An evaluation in vitro of PARP-1 inhibitors, rucaparib and olaparib, as radiosensitisers for the treatment of neuroblastoma

Background: The radiopharmaceutical I-131-meta-iodobenzylguanidine (I-131-MIBG) is an effective treatment for neuroblastoma. However, maximal therapeutic benefit from I-131-MIBG is likely to be obtained by its combination with chemotherapy. We previously reported enhanced antitumour efficacy of I-131-MIBG by inhibition of the poly(ADP-ribose) polymerase-1 (PARP-1) DNA repair pathway using the phenanthridinone derivative PJ34. Recently developed alternative PARP-1 inhibitors have greater target specificity and are expected to be associated with reduced toxicity to normal tissue. Therefore, our purpose was to determine whether the more specific PARP-1 inhibitors rucaparib and olaparib enhanced the efficacy of X-radiation or I-131-MIBG. Methods: Radiosensitisation of SK-N-BE(2c) neuroblastoma cells or noradrenaline transporter gene-transfected glioma cells (UVW/NAT) was investigated using clonogenic assay. Propidium iodide staining and flow cytometry was used to analyse cell cycle progression. DNA damage was quantified by the phosphorylation of H2AX (gamma H2AX). Results: By combining PARP-1 inhibition with radiation treatment, it was possible to reduce the X-radiation dose or I-131-MIBG activity concentration required to achieve 50 % cell kill by approximately 50 %. Rucaparib and olaparib were equally effective inhibitors of PARP-1 activity. X-radiation-induced DNA damage was significantly increased 2 h after irradiation by combination with PARP-1 inhibitors (10-fold greater DNA damage compared to untreated controls; p < 0.01). Moreover, combination treatment (i) prevented the restitution of DNA, exemplified by the persistence of 3-fold greater DNA damage after 24 h, compared to untreated controls (p < 0.01) and (ii) induced greater G(2)/M arrest (p < 0.05) than either single agent alone. Conclusion: Rucaparib and olaparib sensitise cancer cells to X-radiation or I-131-MIBG treatment. It is likely that the mechanism of radiosensitisation entails the accumulation of unrepaired radiation-induced DNA damage. Our findings suggest that the administration of PARP-1 inhibitors and I-131-MIBG to high risk neuroblastoma patients may be beneficial.