Abstract 3453: Selective sensitivity of Ewing's Sarcoma to the PARP inhibitor BMN 673.
Authors: Singh Arun , Chmielowski Bartosz , Jia Weiping , Clarke Kareem , O'Brien Neil , Dering Judy , Conklin Dylan , Shen Yuqiao , Post Leonard , Finn Richard S. , Slamon Dennis J. 
Published: August 2014

Abstract
Background: A principal function of Poly-ADP-Ribose Polymerase (PARP) proteins is to help orchestrate the DNA damage response by recruiting a multitude of proteins to sites of DNA damage. PARP inhibitors have shown particular promise in settings where a preexisting DNA damage repair defect exists, such as in tumors that lack a functional homologous recombination (HR) pathway. PARPs also have roles in epigenetic modulation and PARP inhibition has been shown to affect the cell cycle and cellular stress response. As such, PARP inhibitors may be efficacious in tumors besides those known to have HR defects. We hypothesized that subsets of bone and soft tissue sarcomas could potentially be selectively sensitive to BMN 673, an oral, selective PARP-1/2 inhibitor currently in phase 1 trials (AACR-NCI-EORTC Abstract #B-64, #B-67, 2011).
Methods: We determined the IC50G of BMN 673 across 50 human soft tissue and bone sarcoma cell lines representing 5 main histologies by exposing the lines to six BMN 673 concentrations over a 6-day incubation and using direct cell counts. Synergy was determined by calculating cell growth inhibition in cell lines exposed to BMN 673 along with gemcitabine and SN-38. Translocations in Ewings sarcoma lines were available from public sources. In vivo response was evaluated by monitoring the changes in tumor volume of established subcutaneous xenografts in 6-8wk old CD-1 athymic, nude mice treated daily with BMN 673 by gavage. Baseline gene expression profiles of the cell lines were generated using the Agilent microarray platform.
Results: BMN 673 had a potent anti-proliferative activity in low nanomolar range with 20/24 Ewing's sarcoma lines demonstrating IC50G&lt;24 nM. The IC50G of Ewing's lines to BMN 673 (Mean IC50G: 10.4 nM; n=24 lines) was significantly lower compared to osteosarcoma (Mean IC50G: 108.6 nM, n=11; p=0.0001) and liposarcoma (Mean IC50G: 197.4 nM, n=4; p=0.0001) but not statistically significant compared to rhabdomyosarcoma (Mean IC50G: 19.8 nM, n=9; p=0.18). Ewing's sarcoma cells harboring EWS-FLI1 and EWS-ERG translocations both appear to be highly sensitive to BMN 673 (EWS-FLI1: Mean IC50G: 10.8 nM, n=18; EWS-ERG: Mean IC50G: 9.0 nM, n=5, p=0.61). PARP1 mRNA levels were not different among the very sensitive and the moderately sensitive Ewings lines. Combination of BMN 673 with gemcitabine or SN-38 generated tumor growth inhibition more potent than either agent alone. An in vivo dose-dependent sensitivity to BMN 673 was noted in xenograft studies using 3 Ewing's and 1 osteosarcoma cell line. Microarray analyses for predictors of response/resistance are currently ongoing.
Conclusions: A large panel of Ewings sarcoma cell lines demonstrate selective in vitro and in vivo sensitivity to BMN 673, this response may be augmented by the addition of cytotoxic chemotherapy.
Citation Format: Arun Singh, Bartosz Chmielowski, Weiping Jia, Kareem Clarke, Neil O'Brien, Judy Dering, Dylan Conklin, Yuqiao Shen, Leonard Post, Richard S. Finn, Dennis J. Slamon. Selective sensitivity of Ewing's Sarcoma to the PARP inhibitor BMN 673. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3453. doi:10.1158/1538-7445.AM2013-3453