Anthony J. Chalmers

This article will present the rationale for combining chemical inhibitors of the DNA repair enzyme poly(ADP-ribose) polymerase (PARP) with conventional cytotoxic agents to improve the treatment of glioblastoma. After a brief review of the current therapeutic options for these aggressive tumours, the possible reasons for their resistance to radiation and chemotherapy will be discussed, highlighting the important role of DNA damage response pathways in many key resistance mechanisms. The dose-limiting toxicities associated with radiation and chemotherapy treatment will be described in order to illustrate the importance of tumour specificity in any attempt to increase the effectiveness of conventional treatments. There will then be a summary of the reasons why targeting DNA repair pathways might achieve tumour specific sensitization. After a brief summary of the key DNA damage response pathways, the biology, biochemistry and pharmacology of PARP and the existing PARP inhibitors will be presented. The major part of the review will cover the effects of combining PARP inhibitors with radiation and chemotherapy in vitro and in vivo, commenting on the underlying mechanisms and indicating where the data are predictive of tumour specific sensitization. Finally, we will consider specific scenarios where PARP inhibitors might contribute to the treatment of glioblastoma patients, discuss the challenges and opportunities associated with early phase clinical testing of these agents, and describe the clinical trials that are either underway or in development.

As single agents, chemical inhibitors of poly(ADP-ribose) polymerase (PARP) are nontoxic and have clinical efficacy against BRCA1- and BRCA2-deficient tumors. PARP inhibitors also enhance the cytotoxicity of ionizing radiation and alkylating agents but will only improve clinical outcomes if tumor sensitization exceeds effects on normal tissues. It is unclear how tumor DNA repair proficiency affects the degree of sensitization. We have previously shown that the radiosensitizing effect of PARP inhibition requires DNA replication and will therefore affect rapidly proliferating tumors more than normal tissues. Because many tumors exhibit defective DNA repair, we investigated the impact of double-strand break (DSB) repair integrity on the sensitizing effects of the PARP inhibitor olaparib. Sensitization to ionizing radiation and the alkylating agent methylmethane sulfonate was enhanced in DSB repair–deficient cells. In Artemis^−/− and ATM^−/− mouse embryo fibroblasts, sensitization was replication dependent and associated with defective repair of replication-associated damage. Radiosensitization of Ligase IV^−/− mouse embryo fibroblasts was independent of DNA replication and is explained by inhibition of "alternative" end joining. After methylmethane sulfonate treatment, PARP inhibition promoted replication-independent accumulation of DSB, repair of which required Ligase IV. Our findings predict that the sensitizing effects of PARP inhibitors will be more pronounced in rapidly dividing and/or DNA repair defective tumors than normal tissues and show their potential to enhance the therapeutic ratio achieved by conventional DNA-damaging agents. Mol Cancer Ther; 9(6); 1775–87. ©2010 AACR.

Glioblastoma multiforme (GBM) are the most common primary brain tumor and are resistant to standard therapies. The nondividing nature of normal brain provides an opportunity to enhance the therapeutic ratio by combining radiation with inhibitors of replication-specific DNA repair pathways. Based on our previous findings that inhibition of poly(ADP-ribose) polymerase (PARP) increases radiosensitivity of human glioma cells in a replication-dependent manner and generates excess DNA breaks that are repaired by homologous recombination (HR), we hypothesized that inhibition of HR would amplify the replication-specific radiosensitizing effects of PARP inhibition. Specific inhibitors of HR are not available, but the heat shock protein 90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) has been reported to inhibit HR function. The radiosensitizing effects of 17-AAG and the PARP inhibitor olaparib were assessed, and the underlying mechanisms explored. 17-AAG down-regulated Rad51 and BRCA2 protein levels, abrogated induction of Rad51 foci by radiation, and inhibited HR measured by the I-Sce1 assay. Individually, 17-AAG and olaparib had modest, replication-dependent radiosensitizing effects on T98G glioma cells. Additive radiosensitization was observed with combination treatment, mirrored by increases in γH2AX foci in G₂-phase cells. Unlike olaparib, 17-AAG did not increase radiation sensitivity of Chinese hamster ovary cells, indicating tumor specificity. However, 17-AAG also enhanced radiosensitivity in HR-deficient cells, indicating that its effects were only partially mediated by HR inhibition. Additional mechanisms are likely to include destabilization of oncoproteins that are up-regulated in GBM. 17-AAG is therefore a tumor-specific, replication-dependent radiosensitizer that enhances the effects of PARP inhibition. This combination has therapeutic potential in the management of GBM. [Mol Cancer Ther 2009;8(8):2243–54]

Since many anti-cancer agents act by inflicting DNA damage on tumour cells, there is increasing interest in the use of inhibitors of DNA repair to increase the cytotoxicity of these agents. Poly(ADP-ribose) polymerase (PARP) is an abundant nuclear enzyme that binds to sites of DNA damage and promotes repair by modifying a number of key proteins. Potent and specific inhibitors of PARP are available; these have been shown to increase the cytotoxicity of a range of anti-cancer agents including temozolomide, irinotecan and radiation. Data from laboratory studies on human tumour cell lines, pre-clinical studies including tumour xenograft models and early phase clinical testing in human subjects are discussed. Pre-clinical and early clinical testing indicates that PARP inhibitors are extremely well tolerated. As single agents they have activity against BRCA1- and BRCA2-deficient cancers, and in combination they increase the cytotoxic effects of certain chemotherapy agents. In order for PARP inhibitors to improve outcomes for patients, their sensitizing effects must be tumour specific. Early clinical data indicate that systemic toxicity may be exacerbated, so future trials must address this issue. The mechanism of action of PARP inhibitors in combination with cytotoxic agents is also uncertain. Among BRCA-deficient cancers, mechanisms of inherent and acquired resistance to PARP inhibitors are under investigation. Combining these agents with radiotherapy appears promising but designing clinical trials to test the efficacy and toxicity of this combination is problematic. A particularly promising role for PARP inhibitors in the treatment of malignant brain tumours is outlined.

Current methods for quantifying effects of DNA repair modifiers on radiation sensitivity assume a constant effect independent of the radiation dose received. The aim of this study was to develop and evaluate a modelling strategy by which radiation dose dependent effects of DNA repair inhibitors on clonogenic survival might be identified and their significance assessed. An indicator model that allowed quantification of the Sensitiser Effect on Radiation response as a function of Dose (SERD) was developed. This model was fitted to clonogenic survival data derived from human tumour and rodent fibroblast cell lines irradiated in the presence and absence of chemical inhibitors of poly(ADP-ribose) polymerase (PARP) activity. PARP inhibition affected radiation response in a cell cycle and radiation dose dependent manner, and was also associated with significant radiation-independent effects on clonogenic survival. Application of the SERD method enabled identification of components of the radiation response that were significantly affected by PARP inhibition and indicated the magnitude of the effects on each component. The proposed approach improves on current methods of analysing effects of DNA repair modification on radiation response. Furthermore, it may be generalised to account for other parameters such as proliferation or dose rate to enable its use in the context of fractionated or continuous radiation exposures.