Abstract 870: Radiation-induced PD-L1 upregulation can be detected by Zr-89-PD-L1 PET/CT in the tumor micro-environment of murine HPV positive HNSCC model and melanoma model

Background: Radiation therapy (RT) can induce upregulation of programmed death ligand 1 (PD-L1) on tumor cells or myeloid cells, which might affect the response to RT with or without anti-PD-1/PD-L1 blockade. RT-induced PD-L1 expression during therapy could be a predictive marker for the therapy response, however, serial biopsies to monitor its distribution may be unreliable because its expression is heterogeneous. Therefore, non-invasive imaging of tumor PD-L1 expression could be more helpful. Materials and Methods: Two different murine tumor models (MEER and B16F10) were established in two locations, at the back of the neck and at the right flank of C57BL/6 mouse. Then fractionated RT (2Gyx4 or 2GyX10) with or without anti PD-1 therapy was delivered only to the neck tumor. PD-L1 expression was measured by PET/CT and biodistrubtion with Zr-89-DFO-anti-mouse PD-L1 monoclonal Ab (clone 10F.9G2) and the results were corroborated by flow cytometric analysis and immunohistochemistry. PET/CT imaging and biodistributions were performed 48 or 96 hours after tracer-injection. The change of PD-L1 expression on irradiated neck tumor was evaluated using non-irradiated flank tumor as a control. Endothelial cell morphology in tumor vessels was also analyzed by CD31 staining to determine whether RT-induced increased permeability of tumor vessels might result in increased non-specific tracer accumulations in the irradiated tumors. Results: PET/CT imaging and biodistribution study of ex-vivo tracer uptake values demonstrated significant increased tracer uptake in irradiated neck tumor compared to non-irradiated flank tumor in either case of MEER and B16F10. Tracer uptakes in the spleens were significantly decreased if high dose of non-labeled anti-mouse PD-L1 monoclonal Ab was given in advance before tracer-injection, which indicates the tracer is fully functional. Flow cytometry and immunohistochemistry showed PD-L1 upregulation in both irradiated tumors corroborating PET/CT imaging of RT-induced PD-L1 upregulation. Flow cytometry also revealed RT-induced PD-L1 upregulation on myeloid cells is more prominent than that of tumor cells in both MEER and B16F10. CD31-positive staining in the irradiated tumors was not different from that of non-irradiated tumors, which shows RT did not induce alterations in tumor blood vessels. PD-L1 upregulation of MEER was not seen unless it was performed 2GyX10 RT, however, that of B16F10 could be seen at an earlier time-point of 2GyX4. Combination therapy of anti PD-1 with RT did not show further PD-L1 upregulation compared to RT without anti PD-1 therapy. Conclusion: RT-induced PD-L1 upregulation in different tumor models of MEER and B16F10 was identified by PET/CT using Zr-89 labeled PD-L1 monoclonal Ab and its validity was corroborated by flow cytometric analysis and immunohistochemistry. Citation Format: Masahiro Kikuchi, Raghvendra M Srivastava, David A Clump, Julio A Diaz-Perez, Lingyi Sun, Dexing Zeng, W. Barry Edwards, Carolyn J Anderson, Robert L Ferris. Radiation-induced PD-L1 upregulation can be detected by Zr-89-PD-L1 PET/CT in the tumor micro-environment of murine HPV positive HNSCC model and melanoma model [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 870. doi:10.1158/1538-7445.AM2017-870