Resetting the tumor microenvironment to favor anti-tumor immunity after local ablation.

2561 Background: Percutaneous tumor ablation is a non-surgical method of tumor destruction that leaves necrotic tumor debris in situ. Tumor associated antigens released after ablation have the potential to initiate a systemic anti-tumor immune response, however the hostile tumor microenvironment hinders antigen presentation and T cell activity. We hypothesized that resetting the tumor microenvironment with oral sodium bicarbonate to decrease tumor acidity and low-dose cyclophosphamide to deplete pro-tumor immune cells would improve the ability of ablation to initiate anti-tumor immunity. Methods: Tumor growth, overall survival, and metastatic burden was assessed in orthotopic tumor models of triple-negative breast cancer (67NR, 4T1, and E0771). Tumor ablation was performed on palpable tumors using percutaneous ethanol injection (PEI) with 6% ethylcellulose to improve retention in the tumor. Surgical excision was used as a negative control to test the role of in situ tumor debris. Before ablation mice were placed on 200 mM of sodium bicarbonate (SB) in their drinking water and received a single intraperitoneal injection of 200 mg/kg of cyclophosphamide (CP). Mice surviving to 60 days after tumor implant without a primary tumor or signs of metastases were considered "cured" and re-challenged with 50e5 tumor cells in the contralateral mammary pad. T cell dependance was assessed with in vivo CD8 depletions. Results: The combination of PEI+SB+CP produced a potent anti-tumor response, curing a majority of mice (5/7 of E0771, 8/12 of 67NR, 7/12 of 4T1). No mice were cured using PEI alone, SB alone, CP alone, or any combination of two therapies (0/51 of E0771, 0/73 of 67NR, 0/75 of 4T1,). Re-challenge tumor growth was hindered in mice cured with PEI+SB+CP. Mice receiving PEI+SB+CP had significantly less metastases and lived longer than mice receiving surgical excision alone or surgical excision with SB+CP. Additionally the anti-metastatic response of PEI+SB+CP was undone when CD8+ T cells were depleted. Conclusions: Here the anti-tumor response of local ablation produced by PEI was enhanced by priming the tumor with low-dose CP and oral SB in metastatic breast cancer. These results suggest that tumor ablation with CP and SB can create a T cell dependent, personalized immune response to a tumor using only low-cost, easily accessible supplies, and the host’s own tumor.