Cancer Research and Treatment

Original Article

Low-Dose Cyclophosphamide Enhances the Tumoricidal Effects of 5-Day Spacing Stereotactic Ablative Radiotherapy by Boosting Antitumor Immunity: Hyunkyung Kim, Seok-Joo Chun, Sojung Sun, Haeun Cho, Tae-Jin Kim, Yoon-Jin Lee, Eui Kyu Chie, Kwangmo Yang, Mi-Sook Kim; Received August 21, 2024 Accepted November 7, 2024 Published online November 8, 2024; DOI: https://doi.org/10.4143/crt.2024.807 [Epub ahead of print]

Abstract PDF Supplementary Material PubReader ePub: 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.

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Review Article

Role of HIF-1α in the Responses of Tumors to Radiotherapy and Chemotherapy: Chang W Song, Hyunkyung Kim, Mi-Sook Kim, Heon J Park, Sun-Ha Paek, Stephanie Terezakis, L Chinsoo Cho; Cancer Res Treat. 2025;57(1):1-10. Published online June 5, 2024; DOI: https://doi.org/10.4143/crt.2024.255

Abstract PDF PubReader ePub

Tumor microenvironment is intrinsically hypoxic with abundant hypoxia-inducible factors-1α (HIF-1α), a primary regulator of the cellular response to hypoxia and various stresses imposed on the tumor cells. HIF-1α increases radioresistance and chemoresistance by reducing DNA damage, increasing repair of DNA damage, enhancing glycolysis that increases antioxidant capacity of tumors cells, and promoting angiogenesis. In addition, HIF-1α markedly enhances drug efflux, leading to multidrug resistance. Radiotherapy and certain chemotherapy drugs evoke profound anti-tumor immunity by inducing immunologic cell death that release tumor-associated antigens together with numerous pro-immunological factors, leading to priming of cytotoxic CD8+ T cells and enhancing the cytotoxicity of macrophages and natural killer cells. Radiotherapy and chemotherapy of tumors significantly increase HIF-1α activity in tumor cells. Unfortunately, HIF-1α effectively promotes various immune suppressive pathways including secretion of immune suppressive cytokines, activation of myeloid-derived suppressor cells, activation of regulatory T cells, inhibition of T cells priming and activity, and upregulation of immune checkpoints. Consequently, the anti-tumor immunity elevated by radiotherapy and chemotherapy is counterbalanced or masked by the potent immune suppression promoted by HIF-1α. Effective inhibition of HIF-1α may significantly increase the efficacy of radiotherapy and chemotherapy by increasing radiosensitivity and chemosensitivity of tumor cells and also by upregulating anti-tumor immunity.

Citations

Citations to this article as recorded by

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Molecular Pharmaceutics.2025; 22(4): 2142. CrossRef

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