Abstract 2258: CD73 targeting therapy enhances the anti-tumor effect of radiation therapy in rectal cancer
Authors: Tsukui Hidenori , Horie Hisanaga , Saito Akira , Koinuma Koji , Ohsawa Hideyuki , Yamaguchi Hironori , Sakuma Yasunaru , Hosoya Yoshinori , Yoshimura Kotaro , Lefor Alan , Sata Naohiro , Kitayama Joji 
Published: September 2020

Abstract
[Background and Purpose]Extracellular adenosine exerts strong immunosuppressive activity and is considered as an important metabolic immune-checkpoint molecule. CD73 (ecto-5'-nucleotidase) is the rate-limiting enzyme to produce adenosine, and high CD73 expression in tumor tissue is linked to poor prognosis. However, the role of CD73/adenosine signaling in the response against radiation therapy (RT) is less understood. Here, we examined the anti-tumor effects of the combination of CD73 blockade and RT.
[Method]LuM1, a highly metastatic subtype of colon26 (murine colon cancer cell) were subcutaneously (sc) inoculated in Balb/c mouse. At day 12,14 and16, 4Gy RT was locally applied selectively to sc tumor with lead shielding of other body. Blocking CD73 mAb (200μg/body) was intraperitoneally administrated at day 12,14,16,19,22 and 25. The mice were sacrificed at day 28 and the growth of irradiated tumor and non-irradiated lung metastasis was evaluated. For evaluating immune function, tumor and spleen were excised at day 18. In addition, expression of CD73 in 64 human rectal cancer after chemoradiotherapy (CRT) was examined with immunohistochemistry and evaluated for the impact on patient outcome.
[Result]Flowcytometric analysis showed that LuM1 positively expressed CD73 which was significantly enhanced by 10Gy RT in vitro. Local RT (4Gy x 2) also enhanced the expression of CD73 in LuM1 of sc tumor (MFI: Median=177, 120-234 vs 65.5, 24.3-72.3, n= 5, p=0.0079). Local RT caused significant delay in growth of sc tumor. However, the addition of anti-CD73 mAb further reduced the tumor weight as compared with isotype control (Μ=0.15g, 0.004-0.29g vs 0.34g, 0.1-0.73g, n=8, p=0.0379). Interestingly, the number of metastases in non-irradiated lung was also significantly decreased by anti-CD73mAb (Μ=1, 0-30 vs 12, 1-70, n=8, p=0.040).The percentage of IFNγ (+)/CD4 (+) cells and IFNγ (+)/CD8a (+) cells of splenocytes were increased in RT +Anti-CD73 groups compared to RT +Anti-Isotype group. (Μ=9.95%, 7.71-16.6% vs 3.64%, 1.01-11.7%, n=6, p=0.0022 and Μ=15.5%, 11.9-24.1% vs 5.98%, 1.51-16.6%, n=6, p=0.041).In addition, the percentage of IFNγ (+)/CD4 (+) cells of tumor infiltrating lymphocytes was increased in RT +Anti-CD73 groups compared to RT +Anti-Isotype group. (Μ=12.2%, 9.77-18.2% vs 7.09%, 1.55-9.41%, n=6, p=0.0022).Among the patients who received CRT, 13 patients with high expression level of CD73 both in remnant tumor cells and stroma showed significantly worse recurrence free survival and overall survival than other 51 patients (p=0.0059 and p=0.0077).
[Conclusion]RT enhances the tissue expression of CD73, which can provide large amounts of adenosine in irradiated tumor tissue and may seriously suppress the local immune response and impair the response against RT. Our results suggest that functional blockade of CD73 can enhance not only direct but also abscopal effects of RT possibly through T cell activations, which may improve the outcome of the patients with advanced rectal cancer.
Citation Format: Hidenori Tsukui, Hisanaga Horie, Akira Saito, Koji Koinuma, Hideyuki Ohsawa, Hironori Yamaguchi, Yasunaru Sakuma, Yoshinori Hosoya, Kotaro Yoshimura, Alan Lefor, Naohiro Sata, Joji Kitayama. CD73 targeting therapy enhances the anti-tumor effect of radiation therapy in rectal cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2258.