Abstract PD2-01: Exploring CDK4/6 inhibitor therapy response and drug resistance development at the single cell level in metastatic breast cancer CTCs
Authors: Trumpp Andreas , Becker Lisa , Fletcher Michael , Würth Roberto , Thippeswamy Anand , Haas Simon , Velten Lars , Michel Laura , Marme Frederik , Jauch Sarah , Becker Mirjam , Steinmetz Lars , Schneeweiss Andreas , Zapatka Marc , Lichter Peter , Sprick Martin 
Published: March 2020

Abstract
Although the survival of patients diagnosed with breast cancer has greatly improved over the last decades, the metastatic stage of the disease still remains incurable. Blood circulating tumor cells (CTCs) are assumed to be the source for metastatic onset and the enumeration of CTCs has already proven its prognostic value for survival in metastatic breast cancer (mBC) patients. However, the biological profiles of CTCs are so far not considered in therapy selection, but hold the potential to display the disease stage in a minimally invasive procedure. Here we serially collect liquid biopsies of mBC patients throughout their course of therapy with the CDK4/6 inhibitor palbociclib to monitor treatment response and drug resistance development. To this end, we combine transcriptomic, genomic and proteomic analyses of primary CTCs on bulk but also on the single cell level. Furthermore, we generated patient-specific, in vitro expanded CTC cell lines to intensively study mechanisms of drug resistance and identify new therapeutic targets by drug sensitivity assays. By analyzing mBC CTCs of serial liquid biopsies we monitored cell cycle arrest upon palbociclib treatment and drug resistance development. Drug sensitivity and successive drug resistance can be observed in primary and in vitro expanded, patient-specific CTCs. As revealed by RNA-seq of bulk and single CTCs, acquired resistance to palbociclib in mBC CTCs is associated with altered transcriptional programs including RB transcriptional corepressor 1 (RB1) function and can be linked to a loss-of-function mutation in the RB1 gene. Interestingly, apart from altered RB1 function, we can identify a global transcriptional transition from palbociclib sensitivity to resistance. Based on the transcriptomic information, pathways that are upregulated in resistant CTCs were determined and the patient-derived CTC cell lines were used to validate and specifically target tumor cells that are resistant to CDK4/6 inhibitors. Using drug screening assays on the CTC-derived cell lines, we identified potential drugs that in combination with palbociclib could circumvent the acquired resistance and help to prolong patient survival. Our study indicates that information about the biological state of serially collected CTCs can help to earlier detect the onset of resistance and should be included in clinical disease management. This will help to spare the patient severe side effects from ineffective drug treatment.
Citation Format: Andreas Trumpp, Lisa Becker, Michael Fletcher, Roberto Würth, Anand Thippeswamy, Simon Haas, Lars Velten, Laura Michel, Frederik Marme, Sarah Jauch, Mirjam Becker, Lars Steinmetz, Andreas Schneeweiss, Marc Zapatka, Peter Lichter, Martin Sprick. Exploring CDK4/6 inhibitor therapy response and drug resistance development at the single cell level in metastatic breast cancer CTCs [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 PD2-01.