Abstract 3212: Combining Toca 511 and 5-fluorocytosine with αPD-1 or αCTLA-4 antibody significantly reduces tumor burden compared to either checkpoint inhibitor alone or in combination in a subcutaneous mouse glioma model

Toca 511 (vocimagene amiretrorepvec), a retroviral replicating vector encoding an optimized yeast cytosine deaminase (CD) protein, selectively replicates and spreads in malignant cells. In infected cells, CD enzyme is expressed and converts 5-FC (5-fluorocytosine, an oral anti-fungal drug) to 5-FU, leading to both direct tumor cytotoxicity and extended immunotherapeutic effects. As of 9/25/2015, 116 patients with recurrent high grade glioma have been treated with Toca 511 & Toca FC in 3 ongoing Phase 1 clinical trials (NCT01156584, NCT01470794, NCT01985256), in which potential benefits were found, including extended overall survival and a favorable safety profile. The purpose of this study was to determine if combinations of Toca 511+5-FC with αPD-1, αCTLA-4, or both, are therapeutically useful in a syngeneic mouse model, and for which combinations we could see meaningful variations in tumor infiltrating immune cells. 2×10⁁6 Tu-2449SC glioma cells (2% pretransduced with Toca 511 to provide a uniform starting inoculum) were implanted on the right flanks of B6C3F1 mice and treatment initiated when tumors reached ≈100mm3. 5-FC treated mice received 500 mg/kg/day IP (5 days per week). αPD-1 was administered at 300μg/mouse IP followed by 3 maintenance doses every 3 days of 200μg/mouse. αCTLA-4 (100μg/mouse, IP) was administered every 3 days (total of 6 treatments). Isotype antibodies were administered to control groups. The study was terminated after 3 cycles of 5-FC. Tumors and spleens were collected and analyzed by flow cytometry for 26 unique immune cell populations. Tumor burden was significantly reduced with the addition of Toca 511+5-FC to either antibody treatment alone or in combination compared to antibody treatment groups. Treatment with Toca 511 and 5-FC alone was highly effective at controlling tumor growth, therefore, additive treatment with αPD-1, αCTLA-4, or αPD-1 + αCTLA-4 did not significantly further reduce tumor burden. The combination of αPD-1 and αCTLA-4, but not either antibody alone, significantly reduced tumor burden. Treatment with 5-FC significantly reduced tumor associated macrophages as well as myeloid derived suppressor cells and increased CD4+ T cell populations. Of the T cell populations that were increased in the tumors, a greater percentage of T cells were producing IFNγ. Combining Toca 511+5-FC with αPD-1, αCTLA-4 or the combination of antibodies significantly reduced tumor growth compared to antibody treatments alone. Tumor associated immune cell population analysis also revealed that Toca 511+5-FC altered the immune cell populations in the tumor to generate a more permissive environment for immune activation and anti-tumor immune response. Toca 511 & Toca FC may be broadly applicable in combination with checkpoint inhibitors to help activate the immune system. Citation Format: Leah Mitchell, Fernando Lopez Espinoza, Kader Yagiz, Daniel Mendoza, Maria Rodriguez-Aguirre, Sean Mitchell, Douglas J. Jolly, Joan M. Robbins. Combining Toca 511 and 5-fluorocytosine with αPD-1 or αCTLA-4 antibody significantly reduces tumor burden compared to either checkpoint inhibitor alone or in combination in a subcutaneous mouse glioma model. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3212.