Non-small cell lung cancer (NSCLC) is the leading cause of cancer-related deaths in the US. Treatment most commonly relies on ionizing radiation (IR) and platinum-based DNA damaging agents, but long-term survival is poor and patients tend to suffer chronic side-effects due to the high radiation dose to the surrounding normal tissues. Therefore, new treatments are needed that can be used in combination lower radiation doses.

We have recently reported that low, non-cytotoxic doses Poly ADP ribose polymerase inhibitors (PARPi) Talazoparib in combination with DNA methyltransferase inhibitors (DNMTi) Decitabine (DAC) or azacytidine (AZA) significantly increase cytotoxicity in acute myeloid leukemia and breast cancer models in vitro and anti-tumor effects in vivo. Simultaneous administration of both inhibitors result in increased PARP binding in DNA, leading to higher levels of DNA double strand breaks (DSBs), yielding increased cytotoxicity, compared with each agent treatment alone.

We first studied the efficacy of Talazoparib and AZA combination therapy in multiple NSCLC cell lines (A549, H358 and H838) in vitro through colony forming assays. Results showed, compared to single agent treatments, combination drug treatment significantly decreased colony formation. Cell viability was also significantly decreased with the drug combination in MTS assays (P<0.05). These results also showed synergistic activity between the two drugs, with a combination index of less than 1 for all tested NSCLC cell lines. Importantly, the drug combination increased PARP trapping in chromatin and DSB formation, as measured by immunofluorescent staining for γH2AX. We next determined whether AZA/Talazoparib in combination with a single dose of radiation (2Gy) had an increased anti-cancer effect compared to each modality alone in in vivo mouse xenografts of NSCLC A549. While the combination of PARPi+DNMTi with radiation treatment decreased tumor growth, compared to PARPi/DNMTi alone or RT alone, no significant enhancement with radiation was observed. We next determined whether low doses of fractionated IR (2Gy 3 fractions) would improve the efficacy of DNTMi and PARPi combination treatment. In vitro studies with colony forming assays of NSCLC cell line A549 show that IR doses in combination with PARPi and DNMTi decrease clonogenicity, compared with non-IR controls. Furthermore, A549 xenografts were treated with the drug combination and then irradiated 1 week later with 2Gy daily for 3 consecutive days. Mice treated with the drug combination followed by IR had significant decreases in tumor volume and survival (P<0.05). This suggests that low doses of PARPi and DNMTi therapy in combination with low dose IR can potentially target NSCLC tumors. This represents a novel treatment approach for NSCLC patients that may reduce chronic side-effects of high dose IR.

Citation Format: Christopher Biondi, Daniel Fontaine, Lora Stojanovic, Pratik Nagaria, Rena Lapidus, Eun Yong Choi, Javed Mahmood, Stephen Baylin, Feyruz V. Rassool. Enhancing the therapeutic effects of PARP inhibitors in combination DNA methyl transferase inhibitors, using low doses of ionizing radiation in non small cell lung cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1422. doi:10.1158/1538-7445.AM2017-1422