Abstract 1080: Altered response to BET-bromodomain inhibitors JQ1 and I-BET-762 targeting c-Myc in erdafitinib-resistant endometrial carcinoma cell line AN3 CA
Authors: VAN RIEL Wilhelmina E , Melis Janneke J. , Vogels Demi H. , Mulder Winfried R. , Kooijman Jeffrey J. , Buijsman Rogier C. , Zaman Guido J. 
Published: July 2021

Abstract
The fibroblast growth factor receptors (FGFRs) 1-4 are receptor tyrosine kinases (RTKs) involved in activation of essential cellular processes such as differentiation, proliferation and migration. Since alterations in FGFRs are common in multiple cancers, including breast cancer, non-small cell lung cancer and endometrial cancer, several kinase inhibitors targeting FGFRs are in clinical development. Erdafitinib, a pan-FGFR inhibitor, has been approved as second-line treatment of locally advanced or metastatic urothelial carcinoma harboring genetic FGFR2 or FGFR3 alterations. Although erdafitinib is very effective against these tumors, progression-free survival lasts only a few months, indicating that resistance also develops fast. Insight into the evolved resistance mechanism is crucial for the development of improved therapies. In this study we generated cell lines resistant to erdafitinib by prolonged culturing of the endometrial carcinoma cell line AN3 CA, harboring FGFR2 gain-off-function mutation N549K, to increasing doses of erdafitinib. To gain insight into the developed resistance, the expression of genes that have previously been reported to be involved in resistance against FGFR inhibitors was analyzed by qPCR and immunoblot. In addition, anti-proliferative effects of target inhibition by small molecules was evaluated. To get an unbiased view on altered gene expression, RNA sequencing (RNA-seq) was performed, followed by Gene-Set-Enrichment Analysis (GSEA). Alterations in FGFR1-4 and cancer hotspot gene sequences were detected by DNA sequencing. Occurrence of resistance to erdafitinib was confirmed in proliferation assays by a decreased response of the erdafitinib-resistant cell lines compared to the parental line. In addition, cross-resistance to other FGFR inhibitors infigratinib, pemigatinib, derazantinib and AZD4547 was observed. Although mRNA expression was altered for several RTKs previously reported to be involved in resistance to FGFR inhibitors, such as EGFR, ERBB2/3 and c-MET, involvement in resistance to erdafitinib could be excluded, as no change in response was observed in proliferation assays with their associated targeted inhibitors. RNA-seq and GSEA indicated upregulation of c-Myc target genes in erdafitinib-resistant cell lines. The involvement of c-Myc in the developed resistance was further confirmed by increased response to BET-bromodomain inhibitors JQ1 and I-BET-762, which indirectly target c-Myc. Furthermore, DNA sequencing identified novel mutations in coding regions of FGFR1 and KRAS genes. In conclusion, we show that multiple factors contribute to the development of resistance against erdafitinib in an FGFR2-mutant endometrial carcinoma cell line. BET-bromodomain inhibitors are of potential interest as therapeutic agents to overcome resistance against FGFR inhibitors.
Citation Format: Wilhelmina E VAN RIEL, Janneke J. Melis, Demi H. Vogels, Winfried R. Mulder, Jeffrey J. Kooijman, Rogier C. Buijsman, Guido J. Zaman. Altered response to BET-bromodomain inhibitors JQ1 and I-BET-762 targeting c-Myc in erdafitinib-resistant endometrial carcinoma cell line AN3 CA [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1080.