Abstract NT-094: TARGETED INHIBITION OF HSP90 IMPAIRS DNA-DAMAGE RESPONSE PROTEINS AND INCREASES THE SENSITIVITY OF OVARIAN CARCINOMA CELLS TO PARP INHIBITORS
Authors: Gabbasov Rashid , Benrubi I. Daniel , O'Brien Shane W. , Krais John J. , Johnson Neil , Litwin Samuel , Connolly Denise C. 
Published: June 2021

Abstract
Poly (ADP-ribose) Polymerase inhibitors (PARPi) are a promising class of inhibitors for the treatment of high grade serous ovarian carcinoma (HGSOC). The greatest activity of these agents is seen in patients with defects in DNA damage repair (DDR) mechanisms, including mutation or epigenetic inactivation of BRCA1 and BRCA2 genes and alterations in expression and/or function of DNA repair genes/proteins. PARPi are approved as second-line and maintenance therapies in recurrent HGSOCs. Notably, clinical trials have shown that single agent PARPi show activity in a significant number of HGSOC patients in the absence of alterations in BRCA genes, particularly in patients with platinum sensitive disease, possibly those with tumors exhibiting defects in homologous recombination (HR), or ‘BRCAness'. To extend the benefit of these agents beyond patients with inherent defects in HR, we wished to test the idea that combination of PARPi with agents that functionally abrogate HR could extend the benefit of PARPi's. An attractive molecular target for this purpose is heat shock protein 90 (HSP90) based on its essential role in mediating the maturation and stability of several key proteins required for the DDR. The goal of this study was to test the hypothesis that targeted inhibition of HSP90 with a small-molecule inhibitor ganetespib (STA-9090) would sensitize non-BRCA mutant OC cells to the PARPi talazoparib (BMN-673). To test this hypothesis, we used established OC cell lines (OVCAR3, UWB1.289), and novel OC cells lines (OC-38, OC-1) derived in our laboratory from de-identified tumors isolated from patients with HGSOC. Cells were treated with talazoparib and ganetespib alone and in combination, and the effects of drug treatment on expression of DDR proteins, ionizing radiation (IR)-induced RAD51+ and γH2AX+ foci, and cell viability was assessed. Ganetespib treatment led to dose- and time-dependent depletion of HSP90 client proteins participating in DDR including BRCA1 and 2, MRE11, CDK1, CHK1, RAD51. Treatment with ganetespib also led to a significant reduction in the percentage of RAD51+ nuclei following IR. The quantity of DNA double-strand break marker γH2Ax foci/nucleus decreased over time in vehicle treated, but not in ganetespib-treated cells. We next conducted a comprehensive analysis of cytotoxicity in cells treated with ganetespib and talazoparib alone, in combination and at differing molar ratios. Combination indexes (CI) were calculated to assess additive, synergistic and antagonistic effects using the methods of Greco plus the bootstrap to determine statistical significance. Ganetespib sensitized BRCA1-null UWB1.289 cells to the effects of talazoparib (CI=0.73, p&lt;0.0005). Among the non-BRCA mutant cell lines analyzed, the combination of ganetespib and talazoparib were synergistic in some patient-derived cell lines, but antagonistic in others. Together, our data suggest that ganetespib effectively disrupts critical DDR pathway proteins in HGSOC cells and may sensitize non-BRCA-mutant OC cells to PARPi. From clinical perspective, this implicates the potential of sensitization of some HGSOC patients without HR pathway alterations to PARPi, and potentially other DNA-damage inducing agents.
Citation Format: Rashid Gabbasov, I. Daniel Benrubi, Shane W. O'Brien, John J. Krais, Neil Johnson, Samuel Litwin, Denise C. Connolly. TARGETED INHIBITION OF HSP90 IMPAIRS DNA-DAMAGE RESPONSE PROTEINS AND INCREASES THE SENSITIVITY OF OVARIAN CARCINOMA CELLS TO PARP INHIBITORS [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr NT-094.