Retroviral Replicating Vector Research Articles

Abstract 1417: IRF8 driven reprogramming of the immune microenvironment enhances anti-tumor adaptive immunity and reduces immunosuppression in murine glioblastoma

Abstract Glioblastoma (GBM) has an extremely immunosuppressive microenvironment, primarily mediated by the recruitment of immature myeloid cells (IMCs) to the tumor. IMCs suppress T cell function and dampen the anti-tumor response. In this study, we sought to “reprogram” IMCs into antigen-presenting cells (APCs) to enhance anti-tumor T cell responses. To induce IMC differentiation into APCs, we utilized a retroviral replicating vector (RRV) expressing IRF8, a master regulator of type 1 conventional dendritic cell (cDC1) development. We evaluated the impact of IRF8 RRV on in vivo tumor growth kinetics, survival, immunosuppression, and immune microenvironment. Because RRV transduction requires the proliferation of target cells, we assessed the proliferative capacity of myeloid cells in murine and human GBM samples. We utilized the SB28 cell line, a poorly immunogenic murine GBM model. We pre-transduced SB28 cells with IRF8 RRV and implanted them intracranially into syngeneic C57BL/6 mice; controls expressed an empty vector. Intratumoral myeloid cells were proliferative in both murine and human samples. In vivo, tumor and myeloid cells are infected proportionately to their respective levels of proliferation. Mice with IRF8 RRV pre-transduced intracranial tumors had significantly longer survival (median 28 days) compared to controls (median 19 days) (p <0.0001). Additionally, tumor growth was delayed in IRF8 RRV mice. IRF8 RRV mice had significant enrichment of dendritic cells (p = 0.0145), cDC1s (p < 0.0001), T cells (p = 0.0025) and CD8+ T cells (p < 0.0001). Both monocytic and granulocytic IMCs had decreased expression of the immunosuppressive markers Arginase 1 and IDO1. Further, ex vivo co-culture of intra-tumoral myeloid cells with naïve T cells showed a functional decrease in immunosuppression in IRF8 RRV tumors compared to controls (p < 0.0001). To study the mechanism of action, we assessed whether IRF8 transduction of tumor cells alone or both tumor and immune cells, was necessary for the observed survival benefit. To inhibit RRV spread from infected cells to other cells, we treated mice with azidothymidine, a reverse transcription inhibitor. A robust survival benefit and tumor growth inhibition were seen only in mice where the RRV could spread freely (p = 0.0005). This finding shows that both tumor and immune cells must be infected to cause the phenotype and suggests that reprogramming immune cells is required for therapeutic efficacy. Our novel approach used RRV to deliver a myeloid transcriptional regulator in intracerebral GBM. This therapy achieved significant survival benefits and increased intratumoral cytotoxic T cells and APCs, while decreasing myeloid immunosuppression. Further studies will expand upon the mechanism of action and test combination therapies. Citation Format: Megan Montoya, Sara A. Collins, Pavlina Chuntova, Noriyuki Kasahara, Hideho Okada. IRF8 driven reprogramming of the immune microenvironment enhances anti-tumor adaptive immunity and reduces immunosuppression in murine glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1417.

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.

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

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

EXTH-34. LOCAL DELIVERY OF AN IL-15 SUPERAGONIST USING A REPLICATING RETROVIRUS SYNGERIZES WITH TEMOZOLOMIDE TO INCREASE LYMPHOCYTE INFILTRATION AND SURVIVAL IN TWO POORLY IMMUNOGENIC GLIOBLASTOMA MOUSE MODELS

Abstract Glioblastoma (GBM) leads to systemic and local immunosuppression and immunotherapies have had limited clinical success. We evaluated the treatment efficacy of RLI (receptor-linker-IL-15), 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 poorly immunogenic with low-mutational burden and known resistance to immunotherapy, similar to human GBM. RRV-RLI replicated and spread in cultured SB28 murine GBM cells with robust production of functional RLI (165.4 ± 5.3 ng/mL). Stereotactic injection of RRV-RLI into pre-established intracerebral Tu2449 tumors led to complete eradication of intracerebral tumors with long term survival (median not reached) in > 85% of treated mice vs. median survival of 12.5 days in control mice (p=0.001). Treated mice demonstrated significantly increased CD8 T cell infiltration. These mice also demonstrated immunologic memory to intracranial rechallenge. In SB28, RRV-RLI delivery into intracerebral tumors reduced tumor growth on bioluminescence imaging, and increased median survival compared to controls (55 vs. 19 days, p=0.002), with long-term survival in 12% of treated mice. Combining RRV-RLI treatment with systemically administered temozolomide (TMZ) in the SB28 model provided long term survival (median not reached) in >60% of treated mice. Immunologic analysis of treated tumors revealed a significant increase in CD8 T cell (2.1% to 20.5% of CD45 cells) and Natural Killer cell (1.4% to 9.6% of CD45 cells )infiltration in RLI-RRV+TMZ mice vs. PBS+TMZ mice. RLI was not detected in the blood of treated mice, and tumor-localized RRV-RLI gene delivery showed no adverse systemic immune effects. In summary, RRV-mediated RLI immunotherapy results in immunostimulatory changes that are further potentiated by systemically administered temozolomide. This tumor-localized gene therapy has the potential to synergize with standard of care treatment to reverse the immunosuppressed GBM tumor microenvironment and provide a significant treatment benefit.

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.

Retroviral Replicating Vector Toca 511 (Vocimagene Amiretrorepvec) for Prodrug Activator Gene Therapy of Lung Cancer.

Therapeutic efficacy of retroviral replicating vector (RRV)-mediated prodrug activator gene therapy has been demonstrated in a variety of tumor models, but clinical investigation of this approach has so far been restricted to glioma and gastrointestinal malignancies. In the present study, we evaluated replication kinetics, transduction efficiency, and therapeutic efficacy of RRV in experimental models of lung cancer. RRV delivering GFP as a reporter gene showed rapid viral replication in a panel of lung cancer cells in vitro, as well as robust intratumoral replication and high levels of tumor transduction in subcutaneous and orthotopic pleural dissemination models of lung cancer in vivo. Toca 511 (vocimagene amiretrorepvec), a clinical-stage RRV encoding optimized yeast cytosine deaminase (yCD) which converts the prodrug 5-fluorocytosine (5-FC) to the active drug 5-fluorouracil (5-FU), showed potent cytotoxicity in lung cancer cells upon exposure to 5-FC prodrug. In vivo, Toca 511 achieved significant tumor growth inhibition following 5-FC treatment in subcutaneous and orthotopic pleural dissemination models of lung cancer in both immunodeficient and immunocompetent hosts, resulting in significantly increased overall survival. This study demonstrates that RRV can serve as highly efficient vehicles for gene delivery to lung cancer, and indicates the translational potential of RRV-mediated prodrug activator gene therapy with Toca 511/5-FC as a novel therapeutic strategy for pulmonary malignancies.

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

Abstract 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

Abstract 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 > 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.

CTNI-03. EXTENDED BENEFITS IN PATIENTS WITH RECURRENT HIGH GRADE GLIOMA THAT CONTINUOUSLY RECEIVE TOCA FC AFTER TOCA 511 TREATMEN

Abstract Toca 511 (vocimagene amiretrorepvec) is an investigational nonlytic, retroviral replicating vector (RRV) constructed with a codon-optimized yeast cytosine deaminase (CD) gene. Toca 511 infects cancer cells, and stably delivers CD gene whose protein product converts courses of the prodrug TocaFC (5-fluorocytosine) into 5-fluorouracil (5-FU). Several phase 1 studies and a phase 2/ 3 clinical study tested Toca511 in combination with TocaFC in patients undergoing planned resection for recurrent glioblastoma (rGBM) or anaplastic astrocytoma (rAA). The last trial was completed in 2019, and although the Phase 2/3 study did not meet the primary objective, patients who appeared to benefit from the treatment elected to continue TocaFC through an expanded access or compassionate use pathway. Seven patients continued on treatment: 3 men and 4 women; 5 with rGBM, 2 with rAA. Duration of treatment with TocaFC ranges from 29 months to 7 years and 5 months (average 49.2 months). Six patients who are still on active treatment with TocaFC have either stable disease or complete response. TocaFC is well tolerated in these patients, typical side effects include diarrhea. Some patients with rGBM or rAA appear to benefit from extended TocaFC treatment after Toca511. Individual cases will be discussed.

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

Abstract 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 > 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.

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

Abstract 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.

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

Abstract 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< 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< 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.

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.

Historic Overview of Genetic Engineering Technologies for Human Gene Therapy.

The concepts of gene therapy were initially introduced during the 1960s. Since the early 1990s, more than 1900 clinical trials have been conducted for the treatment of genetic diseases and cancers mainly using viral vectors. Although a variety of methods have also been performed for the treatment of malignant gliomas, it has been difficult to target invasive glioma cells. To overcome this problem, immortalized neural stem cell (NSC) and a nonlytic, amphotropic retroviral replicating vector (RRV) have attracted attention for gene delivery to invasive glioma. Recently, genome editing technology targeting insertions at site-specific locations has advanced; in particular, the clustered regularly interspaced palindromic repeats/CRISPR-associated-9 (CRISPR/Cas9) has been developed. Since 2015, more than 30 clinical trials have been conducted using genome editing technologies, and the results have shown the potential to achieve positive patient outcomes. Gene therapy using CRISPR technologies for the treatment of a wide range of diseases is expected to continuously advance well into the future.

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

Abstract 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).

Novel Semi-Replicative Retroviral Vector Mediated Double Suicide Gene Transfer Enhances Antitumor Effects in Patient-Derived Glioblastoma Models.

As glioblastomas are mostly localized infiltrative lesions, gene therapy based on the retroviral replicating vector (RRV) system is considered an attractive strategy. Combinations of multiple suicide genes can circumvent the limitations associated with each gene, achieving direct and synergistic cytotoxic effects, along with bystander cell killing. In this study, we constructed a semi-and pseudotyped-RRV (sp-RRV) system harboring two suicide genes—herpes simplex virus type 1 thymidine kinase (TK) and yeast cytosine deaminase (CD)—to verify the dissemination and antitumor efficacy of our sp-RRV system (spRRVe-sEF1α-TK/sRRVgp-sEF1α-CD) in seven patient-derived glioblastoma stem-like cells (GSCs). Flow cytometry and high-content analysis revealed a wide range of transduction efficiency and good correlation between the delivery of therapeutic genes and susceptibility to the prodrugs ganciclovir and 5-fluorocytosine in patient-derived GSCs in vitro. Intra-tumoral delivery of spRRVe-sEF1α-TK/sRRVgp-sEF1α-CD, combined with prodrug treatment, synergistically inhibited cell proliferation and angiogenesis while increasing apoptosis and the depletion of tumor-associated macrophages in orthotopic glioblastoma xenografts. Genomic profiling of patient-derived GSCs revealed that the key genes preventing sp-RRV infection and transmission were associated with cell adhesion, migration, development, differentiation, and proliferation. This is the first report demonstrating that a novel sp-RRV-mediated TK/CD double suicide gene transfer system has high oncolytic power against extremely heterogeneous and treatment-refractory glioblastomas.

Abstract 4771: The addition of toca 511 and 5-FC to temozolomide improves response in a temozolomide-resistant murine glioblastoma model and correlates with toca 511 dose

Abstract Background: Toca 511 (vocimagene amiretrorepvec), an amphotropic retroviral replicating vector (RRV), can successfully and safely deliver a functional, optimized yeast cytosine deaminase (CD) gene to tumors. Within infected cells, CD converts 5-fluorocytosine (5-FC) to the anti-cancer drug 5-fluorouracil (5-FU). The combination of Toca 511 with oral extended release 5-FC (Toca FC), is currently being evaluated in a randomized phase III clinical trial for recurrent high grade glioma (glioblastoma (GBM) and anaplastic astrocytoma) (NCT02414165, Toca 5). Temozolomide (TMZ), in combination with radiation therapy, is the standard of care used for first-line chemotherapy treatment of patients with GBM, the most common and aggressive form of primary brain cancer. Previously, we have shown that: (1) Toca 511/5-FC treatment provides durable response in a syngeneic murine glioma model and supports anti-tumor immune memory; (2) The combination of TMZ and Toca 511/5-FC had synergistic efficacy in a TMZ-sensitive human glioma nude mouse model; (3) TMZ did not inhibit the efficacy of Toca 511/5-FC in a TMZ resistant murine glioma syngeneic model; (4) Toca 511/5-FC caused significant radiosensitization in a radioresistant murine glioma model. Results: To assess the interaction of TMZ with escalating doses of Toca 511 (as defined by percent of tumor transduction by RRV), an orthotopic TMZ-resistant murine glioma model, Tu-2449, was utilized. These results show that moderate levels of tumor transduction of Toca 511 (30% - 50%) with 5-FC treatment prolongs survival in the presence of TMZ compared with lower transduction rates (10%). Additionally, mice treated with Toca 511/5-FC and TMZ are being tested for the establishment of anti-tumor immune memory. Conclusion: These results demonstrate that (1) survival correlates with the transduction levels of Toca 511 when combined with temozolomide in the Tu-2449 orthotopic glioma model and (2) that this combination may support anti-tumor immune memory. These studies along with prior work support evaluation of the combination of Toca 511/5-FC with temozolomide in patients with newly diagnosed GBM (NRG-BN006). Citation Format: Maria E. Rodriguez-Aguirre, Sophie Viaud, Daniel Mendoza, Tiffany Montellano, Derek G. Ostertag, Harry Gruber, Douglas Jolly, Cornelia Bentley. The addition of toca 511 and 5-FC to temozolomide improves response in a temozolomide-resistant murine glioblastoma model and correlates with toca 511 dose [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4771.

PD-L1 checkpoint blockade delivered by retroviral replicating vector confers anti-tumor efficacy in murine tumor models.

Immune checkpoint inhibitors (CPIs) are associated with a number of immune-related adverse events and low response rates. We provide preclinical evidence for use of a retroviral replicating vector (RRV) selective to cancer cells, to deliver CPI agents that may circumvent such issues and increase efficacy. An RRV, RRV-scFv-PDL1, encoding a secreted single chain variable fragment targeting PD-L1 can effectively compete with PD-1 for PD-L1 occupancy. Cell binding assays showed trans-binding activity on 100% of cells in culture when infection was limited to 5% RRV-scFv-PDL1 infected tumor cells. Further, the ability of scFv PD-L1 to rescue PD-1/PD-L1 mediated immune suppression was demonstrated in a co-culture system consisting of human-derived immune cells and further demonstrated in several syngeneic mouse models including an intracranial tumor model. These tumor models showed that tumors infected with RRV-scFv-PD-L1 conferred robust and durable immune-mediated anti-tumor activity comparable or superior to systemically administered anti-PD-1 or anti PD-L1 monoclonal antibodies. Importantly, the nominal level of scFv-PD-L1 detected in serum is ∼50–150 fold less than reported for systemically administered therapeutic antibodies targeting immune checkpoints. These results support the concept that RRV-scFv-PDL1 CPI strategy may provide an improved safety and efficacy profile compared to systemic monoclonal antibodies of currently approved therapies.

CADD-23. PD-L1 CHECKPOINT BLOCKADE USING A SINGLE-CHAIN VARIABLE FRAGMENT TARGETING PD-L1 DELIVERED BY RETROVIRAL REPLICATING VECTOR (TOCA 521) ENHANCES ANTI-TUMOR EFFECT IN CANCER MODELS

Conventional interventions for treating glioblastoma (GBM) patients has had limited success, with a median overall survival of 15–17 months. Recently, immune checkpoint inhibitors (CPIs) showed long-term response rates from 20–30% in some tumors, but no consistent clinical benefit with these agents has been demonstrated so far in GBM patients. We propose and provide preliminary evidence for a strategy using retroviral replicating vectors (RRV) to deliver CPI agents selectively to cancer cells that may circumvent such issues. An RRV encoding a single-chain variable fragment targeting PD-L1 (RRV-scFv-PDL1, Toca 521) binds to both mouse and human PD-L1 by competitive ELISA and competes for target occupancy with a commercially available monoclonal antibody against cell surface PD-L1. A dose-dependent bystander effect is observed with scFv PD-L1 protein expressed from RRV-scFv-PDL1 infected tumor cells showing saturated receptor binding to the cell surface PD-L1 of bystander cells when co-cultured with as low as 10% scFv PD-L1 expressing cells. In addition, the immune functional activity of scFv PD-L1 to reverse PD-1/PD-L1 mediated immune suppression was observed in a co-culture system in vitro and further supported by in vivo mouse models. Such models included a syngeneic orthotopic glioma showing that tumors infected with RRV-scFv-PDL1 conferred robust and durable immune-mediated antitumor activity superior to systemically administered anti-PD-1/anti-PD-L1 monoclonal antibodies. These results support the concept that RRV-scFv-PDL1 CPI strategy may provide an improved safety and efficacy profile compared to systemic monoclonal antibodies. The superior anti-tumor activity of RRV-scFv-PDL1 may be due to consistent high levels of scFv PD-L1 within the tumor microenvironment. This localized delivery approach with less concern for autoimmune adverse events may be therapeutically beneficial as an immuno-oncology agent either alone or in combination.