Abstract PO3-18-08: Rational therapeutic combination of Bromodomain and Extra-Terminal domain (BET) inhibitor and Fibroblast Growth Factor Receptor (FGFR) inhibitor for treatment of invasive lobular carcinoma

Background: Invasive lobular carcinoma (ILC) is the second most pervasive subtype after invasive ductal carcinoma (IDC), accounting for approximately 10-15% of all breast cancers. It is characterized by loss of E-cadherin expression and non-adherent tumor cells that invade the stroma in a “single-file” pattern. Women with ILC are typically diagnosed at an older age and later stage with ER-positive disease. ILC is more likely to exhibit late recurrence and metastasize to the gastrointestinal tract and urogenital tract compared with IDC. It is routinely treated with anti-endocrine therapy and chemotherapy, however, while not entirely chemo-refractory, displays a poor response to chemotherapy compared with IDC. As such, options for recurrent disease are limited and there is an urgent need to develop tailored therapy for ILC, especially for those patients that recur. The family of bromodomain and extra-terminal domain (BET) proteins, comprising BRD2/3/4/T, are epigenetic readers that bind to acetylated lysine residues on histones and recruit transcription factors to drive the expression of oncogenes. Previously. we discovered that BRD3 is a marker of poor prognosis in ILC and there is emerging evidence that BET inhibitors (iBET) are effective in diverse types of breast cancer. Here, we investigated the therapeutic potential of iBET in ILC, alone and in combination with FGFR inhibitors. Methods: IC50s for a panel of iBET using two typical ILC cell lines MDA-MB-134VI (MM134) and SUM44PE (SUM44) were determined. RNA-sequencing and Genexplain analysis was applied to reveal transcriptional networks, master regulators and potential resistance mechanisms. BRD3 and FGFR3 were knocked down using siRNA to evaluate their function in ILC cell lines. Furthermore, we utilized ILC cell-derived xenograft (CDX) models in SCID-beige mice established by mammary intraductal (MIND) implantation to evaluate the therapeutic potency of iBET alone or in combination with fibroblast growth factor receptor (FGFR) inhibitor in vivo. Results: We demonstrated that iBET significantly inhibited ILC cell growth in both 2D and 3D culture, with the greatest potency demonstrated by JQ1 and Mivebresib (ABBV-075). RNA-sequencing revealed dysregulated pathways in cell cycle division, DNA damage, apoptosis and MAPK signaling following iBET treatment. Reverse engineering of transcriptional profiles using Genexplain revealed that FGFR3 is a significantly upregulated master regulator (MTR) among 142 MTRs across both cell lines and both iBETs. Upregulation of FGFR3 after iBET treatment was verified at the protein level by Western blotting. We also show that BRD3 and FGFR3 knockdown significantly inhibited cell growth, which supports the key role both play in ILC progression. Further, we analyzed the iBET therapeutic effect when combined with the FGFR inhibitor, erdafitinib, as a strategy to overcome potential resistance due to FGFR upregulation post iBET treatment. This revealed that the combination of iBET and erdafitinib could inhibit ILC cell growth more effectively compared to using either agent alone. Furthermore, our in vivo study showed that JQ1 could inhibit tumor growth in a SUM44-MIND model and alleviate metastasis to peritoneum, bone and ovary compared with the vehicle group. Moreover, we also assessed the combination of mivebresib and erdafitinib in vivo. This revealed that iBET and an FGFR inhibitor work synergistically to decrease tumour burden and metastatic potential in both MM134-MIND and SUM44-MIND models. Conclusion: Our results provide evidence that iBET, either alone or in combination with erdafitinib, is remarkably effective at inhibiting ILC growth, both in vitro and in vivo and represents a rational therapeutic strategy for recurrent ILC patients in the future. Citation Format: Binbin Gao, Elspeth Ward, Anna Blümel, Emer Conroy, Rachel Moore, Grainne Cremin, Rachel Bleach, Kathryn Haley, Tríona Ní Chonghaíle, Andreas Lindner, Jochen Prehn, Yi Zhang, Idalia Cruz, Leena Hilakivi-Clarke, Georgios Sflomos, Cathrin Brisken, William Gallagher, Darran O'Connor. Rational therapeutic combination of Bromodomain and Extra-Terminal domain (BET) inhibitor and Fibroblast Growth Factor Receptor (FGFR) inhibitor for treatment of invasive lobular carcinoma [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-18-08.