Inhibitors of poly(ADP-ribose) glycohydrolase (PARG) exhibit single agent therapeutic activity and sensitize glioblastoma cells to ionizing radiation

Abstract Objective The lack of an effective therapy for glioblastoma (GBM) largely results from the intrinsic resistance of GBM cells. The radiosensitizing activity of inhibitors of poly(ADP-ribose) polymerases (PARPs) highlights the important role of poly(ADP-ribose) (PAR) in the DNA damage response. In contrast to PARPs, inhibition of poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for degrading PAR chains, has shown single agent therapeutic activity in non-glioma cancer cells. This work aims to validate the therapeutic potential of PARG inhibitors (PARGi) in GBM. Results Baseline PAR levels were found to vary between different primary and commercial GBM cells, with PARylation increasing upon exposure of cells to ionizing radiation (IR), as expected. Target engagement of a novel PARGi, PDD00017273, was confirmed by the accumulation of nuclear PAR in treated cells. Inhibitor specificity was demonstrated using an inactive control compound and by combining PARGi with the PARP inhibitor olaparib, which blocked the effect. Single agent treatment with PARGi reduced the clonogenic survival of GBM cells in a concentration-dependent manner. Importantly, PARGi also sensitized GBM cells to IR (sensitizer enhancement ratios, SER, ≥ 1.40) Conclusion In contrast to PARP inhibitors, novel PARGi exhibit single agent activity against a panel of GBM cell lines, and also show robust radiosensitizing activity. PARGi therefore have therapeutic potential in this cancer of unmet need.