Abstract 626: Bromodomain inhibition in ovarian cancer and the tumor microenvironment to improve PARP inhibitor response
Authors: Wilson Andrew J. , Hoover Alyssa , Harris Whitney , Liu Esther , Khabele Dineo , Yull Fiona 
Published: August 2018

Background: Ovarian cancer is the most lethal gynecologic malignancy. While women with BRCA-deficient tumors show sensitivity to PARP inhibitors (PARPi), new treatment options are needed for PARPi-resistant tumors. An emerging strategy to improve PARPi response is combination therapy with epigenetic drugs. A newly recognized epigenetic drug target in ovarian cancer is the bromodomain and extraterminal (BET) protein family. BET proteins such as BRD4 promote oncogenic transcription of progrowth and survival genes, including the established link between inflammation and cancer, nuclear factor-kappaB (NF-κB). A complementary strategy to targeting cancer cells with cytotoxic drugs is to activate normal immune processes in the tumor microenvironment (TME). In syngeneic mouse ovarian cancer models, we have shown that M2-like protumor macrophages are a prominent component of the TME, and that NF-κB inhibition reduces the M2 population. Thus, BET inhibitors (BETi) have the potential to induce transcriptional reprogramming in both tumors and macrophages for therapeutic benefit.

Objective: To determine the cellular and molecular effects of combining BETi and PARPi in mouse ovarian cancer and peritoneal macrophage cell lines.

Methods: Cultured wild-type and CRISPR-modified (TP53 and TP53/BRCA2 knockout) ID8 mouse ovarian cancer cells, and PMJ2-PC mouse peritoneal macrophages, were treated with vehicle, the PARPi olaparib, the first-in-class BETi JQ1 or the JQ1/olaparib combination for 24-72h. Sulforhodamine B (SRB) assays assessed cell growth. Immunofluorescence assays assessed adherent cell number, DNA damage (pH2AX) and cell cycle indices. Protein levels of pH2AX and the apoptosis marker cleaved PARP were assessed by Western blot. NF-κB activity was measured by luciferase assays of a transiently transfected reporter plasmid.

Results: Combined JQ1 and olaparib treatment synergistically reduced cell growth in SRB assays in wild-type and TP53 knockout ID8 cells. TP53/BRCA2 knockout cells showed greater responses to PARPi alone and no synergism was observed. Consistent with these results, the JQ1/olaparib combination cooperatively reduced the number of adherent cells and cells in S phase, and increased the G0/G1 population, DNA damage and apoptosis. In contrast, the drug combination had minimal effects on DNA damage or apoptosis in PMJ2-PC macrophages, while NF-κB activity was reduced in both cancer cells and macrophages.

Conclusions: BETi sensitize mouse ovarian cancer cells to the cytotoxic effects of PARPi. Combined drug treatment also has potential to inhibit NF-κB in both cancer cells and macrophages. Our novel immunomodulatory strategy will be tested in ID8 syngeneic ovarian cancer models. We believe BETi combination treatment could expand the use of PARPi in ovarian cancer patients, with the potential to benefit a substantial number of women with this devastating disease.

Citation Format: Andrew J. Wilson, Alyssa Hoover, Whitney Harris, Esther Liu, Dineo Khabele, Fiona Yull. Bromodomain inhibition in ovarian cancer and the tumor microenvironment to improve PARP inhibitor response [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 626.