IRF8-driven reprogramming of the immune microenvironment enhances anti-tumor adaptive immunity and reduces immunosuppression in murine glioblastoma

Background

Glioblastoma (GBM) has a highly immunosuppressive tumor immune microenvironment (TIME), largely mediated by myeloid-derived suppressor cells (MDSCs). Here, we utilized a retroviral replicating vector (RRV) to deliver Interferon Regulatory Factor 8 (IRF8), a master regulator of type 1 conventional dendritic cell (cDC1) development, in a syngeneic murine GBM model. We hypothesized that RRV-mediated delivery of IRF8 could "reprogram" intratumoral MDSCs into antigen-presenting cells (APCs) and thereby restore T-cell responses.

Methods

Effects of RRV-IRF8 on survival and tumor growth kinetics were examined in the SB28 murine GBM model. Immunophenotype was analyzed by flow cytometry and gene expression assays. We assayed functional immunosuppression and antigen presentation by ex vivo T-cell-myeloid co-culture.

Results

Mice with RRV-IRF8 pre-transduced intracerebral tumors had significantly longer survival and slower tumor growth compared to controls. RRV-IRF8 treated tumors exhibited significant enrichment of cDC1s and CD8+ T-cells. Additionally, myeloid cells derived from RRV-IRF8 tumors showed decreased expression of the immunosuppressive markers Arg1 and IDO1 and demonstrated reduced suppression of naïve T-cell proliferation in ex vivo co-culture, compared to controls. Furthermore, DCs from RRV-IRF8 tumors showed increased antigen presentation compared to those from control tumors. In vivo treatment with azidothymidine (AZT), a viral replication inhibitor, showed that IRF8 transduction in both tumor and non-tumor cells is necessary for survival benefit, associated with a reprogrammed, cDC1- and CD8 T-cell-enriched TIME.

Conclusions

Our results indicate that reprogramming of glioma-infiltrating myeloid cells by in vivo expression of IRF8 may reduce immunosuppression and enhance antigen presentation, achieving improved tumor control.

Targeted Suicide Gene Therapy with Retroviral Replicating Vectors for Experimental Canine Cancers

Cancer in dogs has increased in recent years and is a leading cause of death. We have developed a retroviral replicating vector (RRV) that specifically targets cancer cells for infection and replication. RRV carrying a suicide gene induced synchronized killing of cancer cells when administered with a prodrug after infection. In this study, we evaluated two distinct RRVs derived from amphotropic murine leukemia virus (AMLV) and gibbon ape leukemia virus (GALV) in canine tumor models both in vitro and in vivo. Despite low infection rates in normal canine cells, both RRVs efficiently infected and replicated within all the canine tumor cells tested. The efficient intratumoral spread of the RRVs after their intratumoral injection was also demonstrated in nude mouse models of subcutaneous canine tumor xenografts. When both RRVs encoded a yeast cytosine deaminase suicide gene, which converts the prodrug 5-fluorocytosine (5-FC) to the active drug 5-fluorouracil, they caused tumor-cell-specific 5-FC-induced killing of the canine tumor cells in vitro. Furthermore, in the AZACF- and AZACH-cell subcutaneous tumor xenograft models, both RRVs exerted significant antitumor effects. These results suggest that RRV-mediated suicide gene therapy is a novel therapeutic approach to canine cancers.

In vivo selection of anti-HIV-1 gene-modified human hematopoietic stem/progenitor cells to enhance engraftment and HIV-1 inhibition

Hematopoietic stem/progenitor cell (HSPC)-based anti-HIV-1 gene therapy holds great promise to eradicate HIV-1 or to provide long-term remission through a continuous supply of anti-HIV-1 gene-modified cells without ongoing antiretroviral therapy. However, achieving sufficient engraftment levels of anti-HIV gene-modified HSPC to provide therapeutic efficacy has been a major limitation. Here, we report an in vivo selection strategy for anti-HIV-1 gene-modified HSPC by introducing 6-thioguanine (6TG) chemoresistance through knocking down hypoxanthine-guanine phosphoribosyl transferase (HPRT) expression using RNA interference (RNAi). We developed a lentiviral vector capable of co-expressing short hairpin RNA (shRNA) against HPRT alongside two anti-HIV-1 genes: shRNA targeting HIV-1 co-receptor CCR5 and a membrane-anchored HIV-1 fusion inhibitor, C46, for efficient in vivo selection of anti-HIV-1 gene-modified human HSPC. 6TG-mediated preconditioning and in vivo selection significantly enhanced engraftment of HPRT-knockdown anti-HIV-1 gene-modified cells (>2-fold, p < 0.0001) in humanized bone marrow/liver/thymus (huBLT) mice. Viral load was significantly reduced (>1 log fold, p < 0.001) in 6TG-treated HIV-1-infected huBLT mice compared to 6TG-untreated mice. We demonstrated that 6TG-mediated preconditioning and in vivo selection considerably improved engraftment of HPRT-knockdown anti-HIV-1 gene-modified HSPC and repopulation of anti-HIV-1 gene-modified hematopoietic cells in huBLT mice, allowing for efficient HIV-1 inhibition.

Therapeutic Efficacy of Prodrug Activator Gene Therapy Using Retroviral Replicating Vectors for Human Ovarian Cancer

BACKGROUND/AIM

Retroviral replicating vectors (RRV) have exhibited efficient tumor transduction and improved therapeutic benefits in a variety of cancer models. In this study, we validated two RRV created from amphotropic murine leukemia virus (AMLV) and gibbon ape leukemia virus (GALV), which use different cell receptors for virus entry, in human ovarian cancer (OC) cells.

MATERIALS AND METHODS

Expression levels of the receptors for AMLV (PiT-2) and GALV (PiT-1) in human OC cell lines (A2780, Caov3, RMG-1, SKOV-3), fibroblasts and HEK293 cells were evaluated using quantitative RT-PCR. In vitro RRV-GFP replication was monitored using flow cytometry, and cytotoxicity quantitated using AlamarBlue assay after 5-fluorocytosine treatment of OC cells transduced with RRV expressing the yeast cytosine deaminase prodrug activator gene. In vivo antitumor effect of RRV-mediated prodrug activator gene therapy was investigated in a SKOV-3 subcutaneous tumor model.

RESULTS

Quantitative RT-PCR analysis revealed high expression levels of PiT-2 (AMLV receptor) and PiT-1 (GALV receptor) in the RMG-1 and SKOV3 OC cell lines, compared with their levels in non-malignant cells. In RMG-1 and SKOV3 cells, both RRV showed highly efficient RRV replication and spread leading to over 90% transduction by Days 10-13. Additionally, both RRV that express the yeast cytosine deaminase gene demonstrated effective cell killing of RMG-1 and SKOV-3 cells upon treatment with the prodrug 5-fluorocytosine. Notably, RRV-mediated prodrug activator gene therapy showed significant inhibition of subcutaneous SKOV-3 tumor growth in nude mice.

CONCLUSION

RRV-mediated prodrug activator gene therapy may be used for treating PiT-expressing human OC.

Retroviral Replicating Vectors Mediated Prodrug Activator Gene Therapy in a Gastric Cancer Model

Retroviral replicating vectors (RRVs) selectively replicate and can specifically introduce prodrug-activating genes into tumor cells, whereby subsequent prodrug administration induces the death of the infected tumor cells. We assessed the ability of two distinct RRVs generated from amphotropic murine leukemia virus (AMLV) and gibbon ape leukemia virus (GALV), which infect cells via type-III sodium-dependent phosphate transporters, PiT-2 and PiT-1, respectively, to infect human gastric cancer (GC) cells. A quantitative RT-PCR showed that all tested GC cell lines had higher expression levels of PiT-2 than PiT-1. Accordingly, AMLV, encoding a green fluorescent protein gene, infected and replicated more efficiently than GALV in most GC cell lines, whereas both RRVs had a low infection rate in human fibroblasts. RRV encoding a cytosine deaminase prodrug activator gene, which converts the prodrug 5-flucytosine (5-FC) to the active drug 5-fluorouracil, showed that AMLV promoted superior 5-FC-induced cytotoxicity compared with GALV, which correlated with the viral receptor expression level and viral spread. In MKN-74 subcutaneous xenograft models, AMLV had significant antitumor effects compared with GALV. Furthermore, in the MKN-74 recurrent tumor model in which 5-FC was discontinued, the resumption of 5-FC administration reduced the tumor volume. Thus, RRV-mediated prodrug activator gene therapy might be beneficial for treating human GC.

CSIG-05. A GABP DOMINANT NEGATIVE APPROACH TO THE REVERSAL OF TUMOR IMMORTALITY

Immortality is a fundamental hallmark of human cancer cells and therapeutic reversal is of great interest. Telomerase Reverse Transcriptase promoter (TERTp) mutations reactivate TERT expression, the rate limiting step in telomerase activity and cellular immortality. TERTp mutations are the most common non-coding mutation across all cancer types, including glioblastoma (GBM), oligodendroglioma, medulloblastoma, and high-grade meningioma. While prior telomerase therapies lack tumor selectivity and are poorly tolerated, TERTp mutations and their regulation offer a unique opportunity for tumor specific reversal of cellular immortality. The mutations generate a de novo E26 Transformation Specific (ETS) binding motif that recruits the GA-Binding Protein (GABP) multimeric complex to reactivate TERT expression. Initially we found a GABP tetramer reactivates the mutant TERTp, suggesting targeting only the tetramer may induce telomere shortening and tumor cell death. However, through knocking out this tetramer in cell cultures, we found that the GABP dimer is upregulated and maintains TERT expression. In the absence of both the dimer and one tetramer isoform, many tumor cells senesce or undergo cell death. However, some tumor cells escape this fate via upregulation of a second GABP tetrameric paralogue that maintains TERT expression. Therefore, to block all GABP complexes with a single approach, we devised a transactivation domain null, putative dominant negative GABP subunit that ultimately could be delivered to patients via a replicating retroviral system. Introduction of the construct into TERTp mutant glioma cells significantly reduced TERT expression while not altering expression in TERTp wildtype GBM cells, suggesting tumor selectivity. TERT expression was also reduced in a second TERTp mutant tumor type. We are currently investigating the long-term effects of the dominant negative GABP on telomere length and tumor immortality. Dominant negative GABP is a promising candidate for tumor selective reversal of cellular immortality by inhibiting mutant TERTp activation by all GABP multimeric forms.

MEDB-59. A draft atlas of medulloblastoma cellular evolution under therapy

How standard care shapes the cellular composition of recurrent medulloblastoma (MB), if therapy selects for specific tumor or immune cell types, is unknown. We report the pilot phase of our ongoing effort to profile human longitudinal MB specimens via single-cell transcriptomics and epigenetics. We profiled 11 diagnostic and eight recurrent specimens from 19 subjects via single-nucleus RNA sequencing (snRNA-seq), and four subjects via single-nucleus assay for transposase-accessible chromatin. Specimens from select subjects were also profiled to assess genome-wide enhancer activity via single-nucleus cleavage-under-targets and tagmentation. We found an upregulation of the DNA-damage response, RNA translation, WNT and NOTCH signaling in recurrent specimens. The percentages of stem-like cells increased by over two-fold at recurrence. We found that microglia and oligodendrocyte-lineage cells were the most abundant non-malignant tumor-associated cell types, representing 2%-10% of cells profiled. Microglia abundances were relatively stable across molecular subtypes, and when comparing primary to recurrent tumors. There was a moderate, but statistically significant, increase in oligodendrocyte abundance in SSH and WNT tumors, compared to Group 3/4 tumors. We compared gene expression in tumor cells with public snRNA-seq from developing human cerebella (PCW 9-21). Combined Group-3/4 cell analysis supports a common lineage hierarchy, with an enrichment for unipolar brush-cell and Purkinje-cell phenotypes found in Group-4 tumors. All Group-3/4 cases contained cycling cells expressing markers of PAX2+ interneuron progenitors, most cycling cells had this phenotype. All specimens contained populations of non-cycling granule-cell progenitor-like cells. We performed single-cell co-expression receptor/ligand analysis to infer paracrine signaling between tumor and non-malignant cell types. This identified both tumor cells and microglia as sources of growth factors, pro-inflammatory cytokines, and pro-apoptotic ligands. Non-malignant oligodendrocyte-lineage cells uniquely expressed IL6-family cytokines, pleiotrophin, and class-III semaphorins. These studies shed light on the cellular heterogeneity of MB and the effect of standard therapy in shaping composition at recurrence.

EXTH-65. INSERTION OF MICRORNA TARGET SEQUENCES INTO RETROVIRAL REPLICATING VECTORS EFFECTIVELY RESTRICTS TRANSGENE EXPRESSION AND VIRAL REPLICATION IN HUMAN HEMATOPOIETIC STEM AND PROGENITOR CELLS

Prodrug activator gene therapy with a retroviral replicating vector (RRV) has shown a highly favorable safety profile and long-term survival in early-phase trials for recurrent high-grade glioma. Overall endpoints were not met in a recent Phase 3 trial, but highly statistically significant survival was observed in prespecified patient subgroups as compared to randomized matched control patients receiving standard-of-care treatments, and further clinical evaluation is being focused on these subgroups. Additional strategies to enhance therapeutic potency may require two or more RRVs to deliver multiple transgenes simultaneously, but RRVs encoated with the same envelope protein will compete for the same cellular receptors, interfering with efficient co-infection. The current clinical vector (formerly Toca511, now DB107) is encoated by amphotropic murine leukemia virus (MLV) envelope, which binds to inorganic phosphate transporter PiT-2/SLC20A2. To switch RRV tropism, we developed RRVs pseudotyped with a heterologous envelope from Gibbon ape leukemia virus (GALV), which utilizes an alternative phosphate transporter, PiT-1/SLC20A1, for cell entry. Efficient co-infection of established and primary human glioblastoma cells with MLV- and GALV-pseudotyped RRV was achieved, without receptor competition. However, human hematopoietic stem/progenitor cells (HSPC) also express high levels of PiT-1, which may increase potential genotoxicity of GALV-pseudotyped RRV. Accordingly, as a novel strategy to restrict gene expression and replication in HSPC, we have developed and tested new RRV designs incorporating microRNA target sequences (miRT). Insertion of miRT recognized by hematopoietic lineage-specific microRNA-142-3p resulted in complete suppression of RRV replication in primary human CD34+ HSPC, with the percentage of infected cells below 1% after vector inoculation at multiplicities of infection (MOI) 0.05 and 0.1, and remaining below 3% even after inoculation at MOI 0.5 and 1.0. In vivo models have been developed for on-going studies to evaluate miRT-mediated restriction strategies for avoidance of potential genotoxicity by RRV in normal hematopoietic cells.

EXTH-33. RETROVIRAL REPLICATING VECTORS PSEUDOTYPED WITH GIBBON APE LEUKEMIA VIRUS ENVELOPE FOR PRODRUG ACTIVATOR GENE THERAPY IN PRECLINICAL GLIOMA MODELS

Amphotropic retroviral replicating vector (RRV) Toca 511, expressing the yeast cytosine deaminase (CD) prodrug activator gene, showed promising evidence of therapeutic benefit and increased survival in early-phase trials for recurrent high-grade glioma. While a multi-center Phase 3 trial did not meet its overall endpoints, highly statistically significant survival was observed within predetermined patient subgroups compared to matched randomized control patients, and clinical investigation is on-going. Hence it is worthwhile to consider strategies aimed at enhancing therapeutic efficacy, such as delivering combinations of multiple transgenes. However, RRVs encoated with the same envelope compete for the same cancer cell surface receptors. We have now developed novel RRV encoated (‘pseudotyped’) with a heterologous envelope derived from Gibbon ape leukemia virus (GALV), which utilizes a different cell surface receptor from the native amphotropic retrovirus envelope for cellular entry. RRV(GALV) vectors expressing either GFP or HSV thymidine kinase (TK) were constructed, and efficient replication and transgene expression was observed in &gt; 90% of both established and primary human glioblastoma cells within 14 days after initial infection at 0.01 (1%) multiplicity of infection (MOI). Genomic stability of RRV(GALV) vectors was also confirmed over prolonged propagation. Established and primary human glioblastoma cells infected with RRV(GALV)-TK vector showed ≥ 50%-90% reduction in cell viability after exposure to Ganciclovir prodrug in the range of 1µM-100µM for 5 days, as compared to uninfected control cells or cells infected with RRV(GALV)-GFP control vector. Furthermore, dual infection with RRV(GALV)-TK and amphotropic RRV-CD (Toca 511) resulted in synergistic cytotoxicity upon simultaneous exposure to their respective prodrugs. Further data will be presented from on-going studies evaluating these vectors in intracerebral glioblastoma models. These results indicate that GALV envelope-pseudotyped RRV can efficiently deliver prodrug activator gene therapy in experimental glioma models, and open the door to combinatorial gene therapy regimens with this vector platform.

EXTH-13. LOCAL DELIVERY OF AN IL-15 SUPERAGONIST USING A REPLICATING RETROVIRUS SIGNIFICANTLY IMPROVES SURVIVAL AND LYMPHOCYTE INFILTRATION IN POORLY IMMUNOGENIC MURINE GLIOBLASTOMA MODELS

Glioblastoma (GBM) leads to severe systemic and local immunosuppression, and immunotherapies have had limited clinical success. Here, we evaluated the treatment efficacy of RLI, a superagonist of T-cell activator IL-15, delivered to tumor cells using a tumor-selective retroviral replicating vector (RRV) in the syngeneic murine SB28 and Tu2449 GBM models, which are both engineered to be poorly immunogenic with low-mutational burden and known resistance to immunotherapy, and hence more accurate biomimetic models of human GBM. RRV-RLI replicated and spread effectively in cultured murine GBM cells with robust production of functional RLI (165.4 ± 5.3 ng/mL). Stereotactic injection of RRV-RLI into pre-established intracerebral SB28 tumors significantly reduced tumor growth on bioluminescent imaging, and increased median survival compared to control mice (55 vs. 19 days, p=0.002), leading to long-term survival in 12% of treated mice. In the Tu2449 model, imaging results showed complete eradication of intracerebral tumors after RRV-RLI treatment, with long-term survival (median not reached) in &gt; 85% of treated mice, compared to a median survival of 12.5 days in control mice (p=0.001). RRV-RLI treated tumors showed significantly increased CD8 T-cell infiltration, without altering immunosuppressive cell populations. Similarly, broad anti-tumor inflammatory changes, including increased expression of genes involved in T-cell activation and killing, were observed in the NanoString nCounter platform using a 770-gene panel representing various immune cell types. Notably, RLI was not detected in the blood of treated mice, and tumor-localized RRV-RLI gene delivery showed no adverse systemic immune effects in either model. In summary, RRV-mediated RLI immunotherapy results in immunostimulatory and pro-inflammatory changes to the tumor microenvironment and achieves a significant survival benefit in two poorly immunogenic syngeneic murine models of GBM. This tumor-localized immunomodulatory gene therapy has the potential to safely reverse the T-cell depleted immunophenotype of GBM.

Clinical development of retroviral replicating vector Toca 511 for gene therapy of cancer

INTRODUCTION

The use of tumor-selectively replicating viruses is a rapidly expanding field that is showing considerable promise for cancer treatment. Retroviral replicating vectors (RRV) are unique among the various replication-competent viruses currently being investigated for potential clinical utility, because they permanently integrate into the cancer cell genome and are capable of long-term persistence within tumors. RRV can mediate efficient tumor-specific delivery of prodrug activator genes, and subsequent prodrug treatment leads to synchronized cell killing of infected cancer cells, as well as activation of antitumor immune responses.

AREAS COVERED

Here we review preclinical studies supporting bench-to-bedside translation of Toca 511, an optimized RRV for prodrug activator gene therapy, the results from Phase I through III clinical trials to date, and potential future directions for this therapy as well as other clinical candidate RRV.

EXPERT OPINION

Toca 511 has shown highly promising results in early-stage clinical trials. This vector progressed to a registrational Phase III trial, but the results announced in late 2019 appeared negative overall. However, the median prodrug dosing schedule was not optimal, and promising possible efficacy was observed in some prespecified subgroups. Further clinical investigation, as well as development of RRV with other transgene payloads, is merited.

Unique challenges for glioblastoma immunotherapy-discussions across neuro-oncology and non-neuro-oncology experts in cancer immunology. Meeting Report from the 2019 SNO Immuno-Oncology Think Tank

Cancer immunotherapy has made remarkable advances with over 50 separate Food and Drug Administration (FDA) approvals as first- or second-line indications since 2015. These include immune checkpoint blocking antibodies, chimeric antigen receptor-transduced T cells, and bispecific T-cell-engaging antibodies. While multiple cancer types now benefit from these immunotherapies, notable exceptions thus far include brain tumors, such as glioblastoma. As such, it seems critical to gain a better understanding of unique mechanistic challenges underlying the resistance of malignant gliomas to immunotherapy, as well as to acquire insights into the development of future strategies. An Immuno-Oncology Think Tank Meeting was held during the 2019 Annual Society for Neuro-Oncology Scientific Conference. Discussants in the fields of neuro-oncology, neurosurgery, neuro-imaging, medical oncology, and cancer immunology participated in the meeting. Sessions focused on topics such as the tumor microenvironment, myeloid cells, T-cell dysfunction, cellular engineering, and translational aspects that are critical and unique challenges inherent with primary brain tumors. In this review, we summarize the discussions and the key messages from the meeting, which may potentially serve as a basis for advancing the field of immune neuro-oncology in a collaborative manner.

THER-06. THERAPEUTIC EFFICACY OF RRV-MEDIATED PRODRUG ACTIVATOR GENE THERAPY IN CLINICAL TRIALS OF RECURRENT HIGH-GRADE GLIOMA AND IN MURINE ORTHOTOPIC MODELS OF INTRACEREBRAL GLIOMA AND INTRACEREBELLAR MEDULLOBLASTOMA

Toca 511, a clinical-stage tumor-selective retroviral replicating vector (RRV), encodes optimized yeast cytosine deaminase (CD), which converts the prodrug 5-fluorocytosine (5-FC) to the active drug 5-fluorouracil (5-FU) within infected cancer cells. In preclinical models of intracerebral glioblastoma, 5-FU generated locally by Toca 511 (RRV-CD) prodrug activator gene therapy has also been shown to kill immunosuppressive myeloid cells in the tumor microenvironment, leading to anti-cancer immune activation and long-term survival. Early-phase clinical trials of Toca 511 in recurrent high-grade glioma showed highly promising evidence of therapeutic benefit, leading to a Phase III trial completed in late 2019 (n=400 patients, randomized 1:1 vs. standard chemotherapy), which appeared to show negative results overall. However, additional analysis showed possible efficacy in prespecified subgroups, and further clinical investigation is being pursued. In preclinical studies, we have also evaluated RRV for use in medulloblastoma, the most common malignant tumor of the pediatric nervous system. Both established and primary human medulloblastoma cell lines supported efficient RRV replication in vitro, with spread to >90% of cells by day 10 post-inoculation, and RRV-CD-transduced medulloblastoma cells showed significant dose-dependent reduction of viability upon exposure to 5-FC, compared to controls. In an intracerebellar HDMB03 medulloblastoma model, RRV-CD-treated mice exhibited long-term survival while on sequential cycles of 5-FC prodrug, until prodrug treatment was stopped, after which 25% long-term survival was observed (median survival 110 days) as compared to controls (median survival 28 days, 100% lethality) (p=0.00007). These results support further evaluation of RRV-mediated prodrug activator gene therapy for pediatric brain tumors.

EXTH-17. LOCAL DELIVERY OF CYTOKINES AND SYNTHETIC IMMUNOMODULATORS INCREASES T CELL INFILTRATION AND SIGNIFICANTLY IMPROVES SURVIVAL IN A POORLY IMMUNOGENIC MODEL OF GLIOBLASTOMA

BACKGROUND

Severe local and systemic immune suppression in glioblastoma (GBM) contributes to the failure of single-agent immunotherapies in clinical trials. In this study, we evaluated the efficacy of locally delivered combination immunotherapy in a poorly immunogenic murine GBM model.

METHODS

Immunomodulators used in these studies included: IL-15 and IL-7 (T cell activation), LIGHT (T cell tumor infiltration), FLT3L (dendritic cell maturation/proliferation), a surface T cell engager (T cell killing of tumor cells), and a bispecific PD-L1/T cell engager (T cell killing targeted to PD-L1-expressing cells). We first assessed T cell-mediated cytotoxicity in vitro against SB28, a poorly immunogenic murine GBM cell line, after expressing these immunomodulators in combination. Next, tumor growth inhibition in vivo was evaluated in syngeneic C57BL/6 mice, initially by establishment of intracranial tumors with pre-transduced SB28 cells, and subsequently by delivering these immunomodulators to pre-established naïve SB28 tumors using neural stem cells (NSCs) and retroviral replicating vectors (RRV).

RESULTS

SB28 cells transduced with immunomodulators activated dose-dependent T cell-mediated cytotoxicity in vitro. Mice with pre-transduced intracranial SB28 gliomas showed significantly longer survival (minimum survival: 60 days, long-term survival in 57% of mice) vs. control mice (median survival: 20 days) (p&lt; 0.001), and significantly increased tumor infiltration of CD4+ and CD8+ T cells. NSC- and RRV-mediated immunomodulator delivery to pre-established SB28 gliomas also resulted in significantly increased survival of treated mice vs. controls (median survival: 31 days vs. 22 days, p&lt; 0.001). Immunomodulator-treated tumors again showed significantly increased infiltration of CD4+ and CD8+ T cells, along with decreased CD11b+ cell infiltration.

CONCLUSIONS

A novel combination therapy for GBM immunotherapy activated T cell killing of SB28 GBM cells in vitro and achieved a significant survival benefit in vivo, associated with anti-tumor alterations to the GBM tumor microenvironment. Further studies to optimize the efficiency of combinatorial immunomodulator delivery are currently underway.

EXTH-44. DEVELOPMENT OF RETROVIRAL REPLICATING VECTORS ENCODING SEQUENCE-OPTIMIZED NITROREDUCTASE FOR PRODRUG ACTIVATOR GENE THERAPY IN HUMAN GLIOMA MODELS

Early-phase clinical trials of Toca 511, a retroviral replicating vector (RRV) expressing an optimized form of yeast cytosine deaminase for prodrug activator gene therapy, showed highly promising evidence of therapeutic benefit and increased survival in recurrent high-grade glioma, but an international Phase 3 trial failed to meet primary and secondary endpoints. Nonetheless, promising results observed within predetermined subgroups, combined with the highly favorable safety profile of this approach, make it worthwhile to consider alternative prodrug-activating enzymes in order to increase therapeutic efficacy. In this study, we employed RRV to deliver modified versions of another prodrug activator gene, E.coli nitroreductase (NTR), which activates the prodrug CB1954 to generate a potent bifunctional alkylating agent. We constructed RRV encoding two different isoforms of wild-type E.coli NTR genes (NfsA, NfsB) as well as variants of both isoforms extensively modified to optimize human codon usage and vector stability. Further efforts to improve the potency of these prodrug activator enzymes included the construction of additional new vectors incorporating point mutations at the active site which have previously been demonstrated to improve prodrug conversion efficiency. NTR transgene insertion did not affect RRV replication, which resulted in increasing NTR expression over time in human glioma cells in culture. Sequence optimization resulted in enhanced genomic stability of the vectors upon serial passage and also in significantly reduced cell viability in vitro on treatment of fully transduced U87 and U87-EGFRviii glioma cells with CB1954 prodrug. In orthotopic U87-EGFRviii intracerebral tumor models, stereotactic intratumoral injection of sequence-optimized RRV-NTR and repeated cycles of prodrug treatment resulted in rapid and significant reduction of tumor burden as monitored by bioluminescence imaging, leading to prolonged survival benefit. These data indicate that improved therapeutic efficacy can be achieved with optimized NTR prodrug activator gene therapy delivered by RRV in experimental glioma models.

Introduction to immunotherapy for brain tumor patients: challenges and future perspectives

Malignant gliomas, including glioblastoma (GBM) as the most aggressive type of adult CNS tumors, are notoriously resistant to current standard of care treatments, including surgery, systemic chemotherapy, and radiation therapy (RT). This lack of effective treatment options highlights the urgent need for novel therapies, including immunotherapies. The overarching goal of immunotherapy is to stimulate and activate the patient's immune system in a targeted manner to kill tumor cells. The success of immunotherapeutic interventions in other cancer types has led to interest in and evaluation of various experimental immunotherapies in patients with malignant gliomas. However, these primary malignant brain tumors present a challenge because they exist in a vital and sensitive organ with a unique immune environment. The challenges and current status of experimental immunotherapeutic approaches, including vaccines, immune-checkpoint blockade, chimeric antigen receptor T-cell therapy, and oncolytic viruses will be discussed, as well as the potential for combinatorial therapies.

Efficient Prodrug Activator Gene Therapy by Retroviral Replicating Vectors Prolongs Survival in an Immune-Competent Intracerebral Glioma Model

Prodrug activator gene therapy mediated by murine leukemia virus (MLV)-based retroviral replicating vectors (RRV) was previously shown to be highly effective in killing glioma cells both in culture and in vivo. To avoid receptor interference and enable dual vector co-infection with MLV-RRV, we have developed another RRV based on gibbon ape leukemia virus (GALV) that also shows robust replicative spread in a wide variety of tumor cells. We evaluated the potential of GALV-based RRV as a cancer therapeutic agent by incorporating yeast cytosine deaminase (CD) and E. coli nitroreductase (NTR) prodrug activator genes into the vector. The expression of CD and NTR genes from GALV-RRV achieved highly efficient delivery of these prodrug activator genes to RG-2 glioma cells, resulting in enhanced cytotoxicity after administering their respective prodrugs 5-fluorocytosine and CB1954 in vitro. In an immune-competent intracerebral RG-2 glioma model, GALV-mediated CD and NTR gene therapy both significantly suppressed tumor growth with CB1954 administration after a single injection of vector supernatant. However, NTR showed greater potency than CD, with control animals receiving GALV-NTR vector alone (i.e., without CB1954 prodrug) showing extensive tumor growth with a median survival time of 17.5 days, while animals receiving GALV-NTR and CB1954 showed significantly prolonged survival with a median survival time of 30 days. In conclusion, GALV-RRV enabled high-efficiency gene transfer and persistent expression of NTR, resulting in efficient cell killing, suppression of tumor growth, and prolonged survival upon CB1954 administration. This validates the use of therapeutic strategies employing this prodrug activator gene to arm GALV-RRV, and opens the door to the possibility of future combination gene therapy with CD-armed MLV-RRV, as the latter vector is currently being evaluated in clinical trials.

STEM-24. THERAPEUTIC APPROACH OF STEM CELL CARRYING RETROVIRAL REPLICATING VECTORS IN HUMAN GLIOMA MODEL

Toca 511, an improved retroviral replicating vectors (RRVs) expressing a codon-optimized cytosine deaminase, has shown highly promising evidence of therapeutic benefit in preclinical and clinical studies for gene therapy of glioma. In the present study, we engineered human mesenchymal stem cells (MSC) as tumor-homing cellular carriers that produce and release RRV, and evaluated the effect of this mode of virus delivery on the time course of intratumoral RRV dissemination. Human MSC isolates were engineered to produce RRV (MSC-RRV). Cytotoxicity assays confirmed efficient prodrug activator function in U-87 glioma cells transduced with vectors produced from MSC-RRV. To evaluate intratumoral migration activity and tumor-homing migration activity in vivo, individual MSC isolates were injected either directly into human glioma xenografts, or into the contralateral brain hemisphere. MSC-RRV isolates showing the highest levels of intratumoral migration activity were selected, and the efficiency of intratumoral dissemination and tumoricidal activity achieved by these isolates was compared against injection of RRV virus preparations in subcutaneous glioma models. Compared to virus injection, MSC-RRV achieved 1.6x-higher levels of tumor transduction (p< 0.05), and 2x-reduced tumor growth (p=0.018) at earlier time points. In short-term survival studies using lower doses of MSC-RRV cells vs. RRV virus injected 14 days post-establishment of intracranial U-87 gliomas, a small but statistically significant prolongation of median survival was seen with intracranial tumors treated with MSC-RRV as compared to RRV (26 vs. 20 days, p< 0.05) after only a single cycle of 5-FC prodrug. Thus, MSC can be employed as mobile tumor-homing RRV-producer cells, which release RRV as they migrate to tumor foci in vivo and actively penetrate into individual tumor masses, resulting in more rapid tumor transduction and earlier therapeutic efficacy, which may be advantageous particularly for multi-focal and metastatic disease. This study was funded by the California Institute for Regenerative Medicine (TR2-01791).

Durable complete responses in some recurrent high-grade glioma patients treated with Toca 511 + Toca FC

Background

Vocimagene amiretrorepvec (Toca 511) is an investigational gamma-retroviral replicating vector encoding cytosine deaminase that, when used in combination with extended-release 5-fluorocytosine (Toca FC), results preclinically in local production of 5-fluorouracil, depletion of immune-suppressive myeloid cells, and subsequent induction of antitumor immunity. Recurrent high-grade glioma (rHGG) patients have a high unmet need for effective therapies that produce durable responses lasting more than 6 months. In this setting, relapse is nearly universal and most responses are transient.

Methods

In this Toca 511 ascending-dose phase I trial (NCT01470794), HGG patients who recurred after standard of care underwent surgical resection and received Toca 511 injected into the resection cavity wall, followed by orally administered cycles of Toca FC.

Results

Among 56 patients, durable complete responses were observed. A subgroup was identified based on Toca 511 dose and entry requirements for the follow-up phase III study. In this subgroup, which included both isocitrate dehydrogenase 1 (IDH1) mutant and wild-type tumors, the durable response rate is 21.7%. Median duration of follow-up for responders is 35.7+ months. As of August 25, 2017, all responders remain in response and are alive 33.9+ to 52.2+ months after Toca 511 administration, suggesting a positive association of durable response with overall survival.

Conclusions

Multiyear durable responses have been observed in rHGG patients treated with Toca 511 + Toca FC in a phase I trial, and the treatment will be further evaluated in a randomized phase III trial. Among IDH1 mutant patients treated at first recurrence, there may be an enrichment of complete responders.

A CK1α Activator Penetrates the Brain and Shows Efficacy Against Drug-resistant Metastatic Medulloblastoma

PURPOSE

Although most children with medulloblastoma are cured of their disease, Sonic Hedgehog (SHH) subgroup medulloblastoma driven by TRP53 mutations is essentially lethal. Casein kinase 1α (CK1α) phosphorylates and destabilizes GLI transcription factors, thereby inhibiting the key effectors of SHH signaling. We therefore tested a second-generation CK1α activator against TRP53-mutant, MYCN-amplified medulloblastoma.

EXPERIMENTAL DESIGN

The ability of this CK1α activator to block SHH signaling was determined in vitro using GLI reporter cells, granular precursor primary cultures, and PATCHED1 (PTCH1)-mutant sphere cultures. While in vivo efficacy was tested using 2 different medulloblastoma mouse models: PTCH1 and ND2:SMOA1. Finally, the clinical relevance of CK1α activators was demonstrated using a TRP53-mutant, MYCN-amplified patient-derived xenograft.

RESULTS

SSTC3 inhibited SHH activity in vitro, acting downstream of the vismodegib target SMOOTHENED (SMO), and reduced the viability of sphere cultures derived from SHH medulloblastoma. SSTC3 accumulated in the brain, inhibited growth of SHH medulloblastoma tumors, and blocked metastases in a genetically engineered vismodegib-resistant mouse model of SHH medulloblastoma. Importantly, SSTC3 attenuated growth and metastasis of orthotopic patient-derived TRP53-mutant, MYCN-amplified, SHH subgroup medulloblastoma xenografts, increasing overall survival.

CONCLUSIONS

Using a newly described small-molecule, SSTC3, we show that CK1a activators could address a significant unmet clinical need for patients with SMO inhibitor-resistant medulloblastoma, including those harboring mutations in TRP53.

Efficient tumor transduction and antitumor efficacy in experimental human osteosarcoma using retroviral replicating vectors

Retroviral replicating vectors (RRVs) have achieved efficient tumor transduction and enhanced therapeutic benefit in a wide variety of cancer models. Here, we evaluated two different RRVs derived from amphotropic murine leukemia virus (AMLV) and gibbon ape leukemia virus (GALV), which utilize different cellular receptors (PiT-2 and PiT-1, respectively) for viral entry, in human osteosarcoma cells. Quantitative RT-PCR showed that low levels of expression of both receptors were observed in normal and non-malignant cells. However, high PiT-2 (for AMLV) and low PiT-1 (for GALV) expression was observed in most osteosarcoma cell lines. Accordingly, AMLV expressing the green fluorescent protein gene infected and replicated more efficiently than GALV in most osteosarcoma cell lines. Furthermore, RRVs expressing the cytosine deaminase prodrug activator gene showed differential cytotoxicity that correlated with the results of viral spread. AMLV-RRV-mediated prodrug activator gene therapy achieved significant inhibition of subcutaneous MG-63 tumor growth over GALV in nude mice. These data indicate that AMLV vectors predominate over GALV in human osteosarcoma cells. Moreover, our findings support the potential utility of the two RRVs in personalized cancer virotherapy on the basis of receptor expression.

Abstract 5927: Assessing the efficacy of a novel gene therapy approach for treating ovarian cancer: Combination RRV-mediated prodrug activated suicide gene therapy

Introduction/Aim: Retroviral replicating vectors (RRVs) afford a unique strategy for gene therapy targeting cancer cells. By “activating” RRVs with prodrug activator suicide genes, these RRVs become strong gene delivery vehicles that replicate selectively in cancer cells, and stably persist through proviral genomic integration, leading to inducible cell death upon prodrug administration. This strategy has shown highly promising clinical results in early phase trials (published), and is currently being evaluated in an international Phase III clinical trial for patients with recurrent high-grade glioma. Recently, we have also applied RRV-mediated gene therapy to preclinical models of ovarian cancer. The aim of this current study was to evaluate feasibility and efficacy of combination suicide gene therapy using two RRVs encoated (‘pseudotyped') with different envelope proteins and delivering different prodrug activators for the treatment of ovarian cancer.

Methods: Viral spread was monitored in established and primary patient-derived ovarian cancer cell lines (SKOV3, OCI-P5X, OCI-C5X) over the course of ≥27 days using amphotropic murine leukemia virus (AMLV) envelope- and gibbon ape leukemia virus (GALV) envelope-pseudotyped RRVs delivering either fluorescent protein reporter genes (GFP, mStrawberry) or prodrug activator genes (yeast cytosine deaminase, E. coli nitroreductase), either individually or in combination, at various multiplicities of infection (MOI). Flow cytometry for GFP, mStrawberry, and Gag viral protein was used to determine transduction levels and transgene expression levels of RRVs at serial time points. Each therapeutic RRV was tested by MTS assay for its ability to induce cytotoxicity upon exposure to different concentrations prodrug. To determine viral integration and vector stability, PCR of genomic DNA extracted from infected cells was performed using RRV specific primers, and qRT-PCR was performed to determine vector copy number per cell.

Results: All ovarian cancer cell lines tested could be &gt;80% transduced by AMLV- and GALV-pseudotyped RRVs, both individually and in combination, within 6-24 days even after initial inoculation at MOI 0.001, with replication kinetics varying between different cell lines. Both RRVs could stably integrate into the target cell genome without evidence of transgene deletion. MTS assays demonstrated that prodrug activated cytotoxicity led to efficient killing of RRV-transduced ovarian cancer cells in vitro, with synergistic cytotoxicity observed at lower prodrug concentrations after combined treatment.

Conclusion: RRV-mediated prodrug activator gene therapy may be an effective approach for efficiently and selectively inducing cytotoxicity in ovarian cancer cells. These data support further evaluation of this strategy through translational studies in vivo.

Citation Format: James Grosso, Sara Collins, Aki Inagaki, Brian Slomovitz, Noriyuki Kasahara. Assessing the efficacy of a novel gene therapy approach for treating ovarian cancer: Combination RRV-mediated prodrug activated suicide gene therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5927.

Retroviral replicating vector-mediated gene therapy achieves long-term control of tumor recurrence and leads to durable anticancer immunity

Background

Prodrug-activator gene therapy with Toca 511, a tumor-selective retroviral replicating vector (RRV) encoding yeast cytosine deaminase, is being evaluated in recurrent high-grade glioma patients. Nonlytic retroviral infection leads to permanent integration of RRV into the cancer cell genome, converting infected cancer cell and progeny into stable vector producer cells, enabling ongoing transduction and viral persistence within tumors. Cytosine deaminase in infected tumor cells converts the antifungal prodrug 5-fluorocytosine into the anticancer drug 5-fluorouracil, mediating local tumor destruction without significant systemic adverse effects.

Methods

Here we investigated mechanisms underlying the therapeutic efficacy of this approach in orthotopic brain tumor models, employing both human glioma xenografts in immunodeficient hosts and syngeneic murine gliomas in immunocompetent hosts.

Results

In both models, a single injection of replicating vector followed by prodrug administration achieved long-term survival benefit. In the immunodeficient model, tumors recurred repeatedly, but bioluminescence imaging of tumors enabled tailored scheduling of multicycle prodrug administration, continued control of disease burden, and long-term survival. In the immunocompetent model, complete loss of tumor signal was observed after only 1-2 cycles of prodrug, followed by long-term survival without recurrence for >300 days despite discontinuation of prodrug. Long-term survivors rejected challenge with uninfected glioma cells, indicating immunological responses against native tumor antigens, and immune cell depletion showed a critical role for CD4+ T cells.

Conclusion

These results support dual mechanisms of action contributing to the efficacy of RRV-mediated prodrug-activator gene therapy: long-term tumor control by prodrug conversion-mediated cytoreduction, and induction of antitumor immunity.

Toca 511 gene transfer and treatment with the prodrug, 5-fluorocytosine, promotes durable antitumor immunity in a mouse glioma model

Background.

Toca 511 (vocimagene amiretrorepvec) is a retroviral replicating vector encoding an optimized yeast cytosine deaminase (CD). Tumor-selective expression of CD converts the prodrug, 5-fluorocytosine (5-FC), into the active chemotherapeutic, 5-fluorouracil (5-FU). This therapeutic approach is being tested in a randomized phase II/III trial in recurrent glioblastoma and anaplastic astrocytoma (NCT0241416). The aim of this study was to identify the immune cell subsets contributing to antitumor immune responses following treatment with 5-FC in Toca 511–expressing gliomas in a syngeneic mouse model.

Methods.

Flow cytometry was utilized to monitor and characterize the immune cell infiltrate in subcutaneous Tu-2449 gliomas in B6C3F1 mice treated with Toca 511 and 5-FC.

Results.

Tumor-bearing animals treated with Toca 511 and 5-FC display alterations in immune cell populations within the tumor that result in antitumor immune protection. Attenuated immune subsets were exclusive to immunosuppressive cells of myeloid origin. Depletion of immunosuppressive cells temporally preceded a second event which included expansion of T cells which were polarized away from Th2 and Th17 in the CD4+ T cell compartment with concomitant expansion of interferon gamma–expressing CD8+ T cells. Immune alterations correlated with clearance of Tu-2449 subcutaneous tumors and T cell–dependent protection from future tumor challenge.

Conclusions.

Treatment with Toca 511 and 5-FC has a concentrated effect at the site of the tumor which causes direct tumor cell death and alterations in immune cell infiltrate, resulting in a tumor microenvironment that is more permissive to establishment of a T cell mediated antitumor immune response.

Pathogens and glioma: a history of unexpected discoveries ushering in novel therapy

In the late 19th century, Dr. William B. Coley introduced the theory that infections may aid in the treatment of malignancy. With the creation of Coley’s toxin, reports of remission during viral illnesses for systemic malignancies soon emerged. A few decades after this initial discovery, Austrian physicians performed intravascular injections of Clostridium to induce oncolysis in patients with glioblastoma. Since then, suggestions between improved survival and infectious processes have been reported in several patients with glioma, which ultimately marshaled the infamous use of intracerebral Enterobacter. These early observations of tumor regression and concomitant infection piloted a burgeoning field focusing on the use of pathogens in molecular oncology.

A Retroviral Replicating Vector Encoding Cytosine Deaminase and 5-FC Induces Immune Memory in Metastatic Colorectal Cancer Models

Treatment of tumors with Toca 511, a gamma retroviral replicating vector encoding cytosine deaminase, followed by 5-fluorocytosine (5-FC) kills tumors by local production of 5-fluorouracil (5-FU). In brain tumor models, this treatment induces systemic anti-tumor immune responses and long-term immune-mediated survival. Phase 1 Toca 511 and Toca FC (extended-release 5-FC) clinical trials in patients with recurrent high-grade glioma show durable complete responses and promising survival data compared to historic controls. The work described herein served to expand on our earlier findings in two models of metastatic colorectal carcinoma (mCRC). Intravenous (i.v.) delivery of Toca 511 resulted in substantial tumor-selective uptake of vector into metastatic lesions. Subsequent treatment with 5-FC resulted in tumor shrinkage, improved survival, and immune memory against future rechallenge with the same CT26 CRC cell line. Similar results were seen in a brain metastasis model of mCRC. Of note, 5-FC treatment resulted in a significant decrease in myeloid-derived suppressor cells (MDSCs) in mCRC tumors in both the liver and brain. These results support the development of Toca 511 and Toca FC as a novel immunotherapeutic approach for patients with mCRC. A phase 1 study of i.v. Toca 511 and Toca FC in solid tumors, including mCRC, is currently underway (NCT02576665).

Phase 1 trial of vocimagene amiretrorepvec and 5-fluorocytosine for recurrent high-grade glioma

Toca 511 (vocimagene amiretrorepvec) is an investigational nonlytic, retroviral replicating vector (RRV) that delivers a yeast cytosine deaminase, which converts subsequently administered courses of the investigational prodrug Toca FC (extended-release 5-fluorocytosine) into the antimetabolite 5-fluorouracil. Forty-five subjects with recurrent or progressive high-grade glioma were treated. The end points of this phase 1, open-label, ascending dose, multicenter trial included safety, efficacy, and molecular profiling; survival was compared to a matching subgroup from an external control. Overall survival for recurrent high-grade glioma was 13.6 months (95% confidence interval, 10.8 to 20.0) and was statistically improved relative to an external control (hazard ratio, 0.45; P = 0.003). Tumor samples from subjects surviving more than 52 weeks after Toca 511 delivery disproportionately displayed a survival-related mRNA expression signature, identifying a potential molecular signature that may correlate with treatment-related survival rather than being prognostic. Toca 511 and Toca FC show excellent tolerability, with RRV persisting in the tumor and RRV control systemically. The favorable assessment of Toca 511 and Toca FC supports confirmation in a randomized phase 2/3 trial (NCT02414165).

Epithelial membrane protein-2 (EMP2) promotes angiogenesis in glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most aggressive malignant brain tumor and is associated with an extremely poor clinical prognosis. One pathologic hallmark of GBM is excessive vascularization with abnormal blood vessels. Extensive investigation of anti-angiogenic therapy as a treatment for recurrent GBM has been performed. Bevacizumab, a monoclonal anti-vascular endothelial growth factor A (VEGF-A), suggests a progression-free survival benefit but no overall survival benefit. Developing novel anti-angiogenic therapies are urgently needed in controlling GBM growth. In this study, we demonstrate tumor expression of epithelial membrane protein-2 (EMP2) promotes angiogenesis both in vitro and in vivo using cell lines from human GBM. Mechanistically, this pro-angiogenic effect of EMP2 was partially through upregulating tumor VEGF-A levels. A potential therapeutic effect of a systemic administration of anti-EMP2 IgG1 on intracranial xenografts was observed resulting in both significant reduction of tumor load and decreased tumor vasculature. These results suggest the potential for anti-EMP2 IgG1 as a promising novel anti-angiogenic therapy for GBM. Further investigation is needed to fully understand the molecular mechanisms how EMP2 modulates GBM pathogenesis and progression and to further characterize anti-EMP2 therapy in GBM.

A murine model of suicide gene therapy in the brainstem demonstrates efficacy without toxic inflammation, suggesting possible use as a treatment for diffuse intrinsic pontine glioma (DIPG)

Diffuse intrinsic pontine glioma (DIPG) is a rare pediatric brainstem tumor that is the leading cause of death among pediatric brain malignancies. Despite over 200 clinical trials, no significant progress has been made, leading our lab to consider suicide gene therapy. The brainstem has particularly sensitive anatomy, creating the potential for toxic inflammation. Furthermore, the clinical standard of care for DIPG includes conventional radiation therapy (RT) followed by dexamethasone (DEX), each of which can alter the efficacy of gene therapy. In this study we found that a variety of gliomas, including human DIPG and murine glioma cell lines, are infectable in vitro with a replication-competent Moloney murine leukemia virus (MoMLV) (currently in Phase 2/3 clinical trials) and susceptible to ganciclovir (GCV)-mediated killing via the HSV thymidine kinase (tk) suicide gene. A B16 melanoma cell line expressing HSV tk implanted into the brainstem of mice, extended median survival with GCV therapy from 13 days to 38.5 days (p=.0006) with no signs of toxic inflammation. Pre-treatment with RT and DEX did not affect therapeutic efficacy. We have shown that GCV/TK therapy is mediated by NK and CD8 cells in subcutaneous tumors, and we are investigating whether a similar immunologic mechanism is involved in this brainstem model. These preclinical data will pave the way for novel clinical trials for the treatment of DIPG.

Abstract B03: Development of retroviral replicating vectors expressing codon-optimized nitroreductase for prodrug activator gene therapy in human glioma models

Our studies to date have demonstrated dramatic survival benefit when tumor-selective retroviral replicating vectors (RRV) are employed for gene therapy in a variety of preclinical cancer models. RRV-mediated prodrug activator gene therapy using yeast cytosine deaminase (RRV-CD; “Toca 511”) has been evaluated in multi-center Phase I ascending dose trials in patients with recurrent high grade glioma (http://www. clinicaltrials.gov: NCT01156584, NCT01985256, NCT01470794), and based on highly promising evidence of therapeutic benefit, a registrational Phase IIB/III trial has recently been initiated at multiple sites in the United States and Canada (NCT02414165).

Translational development of further RRV-based therapeutic agents is also on-going, and we have now developed an RRV encoding E.coli nitroreductase (NTR), a prodrug activator enzyme which converts CB1954 to a potent bifunctional alkylating agent. We constructed RRV encoding wild-type E.coli NTR genes (RRV-NfsA, RRV-NfsB) as well as NTR variants extensively modified to optimize human codon usage and vector stability (RRV-NAO, RRV-NBO). NTR transgene insertion did not affect vector replication, which resulted in increasing NTR expression over time in U87 human glioma cultures for all vectors, but sequence optimization significantly increased genomic stability of the RRV-NAO and RRV-NBO vectors over serial passage. U87 cells fully transduced with the optimized vectors showed higher levels of NTR protein and increased levels of enzymatic activity compared with cells transduced with wild-type vectors. In vitro cytotoxicity was examined by MTS assay after CB1954 treatment of fully transduced U87 cells. Viability was reduced by &gt;80% within 48 hrs in cells transduced with RRV-NAO, which showed the most potent cell killing efficiency and bystander effect among all vectors tested. Significant reduction in luminescence and inhibition of tumor growth was observed in subcutaneous U87-FLuc2 tumors initiated with 2% RRV-NAO transduction followed by intraperitoneal administration of CB1954. In intracerebral U87-FLuc2 orthotopic tumor models, stereotactic intratumoral injection of RRV-NAO and repeated cycles of prodrug treatment also resulted in significant luminescence reduction, and achieved prolonged survival benefit.

These results indicate that we have been successful in developing an improved prodrug activator gene with therapeutic efficacy when delivered by RRV in experimental models of human glioma. Further studies are aimed at translational development of improved RRV-NTR vectors for future clinical use both as a stand-alone therapy and in combination with RRV-CD (Toca 511).

Citation Format: Sara Collins, Akihito Inagaki, Mark Tangney, Noriyuki Kasahara. Development of retroviral replicating vectors expressing codon-optimized nitroreductase for prodrug activator gene therapy in human glioma models. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr B03.

Abstract B36: Compassionate use of an oncolytic adenovirus in a Stage IV treatment-refractory ovarian cancer patient

Ovarian cancer remains the most common cause of death due to gynecologic cancers, and due to its ambiguous symptomatology, diagnosis often occurs at late stages, and prognosis is poor. This case report describes the compassionate use of an oncolytic adenovirus, ONCOS-102, in a 46-year-old female with Stage IV treatment-refractory serous ovarian cancer presenting with metastases to the groin and left breast, peritoneal carcinomatosis, and pleural carcinomatosis. The patient was initially diagnosed in 2007 with a low-grade serous adenocarcinoma of the ovary (pT3, pN1, pMx, G1), c-kit-positive, ER-positive, HER2/EGFR-negative, BRAF -negative and KRAS mutation-negative. Since then, the patient had received multimodal chemotherapy followed by multiple courses of maintenance therapy, with treatments including Bevacizumab (Avastin), Diflunisal/ASA, PAS/ASA, Abraxane, Tamoxifen, Alimta, Gemcitabine, and Topotecan. The patient had also received abandoned radiotherapy and dendritic cell vaccine immunization. Oncos Therapeutics provided ONCOS-102 for compassionate use, based on the rationale that there was no satisfactory alternative treatment, and she was unable to obtain the drug under another IND or clinical trial protocol. ONCOS-102 (Ad5/3-D24-GMCSF) is an oncolytic adenovirus that, due to a 24-bp deletion in its E1A gene, selectively replicates in cancer cells with a defective Rb pathway, and its Ad5 fiber knob protein is replaced with that of Ad3 to facilitate entry into tumor cells. In addition, the virus is armed with a transgene encoding granulocyte/macrophage colony-stimulating factor (GM-CSF). Thus, ONCOS-102 is hypothesized to mediate therapeutic benefit through multiple mechanisms, including direct oncolysis of tumor cells, triggering necrosis/apoptosis via vascular disruption, and activation of innate and adaptive immunity, leading to the induction of a systemic anti-tumor response. Thus, oncolytic virotherapy can be viewed as a form of in situ vaccination, and to further potentiate its effects, our single-patient compassionate use protocol further incorporated a low priming dose of cyclophosphamide (250 mg/m2 IV bolus) for Treg depletion, and a dual checkpoint inhibitor treatment regimen with ipilimumab (3 mg/kg) and nivolumab (1 mg/kg). Approval for the use of this protocol in this patient was obtained from the FDA, as well as the University of Miami's local Institutional Review Board (IRB) and Institutional Biosafety Committee (IBC). The patient passed pre-treatment evaluations and informed consent was obtained for administration of the virus, which was performed following the cyclophosphamide “priming” regimen at a virus dose of 3 x 10^11 VP by intralesional injection into accessible tumors. No product-related adverse events have been observed, and further clinical results will be presented.

Citation Format: Katharina Feister, Sara Collins, Noriyuki Kasahara, Brian Slomovitz. Compassionate use of an oncolytic adenovirus in a Stage IV treatment-refractory ovarian cancer patient. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr B36.

Abstract A43: Therapeutic efficacy of retroviral replicating vectors in orthotopic models of breast cancer metastasis to the brain

Our studies to date have demonstrated that retroviral replicating vectors (RRV) are capable of highly efficient replication in cancer cells in vitro and in vivo, associated with high levels of tumor-selective gene transfer and significantly enhanced survival benefit when employed for prodrug activator (“suicide”) gene therapy in a variety of cancer models. Clinical testing of RRV-mediated gene therapy using the yeast cytosine deaminase (CD) prodrug activator gene, which mediates conversion of the prodrug 5-fluorocytosine (5-FC) to the anti-cancer drug, 5-fluorouracil (5-FU), is currently underway in multi-center clinical trials being conducted in the United States for patients with recurrent high-grade glioma. In the present study, our goal has been to establish the feasibility of also applying this novel gene therapy strategy to secondary brain tumors arising from metastasis of systemic cancers to the CNS, which occur 5-10 times more frequently than primary brain tumors. Moreover, brain metastases are often accompanied by neurological deficits, which deprive patients of their quality of life. In particular, brain-metastatic breast cancer frequently arises from highly aggressive, treatment-refractory, ‘triple-negative' (ER(-), PR(-), HER2(-)) cells, is associated with a dismal prognosis of 4-6 months, and as treatment options are largely palliative, is the main cause of death in half of these patients. Hence this condition represents an unmet medical need.

We first examined the replication kinetics of RRV expressing the GFP reporter gene (AC3-emd) in human and murine breast cancer cell lines by flow cytometry. The replication kinetics of AC3-emd in the triple-negative human breast cancer cell line MDA-MB-231-BR, and in the murine breast cancer cell line JC, showed robust replication activity over time at both MOI = 0.01 and 0.1, resulting in high levels of transduction within 1-2 weeks. Next, we tested in vitro cytotoxicity by MTS assay after 5-FC treatment of 231BR and JC cells that had been fully transduced with RRV expressing the CD prodrug activator gene (clinical-grade vector AC3-yCD2, also designated “Toca 511”). In both of the AC3-yCD2-transduced cell lines, cell viability was reduced approximately 70-85% even after exposure to 0.1 mM 5-FC treatment, and complete cell killing was observed after 6 days (231-BR) or 4 days (JC) exposure to 1 mM 5-FC treatment. We then examined in vivo replication kinetics of AC3-emd in intracranial 231-BR xenograft models. Intratumoral injection of 10e6 AC3-emd on Day 7 after tumor implantation was confirmed to achieve &gt;80% transduction efficiency by flow cytometry on Day 10 post-injection. In survival studies, animals treated with AC3-yCD2+5-FC prodrug activator gene therapy showed statistically significant (p&lt;0.0001) survival benefit compared to the AC3-yCD2+PBS control group. These data provide preclinical validation for a new Phase I dose escalation trial to evaluate RRV-mediated prodrug activator gene therapy in CNS-metastatic breast cancer, as well as other types of metastatic cancers (TOCA6 trial: clinicaltrials.gov NCT02576665), planned for initiation at the University of Miami, Cleveland Clinic, and UCLA, in mid-2016.

Citation Format: Akihito Inagaki, Sara Collins, Suzanne Matsuura, Noriyuki Kasahara. Therapeutic efficacy of retroviral replicating vectors in orthotopic models of breast cancer metastasis to the brain. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr A43.

Abstract A42: Alternate pseudotypes overcome receptor interference and enable combination suicide gene therapy with retroviral replicating vectors

Retroviral replicating vectors (RRVs) have been shown to achieve efficient tumor transduction and enhanced therapeutic benefit in a wide variety of cancer models. An amphotropic murine leukemia virus (MLV)-based RRV encoding the yeast cytosine deaminase (CD) prodrug activator gene, designated Toca 511 (vocimagene amiretrorepvec), is now being investigated in combination with Toca FC (extended-release 5-FC) in multi-center Phase IIB / III clinical trials for patients with recurrent high-grade glioma (clinicaltrials.gov: NCT02414165). RRV-mediated prodrug activator gene therapy represents the ultimate form of “intracellular” chemotherapy, generated selectively and directly from within the infected cancer cells themselves, without incurring systemic toxicity. Moreover, preclinical data support subsequent activation of the immune system selectively against the cancer. We further propose that combination with additional therapies may be desirable to optimize treatment outcomes. Here we first evaluated two different RRVs derived from MLV and gibbon ape leukemia virus (GALV), and expressing either the same fluorescent reporter gene (MLV-GFP and GALV-GFP, respectively), or different reporter genes (MLV-GFP vs. GALV-RFP; MLV-RFP vs. GALV-GFP), in different types of human cancer cells. Individually, RRVs expressing the same reporter gene efficiently infected human glioma, prostate cancer, and ovarian cancer cells, and showed efficient replication and spread in culture. When marked with different fluorescent reporter genes, it was found that MLV-GFP can spread in cells that had been pre-transduced with GALV-RFP, but not in MLV-RFP pre-transduced cells. Similarly, GALV-GFP can spread in MLV-RFP pre-transduced cells, but not in GALV-RFP pre-transduced cells. This mutually exclusive infection pattern is likely due to the phenomenon of receptor interference, which occurs when vectors derived from the same viral strain compete for binding to cell surface receptors, resulting in superinfection resistance. Notably, however, replication and spread of either RRV in culture was not affected by pre-transduction with RRV derived from a different strain, indicating that this approach could overcome receptor interference. In order to investigate the effect of combined prodrug-dependent cell killing in vitro, cells were then transduced with MLV- or GALV-based RRV expressing the yeast cytosine deaminase suicide gene (MLV-CD, GALV-CD), which converts the prodrug 5-fluorocytosine (5-FC) into the active drug 5-fluorouracil (5-FU), or with RRV expressing the Herpes thymidine kinase gene (MLV-TK, GALV-TK) which converts Ganciclovir (GCV) to GCV-monophosphate, either individually or in combination. In vitro cytocidal effects obtained by combining different prodrug activator genes were significantly greater when these genes were delivered with RRV derived from two different strains. These data indicate the potential utility of using RRV pseudotyped with envelopes derived from different viral strains to overcome receptor interference leading to superinfection resistance, and achieve effective combined prodrug activator gene therapy.

Citation Format: James Grosso, Sara Collins, Akihito Inagaki, Suzanne Matsuura, Brian Slomovitz, Tan Ince, Noriyuki Kasahara. Alternate pseudotypes overcome receptor interference and enable combination suicide gene therapy with retroviral replicating vectors. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr A42.

Factors in the Selection of Surface Disinfectants for Use in a Laboratory Animal Setting

Because surface disinfectants are an important means of pathogen control within laboratory animal facilities, these products must have an appropriate spectrum of antimicrobial activity. However, many other factors must also be considered, including effects on human health, environmental safety, and animal behavior. Aqueous solutions of sodium hypochlorite often are considered to be the 'gold standard' for surface disinfection, but these products can be corrosive, caustic, and aversive in odor. This study was designed to identify disinfectants that are as effective as hypochlorite solutions but more acceptable for use in a laboratory animal setting. An antiviral disinfectant-efficacy assay was developed by using viral vectors that expressed green fluorescence protein as surrogates for wild-type viruses of concern in laboratory animals. Efficacy testing revealed that most of the products were highly effective when used against viral vectors in suspension. However, when the disinfectants were challenged by buffering virus in protein or drying virus on nonporous surfaces, the hypochlorite and peroxymonosulfate products performed the best. Review of safety data sheets for the agents indicated that a peroxide-based product was considerably safer than the other products tested and that the pH of most products was not conducive to disposal down a drain. Behavioral testing of Swiss Webster, C57Bl/6, and BALB/c mice showed that the hypochlorite- and peroxide-based products were clearly aversive, given that the mice consistently avoided these products. All of these factors must be considered when choosing the appropriate disinfectant.

Extensive Replication of a Retroviral Replicating Vector Can Expand the A Bulge in the Encephalomyocarditis Virus Internal Ribosome Entry Site and Change Translation Efficiency of the Downstream Transgene

We have developed retroviral replicating vectors (RRV) derived from Moloney murine gammaretrovirus with an amphotropic envelope and an encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES)-transgene cassette downstream of the env gene. During long-term (180 days) replication of the vector in animals, a bulge of 7 adenosine residues (A's) in the J-K bifurcation domain sometimes serially added A's. Therefore, vectors with 4-12 A's in the A bulge in the J-K bifurcation domain were generated, and the impact of the variants on transgene protein expression, vector stability, and IRES sequence upon multiple infection cycles was assessed in RRV encoding yeast-derived cytosine deaminase and green fluorescent protein in vitro. For transgene protein expression, after multiple infection cycles, RRV-IRES with 5-7 A's gave roughly comparable levels, 4 and 8 A's were within about 4-5-fold of the 6 A's, whereas 10 and 12 A's were marked lower. In terms of stability, after 10 infection cycles, expansion of A's appeared to be a more frequent event affecting transgene protein expression than viral genome deletions or rearrangement: 4 and 5 A's appeared completely stable; 6, 7, and particularly 8 A's showed some level of expansion in the A bulge; 10 and 12 A's underwent both expansion and transgene deletion. The strong relative translational activity of the 5 A's in the EMCV IRES has not been reported previously. The 5A RRV-IRES may have utility for preclinical and clinical applications where extended replication is required.

Comparison of Retinal Sensitivity between Professional Soccer Players and Non-athletes

The purpose of the study was to compare the peripheral retinal sensitivity of the visual field between professional soccer players and age-gender matched non-athlete subjects. All participants underwent a complete eye evaluation. The visual field was evaluated with the achromatic program 60-4 from the Humphrey automated perimetry. The binocular visual field was created with the best location model. It was divided into 4 quadrants (left superior, right superior, left inferior, and right inferior) and compared between groups. The study group comprised 29 professional male football players and the control group comprised 26 age-matched male non-athletes. Mean age was 25.8±4.7 years in the study group and 26.3±5.1 for controls. The average of retina sensitivity in the left inferior and right inferior quadrants was higher in the study group (27.2±1.2 dB and 27.0±1.4 dB) as compared to controls (26.1±1.9 dB and 25.5±2.1 dB). (Student's t test, P=0.011 and P=0.004, respectively). In this small cohort, professional soccer players presented higher retina sensitivity in the inferior quadrants when compared to non-athletes.