Radiosensitization by erlotinib and veliparib in esophageal squamous cell cancer.

Abstract

70 Background: Dysregulation of the HER/EGFR family is identified in esophageal cancers and confers resistance and inferior survival rates. In addition to their unique selectivity in killing DNA repair-deficient tumors, poly-ADP ribose polymerase inhibitors (PARPi) can enhance radiation-induced cytotoxicity. We and others have also previously demonstrated attenuation of DNA repair capacity in HER-inhibited cells to induce a contextual synthetic lethal interaction with PARPi. We thus hypothesized that erlotinib, a tyrosine kinase inhibitor directed against HER1/HER2, could induce a transient DNA repair deficit and subsequently increase DNA damage with the PARPi veliparib in esophageal cancer cells while increasing tumor radiation (RT) sensitivity. Methods: Esophageal SCC cell lines (KYSE-30, KYSE-410, and OE-21) were treated with combinations of vehicle, erlotinib, veliparib (a PARP1/2 inhibitor), and RT. DNA damage and repair and signaling proteins were assessed by immunofluorescence staining of cells for DNA damage and repair foci and/or western blot analysis. Cell viability and cytotoxicity were determined via cell proliferation assays and colony formation assays, respectively. Tumor growth delay was assessed in mice bearing esophageal tumor xenografts. Results: Consistent with our hypotheses, erlotinib increased γ-H2AX foci, a marker of DNA double strand breaks (DSBs), in all three esophageal SCC tumor cells. This coincided with reduced DSB-repair capacity as assessed via RAD51 foci and pDNA-PK. Triple combination with erlotinib, veliparib, and RT demonstrated enhanced cytotoxicity. A subsequent increase in Annexin positive cells was observed, indicative of activation of the apoptotic response. Importantly, the triple combination was most effective in suppressing the growth of esophageal tumor xenografts in vivo (2– to 5.5–fold lower tumor volume) relative to other treatment groups. Furthermore, we observed reductions in tumor volume from baseline with this triple combination in 3/6 mice. Conclusions: Thus, the combination of erlotinib, PARPi, and RT can be an innovative and effective treatment strategy to enhance the therapeutic ratio and improve outcomes in esophageal SCC cancer patients.