DNA依赖性蛋白激酶亚单位（DNA-PKcs）作为电离辐射（IR）诱导的DNA损伤修复的关键因子，其被IR诱导的丝/苏氨酸位点自磷酸化过程的损害会影响NHEJ修复通路的进行，从而增加细胞对IR的敏感性。我们在前期研究中筛选并首次证实融合多肽BTW3可以抑制DNA-PKcs 的自主磷酸化，从而损害DNA修复并增加了肿瘤细胞的放射敏感性。在本研究中，我们通过离体及在体实验去阐明小分子多肽抑制DNA-PKcs自磷酸化的机制及其生化学特性，然后通过开发新的作用靶点和（或）联合抑制多个自磷酸化位点等方式来优化多肽放射增敏的效果从而使其作用最大化，也研究不同遗传背景的人肿瘤细胞对多肽放射增敏效果的影响。最后我们将小分子多肽连接能靶向肿瘤血管的NGR肽，将其应用在裸鼠移植瘤模型中鉴定其对肿瘤的靶向性及放射增敏效果以及相关机制，从而明确多肽在体内水平对肿瘤的放射增敏作用，为临床上提供新型的靶向放射增敏剂。

DNA-dependent protein kinase subunit (DNA-PKcs) is one of the critical elements involved in ionizing radiation (IR)-induced DNA damage and repair pathways. DNA-PKcs is activated by autophosphorylation on multiple serine or threonine sites in response to IR. Inhibiting its autophosphorylation will affect the None-Homologous End Joining pathway of DNA double-strand break repair and increase the sensitivity of cells to IR. In our previous study, we screened for small inhibitory peptides and identified a peptide (named BTW3) that can inhibit DNA-PKcs autophosphorylation at the ABCDE cluster. BTW3 is capable of inhibiting DNA-PKcs activation and increase cellular radiosensitivity. In this study, we aim to investigate the molecular mechanism of the small inhibitory peptide and to assess the pharmacokinetics and toxicities of the peptide. We will also optimize the radiosensitizing effect of the peptide by identifying new targets and/or combining inhibition of multiple autophosphorylation sites of DNA-PKcs. We will also study the impact of different genetic background of tumor cells on the radiosensitization effect of the peptides. Further, we will conjugate the small peptide with the cyclic tumor-homing peptide, CNGRC, which can target tumor blood vessel cells and has the abilities for cell internalization. That will provide specific in vivo tumor targeting. Finally, we will utilize a human tumor xenograft model in nude mice to characterize the in vivo activity of the radiosensitizing NGR-peptides. Completion of this project will provide strong evidence both in vitro and in vivo for the novel approach to molecular-targeted radiosensitization.