Uveal melanoma (UM) is the most common primary intraocular tumor in adults. Despite of important progress in the local therapy, high radioresistance in primary tumor and chemoresistance in metastatic disease are the major obstacles for UM therapy. Therefore, strategies to overcome resistance to radiation or chemotherapy in UM are urgently needed. In this study, we found that phosphorylation of DNA-PKcs, which is the key factor of non-homologous end joining (NHEJ) pathway, was remarkably overexpressed in ionizing radiation (IR)- and Selumetinib resistant UM cells. Increased amount of NHEJ events were also observed in resistant UM cells. Inhibition of DNA-PKcs by NU7441 significantly impaired DNA repair and re-sensitized resistant UM cells to radiation and Selumetinib both in vitro and in vivo. The results demonstrate increased DNA double strand break repair as a mechanism of resistance to ionizing radiation and Selumetinib, and identify DNA-PKcs as a promising target for radio-and chemotherapy in UM patients. (C) 2019 Elsevier Inc. All rights reserved.