DNA-PK inhibitor AZD7648 is a more portent radiosensitizer than PARP inhibitor Olaparib in BRCA1/2 deficient tumors.

The effectiveness of radiotherapy depends on the sensitivities of 'normal' and cancer cells to the administered radiation dose. Increasing the radiosensitivity of cancers by inhibiting DNA damage repair is a goal of much current research, however success depends on avoiding concomitant sensitization of normal tissues inevitably irradiated during therapy. In this study we investigated the mechanisms of radiosensitization for DNA-PK and PARP inhibitors by examining the impacts on proliferating vs quiescent cell populations. Experiments were performed in BRCA1/2null and wild-type parental cancer models in vitro and in vivo. Overall AZD7648 has greater radiosensitizing activity relative to Olaparib, with BRCA2-deficient models showing the greatest sensitivity. However, DNA-PK inhibitor AZD7648 also produced greater toxicity in all irradiated mice. While both DNA-PK and PARP inhibition sensitizes wild type tumor cells to radiation, in BRCA1/2 deficient cells PARP inhibition by Olaparib had limited radiosensitization capacity. Quiescent cells are more radioresistant than proliferating cells, and these were also effectively sensitized by AZD7648 while Olaparib was unable to increase radiation-induced cell kill, even in BRCA1/2null cells. These findings underscore the distinct mechanisms of radiosensitization for DNA-PK and PARP inhibitors. While DNA-PK inhibitors are able to target both proliferating and non-proliferating tumor cells for greater overall anti-cancer benefit, their application is limited by exacerbation of normal tissue toxicities. Conversely, PARP inhibitors exhibit selective activity for proliferating cells, providing a mechanism for targeting activity to cancers, but due to poor activity in non-proliferating cells they have an overall reduced impact on tumor growth control. This study highlights the importance of creating a therapeutic ratio with DNA damage repair inhibition radiation sensitizing strategies.

DNA-PK inhibition extends the therapeutic effects of Top2 poisoning to non-proliferating cells, increasing activity at a cost.

Type II topoisomerase (Top2) poisoning therapy is used to treat a broad range of cancers via induction of double strand breaks (DSBs) in cells undergoing replication and transcription. Preventing the repair of DSBs via inhibition of DNA-PK, an inhibitor of non-homologous end-joining (NHEJ), increases cell kill with Top2 poisons and has led to the initiation of several clinical trials. To elucidate the cellular mechanisms leading to synergistic activity of dual DNA-PK/Top2 inhibition we looked at their effects in cycling versus non-cycling cells, in 3D spheroids and in xenograft models. Combined DNA-PK/Top2 inhibition was found to not only increase the cell kill in proliferating cells, the cell population that is typically most vulnerable to Top2 poisoning, but also in non-proliferative but transcriptionally active cells. This effect was observed in both cancer and normal tissue models, killing more cells than high concentrations of etoposide alone. The combination treatment delayed tumor growth in mice compared to Top2 poisoning alone, but also led to increased toxicity. These findings demonstrate sensitization of Top2ß-expressing, non-cycling cells to Top2 poisoning by DNA-PK inhibition. Expansion of the target cell population of Top2 poison treatment to include non-proliferating cells via combination with DNA damage repair inhibitors has implications for efficacy and toxicity of these combinations, including for inhibitors of DNA-PK currently in clinical trial.