Abstract 4585: Radiation combined with checkpoint blockades enhanced antitumor efficacy for osteosarcoma
Authors: Takahashi Yutaka , Yasui Tomohiro , Tamari Keisuke , Minami Kazumasa , Koizumi Masahiko , Seo Yuji , Isohashi Fumiaki , Ohtani Keisuke , Kambe Ryosuke , Ogawa Kazuhiko 
Published: July 2017

Osteosarcoama is one of the common malignancies at bone in children and adolescent. Recent study demonstrated that combination of anti-PD-L1 and anti-CTLA-4 antibodies provided grate control of metastatic osteosarcoma. However, a strong local treatment strategy including surgery or high precision radiation therapy is necessary to elucidate osteosarcoma. Radiation therapy plays an important role in local control for malignant tumors but previous studies demonstrated that radiation enhanced immune response in which not only local tumor regression at irradiated sites but also regression of metastatic tumor outside the radiation field were observed. Although this phenomenon, so called the abscopal effect, is rarely seen, recent studies demonstrated that combination of X-ray irradiation with checkpoint blockades provided higher probability of the abscopal effect for some kind of tumors. However, the effect of x-ray irradiation combined with checkpoint blockade on the abscopal effect for osteosarcoma has been totally unknown. We investigated whether local X-ray irradiation combined with the anti-PD-L1 and anti-CTLA-4 antibodies enhances local and distant antitumor efficacy for osteosarcoma.

LM8 mouse osteosarcoma cells were inoculated into both legs of C3H mice. Mice were treated by 10 Gy X-ray irradiation alone to the tumor in the one side

leg (RAD group) at day 12, 150 ug of anti-PD-L1 and anti-CTLA-4 antibodies (P1C4 group) at days 9, 12, and 15, or those combination (COMB group).

Administration of anti-PD-L1 and anti-CTLA-4 antibodies provided tumor growth delay or complete response at day 37 for about 20% of the mice. X-ray irradiation strongly inhibited tumor growth at irradiated tumor but not in unirradiated tumor. On the other hand, the combination therapy provided the strongest tumor growth inhibition not only at irradiated tumor but also at unirradiated tumor for about 89% of the mice. Accordingly, lung metastasis in mice in COMB group was strongly reduced by 97% with the significant survival benefit compared with the mice in P1C4 group. Flow cytometric analysis revealed that mice in COMB group significantly recruited CD8 tumor infiltrating lymphocytes with moderate reduction of regularly T cells (Tregs), thereby increasing the CD8/Treg ratio. Furthermore, quantitative real time PCR showed significant induction of PD-L1 on irradiated LM8 cells in vitro. The radiation-induced upregulations of PD-L1, B7-1, and B7-2, the ligands of CTLA-4, were also confirmed by flow cytometry, indicating that the enhanced efficacy anti-PDL1 and CTLA-4 antibodies may associate with these upregulations by radiation.

These results suggest that X-ray irradiation contributes to the enhancement of

the efficacy for the distant metastasis as well as local control in the treatment of anti-PD-L1 and anti-CTLA-4 antibodies for osteosarcoma. Our data provide a rational to establish a new therapeutic strategy and start up a clinical trial for osteosarcoma.

Citation Format: Yutaka Takahashi, Tomohiro Yasui, Keisuke Tamari, Kazumasa Minami, Masahiko Koizumi, Yuji Seo, Fumiaki Isohashi, Keisuke Ohtani, Ryosuke Kambe, Kazuhiko Ogawa. Radiation combined with checkpoint blockades enhanced antitumor efficacy for osteosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4585. doi:10.1158/1538-7445.AM2017-4585