Abstract 1828: Efficacy and mechanism of radiotherapy combined with fruquintinib and tirelizumab in mCRC

Abstract Background: Immune checkpoint inhibitors (ICIs) targeting programmed cell death protein 1 (PD-1) or programmed death ligand 1(PD-L1) respond well to deficient-microsatellite(dMMR) colorectal cancer and poorly to proficient-microsatellite (pMMR) CRC. Anti-vascular therapy is the standard backline treatment regimen for advanced metastatic colorectal cancer and also potentiates the immunotherapeutic efficacy of CRC by promoting immune cell infiltration and remodeling the tumor immune microenvironment (TIME). However, it is not clear whether combining radiotherapy, anti-vascular and anti-PD-1 can affect the efficacy of pMMR CRC. In this experiment, we investigated the antitumor efficacy of radiotherapy combined with fruquintinib and tirelizumab in pMMR CRC mice. Methods: CT26 cellular hormonal tumor corresponding to pMMR CRC. A mouse model with subcutaneous transplanted tumors is established, divided into control, radiotherapy (IR), fruquintinib + tirelizumab (F+T), and radiotherapy + fruquintinib + tirelizumab (IR+F+T) groups. Immunofluorescence (IF) experiments were conducted to investigate the number and function of tumor vessels. Immunohistochemistry (IHC) and flow cytometry (FC) were utilized to examine immune cell infiltration and alterations within the TIME. Results: Compared to the control group, tumor growth was significantly inhibited after treatment. When compared to IR and F+T, IR+F+T demonstrated a remarkable suppression of tumor growth. The quantitative analysis results showed a significant decrease in Ki-67 positive cells in the target tumors with IR+F+T compared to IR and F+T. The TUNEL results indicated that treatment promoted tumor cell apoptosis, and the effect was further enhanced with triple combinational therapy. Both F+T and IR+F+T repressed CD31 expression and improved the ratio of α-SMA+/CD31+ in tumor tissue, yet there was no significant difference between the two groups. The immunohistochemistry and flow cytometry results demonstrated that triple combinational therapy increased tumor-infiltrating CD8+ T cells and significantly elevated the proportions of CD8, CD69, and CD86. Both F+T and IR+F+T boosted PD-L1 expression, with no significant difference between them. Conclusions: Combined irradiation on top of fruquintinib and tirelizumab treatment enhanced the efficiency in CT26 murine CRC syngeneic tumor model. There was no significant effect of radiotherapy on the anti-angiogenesis of fruquintinib and the promotion of normal vascular function. Irradiation promoted CD8+ T cell and dendritic cell infiltration and activation. Citation Format: Mingsheng Zhang, Qingqing Yu, Huiying Hou, Xiaoting Su, Qin Huang, Hong Qiu, Le Huang, Liang Zhuang, Qiang Fu, Yanmei Zou, Li Sun, Liu Huang, Shunfang Liu, Fei Liu, Xianglin Yuan. Efficacy and mechanism of radiotherapy combined with fruquintinib and tirelizumab in mCRC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1828.