum-resistant group and 11.4 months (95% CI, 4.3-13.2 months) in the platinum-refractory group. The median OS was 15.7 months (95% CI, 12.3-23.8 months) in all patients, with a median OS of 18.5 months (95% CI, 11.3-23.8 months) in the platinum-resistant group and 14.7 months (95% CI, 10.8-33.6 months) in the platinum-refractory group. Most frequent treatment-related adverse events (TRAEs) (any grade, grade 3) were pyrexia (63.0%, 3.7%, respectively) and abdominal pain (51.9%, 7.4%, respectively). There were no grade 4 TRAEs, and no treatment-related discontinuations or deaths.

Conclusions and Relevance

In this phase 2 nonrandomized clinical trial, Olvi-Vec followed by platinum-based chemotherapy with or without bevacizumab as immunochemotherapy demonstrated promising ORR and PFS with a manageable safety profile in patients with PRROC. These hypothesis-generating results warrant further evaluation in a confirmatory phase 3 trial.

Trial Registration

ClinicalTrials.gov Identifier: NCT02759588.

Radiotherapy/Chemotherapy-Immunotherapy for Cancer Management: From Mechanisms to Clinical Implications

Cancer immunotherapy has drawn much attention because it can restart the recognition and killing function of the immune system to normalize the antitumor immune response. However, the role of radiotherapy and chemotherapy in cancer treatment cannot be ignored. Due to cancer heterogeneity, combined therapy has become a new trend, and its efficacy has been confirmed in many studies. This review discussed the clinical implications and the underlying mechanisms of cancer immunotherapy in combination with radiotherapy or chemotherapy, offering an outline for clinicians as well as inspiration for future research.

Combining of Oncolytic Virotherapy and Other Immunotherapeutic Approaches in Cancer: A Powerful Functionalization Tactic

Oncolytic viruses have found a good place in the treatment of cancer. Administering oncolytic viruses directly or by applying genetic changes can be effective in cancer treatment through the lysis of tumor cells and, in some cases, by inducing immune system responses. Moreover, oncolytic viruses induce antitumor immune responses via releasing tumor antigens in the tumor microenvironment (TME) and affect tumor cell growth and metabolism. Despite the success of virotherapy in cancer therapies, there are several challenges and limitations, such as immunosuppressive TME, lack of effective penetration into tumor tissue, low efficiency in hypoxia, antiviral immune responses, and off-targeting. Evidence suggests that oncolytic viruses combined with cancer immunotherapy-based methods such as immune checkpoint inhibitors and adoptive cell therapies can effectively overcome these challenges. This review summarizes the latest data on the use of oncolytic viruses for the treatment of cancer and the challenges of this method. Additionally, the effectiveness of mono, dual, and triple therapies using oncolytic viruses and other anticancer agents has been discussed based on the latest findings.

A dynamical model of combination therapy applied to glioma

Glioma is a human brain tumor that is very difficult to treat at an advanced stage. Studies of glioma biomarkers have shown that some markers are released into the bloodstream, so data from these markers indicate a decrease in the concentration of blood glucose and serum glucose in patients with glioma; these suggest an association between glucose and glioma. This decrease mechanism in glucose concentration can be described by the coupled ordinary differential equations of the early-stage glioma growth and interactions between glioma cells, immune cells, and glucose concentration. In this paper, we propose developing a new mathematical model to explain how glioma cells evolve and survive combination therapy between chemotherapy and oncolytic virotherapy, as an alternative to glioma treatment. In this study, three therapies were applied for analysis, that is, (1) chemotherapy, (2) virotherapy, and (3) a combination of chemotherapy and virotherapy. Virotherapy uses specialist viruses that only attack tumor cells. Based on the simulation results of the therapy carried out, we conclude that combination therapy can reduce the glioma cells significantly compared to the other two therapies. The simulation results of this combination therapy can be an alternative to glioma therapy.

In vitro Characterization of Enhanced Human Immune Responses by GM-CSF Encoding HSV-1-Induced Melanoma Cells

PURPOSE

We studied the innate and adaptive immune response against melanoma cells after JS-1 (wild-type herpes simplex virus 1, wt HSV-1) or Talimogene laherparepvec (T-VEC) infection and evaluated the antitumoral efficacy in human melanoma cells. We analyzed the putative synergistic biological and immunological effects of JS-1 or T-VEC combined with cytostatic drugs in human tumor and immune cells. T-VEC is a genetically modified strain of HSV-1. Genetic modifications (insertion of the granulocyte-macrophage colony-stimulating factor (GM-CSF) gene) were made to attenuate the virus and increase selectivity for cancer cells. In addition to the direct oncolytic effect, we investigated the immune stimulatory effects of T-VEC by comparing it with JS-1. JS-1 is identical T-VEC except for the inserted GM-CSF gene.

MATERIALS AND METHODS

We analyzed the effects of T-VEC and JS-1 with cytostatic drugs in human tumor-immune cell coculture experiments. After coculture, the surface markers CD80, CD83 and CD86 were measured by fluorescence-activated cell sorting and the cytokines, interleukin (IL)-2, IL-6, tumor necrosis factor (TNF)-α and GM-CSF, by enzyme-linked immunosorbent assays. Furthermore, we analyzed the potential of the viruses to induce T cell activation, measured on the basis of CD4, CD8 and CD69. Analysis of these markers and cytokines allows for conclusions to be drawn concerning the maturation of dendritic cells (DCs) and the immunostimulatory effects of the treatment.

RESULTS

We documented increased activation of human cytotoxic T lymphocytes after infection by both HSV-1 strains and treatment with cytostatic drugs without significant differences between T-VEC and JS-1.

CONCLUSION

We demonstrated an immune response as a result of infection with both viruses, but T-VEC was in vitro not stronger than JS-1. The immunostimulatory effects of the viruses could be partially increased by chemotherapy, providing a rationale for future preclinical studies designed to explore T-VEC in combined regimens.

Natural killer cell-based strategies for immunotherapy of cancer

Natural killer (NK) cells are a family of lymphocytes with a natural ability to kill infected, harmed, or malignantly transformed cells. As these cells are part of the innate immunity, the cytotoxic mechanisms are activated upon recognizing specific patterns without prior antigen sensitization. This recognition is crucial for NK cell function in the maintenance of homeostasis and immunosurveillance. NK cells not only act directly toward malignant cells but also participate in the complex immune response by producing cytokines or cross-talk with other immune cells. Cancer may be seen as a break of all immune defenses when malignant cells escape the immunity and invade surrounding tissues creating a microenvironment supporting tumor progression. This process may be reverted by intervening immune response with immunotherapy, which may restore immune recognition. NK cells are important effector cells for immunotherapy. They may be used for adoptive cell transfer, genetically modified with chimeric antigen receptors, or triggered with appropriate antibodies and other antibody-fragment-based recombinant therapeutic proteins tailored specifically for NK cell engagement. NK cell receptors, responsible for target recognition and activation of cytotoxic response, could also be targeted in immunotherapy, for example, by various bi-, tri-, or multi-specific fusion proteins designed to bridge the gap between tumor markers present on target cells and activation receptors expressed on NK cells. However, this kind of immunoactive therapeutics may be developed only with a deep functional and structural knowledge of NK cell receptor: ligand interactions. This review describes the recent developments in the fascinating protein-engineering field of NK cell immunotherapeutics.

A phase 1b study of intraperitoneal oncolytic viral immunotherapy in platinum-resistant or refractory ovarian cancer

OBJECTIVE

Our objective was to assess safety and adverse events associated with intraperitoneal Olvi-Vec virotherapy in patients with platinum-resistant or refractory ovarian cancer (PRROC). Secondary objectives included objective response rate (ORR) per RECIST 1.1 and progression-free survival (PFS).

METHODS

Olvi-Vec is a modified vaccinia virus that causes oncolysis and immune activation. An open-label phase 1b trial using a 3 + 3 dose escalation was conducted. Intraperitoneal Olvi-Vec was given as monotherapy in two consecutive daily doses. Translational analyses included anti-virus antibody levels, viral shedding, circulating tumor cells (CTCs) and T cells.

RESULTS

Twelve patients (median age: 69 years, range: 45-77) with median 5 prior therapies (range: 2-10) and 2 prior platinum lines (range: 1-5) were enrolled. There were three dose level cohorts: 3 × 10⁹ (n = 6), 1 × 10¹⁰ (n = 5), and 2.5 × 10¹⁰ (n = 1) plaque forming units (PFU)/day on two consecutive days. Treatment-related adverse events (TRAEs) included G1/G2 nausea (n = 6), fever (n = 6), abdominal distention (n = 5), and abdominal pain (n = 4). There were no Grade 4 TRAEs, no dose relationship to TRAEs, and no deaths attributed to Olvi-Vec. The ORR was 9% (1/11). Stable disease (SD) was 64% (7/11), and SD ≥15 weeks was 46% (5/11). Median PFS was 15.7 weeks (95%CI: 5.7-34.5), including extended PFS in four patients (23.2, 34.5, 59.4+ and 70.8 weeks). Three patients had extended overall survival (deceased 33.6 months, and alive with disease at 54 and 59 months). CTCs diminished in 6/8 (75%) baseline-positive patients. Immune activation was demonstrated from virus-enhanced tumor infiltration of CD8+ T-cells and activation of tumor-specific T-cells in peripheral blood.

CONCLUSIONS

Oncolytic viral therapy with intraperitoneal Olvi-Vec showed promising safety, clinical activities, and immune activation in patients with PRROC, warranting further clinical investigation.

Treatment of glioblastoma with current oHSV variants reveals differences in efficacy and immune cell recruitment

Oncolytic herpes simplex viruses (oHSVs) have demonstrated efficient lytic replication in human glioblastoma tumors using immunodeficient mouse models, but early-phase clinical trials have reported few complete responses. Potential reasons for the lack of efficacy are limited vector potency and the suppressive glioma tumor microenvironment (TME). Here we compare the oncolytic activity of two HSV-1 vectors, a KOS-strain derivative KG4:T124 and an F-strain derivative rQNestin34.5v.1, in the CT2A and GL261N4 murine syngeneic glioma models. rQNestin34.5v1 generally demonstrated a greater in vivo viral burden compared to KG4:T124. However, both vectors were rapidly cleared from CT2A tumors, while virus remained ensconced in GL261N4 tumors. Immunological evaluation revealed that the two vectors induced similar changes in immune cell recruitment to either tumor type at 2 days after infection. However, at 7 days after infection, the CT2A microenvironment displayed the phenotype of an untreated tumor, while GL261N4 tumors exhibited macrophage and CD4+/CD8+ T cell accumulation. Furthermore, the CT2A model was completely resistant to virus therapy, while in the GL261N4 model rQNestin34.5v1 treatment resulted in enhanced macrophage recruitment, impaired tumor progression, and long-term survival of a few animals. We conclude that prolonged intratumoral viral presence correlates with immune cell recruitment, and both are needed to enhance anti-tumor immunity.

Small molecules baicalein and cinnamaldehyde are potentiators of measles virus-induced breast cancer oncolysis

BACKGROUND

Although the breast cancer mortality has slowed down from 2008 to 2017, breast cancer incidence rate continues to rise and thus, new and/or improved treatments are highly needed. Among them, oncolytic virotherapy which has the ability of facilitating the antitumor adaptive immunity, appears as a promising anticancer therapy. Oncolytic measles virus (MV) is particularly suitable for targeting breast cancer due to the upregulation of MV's receptor nectin-4. Nonetheless, with limited clinical success currently, ways of boosting MV-induced breast cancer oncolysis are therefore necessary. Oncolytic virotherapy alone and combined with chemotherapeutic drugs are two strategic areas with intensive development for the search of anticancer drugs. Considering that baicalein (BAI) and cinnamaldehyde (CIN) have demonstrated antitumor properties against multiple cancers including breast cancer, they could be good partners for MV-based oncolytic virotherapy.

PURPOSE

To assess the in vitro effect of BAI and CIN with MV and assess their combination effects.

METHODS

We examined the combinatorial cytotoxic effect of oncolytic MV and BAI or CIN on MCF-7 breast cancer cells. Potential anti-MV activities of the phytochemicals were first investigated in vitro to determine the optimal combination model. Synergism of MV and BAI or CIN was then evaluated in vitro by calculating the combination indices. Finally, cell cycle analysis and apoptosis assays were performed to confirm the mechanism of synergism.

RESULTS

Overall, the viral sensitization combination modality using oncolytic MV to first infect MCF-7 breast cancer cells followed by drug treatment with BAI or CIN was found to produce significantly enhanced tumor killing. Further mechanistic studies showed that the combinations 'MV-BAI' and 'MV-CIN' display synergistic anti-breast cancer effect, mediated by elevated apoptosis.

CONCLUSION

We demonstrated, for the first time, effective combination of oncolytic MV with BAI or CIN that could be further explored and potentially developed into novel therapeutic strategies targeting nectin-4-marked breast cancer cells.

Tackling HLA Deficiencies Head on with Oncolytic Viruses

Dysregulation of HLA (human leukocyte antigen) function is increasingly recognized as a common escape mechanism for cancers subject to the pressures exerted by immunosurveillance or immunotherapeutic interventions. Oncolytic viruses have the potential to counter this resistance by upregulating HLA expression or encouraging an HLA-independent immunological responses. However, to achieve the best therapeutic outcomes, a prospective understanding of the HLA phenotype of cancer patients is required to match them to the characteristics of different oncolytic strategies. Here, we consider the spectrum of immune competence observed in clinical disease and discuss how it can be best addressed using this novel and powerful treatment approach.

Combining vanadyl sulfate with Newcastle disease virus potentiates rapid innate immune-mediated regression with curative potential in murine cancer models

The avian paramyxovirus, Newcastle disease virus (NDV), is a promising oncolytic agent that has been shown to be safe and effective in a variety of pre-clinical cancer models and human clinical trials. NDV preferentially replicates in tumor cells due to signaling defects in apoptotic and antiviral pathways acquired during the transformation process and is a potent immunostimulatory agent. However, when used as a monotherapy NDV lacks the ability to consistently generate durable remissions. Here we investigate the use of viral sensitizer-mediated combination therapy to enhance the anti-neoplastic efficacy of NDV. Intratumoral injection of vanadyl sulfate, a pan-inhibitor of protein tyrosine phosphatases, in combination with NDV significantly increased the number and activation status of natural killer (NK) cells in the tumor microenvironment, concomitant with increased expression of interferon-β, granulocyte-macrophage colony-stimulating factor, and monocyte chemoattractant protein-1, leading to rapid tumor regression and long-term cures in mice bearing syngeneic B16-F10 melanomas. The anti-tumor efficacy of this combination therapy was abrogated when NK cells were depleted and when interferon-β expression was transiently suppressed. Tumor-specific CD8⁺ T cell responses were not detected, nor were mice whose tumors regressed protected from re-challenge. This suggested efficacy of the combination therapy predominantly relied on the innate immune system. Importantly, efficacy was not limited to melanoma; it was also demonstrated in a murine prostate cancer model. Taken together, these results suggest that combining NDV with vanadyl sulfate potentiates an innate immune response that can potentiate rapid clearance of tumors, with type I interferon signaling and NK cells being important mechanisms of action.

Fighting microbial pathogens by integrating host ecosystem interactions and evolution

Successful therapies to combat microbial diseases and cancers require incorporating ecological and evolutionary principles. Drawing upon the fields of ecology and evolutionary biology, we present a systems-based approach in which host and disease-causing factors are considered as part of a complex network of interactions, analogous to studies of "classical" ecosystems. Centering this approach around empirical examples of disease treatment, we present evidence that successful therapies invariably engage multiple interactions with other components of the host ecosystem. Many of these factors interact nonlinearly to yield synergistic benefits and curative outcomes. We argue that these synergies and nonlinear feedbacks must be leveraged to improve the study of pathogenesis in situ and to develop more effective therapies. An eco-evolutionary systems perspective has surprising and important consequences, and we use it to articulate areas of high research priority for improving treatment strategies.

Effects of oncolytic viruses and viral vectors on immunity in glioblastoma

Glioblastoma (GBM) is regarded as an incurable disease due to its poor prognosis and limited treatment options. Virotherapies were once utilized on cancers for their oncolytic effects. And they are being revived on GBM treatment, as accumulating evidence presents the immunogenic effects of virotherapies in remodeling immunosuppressive GBM microenvironment. In this review, we focus on the immune responses induced by oncolytic virotherapies and viral vectors in GBM. The current developments of GBM virotherapies are briefly summarized, followed by a detailed depiction of their immune response. Limitations and lessons inferred from earlier experiments and trials are discussed. Moreover, we highlight the importance of engaging the immune responses induced by virotherapies into the multidisciplinary management of GBM.

Clinical CAR-T Cell and Oncolytic Virotherapy for Cancer Treatment

Immunotherapy has recently garnered success with the induction of clinical responses in tumors, which are traditionally associated with poor outcomes. Chimeric antigen receptor T (CAR-T) cells and oncolytic viruses (OVs) have emerged as promising cancer immunotherapy agents. Herein, we provide an overview of the current clinical status of CAR-T cell and OV therapies. While preclinical studies have demonstrated curative potential, the benefit of CAR-T cells and OVs as single-agent treatments remains limited to a subset of patients. Combinations of different targeted therapies may be required to achieve efficient, durable responses against heterogeneous tumors, as well as the microenvironment. Using a combinatorial approach to take advantage of the unique features of CAR-T cells and OVs with other treatments can produce additive therapeutic effects. This review also discusses ongoing clinical evaluations of these combination strategies for improved outcomes in treatment of resistant malignancies.

Clinical landscape of oncolytic virus research in 2020

Oncolytic viruses (OVs) are a new class of cancer therapeutics. This review was undertaken to provide insight into the current landscape of OV clinical trials. A PubMed search identified 119 papers from 2000 to 2020 with 97 studies reporting data on 3233 patients. The viruses used, presence of genetic modifications and/or transgene expression, cancer types targeted, inclusion of combination strategies and safety profile were reported. In addition, information on viral bioshedding across the studies, including which tissues or body fluids were evaluated and how virus was detected (eg, PCR, plaque assay or both), is also reported. Finally, the number of studies evaluating antiviral and antitumor humoral and cellular immune responses were noted. We found that adenovirus (n=30) is the most common OV in clinical trials with approximately two-thirds (n=63) using modified or recombinant viral backbones and granulocyte-macrophage colony-stimulating factor (n=24) was the most common transgene. The most common tumors targeted were melanoma (n=1000) and gastrointestinal (GI; n=577) cancers with most using monotherapy OVs given by intratumoral (n=1482) or intravenous (n=1347) delivery. The most common combination included chemotherapy (n=36). Overall, OV treatment-related adverse events were low-grade constitutional and local injection site reactions. Viral shedding was frequently measured although many studies restricted this to blood and tumor tissue and used PCR only. While most studies did report antiviral antibody titers (n=63), only a minority of studies reported viral-specific T cell responses (n=10). Tumor immunity was reported in 48 studies and largely relied on general measures of immune activation (eg, tumor biopsy immunohistochemistry (n=25) and serum cytokine measurement (n=19)) with few evaluating tumor-specific immune responses (n=7). Objective responses were reported in 292 (9%) patients and disease control was achieved in 681 (21.1%) patients, although standard reporting criteria were only used in 53% of the trials. Completed clinical trials not reported in the peer-reviewed literature were not included in this review potentially underestimating the impact of OV treatment. These data provide insight into the current profile of OV clinical trials reporting and identifies potential gaps where further studies are needed to better define the role of OVs, alone and in combination, for patients with cancer.

A Roadmap for the Success of Oncolytic Parvovirus-Based Anticancer Therapies

Autonomous rodent protoparvoviruses (PVs) are promising anticancer agents due to their excellent safety profile, natural oncotropism, and oncosuppressive activities. Viral infection can trigger immunogenic cell death, activating the immune system against the tumor. However, the efficacy of this treatment in recent clinical trials is moderate compared with results seen in preclinical work. Various strategies have been employed to improve the anticancer activities of oncolytic PVs, including development of second-generation parvoviruses with enhanced oncolytic and immunostimulatory activities and rational combination of PVs with other therapies. Understanding the cellular factors involved in the PV life cycle is another important area of investigation. Indeed, these studies may lead to the identification of biomarkers that would allow a more personalized use of PV-based therapies. This review focuses on this work and the challenges that still need to be overcome to move PVs forward into clinical practice as an effective therapeutic option for cancer patients.

Synergistic activation of p53 by actinomycin D and nutlin-3a is associated with the upregulation of crucial regulators and effectors of innate immunity

Actinomycin D and nutlin-3a (A + N) activate p53, partly through induction of phosphorylation on Ser392. The death of A549 cells induced by A + N morphologically resembles inflammation-inducing pyroptosis - cell destruction triggered by activated caspase-1. The treatment with A + N (or camptothecin) strongly upregulated caspase-1 and its two activators: IFI16 and NLRP1, however, caspase-1 activation was not detected. A549 cells may have been primed for pyroptosis, with the absence of a crucial trigger. The investigation of additional innate immunity elements revealed that A + N (or camptothecin) stimulated the expression of NLRX1, STING (stimulator of interferon genes) and two antiviral proteins, IFIT1 and IFIT3. IFI16 and caspase-1 are coded by p53-regulated genes which led us to investigate regulation of NLRP1, NLRX1, STING, IFIT1 and IFIT3 in p53-dependent mode. The upregulation of NLRP1, NLRX1 and STING was attenuated in p53 knockdown cells. The upsurge of the examined genes, and activation of p53, was inhibited by C16, an inhibitor of PKR kinase. PKR was tested due to its ability to phosphorylate p53 on Ser392. Surprisingly, C16 was active even in PKR knockdown cells. The ability of C16 to prevent activation of p53 and expression of innate immunity genes may be the source of its strong anti-inflammatory action. Moreover, cells exposed to A + N can influence neighboring cells in paracrine fashion, for instance, they shed ectodomain of COL17A1 protein and induce, in p53-dependent mode, the expression of gene for interleukin-7. Further, the activation of p53 also spurred the expression of SOCS1, an inhibitor of interferon triggered STAT1-dependent signaling. We conclude that, stimulation of p53 primes cells for the production of interferons (through upregulation of STING), and may activate negative-feedback within this signaling system by enhancing the production of SOCS1.

•

Actinomycin D and nutlin-3a strongly and synergistically activate p53 protein

•

Strongly activated p53 promotes expression of innate immunity genes

•

Strong activation of innate immunity genes can be prevented by C16 compound

•

By inducing SOCS1 protein p53 can prevent overactivation of interferon signaling

•

Strongly activated p53 can send signal to nearby immune cells through interleukin-7

Mathematical Modeling of Oncolytic Virotherapy

Mathematical modeling in biology has a long history as it allows the analysis and simulation of complex dynamic biological systems at little cost. A mathematical model trained on experimental or clinical data can be used to generate and evaluate hypotheses, to ask "what if" questions, and to perform in silico experiments to guide future experimentation and validation. Such models may help identify and provide insights into the mechanisms that drive changes in dynamic systems. While a mathematical model may never replace actual experiments, it can synergize with experiments to save time and resources by identifying experimental conditions that are unlikely to yield favorable outcomes, and by using optimization principles to identify experiments that are most likely to be successful. Over the past decade, numerous models have also been developed for oncolytic virotherapy, ranging from merely theoretic frameworks to fully integrated studies that utilize experimental data to generate actionable hypotheses. Here we describe how to develop such models for specific oncolytic virotherapy experimental setups, and which questions can and cannot be answered using integrated mathematical oncology.

Immunotherapy a New Hope for Cancer Treatment: A Review

Cancer is a major burden of disease worldwide with considerable impact on society. The tide of immunotherapy has finally changed after decades of disappointing results and has become a clinically validated treatment for many cancers. Immunotherapy takes many forms in cancer treatment, including the adoptive transfer of ex vivo activated T cells, oncolytic viruses, natural killer cells, cancer vaccines and administration of antibodies or recombinant proteins that either costimulate cells or block the so-called immune checkpoint pathways. Recently, cancer immunotherapy has received a high degree of attention, which mainly contains the treatments for programmed death ligand 1 (PD-L1), programmed death 1 (PD-1), chimeric antigen receptors (CARs) and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4). Here, this paper reviewed the current understandings of the main strategies in cancer immunotherapy (adoptive cellular immunotherapy, immune checkpoint blockade, oncolytic viruses and cancer vaccines) and discuss the progress in the synergistic design of immune-targeting combination therapies.

Immune Conversion of Tumor Microenvironment by Oncolytic Viruses: The Protoparvovirus H-1PV Case Study

Cancer cells utilize multiple mechanisms to evade and suppress anticancer immune responses creating a “cold” immunosuppressive tumor microenvironment. Oncolytic virotherapy is emerging as a promising approach to revert tumor immunosuppression and enhance the efficacy of other forms of immunotherapy. Growing evidence indicates that oncolytic viruses (OVs) act in a multimodal fashion, inducing immunogenic cell death and thereby eliciting robust anticancer immune responses. In this review, we summarize information about OV-mediated immune conversion of the tumor microenvironment. As a case study we focus on the rodent protoparvovirus H-1PV and its dual role as an oncolytic and immune modulatory agent. Potential strategies to improve H-1PV anticancer efficacy are also discussed.

H-1 Parvovirus as a Cancer-Killing Agent: Past, Present, and Future

The rat protoparvovirus H-1PV is nonpathogenic in humans, replicates preferentially in cancer cells, and has natural oncolytic and oncosuppressive activities. The virus is able to kill cancer cells by activating several cell death pathways. H-1PV-mediated cancer cell death is often immunogenic and triggers anticancer immune responses. The safety and tolerability of H-1PV treatment has been demonstrated in early clinical studies in glioma and pancreatic carcinoma patients. Virus treatment was associated with surrogate signs of efficacy including immune conversion of tumor microenvironment, effective virus distribution into the tumor bed even after systemic administration, and improved patient overall survival compared with historical control. However, monotherapeutic use of the virus was unable to eradicate tumors. Thus, further studies are needed to improve H-1PV's anticancer profile. In this review, we describe H-1PV's anticancer properties and discuss recent efforts to improve the efficacy of H-1PV and, thereby, the clinical outcome of H-1PV-based therapies.

Chemovirotherapeutic Treatment Using Camptothecin Enhances Oncolytic Measles Virus-Mediated Killing of Breast Cancer Cells

Oncolytic virotherapy represents an emerging development in anticancer therapy. Although it has been tested against a variety of cancers, including breast cancer, the efficacy of oncolytic viral vectors delivered as a monotherapy is limited. Enhancing viral oncolytic therapies through combination treatment with anticancer agents is a feasible strategy. In this study, we considered a chemovirotherapeutic approach for treating breast adenocarcinoma using oncolytic measles virus (MV) and the chemotherapeutic agent camptothecin (CPT). Our results demonstrated that co-treatment of MV with CPT yielded enhanced cytotoxicity against breast cancer cells. Low dosage CPT combined with MV was also found to elicit the same therapeutic effect as high doses of CPT. At the lower dosage used, CPT did not inhibit the early stages of MV entry, nor reduce viral replication. Further studies revealed that co-treatment induced significantly enhanced apoptosis of the breast cancer cells compared to either MV or CPT alone. Overall, our findings demonstrate the potential value of MV plus CPT as a novel chemovirotherapeutic treatment against breast cancer and as a strategy to enhance MV oncolytic activity.

Poxviruses as Gene Therapy Vectors: Generating Poxviral Vectors Expressing Therapeutic Transgenes

Treatments with poxvirus vectors can have long-lasting immunological impact in the host, and thus they have been extensively studied to treat diseases and for vaccine development. More importantly, the oncolytic properties of poxviruses have led to their development as cancer therapeutics. Two poxviruses, vaccinia virus (VACV) and myxoma virus (MYXV), have been extensively studied as virotherapeutics with promising results. Vaccinia virus vectors have advanced to the clinic and have been tested as oncolytic therapeutics for several cancer types with successes in phase I/II clinical trials. In addition to oncolytic applications, MYXV has been explored for additional applications including immunotherapeutics, purging of cancer progenitor cells, and treatments for graft-versus-host diseases. These novel therapeutic applications have encouraged its advancement into clinical trials. To meet the demands of different treatment needs, VACV and MYXV can be genetically engineered to express therapeutic transgenes. The engineering process used in poxvirus vectors can be very different from that of other DNA virus vectors (e.g., the herpesviruses). This chapter is intended to serve as a guide to those wishing to engineer poxvirus vectors for therapeutic transgene expression and to produce viral preparations for preclinical studies.

Functional Gene Knockout of NRF2 Increases Chemosensitivity of Human Lung Cancer A549 Cells In Vitro and in a Xenograft Mouse Model

Recent studies point to the evolution of drug resistance in lung cancer as being centered, at least in part, on the upregulation of various genes involved in controlling efflux or drug inactivation. Among the most important of these genes is Nuclear Factor Erythroid 2-Related Factor (NRF2), considered the master regulator of 100–200 target genes involved in cellular responses to oxidative and/or electrophilic stress. With increased focus on the development of combinatorial approaches for cancer treatment, we utilized CRISPR/Cas9 to disable the NRF2 gene in lung cancer cells by disrupting the NRF2 nuclear export signal (NES) domain; phenotypically, the protein is largely blocked from transiting into the nucleus after translation. In tissue culture, cells with this gene knockout were found to have a reduced proliferation phenotype and are more sensitive to chemotherapeutic agents, such as cisplatin and carboplatin. These observations were confirmed in xenograft mouse models wherein the homozygous knockout cells proliferate at a slower rate than the wild-type cells, even in the absence of drug treatment. Tumor growth was arrested for a period of 16 days, with a dramatic decrease in tumor volume being observed in samples receiving the combined action of CRISPR-directed gene editing and chemotherapy.

Oncolytic Viruses on Drugs: Achieving Higher Therapeutic Efficacy

Over the past 20 years there has been a dramatic expansion in the testing of oncolytic viruses (OVs) for the treatment of cancer. OVs are unique biotherapeutics that induce multimodal responses toward tumors, from direct cytopathic effects on cancer cells, to tumor associated blood vessel disruption, and ultimately potent stimulation of anti-tumor immune activation. These agents are highly targeted and can be efficacious as cancer treatments resulting in some patients experiencing complete tumor regression and even cures from OV monotherapy. However, most patients have limited responses with viral replication in tumors often found to be modest and transient. To augment OV replication, increase bystander killing of cancer cells, and/or stimulate stronger targeted anti-cancer immune responses, drug combination approaches have taken center stage for translation to the clinic. Here we comprehensively review drugs that have been combined with OVs to increase therapeutic efficacy, examining the proposed mechanisms of action, and we discuss trends in pharmaco-viral immunotherapeutic approaches currently being investigated.

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.

Please stand by: how oncolytic viruses impact bystander cells

Oncolytic viruses (OVs) do more than simply infect and kill host cells. The accepted mechanism of action for OVs consists of a primary lytic phase and a subsequent antitumor and antiviral immune response. However, not all cells are subject to the direct effects of OV therapy, and it is becoming clear that OVs can also impact uninfected cells in the periphery. This review discusses the effects of OVs on uninfected neighboring cells, so-called bystander effects, and implications for OV therapies alone or in combination with other standard of care chemotherapy.

Lighting a Fire in the Tumor Microenvironment Using Oncolytic Immunotherapy

Oncolytic virus (OV) therapy is potentially a game-changing cancer treatment that has garnered significant interest due to its versatility and multi-modal approaches towards tumor eradication. In the field of cancer immunotherapy, the immunological phenotype of the tumor microenvironment (TME) is an important determinant of disease prognosis and therapeutic success. There is accumulating data that OVs are capable of dramatically altering the TME immune landscape, leading to improved antitumor activity alone or in combination with assorted immune modulators. Herein, we review how OVs disrupt the immunosuppressive TME and can be used strategically to create a "pro-immune" microenvironment that enables and promotes potent, long-lasting host antitumor immune responses.

Potentiating prostate cancer immunotherapy with oncolytic viruses

The clinical effectiveness of immunotherapies for prostate cancer remains subpar compared with that for other cancers. The goal of most immunotherapies is the activation of immune effectors, such as T cells and natural killer cells, as the presence of these activated mediators positively correlates with patient outcomes. Clinical evidence shows that prostate cancer is immunogenic, accessible to the immune system, and can be targeted by antitumour immune responses. However, owing to the detrimental effects of prostate-cancer-associated immunosuppression, even the newest immunotherapeutic approaches fail to initiate the clinically desired antitumour immune reaction. Oncolytic viruses, originally used for their preferential cancer-killing activity, are now being recognized for their ability to overturn cancer-associated immune evasion and promote otherwise absent antitumour immunity. This oncolytic-virus-induced subversion of tumour-associated immunosuppression can potentiate the effectiveness of current immunotherapeutics, including immune checkpoint inhibitors (for example, antibodies against programmed cell death protein 1 (PD1), programmed cell death 1 ligand 1 (PDL1), and cytotoxic T lymphocyte antigen 4 (CTLA4)) and chemotherapeutics that induce immunogenic cell death (for example, doxorubicin and oxaliplatin). Importantly, oncolytic-virus-induced antitumour immunity targets existing prostate cancer cells and also establishes long-term protection against future relapse. Hence, the strategic use of oncolytic viruses as monotherapies or in combination with current immunotherapies might result in the next breakthrough in prostate cancer immunotherapy.

Modelling the spatiotemporal dynamics of chemovirotherapy cancer treatment

Chemovirotherapy is a combination therapy with chemotherapy and oncolytic viruses. It is gaining more interest and attracting more attention in the clinical setting due to its effective therapy and potential synergistic interactions against cancer. In this paper, we develop and analyse a mathematical model in the form of parabolic non-linear partial differential equations to investigate the spatiotemporal dynamics of tumour cells under chemovirotherapy treatment. The proposed model consists of uninfected and infected tumour cells, a free virus, and a chemotherapeutic drug. The analysis of the model is carried out for both the temporal and spatiotemporal cases. Travelling wave solutions to the spatiotemporal model are used to determine the minimum wave speed of tumour invasion. A sensitivity analysis is performed on the model parameters to establish the key parameters that promote cancer remission during chemovirotherapy treatment. Model analysis of the temporal model suggests that virus burst size and virus infection rate determine the success of the virotherapy treatment, whereas travelling wave solutions to the spatiotemporal model show that tumour diffusivity and growth rate are critical during chemovirotherapy. Simulation results reveal that chemovirotherapy is more effective and a good alternative to either chemotherapy or virotherapy, which is in agreement with the recent experimental studies.

Myxoma Virus Optimizes Cisplatin for the Treatment of Ovarian Cancer In Vitro and in a Syngeneic Murine Dissemination Model

A therapeutic approach to improve treatment outcome of ovarian cancer (OC) in patients is urgently needed. Myxoma virus (MYXV) is a candidate oncolytic virus that infects to eliminate OC cells. We found that in vitro MYXV treatment enhances cisplatin or gemcitabine treatment by allowing lower doses than the corresponding IC₅₀ calculated for primary OC cells. MYXV also affected OC patient ascites-associated CD14⁺ myeloid cells, one of the most abundant immunological components of the OC tumor environment; without causing cell death, MYXV infection reduces the ability of these cells to secrete cytokines such as IL-10 that are signatures of the immunosuppressive tumor environment. We found that pretreatment with replication-competent but not replication-defective MYXV-sensitized tumor cells to later cisplatin treatments to drastically improve survival in a murine syngeneic OC dissemination model. We thus conclude that infection with replication-competent MYXV before cisplatin treatment markedly enhances the therapeutic benefit of chemotherapy. Treatment with replication-competent MYXV followed by cisplatin potentiated splenocyte activation and IFNγ expression, possibly by T cells, when splenocytes from treated mice were stimulated with tumor cell antigen ex vivo. The impact on immune responses in the tumor environment may thus contribute to the enhanced antitumor activity of combinatorial MYXV-cisplatin treatment.

Adenovirus-based strategies enhance antitumor capability through p53-mediated downregulation of MGMT in uveal melanoma

Uveal melanoma (UM) is an intractable disease with a low survival rates, despite adequate local treatment, as a result of its metastatic characteristics. Thus, new therapeutic strategies, including combinations of novel gene therapy and traditional chemotherapy, are under investigation to improve long-term prognosis. Dacarbazine or DTIC, an alkylating agent which results in DNA methylation, is most commonly used to treat melanoma but the response is very limited. The O6-methylguanine DNA methyl transferase (MGMT), a DNA repair protein, is involved in chemoresistance in DTIC treatment. We previously investigated a combination of oncolytic adenovirus H101 and the alkylating agent DTIC in the treatment of UM cells in vitro and observed a synergistic antitumor effect. In this study, we validated this result and report an enhanced therapeutic effect in vivo. Our findings also demonstrated that the oncolytic adenovirus H101 decreased MGMT levels via accumulation of p53 overcoming DTIC chemoresistance. Therefore, the clinical therapeutic efficacy of DTIC in the treatment of UM might be improved using this adenovirus-based combination therapy.

Oncolytic virotherapy for urological cancers

Oncolytic virotherapy is a cancer treatment in which replication-competent viruses are used that specifically infect, replicate in and lyse malignant tumour cells, while minimizing harm to normal cells. Anecdotal evidence of the effectiveness of this strategy has existed since the late nineteenth century, but advances and innovations in biotechnological methods in the 1980s and 1990s led to a renewed interest in this type of therapy. Multiple clinical trials investigating the use of agents constructed from a wide range of viruses have since been performed, and several of these enrolled patients with urological malignancies. Data from these clinical trials and from preclinical studies revealed a number of challenges to the effectiveness of oncolytic virotherapy that have prompted the development of further sophisticated strategies. Urological cancers have a range of distinctive features, such as specific genetic mutations and cell surface markers, which enable improving both effectiveness and safety of oncolytic virus treatments. The strategies employed in creating advanced oncolytic agents include alteration of the virus tropism, regulating transcription and translation of viral genes, combination with chemotherapy, radiotherapy or gene therapy, arming viruses with factors that stimulate the immune response against tumour cells and delivery technologies to ensure that the viral agent reaches its target tissue.

Overcoming Barriers in Oncolytic Virotherapy with HDAC Inhibitors and Immune Checkpoint Blockade

Oncolytic viruses (OVs) target and destroy cancer cells while sparing their normal counterparts. These viruses have been evaluated in numerous studies at both pre-clinical and clinical levels and the recent Food and Drug Administration (FDA) approval of an oncolytic herpesvirus-based treatment raises optimism that OVs will become a therapeutic option for cancer patients. However, to improve clinical outcome, there is a need to increase OV efficacy. In addition to killing cancer cells directly through lysis, OVs can stimulate the induction of anti-tumour immune responses. The host immune system thus represents a "double-edged sword" for oncolytic virotherapy: on the one hand, a robust anti-viral response will limit OV replication and spread; on the other hand, the immune-mediated component of OV therapy may be its most important anti-cancer mechanism. Although the relative contribution of direct viral oncolysis and indirect, immune-mediated oncosuppression to overall OV efficacy is unclear, it is likely that an initial period of vigorous OV multiplication and lytic activity will most optimally set the stage for subsequent adaptive anti-tumour immunity. In this review, we consider the use of histone deacetylase (HDAC) inhibitors as a means of boosting virus replication and lessening the negative impact of innate immunity on the direct oncolytic effect. We also discuss an alternative approach, aimed at potentiating OV-elicited anti-tumour immunity through the blockade of immune checkpoints. We conclude by proposing a two-phase combinatorial strategy in which initial OV replication and spread is maximised through transient HDAC inhibition, with anti-tumour immune responses subsequently enhanced by immune checkpoint blockade.

Oncolytic Immunotherapy for Treatment of Cancer

Immunotherapy entails the treatment of disease by modulation of the immune system. As detailed in the previous chapters, the different modes of achieving immune modulation are many, including the use of small/large molecules, cellular therapy, and radiation. Oncolytic viruses that can specifically attack, replicate within, and destroy tumors represent one of the most promising classes of agents for cancer immunotherapy (recently termed as oncolytic immunotherapy). The notion of oncolytic immunotherapy is considered as the way in which virus-induced tumor cell death (known as immunogenic cancer cell death (ICD)) allows the immune system to recognize tumor cells and provide long-lasting antitumor immunity. Both immune responses toward the virus and ICD together contribute toward successful antitumor efficacy. What is now becoming increasingly clear is that monotherapies, through any of the modalities detailed in this book, are neither sufficient in eradicating tumors nor in providing long-lasting antitumor immune responses and that combination therapies may deliver enhanced efficacy. After the rise of the genetic engineering era, it has been possible to engineer viruses to harbor combination-like characteristics to enhance their potency in cancer immunotherapy. This chapter provides a historical background on oncolytic virotherapy and its future application in cancer immunotherapy, especially as a combination therapy with other treatment modalities.

Metabolic alteration--Overcoming therapy resistance in gastric cancer via PGK-1 inhibition in a combined therapy with standard chemotherapeutics

BACKGROUND AND OBJECTIVES

It can be assumed that PGK1 is involved in metastatic spread of gastric carcinomas. Furthermore PGK1 has a proven influence on the characteristics of tumor stem cells. The presence of malignant stem cells, regarding treatment resistance and recurrence, is of considerable importance. We hypothesized that inhibition of PGK1 makes these cells more sensitive to chemotherapeutic agents and therefore mediates an overcome of the existing therapy resistance.

METHODS

All investigations were performed with human gastric adenocarcinoma cell lines. Small hairpin RNA knockdown of PGK1 via adenovirus-shPGK1 was used for PGK1-inhibition. Chemotherapeutic agents were 5-FU and mitomycin. FACS, qRT-PCR, and xCELLigence were performed.

RESULTS

Using the medium-sole-control indicating the highest cell viability and Triton indicating the lowest, mitomycin and 5-FU alone showed a significant decrease in cell viability. The treatment with AdvshPGK1 alone already showed a better decrease. The simultaneous application of chemotherapeutics and adenovirus showed the strongest effect and is comparable to the effect of Triton.

CONCLUSIONS

We showed a significant decrease in cell viability after the simultaneous application of chemotherapeutics and adenovirus. These results suggest that PGK1-inhibition is able to increase the vulnerability of gastric cancer cells and tumor stem cells to overcome the chemotherapeutic therapy resistance.

Promising oncolytic agents for metastatic breast cancer treatment

New therapies for metastatic breast cancer patients are urgently needed. The long-term survival rates remain unacceptably low for patients with recurrent disease or disseminated metastases. In addition, existing therapies often cause a variety of debilitating side effects that severely impact quality of life. Oncolytic viruses constitute a developing therapeutic modality in which interest continues to build due to their ability to spare normal tissue while selectively destroying tumor cells. A number of different viruses have been used to develop oncolytic agents for breast cancer, including herpes simplex virus, adenovirus, vaccinia virus, measles virus, reovirus, and others. In general, clinical trials for several cancers have demonstrated excellent safety records and evidence of efficacy. However, the impressive tumor responses often observed in preclinical studies have yet to be realized in the clinic. In order for the promise of oncolytic virotherapy to be fully realized for breast cancer patients, effectiveness must be demonstrated in metastatic disease. This review provides a summary of oncolytic virotherapy strategies being developed to target metastatic breast cancer.

In vitro screen of a small molecule inhibitor drug library identifies multiple compounds that synergize with oncolytic myxoma virus against human brain tumor-initiating cells

BACKGROUND

Brain tumor-initiating cells (BTICs) are stem-like cells hypothesized to form a disease reservoir that mediates tumor recurrence in high-grade gliomas. Oncolytic virotherapy uses replication-competent viruses to target and kill malignant cells and has been evaluated in clinic for glioma therapy with limited results. Myxoma virus (MyxV) is a safe and highly effective oncolytic virus (OV) in conventional glioma models but, as seen with other OVs, is only modestly effective for patient-derived BTICs. The objective of this study was to determine whether MyxV treatment against human BTICs could be improved by combining chemotherapeutics and virotherapy.

METHODS

A 73-compound library of drug candidates in clinical use or preclinical development was screened to identify compounds that sensitize human BTICs to MyxV treatment in vitro, and synergy was evaluated mathematically in lead compounds using Chou-Talalay analyses. The effects of combination therapy on viral gene expression and viral replication were also assessed.

RESULTS

Eleven compounds that enhance MyxV efficacy were identified, and 6 were shown to synergize with the virus using Chou-Talalay analyses. Four of the synergistic compounds were shown to significantly increase viral gene expression, indicating a potential mechanism for synergy. Three highly synergistic compounds (axitinib, a VEGFR inhibitor; rofecoxib, a cyclooxygenase-2 inhibitor; and pemetrexed, a folate anti-metabolite) belong to classes of compounds that have not been previously shown to synergize with oncolytic viruses in vitro.

CONCLUSIONS

This study has identified multiple novel drug candidates that synergistically improve MyxV efficacy in a preclinical BTIC glioma model.

Viruses for tumor therapy

Oncolytic virotherapy exploits live viruses with selective tropism for cancerous cells and tissues to treat cancer. As discussed here, the field has progressed considerably as a result of both the successes and failures of previous and on-going clinical trials for various cancers. These studies indicate that oncolytic viruses are remarkably safe and more efficacious when virus replication stimulates sustained antitumor immune responses. In the future, virotherapy should be combined with immunomodulatory reagents that target immune tolerance to established cancers.

Oncolytic Poxviruses

Current standard treatments of cancer can prolong survival of many cancer patients but usually do not effectively cure the disease. Oncolytic virotherapy is an emerging therapeutic for the treatment of cancer that exploits replication-competent viruses to selectively infect and destroy cancerous cells while sparing normal cells and tissues. Clinical and/or preclinical studies on oncolytic viruses have revealed that the candidate viruses being tested in trials are remarkably safe and offer potential for treating many classes of currently incurable cancers. Among these candidates are vaccinia and myxoma viruses, which belong to the family Poxviridae and possess promising oncolytic features. This article describes poxviruses that are being developed for oncolytic virotherapy and summarizes the outcomes of both clinical and preclinical studies. Additionally, studies demonstrating superior efficacy when poxvirus oncolytic virotherapy is combined with conventional therapies are described.

High efficiency of alphaviral gene transfer in combination with 5-fluorouracil in a mouse mammary tumor model

Background

The combination of virotherapy and chemotherapy may enable efficient tumor regression that would be unachievable using either therapy alone. In this study, we investigated the efficiency of transgene delivery and the cytotoxic effects of alphaviral vector in combination with 5-fluorouracil (5-FU) in a mouse mammary tumor model (4 T1).

Methods

Replication-deficient Semliki Forest virus (SFV) vectors carrying genes encoding fluorescent proteins were used to infect 4 T1 cell cultures treated with different doses of 5-FU. The efficiency of infection was monitored via fluorescence microscopy and quantified by fluorometry. The cytotoxicity of the combined treatment with 5-FU and alphaviral vector was measured using an MTT-based cell viability assay. In vivo experiments were performed in a subcutaneous 4 T1 mouse mammary tumor model with different 5-FU doses and an SFV vector encoding firefly luciferase.

Results

Infection of 4 T1 cells with SFV prior to 5-FU treatment did not produce a synergistic anti-proliferative effect. An alternative treatment strategy, in which 5-FU was used prior to virus infection, strongly inhibited SFV expression. Nevertheless, in vivo experiments showed a significant enhancement in SFV-driven transgene (luciferase) expression upon intratumoral and intraperitoneal vector administration in 4 T1 tumor-bearing mice pretreated with 5-FU: here, we observed a positive correlation between 5-FU dose and the level of luciferase expression.

Conclusions

Although 5-FU inhibited SFV-mediated transgene expression in 4 T1 cells in vitro, application of the drug in a mouse model revealed a significant enhancement of intratumoral transgene synthesis compared with 5-FU untreated mice. These results may have implications for efficient transgene delivery and the development of potent cancer treatment strategies using alphaviral vectors and 5-FU.

Chemotherapy and Oncolytic Virotherapy: Advanced Tactics in the War against Cancer

Cancer is a traitorous archenemy that threatens our survival. Its ability to evade detection and adapt to various cancer therapies means that it is a moving target that becomes increasingly difficult to attack. Through technological advancements, we have developed sophisticated weapons to fight off tumor growth and invasion. However, if we are to stand a chance in this war against cancer, advanced tactics will be required to maximize the use of our available resources. Oncolytic viruses (OVs) are multi-functional cancer-fighters that can be engineered to suit many different strategies; in particular, their retooling can facilitate increased capacity for direct tumor killing (oncolytic virotherapy) and elicit adaptive antitumor immune responses (oncolytic immunotherapy). However, administration of these modified OVs alone, rarely induces successful regression of established tumors. This may be attributed to host antiviral immunity that acts to eliminate viral particles, as well as the capacity for tumors to adapt to therapeutic selective pressure. It has been shown that various chemotherapeutic drugs with distinct functional properties can potentiate the antitumor efficacy of OVs. In this review, we summarize the chemotherapeutic combinatorial strategies used to optimize virally induced destruction of tumors. With a particular focus on pharmaceutical immunomodulators, we discuss how specific therapeutic contexts may alter the effects of these synergistic combinations and their implications for future clinical use.

Tumor Restrictions to Oncolytic Virus

Oncolytic virotherapy has advanced since the days of its conception but therapeutic efficacy in the clinics does not seem to reach the same level as in animal models. One reason is premature oncolytic virus clearance in humans, which is a reasonable assumption considering the immune-stimulating nature of the oncolytic agents. However, several studies are beginning to reveal layers of restriction to oncolytic virotherapy that are present before an adaptive neutralizing immune response. Some of these barriers are present constitutively halting infection before it even begins, whereas others are raised by minute cues triggered by virus infection. Indeed, we and others have noticed that delivering viruses to tumors may not be the biggest obstacle to successful therapy, but instead the physical make-up of the tumor and its capacity to mount antiviral defenses seem to be the most important efficacy determinants. In this review, we summarize the constitutive and innate barriers to oncolytic virotherapy and discuss strategies to overcome them.

Induction of antiviral genes by the tumor microenvironment confers resistance to virotherapy

Oncolytic viruses obliterate tumor cells in tissue culture but not against the same tumors in vivo. We report that macrophages can induce a powerfully protective antiviral state in ovarian and breast tumors, rendering them resistant to oncolytic virotherapy. These tumors have activated JAK/STAT pathways and expression of interferon-stimulated genes (ISGs) is upregulated. Gene expression profiling (GEP) of human primary ovarian and breast tumors confirmed constitutive activation of ISGs. The tumors were heavily infiltrated with CD68+ macrophages. Exposure of OV-susceptible tumor cell lines to conditioned media from RAW264.7 or primary macrophages activated antiviral ISGs, JAK/STAT signaling and an antiviral state. Anti-IFN antibodies and shRNA knockdown studies show that this effect is mediated by an extremely low concentration of macrophage-derived IFNβ. JAK inhibitors reversed the macrophage-induced antiviral state. This study points to a new role for tumor-associated macrophages in the induction of a constitutive antiviral state that shields tumors from viral attack.

Suppression of antiviral innate immunity by sunitinib enhances oncolytic virotherapy

The use of lytic viruses to preferentially infect and eliminate cancer cells while sparing normal cells is a promising experimental therapeutic approach for treating cancer. However, the efficacy of oncolytic virotherapy is often limited by two innate immunity pathways, the protein kinase PKR and the 2'-5'-oligoadenylate (OAS)/RNase L systems, which are widely present in many but not all tumor cell types. Previously, we reported that the anticancer drug, sunitinib, an inhibitor of VEGF-R and PDGF-R, has off-target effects against both PKR and RNase L. Here we show that combining sunitinib treatments with infection by an oncolytic virus, vesicular stomatitis virus (VSV), led to the elimination of prostate, breast, and kidney malignant tumors in mice. In contrast, either virus or sunitinib alone slowed tumor progression but did not eliminate tumors. In prostate tumors excised from treated mice, sunitinib decreased levels of the phosphorylated form of translation initiation factor, eIF2-α, a substrate of PKR, by 10-fold while increasing median viral titers by 23-fold. The sunitinib/VSV regimen caused complete and sustained tumor regression in both immunodeficient and immunocompetent animals. Results indicate that transient inhibition of innate immunity with sunitinib enhances oncolytic virotherapy allowing the recovery of tumor-bearing animals.

Oncolytic virotherapy in veterinary medicine: current status and future prospects for canine patients

Oncolytic viruses refer to those that are able to eliminate malignancies by direct targeting and lysis of cancer cells, leaving non-cancerous tissues unharmed. Several oncolytic viruses including adenovirus strains, canine distemper virus and vaccinia virus strains have been used for canine cancer therapy in preclinical studies. However, in contrast to human studies, clinical trials with oncolytic viruses for canine cancer patients have not been reported. An 'ideal' virus has yet to be identified. This review is focused on the prospective use of oncolytic viruses in the treatment of canine tumors - a knowledge that will undoubtedly contribute to the development of oncolytic viral agents for canine cancer therapy in the future.