Ku–DNA binding inhibitors modulate the DNA damage response in response to DNA double-strand breaks

The DNA-dependent protein kinase (DNA-PK) plays a critical role in the DNA damage response (DDR) and non-homologous end joining (NHEJ) double-strand break (DSB) repair pathways. Consequently, DNA-PK is a validated therapeutic target for cancer treatment in certain DNA repair-deficient cancers and in combination with ionizing radiation (IR). We have previously reported the discovery and development of a novel class of DNA-PK inhibitors with a unique mechanism of action, blocking the Ku 70/80 heterodimer interaction with DNA. These Ku–DNA binding inhibitors (Ku-DBi's) display nanomolar activity in vitro, inhibit cellular DNA-PK, NHEJ-catalyzed DSB repair and sensitize non-small cell lung cancer (NSCLC) cells to DSB-inducing agents. In this study, we demonstrate that chemical inhibition of the Ku–DNA interaction potentiates the cellular effects of bleomycin and IR via p53 phosphorylation through the activation of the ATM pathway. This response is concomitant with a reduction of DNA-PK catalytic subunit (DNA-PKcs) autophosphorylation at S2056 and a time-dependent increase in H2AX phosphorylation at S139. These results are consistent with Ku-DBi's abrogating DNA-PKcs autophosphorylation to impact DSB repair and DDR signaling through a novel mechanism of action, and thus represent a promising anticancer therapeutic strategy in combination with DNA DSB-inducing agents.