PET imaging of oncolytic VSV expressing the mutant HSV-1 thymidine kinase transgene in a preclinical HCC rat model

Reporter Transgenes for Monitoring the Antitumor Efficacy of Recombinant Oncolytic Viruses

Accurate measurement of tumor size and margins is crucial for successful oncotherapy. In the last decade, non-invasive imaging modalities, including optical imaging using non-radioactive substrates, deep-tissue imaging with radioactive substrates, and magnetic resonance imaging have been developed. Reporter genes play the most important role among visualization tools; their expression in tumors and metastases makes it possible to track changes in the tumor growth and gauge therapy effectiveness. Oncolytic viruses are often chosen as a vector for delivering reporter genes into tumor cells, since oncolytic viruses are tumor-specific, meaning that they infect and lyse tumor cells without damaging normal cells. The choice of reporter transgenes for genetic modification of oncolytic viruses depends on the study objectives and imaging methods used. Optical imaging techniques are suitable for in vitro studies and small animal models, while deep-tissue imaging techniques are used to evaluate virotherapy in large animals and humans. For optical imaging, transgenes of fluorescent proteins, luciferases, and tyrosinases are used; for deep-tissue imaging, the most promising transgene is the sodium/iodide symporter (NIS), which ensures an accumulation of radioactive isotopes in virus-infected tumor cells. Currently, NIS is the only reporter transgene that has been shown to be effective in monitoring tumor virotherapy not only in preclinical but also in clinical studies.

The Evolution and Future of Targeted Cancer Therapy: From Nanoparticles, Oncolytic Viruses, and Oncolytic Bacteria to the Treatment of Solid Tumors

While many classes of chemotherapeutic agents exist to treat solid tumors, few can generate a lasting response without substantial off-target toxicity despite significant scientific advancements and investments. In this review, the paths of development for nanoparticles, oncolytic viruses, and oncolytic bacteria over the last 20 years of research towards clinical translation and acceptance as novel cancer therapeutics are compared. Novel nanoparticle, oncolytic virus, and oncolytic bacteria therapies all start with a common goal of accomplishing therapeutic drug activity or delivery to a specific site while avoiding off-target effects, with overlapping methodology between all three modalities. Indeed, the degree of overlap is substantial enough that breakthroughs in one therapeutic could have considerable implications on the progression of the other two. Each oncotherapeutic modality has accomplished clinical translation, successfully overcoming the potential pitfalls promising therapeutics face. However, once studies enter clinical trials, the data all but disappears, leaving pre-clinical researchers largely in the dark. Overall, the creativity, flexibility, and innovation of these modalities for solid tumor treatments are greatly encouraging, and usher in a new age of pharmaceutical development.

Next Generation Imaging Techniques to Define Immune Topographies in Solid Tumors

In recent years, cancer immunotherapy experienced remarkable developments and it is nowadays considered a promising therapeutic frontier against many types of cancer, especially hematological malignancies. However, in most types of solid tumors, immunotherapy efficacy is modest, partly because of the limited accessibility of lymphocytes to the tumor core. This immune exclusion is mediated by a variety of physical, functional and dynamic barriers, which play a role in shaping the immune infiltrate in the tumor microenvironment. At present there is no unified and integrated understanding about the role played by different postulated models of immune exclusion in human solid tumors. Systematically mapping immune landscapes or "topographies" in cancers of different histology is of pivotal importance to characterize spatial and temporal distribution of lymphocytes in the tumor microenvironment, providing insights into mechanisms of immune exclusion. Spatially mapping immune cells also provides quantitative information, which could be informative in clinical settings, for example for the discovery of new biomarkers that could guide the design of patient-specific immunotherapies. In this review, we aim to summarize current standard and next generation approaches to define Cancer Immune Topographies based on published studies and propose future perspectives.

Optimizing Oncolytic Viral Design to Enhance Antitumor Efficacy: Progress and Challenges

The field of oncolytic virotherapy has seen remarkable advancements in last two decades, leading to approval of the first oncolytic immuno-virotherapy, Talimogene Laherparepvec, for the treatment of melanoma. A plethora of preclinical and clinical studies have demonstrated excellent safety profiles of other oncolytic viruses. While oncolytic viruses show clinical promise in already immunogenic malignancies, response rates are inconsistent. Response rates are even less consistent in immunosuppressed tumor microenvironments like those found in liver, pancreas, and MSI-stable colon cancers. Therefore, the efficacy of oncolytic viruses needs to be improved for more oncolytic viruses to enter mainstream cancer therapy. One approach to increase the therapeutic efficacy of oncolytic viruses is to use them as primers for other immunotherapeutics. The amenability of oncolytic viruses to transgene-arming provides an immense opportunity for investigators to explore different ways of improving the outcome of oncolytic therapy. In this regard, genes encoding immunomodulatory proteins are the most commonly studied genes for arming oncolytic viruses. Other transgenes used to arm oncolytic viruses include those with the potential to favorably modulate tumor stroma, making it possible to image the virus distribution and increase its suitability for combination with other therapeutics. This review will detail the progress made in arming oncolytic viruses with a focus on immune-modulatory transgenes, and will discuss the challenges that need to be addressed for more armed oncolytic viruses to find widespread clinical use.

Engineering and Characterization of Oncolytic Vaccinia Virus Expressing Truncated Herpes Simplex Virus Thymidine Kinase

Oncolytic viruses are a promising class of anti-tumor agents; however, concerns regarding uncontrolled viral replication have led to the development of a replication-controllable oncolytic vaccinia virus (OVV). The engineering involves replacing the native thymidine kinase (VV-tk) gene, in a Wyeth strain vaccinia backbone, with the herpes simplex virus thymidine kinase (HSV-tk) gene, which allows for viral replication control via ganciclovir (GCV, an antiviral/cytotoxic pro-drug). Adding the wild-type HSV-tk gene might disrupt the tumor selectivity of VV-tk deleted OVVs; therefore, only engineered viruses that lacked tk activity were selected as candidates. Ultimately, OTS-412, which is an OVV containing a mutant HSV-tk, was chosen for characterization regarding tumor selectivity, sensitivity to GCV, and the influence of GCV on OTS-412 anti-tumor effects. OTS-412 demonstrated comparable replication and cytotoxicity to VV^tk- (control, a VV-tk deleted OVV) in multiple cancer cell lines. In HCT 116 mouse models, OTS-412 replication in tumors was reduced by >50% by GCV (p = 0.004); additionally, combination use of GCV did not compromise the anti-tumor effects of OTS-412. This is the first report of OTS-412, a VV-tk deleted OVV containing a mutant HSV-tk transgene, which demonstrates tumor selectivity and sensitivity to GCV. The HSV-tk/GCV combination provides a safety mechanism for future clinical applications of OTS-412.

Enhanced Safety and Efficacy of Oncolytic VSV Therapy by Combination with T Cell Receptor Transgenic T Cells as Carriers

Vesicular stomatitis virus (VSV) represents an attractive oncolytic virotherapy platform because of its potent tumor cell-killing and immune-stimulating properties; yet the clinical translation of VSV faces numerous challenges, such as inefficient systemic delivery and severe side effects such as neurotoxicity. We hypothesized that we could overcome these limitations and simultaneously enhance the therapy, by combining VSV with adoptively transferred T cell receptor (TCR) transgenic T cells as carrier cells. We show that CD8⁺ T central memory cells (CD8⁺ T cm) can be efficiently loaded with VSV, they support intracellular virus production, and they can efficiently transfer VSV to tumor cells without compromising their own viability or antitumor reactivity. Loading VSV onto CD8⁺ T cm not only improves the safety compared with systemic administration of naked virus, but this approach also allows for an effective delivery of virus to its tumor target, resulting in an effective combination therapy in NSG mice bearing subcutaneous human acute myeloid leukemia (AML) tumors. We conclude that the combination of potent tumor debulking provided by the oncolytic VSV with the added effector functions afforded by the cytotoxic immune carrier cells results in a potent and safer immunotherapeutic, which can be further developed for clinical translation.

The importance of imaging strategies for pre-clinical and clinical in vivo distribution of oncolytic viruses

Oncolytic viruses (OVs) are an emergent and unique therapy for cancer patients. Similar to chemo- and radiation therapy, OV can lyse (kill) cancer cell directly. In general, the advantages of OVs over other treatments are primarily: a higher safety profile (as shown by less adverse effects), ability to replicate, transgene(s) delivery, and stimulation of a host’s immune system against cancer. The latter has prompted successful use of OVs with other immunotherapeutic strategies in a synergistic manner. In spite of extended testing in pre-clinical and clinical setting, using biologically derived therapeutics like virus always raises potential concerns about safety (replication at non-intended locations) and bio-availability of the product. Recent advent in in vivo imaging techniques dramatically improves the convenience of use, quality of pictures, and amount of information acquired. Easy assessing of safety/localization of the biotherapeutics like OVs became a new potential weapon in the physician’s arsenal to improve treatment outcome. Given that OVs are typically replicating, in vivo imaging can also track virus replication and persistence as well as precisely mapping tumor tissues presence. This review discusses the importance of imaging in vivo in evaluating OV efficacy, as well as currently available tools and techniques.

Recent advances in vesicular stomatitis virus-based oncolytic virotherapy: a 5-year update

Oncolytic virus (OV) therapy is an anti-cancer approach that uses viruses that preferentially infect, replicate in and kill cancer cells. Vesicular stomatitis virus (VSV, a rhabdovirus) is an OV that is currently being tested in the USA in several phase I clinical trials against different malignancies. Several factors make VSV a promising OV: lack of pre-existing human immunity against VSV, a small and easy to manipulate genome, cytoplasmic replication without risk of host cell transformation, independence of cell cycle and rapid growth to high titres in a broad range of cell lines facilitating large-scale virus production. While significant advances have been made in VSV-based OV therapy, room for improvement remains. Here we review recent studies (published in the last 5 years) that address 'old' and 'new' challenges of VSV-based OV therapy. These studies focused on improving VSV safety, oncoselectivity and oncotoxicity; breaking resistance of some cancers to VSV; preventing premature clearance of VSV; and stimulating tumour-specific immunity. Many of these approaches were based on combining VSV with other therapeutics. This review also discusses another rhabdovirus closely related to VSV, Maraba virus, which is currently being tested in Canada in phase I/II clinical trials.

New imaging probes to track cell fate: reporter genes in stem cell research

Cell fate is a concept used to describe the differentiation and development of a cell in its organismal context over time. It is important in the field of regenerative medicine, where stem cell therapy holds much promise but is limited by our ability to assess its efficacy, which is mainly due to the inability to monitor what happens to the cells upon engraftment to the damaged tissue. Currently, several imaging modalities can be used to track cells in the clinical setting; however, they do not satisfy many of the criteria necessary to accurately assess several aspects of cell fate. In recent years, reporter genes have become a popular option for tracking transplanted cells, via various imaging modalities in small mammalian animal models. This review article examines the reporter gene strategies used in imaging modalities such as MRI, SPECT/PET, Optoacoustic and Bioluminescence Imaging. Strengths and limitations of the use of reporter genes in each modality are discussed.