Abstract PO1-23-10: Multi-omics analysis identifies the mechanism in cross-resistance to endocrine therapy and CDK4/6 inhibitor in breast cancer
Authors: Hou Ming-Feng , Li Chung-Liang , Chiang Chih-Po 
Published: May 2024

Abstract
Background
In advanced/metastatic breast cancer (MBC) or recurrent breast cancer on prior endocrine therapy (ET), selective estrogen receptor downregulator (SERD) of fulvestrant and CDK4/6i are the mainstream therapy regimen. In the clinical, fulvestrant combined with CDK4/6i is the second-line therapy for breast cancer (BC) that had progressed during previous endocrine therapy. The alternation of backbone from aromatase inhibitor or tamoxifen (endocrine sensitive) to fulvestrant (endocrine resistance) partially comes from the mechanism of ET resistance. Moreover, in the PALOMA-2 trial, approximately 30% of patients developed into recurrence after two years of CDK4/6i treatment, indicating that the ET backbone may affect CDK4/6i efficacy, bringing an emerging clinical issue of cross-resistance. However, whether the cross-resistance of ET would be more sensitive to CDK4/6i or impair the efficacy of CDK4/6i is still an ambiguous and heterogeneous issue. Also, in CDK4/6i resistance, whether the mechanism of ET resistance is associated with the dysregulation of the cell cycle or activation of other “bypass” signaling pathways is not fully understood. Methods
We use a 3D and 2D culture of ET-sensitive cell S0.5, tamoxifen-resistant cell TAM, and fulvestrant-resistant cell 182R6 as our model. The 3D culture was performed by using the Tools 3d culture plus kit. Drug responses to fulvestrant and ribociclib were conducted by using CCK-8 assay. The potential target and pathway involved in the mechanisms of cross-resistance were analyzed by using multi-omics analysis of sequential window acquisition of all theoretical mass spectra (SWATH-MS), RNA-seq, micro-western array (MWA), traditional western blots, and open access database. Results
In comparison with S0.5, the sensitivity to fulvestrant was indeed reduced in 182R6 cell; however, we found the sensitivity to ribociclib was also reduced in 182R6 cell, which was in contrast to the ribociclib-sensitive TAM cell. The different response to ribociclib between 182R6 and TAM, indicating that cross-resistance between fulvestrant and ribociclib existed in the model of 182R6 and TAM would be another control for 182R6 to find the target with or without cross-resistance. With multi-omics analysis of SWATH-MS, RNA-seq, MWA, western blots, and open access database, we found some unique or high expression of targets and pathways including EGFR, HER2, CDK6, CDK2, MAPK, and TNF-α pathways in cross-resistance R6 cell than in parental and TAM cell. We reconfirmed the above targets by 3D culture and found MAPK signaling pathway of p-ERK was not affected under ribociclib treatment, indicating the important role of the MAPK signaling pathway in ribociclib resistance. Conclusion
To our knowledge, these results provide the first preliminary mechanism of cross-resistance between fulvestrant and ribociclib. We found fulvestrant resistant cell R6 exhibited the phenomenon of cross-resistance to ribociclib, which was in contrast to ribociclib-sensitive TAM cell. We also found some unique or high expression of target and pathway in cross-resistance R6 cells than in parental and TAM cells. In the future, with compound library screening and in vivo animal models, we expect to discover an FDA-approved drug or potential compound to overcome the cross-resistance between fulvestrant and ribociclib.
Citation Format: Ming-Feng Hou, Chung-Liang Li, Chih-Po Chiang. Multi-omics analysis identifies the mechanism in cross-resistance to endocrine therapy and CDK4/6 inhibitor in breast cancer [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 PO1-23-10.