Synergistic activity of DNA damage response kinase inhibitors in combination with the targeted alpha therapy radium-223 dichloride for metastatic castration-resistant prostate cancer

Radium-223 dichloride (Ra-223, Xofigo®) is the first approved α-particle-emitting radionuclide for the treatment of symptomatic bone metastases in patients with castration-resistant prostate cancer (CRPC) with no known evidence of visceral metastases. Ra-223 is a calcium-mimetic that preferentially binds with the bone mineral hydroxyapatite at areas of high bone turnover, such as bone metastases. This highly localized radiotherapy causes a high frequency of unrepairable double-stranded DNA breaks (DSBs), resulting in a potent antitumor effect on bone metastases. Recent evidence has suggested that patients with mutations in the DNA damage response pathway (DDR), may have differential outcomes to Ra-223 treatment (1). DDR comprises a dynamic network of signalling pathways for the maintenance of genomic integrity. Ataxia telangiectasia mutated (ATM) and Rad3-related (ATR) are critical proteins which orchestrate the DDR and their activation is dependent on the type of DNA lesion. ATM is the primary responder to DSBs whilst ATR is activated by a range of lesions including single strand DNA structures at resected ends of DBSs and after replication fork stalling. In this study, we evaluated the impact of combining DDR kinase inhibitors with Ra-223 to investigate whether a greater radiosensitisation response occurs in comparison to standard X-rays in PC3 and DU145 human prostate cell lines and normal prostate epithelial RWPE-1 cells. Cell assays including clonogenic survival, DNA damage assays and flow cytometry were used to assess the effect of DDR kinase inhibitors in combination with ionising radiation. Cells were pre-treated with DDR inhibitors one-hour before exposure to 2Gy X-rays or an equivalent dose of 0.25Gy Ra-223. Our data show that, in all prostate models, DDR kinase inhibitors in combination with Ra-223 significantly enhanced radiosensitivity (p<0.005) response in comparison to combined treatment with X-rays. Furthermore, a greater quantity of residual DSBs at 24 hours post combination treatment was observed after Ra-223 exposure in comparison to X-ray exposure (p<0.001). Promisingly, this combined treatment had minimal effect on RWPE-1 normal cells. Our findings strongly support the combination of DNA damage induction by Ra-223 with DDR kinase inhibitors as a novel potential treatment option for mCRPC patients in order to improve clinical outcome. References: Velho PI, Qazi F, Hassan S, et al. Efficacy of Radium-223 in Bone-metastatic Castration-resistant Prostate Cancer with and Without Homologous Repair Gene Defects. European Urology. 2019; 76: 170-176.