Low-Dose Cyclophosphamide Enhances the Tumoricidal Effects of 5-Day Spacing Stereotactic Ablative Radiotherapy by Boosting Antitumor Immunity.

PURPOSE: This study aimed to investigate the potential role of low-dose cyclophosphamide (Cy) as a radiosensitizer by evaluating its impact on the immune response and the abscopal effect of stereotactic ablative radiotherapy through preclinical models. MATERIALS AND METHODS: CT26 tumors (immunologically hot) and 4T1 tumors (immunologically cold), grown in immunocompetent BALB/c and immunodeficient BALB/c-nude mice, were irradiated with 20 Gy in two fractions with 5-day spacing followed by intraperitoneal injections of 9 mg/kg Cy every 3 days. Immunological changes in CT26 tumors caused by the treatments were assessed using flow cytometry. Changes in the expression of hypoxia-inducible factor-1α (HIF-1α) in tumors were also assessed. Splenocytes and bone marrow-derived dendritic cells (DCs) were exposed to various concentrations of Cy to assess T cell proliferation and DC differentiation. RESULTS: The combination of Cy with radiotherapy (RT+Cy) significantly suppressed tumor growth compared to RT alone in immunocompetent mice, while that effect was not observed in immunodeficient mice. Additionally, RT+Cy effectively induced abscopal effects in hot and cold tumors, with increased CD8+ T cells in blood and tumors. Significantly higher expression levels of granzyme B, interferon γ, and tumor necrosis factor α were observed in RT+Cy group compared to the RT alone group. In vitro data indicated that low-dose Cy promotes DC differentiation. Low-dose Cy suppressed the radiation-induced upregulation of HIF-1α in the tumors. CONCLUSION: Low-dose Cy enhances tumoricidal effects of 5-day spacing high-dose RT by increasing antitumor immune responses.