Targeting DNA double strand break repair with hyperthermia and DNA-PKcs inhibition to enhance the effect of radiation treatment

// Bregje van Oorschot 1 , Giovanna Granata 1 , Simone Di Franco 2 , Rosemarie ten Cate 1 , Hans M. Rodermond 1 , Matilde Todaro 3 , Jan Paul Medema 1 , Nicolaas A.P. Franken 1 1 Laboratory for Experimental Oncology and Radiobiology (LEXOR), Center for Experimental Molecular Medicine, Department of Radiation Oncology, Academic Medical Center, Cancer Genomics Center, Amsterdam, The Netherlands 2 Department of Surgical, Oncological and Stomatological Sciences (DICHIRONS), Cellular and Molecular Pathophysiology Laboratory, University of Palermo, Palermo, Italy 3 Biomedical Department of Internal and Specialistic Medicine (DIBIMIS), University of Palermo, Palermo, Italy Correspondence to: Nicolaas A.P. Franken, email: n.a.franken@amc.uva.nl Keywords: radiation oncology, DNA repair, hyperthermia, double-strand breaks Received: May 10, 2016 Accepted: August 24, 2016 Published: September 01, 2016 ABSTRACT Radiotherapy is based on the induction of lethal DNA damage, primarily DNA double-strand breaks (DSB). Efficient DSB repair via Non-Homologous End Joining or Homologous Recombination can therefore undermine the efficacy of radiotherapy. By suppressing DNA-DSB repair with hyperthermia (HT) and DNA-PKcs inhibitor NU7441 (DNA-PKcs i ), we aim to enhance the effect of radiation. The sensitizing effect of HT for 1 hour at 42°C and DNA-PKcs i [1 μM] to radiation treatment was investigated in cervical and breast cancer cells, primary breast cancer sphere cells (BCSCs) enriched for cancer stem cells, and in an in vivo human tumor model. A significant radio-enhancement effect was observed for all cell types when DNA-PKcs i and HT were applied separately, and when both were combined, HT and DNA-PKcs i enhanced radio-sensitivity to an even greater extent. Strikingly, combined treatment resulted in significantly lower survival rates, 2 to 2.5 fold increase in apoptosis, more residual DNA-DSB 6 h post treatment and a G2-phase arrest. In addition, tumor growth analysis in vivo showed significant reduction in tumor growth and elevated caspase-3 activity when radiation was combined with HT and DNA-PKcs i compared to radiation alone. Importantly, no toxic side effects of HT or DNA-PKcs i were found. In conclusion, inhibiting DNA-DSB repair using HT and DNA-PKcs i before radiotherapy leads to enhanced cytotoxicity in cancer cells. This effect was even noticed in the more radio-resistant BCSCs, which are clearly sensitized by combined treatment. Therefore, the addition of HT and DNA-PKcs i to conventional radiotherapy is promising and might contribute to more efficient tumor control and patient outcome.