Abstract P3-10-06: Combination of BET and FGFR inhibition as a rational therapeutic strategy for invasive lobular breast cancer
Authors: O'Connor Darran P , Walsh Louise , Haley Kathryn , Moran Bruce , Mooney Brian , Madden Stephen , Das Sudipto , Rueda Oscar , Dowling Catriona , Vareslija Damir , Chin Suet-Feung , Linn Sabine , Young Leonie , Jirstrom Karin , Crown John P , Bernards Rene , Caldas Carlos , Gallagher William M , Chonghaile Triona Ni 
Published: March 2020

Invasive lobular carcinoma (ILC) is the second most common type of breast cancer after invasive ductal carcinoma (IDC), accounting for approximately 10-15% of all breast tumors. ILC is characterized by inactivation of E-Cadherin and neoplastic cells that invade the stroma in a "single-file" pattern. Women with ILC are usually older, have used hormone replacement therapy and are more likely to have hormone receptor-positive disease. ILCs have similar survival to IDCs at both five and 10 years, but despite this, the clinical course is distinct: ILCs are three times more likely to metastasize to the peritoneum, gastrointestinal tract, and ovaries and are more frequently bilateral. Therefore, tailored therapeutic options for this distinct, hard-to-treat subtype of breast cancer are required. As part of the RATHER FP7 HEALTH consortium (www.ratherproject.com), we carried out RNA-Seq analysis of 61 primary ILC samples and identified that high expression of the BET family protein Brd3 (uniquely among BRD family members) was associated with poor recurrence free survival (p=0.03, HR 8.63, CI 1.22-60.85). This observation was further validated in the independent METABRIC cohort (n=99), where again, high Brd3 expression (and not other BRD members) was associated with poor recurrence-free survival (p&lt;0.01, HR=3.16, CI 1.24-8.03). Using a two ILC cell lines (SUM44PE and MDA-MB134VI) we found that ILC cells were relatively resistant to the anti-estrogen therapies tamoxifen and fulvestrant compared to those derived from IDC. Next, we tested whether the ILC cell lines were sensitive to BET protein inhibition using the pan-BET family inhibitor JQ1. Interestingly, while JQ1 inhibited cell growth in both ILC cell lines tested, apoptosis was only induced in SUM44PE cells, while MDA-MB134VI cells exhibited G1 arrest. Dynamic BH3 profiling was used to dissect the underlying anti-apoptotic dependencies in each ILC cell type and showed that in the JQ1-resistant MDA-MB134VI cells, survival was dependent on sustained Bcl-Xl expression and transcriptional rewiring inducing Fibroblast Growth Factor Receptor 1 (FGFR1). This increase in FGFR1 was also evident in invasive ductal carcinoma (IDC) cell lines that also did not respond to JQ1. The combination of JQ1 and FGFR1 inhibitor, or JQ1 and the Bcl2/Bcl-Xl/Bcl-W inhibitor navitoclax (ABT-263), were highly effective at inhibiting growth in both 2D and 3D models of ILC and IDC. Interestingly, we found in the RATHER cohort of 61 ILC patients that 20% had FGFR1 amplification suggesting that this therapeutic approach may have utility in a large number of ILC patients. Conclusions: We provide evidence that BETi either alone or in combination with FGFR1 inhibitors or BH3 mimetics may be a useful therapeutic strategy for recurrent ILC patients.

Citation Format: Darran P O'Connor, Louise Walsh, Kathryn Haley, Bruce Moran, Brian Mooney, Stephen Madden, Sudipto Das, Oscar Rueda, Catriona Dowling, Damir Vareslija, Suet-Feung Chin, Sabine Linn, Leonie Young, Karin Jirstrom, John P Crown, Rene Bernards, Carlos Caldas, William M Gallagher, Triona Ni Chonghaile. Combination of BET and FGFR inhibition as a rational therapeutic strategy for invasive lobular breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-10-06.