Impact of Optimized Ku–DNA Binding Inhibitors on the Cellular and In Vivo DNA Damage Response.

Simple Summary: DNA-dependent protein kinase (DNA-PK) is a key player in repairing DNA damage. Ku proteins detect DNA damage and activate DNA-PK. Blocking DNA-PK can make cancer treatments such as radiation more effective. We have developed Ku–DNA binding inhibitors (Ku-DBis) that stop DNA-PK activation, making cancer cells more sensitive to radiation. We recently discovered new Ku-DBis that enter cells better than predecessor compounds, allowing cellular and in vivo analyses. Some non-small cell lung cancer (NSCLC) cells, especially those lacking ATM, were very sensitive to these inhibitors and radiation. However, cells lacking BRCA1 were resistant, which reduced the effectiveness of radiation. In animal studies of NSCLC, Ku-DBi treatment inhibited DNA-PK and enhanced a radiation-dependent decrease in tumor cell proliferation. This is the first time a Ku-targeted inhibitor has been shown to work in living organisms, suggesting their potential application in cancer treatment. Background: DNA-dependent protein kinase (DNA-PK) is a validated cancer therapeutic target involved in DNA damage response (DDR) and non-homologous end-joining (NHEJ) repair of DNA double-strand breaks (DSBs). Ku serves as a sensor of DSBs by binding to DNA ends and activating DNA-PK. Inhibition of DNA-PK is a common strategy to block DSB repair and improve efficacy of ionizing radiation (IR) therapy and radiomimetic drug therapies. We have previously developed Ku–DNA binding inhibitors (Ku-DBis) that block in vitro and cellular NHEJ activity, abrogate DNA-PK autophosphorylation, and potentiate cellular sensitivity to IR. Results and Conclusions: Here we report the discovery of oxindole Ku-DBis with improved cellular uptake and retained potent Ku-inhibitory activity. Variable monotherapy activity was observed in a panel of non-small cell lung cancer (NSCLC) cell lines, with ATM-null cells being the most sensitive and showing synergy with IR. BRCA1-deficient cells were resistant to single-agent treatment and antagonistic when combined with DSB-generating therapies. In vivo studies in an NSCLC xenograft model demonstrated that the Ku-DBi treatment blocked IR-dependent DNA-PKcs autophosphorylation, modulated DDR, and reduced tumor cell proliferation. This represents the first in vivo demonstration of a Ku-targeted DNA-binding inhibitor impacting IR response and highlights the potential therapeutic utility of Ku-DBis for cancer treatment. [ABSTRACT FROM AUTHOR]