Shuichi Kamijima

Radiosensitization of gliomas by intracellular generation of 5-fluorouracil potentiates prodrug activator gene therapy with a retroviral replicating vector

A tumor-selective non-lytic retroviral replicating vector (RRV), Toca 511, and an extended-release formulation of 5-fluorocytosine (5-FC), Toca FC, are currently being evaluated in clinical trials in patients with recurrent high-grade glioma (NCT01156584, NCT01470794 and NCT01985256). Tumor-selective propagation of this RRV enables highly efficient transduction of glioma cells with cytosine deaminase (CD), which serves as a prodrug activator for conversion of the anti-fungal prodrug 5-FC to the anti-cancer drug 5-fluorouracil (5-FU) directly within the infected cells. We investigated whether, in addition to its direct cytotoxic effects, 5-FU generated intracellularly by RRV-mediated CD/5-FC prodrug activator gene therapy could also act as a radiosensitizing agent. Efficient transduction by RRV and expression of CD were confirmed in the highly aggressive, radioresistant human glioblastoma cell line U87EGFRvIII and its parental cell line U87MG (U87). RRV-transduced cells showed significant radiosensitization even after transient exposure to 5-FC. This was confirmed both in vitro by a clonogenic colony survival assay and in vivo by bioluminescence imaging analysis. These results provide a convincing rationale for development of tumor-targeted radiosensitization strategies utilizing the tumor-selective replicative capability of RRV, and incorporation of radiation therapy into future clinical trials evaluating Toca 511 and Toca FC in brain tumor patients.

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

Abstract 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.

Combined Alloreactive CTL Cellular Therapy with Prodrug Activator Gene Therapy in a Model of Breast Cancer Metastatic to the Brain

Purpose: Individual or combined strategies of cellular therapy with alloreactive CTLs (alloCTL) and gene therapy using retroviral replicating vectors (RRV) encoding a suicide prodrug activating gene were explored for the treatment of breast tumors metastatic to the brain. Experimental Design: AlloCTL, sensitized to the HLA of MDA-MB-231 breast cancer cells, were examined in vitro for antitumor functionality toward breast cancer targets. RRV encoding the yeast cytosine deaminase (CD) gene was tested in vivo for virus spread, ability to infect, and kill breast cancer targets when exposed to 5-fluorocytosine (5-FC). Individual and combination treatments were tested in subcutaneous and intracranial xenograft models with 231BR, a brain tropic variant. Results: AlloCTL preparations were cytotoxic, proliferated, and produced IFN-γ when coincubated with target cells displaying relevant HLA. In vivo, intratumorally placed alloCTL trafficked through one established intracranial 231BR focus to another in contralateral brain and induced tumor cell apoptosis. RRV-CD efficiently spread in vivo, infected 231BR and induced their apoptosis upon 5-FC exposure. Subcutaneous tumor volumes were significantly reduced in alloCTL and/or gene therapy–treated groups compared to control groups. Mice with established intracranial 231BR tumors treated with combined alloCTL and RRV-CD had a median survival of 97.5 days compared with single modalities (50–83 days); all experimental treatment groups survived significantly longer than sham-treated groups (median survivals 31.5 or 40 days) and exhibited good safety/toxicity profiles. Conclusion: The results indicate combining cellular and suicide gene therapies is a viable strategy for the treatment of established breast tumors in the brain. Clin Cancer Res; 19(15); 4137–48. ©2013 AACR.

629. Preclinical Evaluation of Prodrug Activator Gene Therapy by Retroviral Replicating Vectors (RRV) in Experimental Models of Breast Cancer CNS Metastasis

Retroviral replicating vectors (RRV) are capable of highly efficient replication in cancer cells in vitro and in vivo. In a variety of cancer models this is associated with high levels of tumor-selective gene transfer and significantly enhanced survival benefit when employed for prodrug activator gene therapy. An RRV encoding a modified yeast cytosine deaminase (CD) prodrug activator gene (Toca 511) in combination with extended-release flucytosine (Toca FC) is under investigation in multi-center ascending-dose trials in patients with recurrent high grade glioma. In the present study, our goal is to establish the feasibility of also applying this novel gene therapy strategy to secondary brain tumors arising from metastasis of systemic cancers. In particular, brain metastases of breast cancer frequently arise from highly aggressive, treatment-refractory, “triple-negative” (ER(-), PR(-), HER2(-)) cells, and are associated with a dismal prognosis of 4-6 months survival. First, RRV encoding GFP (AC3-emd, RRV-GFP) showed robust replication activity in the human breast cancer tumor cell lines MDA-MB-231-BR and JC 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 after 5-FC treatment of MDA-MB-231BR and JC cells infected with an RRV expressing CD (AC3-yCD2, Toca 511). In both of the AC3-yCD2-transduced cell lines, cell viability was reduced approximately 70-85% after exposure to 12.9 mg/mL 5-FC and complete cell killing was observed with 129 mg/mL 5-FC (6 days 5-FC treatment for 231-BR; 4 days for JC). In a 231-BR intracranial xenograft model, intratumoral injection of 10^6 Transducing Units of RRV-GFP resulted in >80% transduction efficiency on Day 10 post-virus injection. In survival studies, animals treated with Toca 511 and 5-FC showed statistically significant (p<0.0001) survival benefit. These preclinical data support the advancement of RRV gene therapy towards clinical investigation in patients with CNS-metastatic breast cancer.

433. Sequence-Optimized Nitroreductase for Retroviral Replicating Vector (RRV) Mediated 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 prodrug activator gene therapy in a variety of preclinical cancer models. RRV-mediated gene therapy using yeast cytosine deaminase (CD) is under investigation in multi-center ascending dose trials in patients with recurrent high grade glioma in the United States. We have further 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 optimisation 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 >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 data 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.

Abstract 5386: Therapeutic efficacy of RMLV vectors in human and murine glioma models

Unique among replicating viruses being developed as oncolytic agents, replicating murine leukemia virus (RMLV) retroviruses propagate without immediate lysis of host cells and maintain viral persistence through stable integration. Due to its intrinsic inability to infect quiescent post-mitotic cells and susceptibility to innate antiviral defenses that exist in normal cells but are inactivated in many cancers, RMLV vector replication has been shown to be highly tumor-selective. We have previously demonstrated that RMLV vectors are capable of highly efficient replication in tumor cells, and we have found significantly enhanced survival benefit in a variety of cancer models in vivo when RMLV vectors are employed for delivery of prodrug activator genes such as yeast cytosine deaminase (yCD), which mediates the conversion of inactive prodrug 5-fluorocytosine (5FC) to the anti-cancer drug, 5-fluorouracil (5FU). In the present study, we tested a newly developed RMLV vector with modified virus backbone sequences and a codon-optimized yCD gene (Toca511). First, we examined in vitro cytotoxicity using human (U87) and murine (TU-2449) glioma cell lines by MTS assay. The cell viability of Toca511-transduced cells was reduced by >80% after 5 or 8 days of exposure to the prodrug, respectively. Second, we examined the therapeutic efficiency of this newly developed vector in subcutaneous U87 human glioblastoma models and TU2449 syngeneic mouse models. Toca511-mediated prodrug activator therapy strongly inhibited tumor growth in both subcutaneous models. Third, therapeutic efficacy was evaluated, and the minimum effective doses of 5FC prodrug and Toca511 vector determined in an intracranial U87 xenograft model. Systemic administration of 5FC resulted in significant inhibition of bioluminescent signals in mice whose tumors had been infected with Toca511 but not in control mice, associated with significant survival benefit even after injecting only 1×1e3 total vector units (p=0.0015). These data suggest that we have been successful in developing an improved retrovirus vector that shows strong therapeutic efficacy when used for prodrug activator gene therapy in both subcutaneous and intracranial xenograft models of human and/or murine glioma. This vector has now entered Phase I/II clinical trials for glioblastoma. Now, with the use of RMLV vectors such as Toca511, the original promise of retrovirus-mediated gene therapy strategies for cancer may finally be fulfilled. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5386. doi:10.1158/1538-7445.AM2011-5386

Abstract 5393: Prodrug activator gene therapy mediated by prostate-specific replicating retrovirus vectors

Replicating murine leukemia virus (RMLV)-based vectors can propagate specifically within actively dividing cancer cells and achieve highly efficient and tumor-selective gene delivery. However, in view of the known potential for retroviral integration-mediated genotoxicity, additional mechanisms to target the virus specifically to cancer cells are desirable. To further improve their tumor specificity and safety profile, we have developed RMLV vectors regulated by two different tissue-specific promoters. We generated GFP-expressing RMLV vectors ACE-emd (wild type), ACE-At-emd (regulated by ARR2PB promoter, a synthetic variant of the probasin promoter which exhibits high specificity for androgen receptor (AR)-positive prostate cancer cells) and ACE-PSES-emd (regulated by PSES promoter, an androgen-independent promoter which is highly active in PSA/PSMA-positive prostate cancer cells, both in the presence and absence of androgen). We also developed ACE-yCD, ACE-At-yCD and ACE-PSES-yCD carrying the yeast cytosine deaminase (yCD) prodrug-activator gene, which converts the nontoxic prodrug 5-fluorocytosine (5FC) into the chemotoxin 5-fluorouracil. These vectors were first in vitro tested in both prostate (LNCaP, MDA PCa 2b and PC-3) and non-prostate (293T) cell lines in the presence and absence of androgen. RMLV vector ACE-At-emd exhibited high specificity and robust replication in AR-positive prostate cancer cell lines, but only in the presence of androgen. RMLV vector ACE-PSES-emd exhibited high specificity for PSA/PSMA-positive prostate cancer cell line both in the presence and absence of androgen. For in vivo studies, male nude mice were inoculated subcutaneously with LNCaP cells (AR-positive, PSA/PSMA-positive) in Matrigel. RMLV vectors expressing GFP were injected intratumorally, and tumors were analyzed by flow cytometry 2 and 4 weeks after vector injection. The results showed that both ARR2PB- and PSES-regulated RMLV vectors achieve efficient gene delivery to LNCaP s.c. tumors in vivo. Cytotoxic activity of RMLV-yCD vectors was examined in vitro in LNCaP cells in the presence and absence of 5FC after virus infection. Cell-specific expression of each vector correlated with reduced cell viability compared to uninfected cells after 5FC treatment. We are now conducting studies to evaluate therapeutic efficacy vs. potential genotoxicity of these vectors for prodrug-activator gene therapy in vivo. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5393. doi:10.1158/1538-7445.AM2011-5393