Showing the Way: Oncolytic Adenoviruses as Chaperones of Immunostimulatory Adjuncts

Differential Susceptibility of Ex Vivo Primary Glioblastoma Tumors to Oncolytic Effect of Modified Zika Virus

Glioblastoma (GBM), the most common primary malignant brain tumor, is a highly lethal form of cancer with a very limited set of treatment options. High heterogeneity in the tumor cell population and the invasive nature of these cells decrease the likely efficacy of traditional cancer treatments, thus requiring research into novel treatment options. The use of oncolytic viruses as potential therapeutics has been researched for some time. Zika virus (ZIKV) has demonstrated oncotropism and oncolytic effects on GBM stem cells (GSCs). To address the need for safe and effective GBM treatments, we designed an attenuated ZIKV strain (ZOL-1) that does not cause paralytic or neurological diseases in mouse models compared with unmodified ZIKV. Importantly, we found that patient-derived GBM tumors exhibited susceptibility (responders) and non-susceptibility (non-responders) to ZOL-1-mediated tumor cell killing, as evidenced by differential apoptotic cell death and cell viability upon ZOL-1 treatment. The oncolytic effect observed in responder cells was seen both in vitro in neurosphere models and in vivo upon xenograft. Finally, we observed that the use of ZOL-1 as combination therapy with multiple PI3K-AKT inhibitors in non-responder GBM resulted in enhanced chemotherapeutic efficacy. Altogether, this study establishes ZOL-1 as a safe and effective treatment against GBM and provides a foundation to conduct further studies evaluating its potential as an effective adjuvant with other chemotherapies and kinase inhibitors.

Zika virus oncolytic activity requires CD8+ T cells and is boosted by immune checkpoint blockade

Glioblastoma multiforme (GBM) is a fatal human cancer in part because GBM stem cells are resistant to therapy and recurrence is inevitable. Previously, we demonstrated Zika virus (ZIKV) targets GBM stem cells and prevents death of mice with gliomas. Here, we evaluated the immunological basis of ZIKV-mediated protection against GBM. Introduction of ZIKV into the brain tumor increased recruitment of CD8⁺ T and myeloid cells to the tumor microenvironment. CD8⁺ T cells were required for ZIKV-dependent tumor clearance because survival benefits were lost with CD8⁺ T cell depletion. Moreover, while anti–PD-1 antibody monotherapy moderately improved tumor survival, when coadministered with ZIKV, survival increased. ZIKV-mediated tumor clearance also resulted in durable protection against syngeneic tumor rechallenge, which also depended on CD8⁺ T cells. To address safety concerns, we generated an immune-sensitized ZIKV strain, which was effective alone or in combination with immunotherapy. Thus, oncolytic ZIKV treatment can be leveraged by immunotherapies, which may prompt combination treatment paradigms for adult patients with GBM.

Spectrum-Wide Exploration of Human Adenoviruses for Breast Cancer Therapy

Oncolytic adenoviruses (Ads) are promising tools for cancer therapeutics. However, most Ad-based therapies utilize Ad type 5 (Ad5), which displays unsatisfying efficiency in clinical trials, partly due to the low expression levels of its primary coxsackievirus and adenovirus receptor (CAR) on tumor cells. Since the efficacy of virotherapy strongly relies on efficient transduction of targeted tumor cells, initial screening of a broad range of viral agents to identify the most effective vehicles is essential. Using a novel Ad library consisting of numerous human Ads representing known Ad species, we evaluated the transduction efficiencies in four breast cancer (BC) cell lines. For each cell line over 20 Ad types were screened in a high-throughput manner based on reporter assays. Ad types featuring high transduction efficiencies were further investigated with respect to the percentage of transgene-positive cells and efficiencies of cellular entry in individual cell lines. Additionally, oncolytic assay was performed to test tumor cell lysis efficacy of selected Ad types. We found that all analyzed BC cell lines show low expression levels of CAR, while alternative receptors such as CD46, DSG-2, and integrins were also detected. We identified Ad3, Ad35, Ad37, and Ad52 as potential candidates for BC virotherapy.

Adenovirus and Immunotherapy: Advancing Cancer Treatment by Combination

Gene therapy with viral vectors has significantly advanced in the past few decades, with adenovirus being one of the most commonly employed vectors for cancer gene therapy. Adenovirus vectors can be divided into 2 groups: (1) replication-deficient viruses; and (2) replication-competent, oncolytic (OVs) viruses. Replication-deficient adenoviruses have been explored as vaccine carriers and gene therapy vectors. Oncolytic adenoviruses are designed to selectively target, replicate, and directly destroy cancer cells. Additionally, virus-mediated cell lysis releases tumor antigens and induces local inflammation (e.g., immunogenic cell death), which contributes significantly to the reversal of local immune suppression and development of antitumor immune responses ("cold" tumor into "hot" tumor). There is a growing body of evidence suggesting that the host immune response may provide a critical boost for the efficacy of oncolytic virotherapy. Additionally, genetic engineering of oncolytic viruses allows local expression of immune therapeutics, thereby reducing related toxicities. Therefore, the combination of oncolytic virus and immunotherapy is an attractive therapeutic strategy for cancer treatment. In this review, we focus on adenovirus-based vectors and discuss recent progress in combination therapy of adenoviruses with immunotherapy in preclinical and clinical studies.

Combining Radiation Therapy With Interferons: Back to the Future

Radiation therapy has been linked to the induction of an intratumoral type I interferon (IFN) response, which positively affects the response to treatment. This has spiked the interest to combine radiation therapy with IFN-based treatment. Interestingly, this combination treatment has been considered previously, since preclinical studies demonstrated a radiosensitizing effect of interferons. As a result, multiple clinical trials have been performed combining radiation therapy with interferons in different tumor types. Although potential benefit has been suggested, the outcomes of the trials are diverse and challenging to interpret. In addition, increased grade ≥3 toxicity frequently resulted in a negative recommendation regarding the combination therapy. The latter appears premature because many studies were small and several aspects of the combination treatment have not yet been sufficiently explored to justify such a definite conclusion. This review summarizes the available literature on this combination therapy, with a focus on IFN-α and IFN-β. Based on preclinical studies and clinical trials, we evaluated the potential opportunities and describe the current challenges. In addition, we identify several issues that should be addressed to fully exploit the potential benefit of this combinatorial treatment approach.

Oncolytic adenovirus: A tool for cancer therapy in combination with other therapeutic approaches

Cancer therapy using oncolytic viruses is an emerging area, in which viruses are engineered to selectively propagate in tumor tissues without affecting healthy cells. Because of the advantages that adenoviruses (Ads) have over other viruses, they are more considered. To achieve tumor selectivity, two main modifications on Ads genome have been applied: small deletions and insertion of tissue- or tumor-specific promoters. Despite oncolytic adenoviruses ability in tumor cell lysis and immune responses stimulation, to further increase their antitumor effects, genomic modifications have been carried out including insertion of checkpoint inhibitors and antigenic or immunostimulatory molecules into the adenovirus genome and combination with dendritic cells and chemotherapeutic agents. This study reviews oncolytic adenoviruses structures, their antitumor efficacy in combination with other therapeutic strategies, and finally challenges around this treatment approach.

Oncolytic viruses and checkpoint inhibitors: combination therapy in clinical trials

Advances in the understanding of cancer immunotherapy and the development of multiple checkpoint inhibitors have dramatically changed the current landscape of cancer treatment. Recent large-scale phase III trials (e.g. PHOCUS, OPTiM) are establishing use of oncolytic viruses as another tool in the cancer therapeutics armamentarium. These viruses do not simply lyse cells to achieve their cancer-killing effects, but also cause dramatic changes in the tumor immune microenvironment. This review will highlight the major vector platforms that are currently in development (including adenoviruses, reoviruses, vaccinia viruses, herpesviruses, and coxsackieviruses) and how they are combined with checkpoint inhibitors. These vectors employ a variety of engineered capsid modifications to enhance infectivity, genome deletions or promoter elements to confer selective replication, and encode a variety of transgenes to enhance anti-tumor or immunogenic effects. Pre-clinical and clinical data have shown that oncolytic vectors can induce anti-tumor immunity and markedly increase immune cell infiltration (including cytotoxic CD8⁺ T cells) into the local tumor microenvironment. This "priming" by the viral infection can change a 'cold' tumor microenvironment into a 'hot' one with the influx of a multitude of immune cells and cytokines. This alteration sets the stage for subsequent checkpoint inhibitor delivery, as they are most effective in an environment with a large lymphocytic infiltrate. There are multiple ongoing clinical trials that are currently combining oncolytic viruses with checkpoint inhibitors (e.g. CAPTIVE, CAPRA, and Masterkey-265), and the initial results are encouraging. It is clear that oncolytic viruses and checkpoint inhibitors will continue to evolve together as a combination therapy for multiple types of cancers.

A compendium of adenovirus genetic modifications for enhanced replication, oncolysis, and tumor immunosurveillance in cancer therapy

In this review, we specifically focus on genetic modifications of oncolytic adenovirus 5 (Ad5)-based vectors that enhance replication, oncolysis/spread, and virus-mediated tumor immunosurveillance. The finding of negative regulation of minor core protein V by SUMOylation led to the identification of amino acid residues, which when mutated increase adenovirus replication and progeny yield. Suppression of Dicer and/or RNAi pathway with shRNA or p19 tomato bushy stunt protein also results in significant enhancement of adenovirus replication and progeny yield. Truncation mutations in E3-19K or i-leader sequence or overexpression of adenovirus death protein (ADP) potently increase adenovirus progeny release and spread without affecting virus yield. Moreover, E3-19K protein, which was found to inhibit both major histocompatibility complex I (MHCI) and MHC-I chain-related A and B proteins (MICA/MICB) expression on the cell surface, protecting infected cells from T-lymphocyte and natural killer (NK) cell attack, may be tailored to selectively target only MHCI or MICA/MICB, or to lose the ability to downregulate both. At last, E3-19K protein may be exploited to deliver tumor-associated epitopes directly to the endoplasmic reticulum for loading MHCI in the transporter associated with antigen processing (TAP)-deregulated cells.

Designer Oncolytic Adenovirus: Coming of Age

The licensing of talimogene laherparepvec (T-Vec) represented a landmark moment for oncolytic virotherapy, since it provided unequivocal evidence for the long-touted potential of genetically modified replicating viruses as anti-cancer agents. Whilst T-Vec is promising as a locally delivered virotherapy, especially in combination with immune-checkpoint inhibitors, the quest continues for a virus capable of specific tumour cell killing via systemic administration. One candidate is oncolytic adenovirus (Ad); it’s double stranded DNA genome is easily manipulated and a wide range of strategies and technologies have been employed to empower the vector with improved pharmacokinetics and tumour targeting ability. As well characterised clinical and experimental agents, we have detailed knowledge of adenoviruses’ mechanisms of pathogenicity, supported by detailed virological studies and in vivo interactions. In this review we highlight the strides made in the engineering of bespoke adenoviral vectors to specifically infect, replicate within, and destroy tumour cells. We discuss how mutations in genes regulating adenoviral replication after cell entry can be used to restrict replication to the tumour, and summarise how detailed knowledge of viral capsid interactions enable rational modification to eliminate native tropisms, and simultaneously promote active uptake by cancerous tissues. We argue that these designer-viruses, exploiting the viruses natural mechanisms and regulated at every level of replication, represent the ideal platforms for local overexpression of therapeutic transgenes such as immunomodulatory agents. Where T-Vec has paved the way, Ad-based vectors now follow. The era of designer oncolytic virotherapies looks decidedly as though it will soon become a reality.

Editorial of the Special Issue: Oncolytic Viruses as a Novel Form of Immunotherapy for Cancer

Oncolytic viruses (OVs), either occurring naturally or through genetic engineering, can selectively infect, replicate in, and kill cancer cells, while leaving normal cells (almost) unharmed [...].