DNAPK inhibition preferentially compromises the repair of radiation-induced DNA double-strand breaks in chronically hypoxic tumor cells in xenograft models

Radiation-induced DNA double strand breaks (DSBs) can be repaired by homologous recombination (HR) and non-homologous end joining (NHEJ). Recently, it has been found that chronic tumor hypoxia compromises HR repair of DNA DSBs, but activates the NHEJ protein DNAPK. We therefore hypothesized that inhibition of DNAPK can preferentially potentiate the sensitivity of chronically hypoxic cancer cells to radiation through contextual synthetic lethality in vivo. In this study, we investigated the impact of DNAPK inhibition by a novel selective DNAPK inhibitor, NU5455, on the repair of radiation-induced DNA DSBs in chronically hypoxic and nonhypoxic cells across a range of xenograft models. We found that NU5455 inhibited DSB repair following radiation in both chronically hypoxic and nonhypoxic tumor cells. Most importantly, the inhibitory effect was more pronounced in chronically hypoxic tumor cells than in nonhypoxic tumor cells. This is the first in vivo study to indicate that DNAPK inhibition may preferentially sensitizes chronically hypoxic tumor cells to radiotherapy, suggesting a broader therapeutic window for transient DNAPK inhibition combined with radiotherapy.