Abstract PR11: Dual blockade of BRD4 and the ATR/WEE1 pathway exploits ARID1A loss in clear-cell ovarian cancers

Purpose: Clear-cell ovarian cancer (CCOC) is often resistant to standard chemotherapy and has limited treatment options in the advanced or recurrent setting. ARID1A is the most prevalent mutation, with approximately 50% of all CCOC harboring this mutation. ARID1A, a member of the SWI/SNF family, regulates transcription and has a major role in the repair of DNA lesions, directly facilitating DNA accessibility on the chromatin or indirectly by facilitating the functions of DNA repair proteins. We propose that CCOCs unique genomic alterations (e.g., ARID1A frameshift or nonsense mutations) will increase dependency on chromatin remodeling and DNA repair (e.g., ATR/CHK1/WEE1) pathways for survival. We hypothesize that combination of low-dose small-molecule inhibitors of the BET family (BRD4i) and the DNA damage repair pathway (ATR/CHK1/WEE1), will especially target ARID1A mutant cancer cells promoting mitotic catastrophe, apoptosis, and tumor regression, sparing normal cells. Method: We compared the sensitivity of BRD4i + ATRi/WEE1i combinations in ARID1A knockdown and knockout cells compared with isogenic parental lines. The effects on transcription, kinase signaling, cell cycle, apoptosis, and DNA damage were evaluated. RNAseq/ATACseq and Reverse Phase Protein Array (RPPA) were also performed. A CCOC preclinical drug development platform was established and BRD4i combinations were evaluated in ARID1A mutant and ARID1A wild-type (ARID1AMUT and ARID1AWT) PDX models. Results: Low-dose BRD4i in combination with DNA damage repair inhibitors, ATRi/WEE1i (BDR4i-ATRi or BRD4i-WEE1i), were synergistic in decreasing survival and colony formation with an increase in apoptosis in ARID1AMUT cells compared to ARID1AWT. BRD4i combinations caused a robust G1 arrest with a decrease pCDC6 levels along with an increase in DNA double-strand breaks in the ARID1AMUT or knockout cells. Combination BRD4i-ATRi showed significant tumor regression and increased overall survival compared to standard chemotherapy or monotherapy in several ARID1AMUT PDX models but not in an ARID1AWT PDX. Among BRD4i-DDRi combinations, BRD4i-ATRi is superior to BRD4i-WEE1i in terms of antitumor effect and drug tolerability. Conclusion: Our studies identify a novel drug combination targeting a genetic alteration (e.g., ARID1A) common in CCOC that is highly effective and tolerable. BRD4i in combination with ATRi or WEE1i was synergistic, decreasing survival, colony formation with an increase in DNA damage and apoptosis in ARID1AMUT models. Using our novel CCOC drug development PDX platform, we demonstrated that BRD4i-ATRi combination therapy is more effective than standard chemotherapy or monotherapy alone with acceptable toxicity in ARID1AMUTPDXs. This abstract is also being presented as Poster B24. Citation Format: Yasuto Kinose, Hyoung Kim, Haineng Xu, Sergey Medvedev, Erin George, Sarah Gitto, Margaret Whicker, Dorothy Hallberg, Lauren Schwartz, Gordon Mills, Victor E. Velculescu, Tian-Li Wang, Ronny Drapkin, Fiona Simpkins. Dual blockade of BRD4 and the ATR/WEE1 pathway exploits ARID1A loss in clear-cell ovarian cancers [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr PR11.