Background: Despite the development of Checkpoint Inhibitor Treatments (Immunotherapy) and Targeted anticancer agents, cytotoxic (chemotherapy) agents remain the mainstay of breast cancer treatments. However, treatment failure is often encountered in breast cancers owing to innate or acquired chemoresistance. Real time monitoring of inherent or incipient chemoresistance is an unmet need to minimize or prevent treatment failures and improve outcomes. However, prior efforts to determine chemoresistance have used tumor tissue or explants and are hence not suitable for repetitive monitoring. We present findings from a large cohort perspective observational study which shows that Circulating Tumor Associated Cells (CTACs) be isolated in sufficient numbers from peripheral blood and can be profiled in vitro for chemoresistance characteristics. Methods: 15 ml of peripheral blood was obtained from 1410 breast cancer patients, of whom 719 were treatment naïve and 691 had received prior systemic therapies. Matched tumor tissue was obtained by a biopsy (post-blood collection) in a subset of 68 cases. Peripheral blood mononuclear cells (PBMCs) were isolated from all blood samples and treated with an epigenetically activating treatment medium which exerts selective cytotoxicity towards non-malignant hematolymphoid cells and allows survival of apoptosis resistant malignant CTACs, which were defined as cells which were EpCAM+, PanCK+ and CD45+/-. Viable Tumor Derived Cells (TDCs) were harvested from biopsied tumor tissue (N=68). CTACs and TDCs were treated in vitro with cytotoxic chemotherapy agents that are used in Standard of Care (SoC) treatment protocols for breast cancer as single agents or in combinations. In 68 cases, concordance in Chemoresistance Profiles (CRP) was determined between CTACs and corresponding TDCs. In 681 pretreated cases, CRP of CTACs evaluated cumulative (innate and acquired) resistance following prior exposure to chemotherapy agents. In 685 therapy naïve cases, CRP of CTACs evaluated innate chemoresistance towards chemotherapy agents. Results: Among the 68 paired samples of CTACs and TDCs, there were 733 unique combinations of CTAC-TDC-drug, among which 366 pairs (50%) were concordant for chemo-resistance, 336 pairs (46%) were concordant for absence of resistance, and 31 pairs (4%) showed absence of concordance leading to a cumulative concordance of 96% in CRP between TDC and CTACs. Among the 681 pretreated cases, resistance towards ≥1 anticancer agents was observed in 67% of the samples. Among the 685 therapy naïve cases, resistance towards ≥1 anticancer agents was observed in 39% of the samples. Conclusion: The present study shows that sufficient CTACs can be harvested from peripheral blood for meaningful non-invasive chemoresistance profiling and that the resistance profiles of CTACs are concordant with that of tumor tissue. The present approach can identify innate as well as acquired chemoresistance and can guide selection of appropriate therapies. This approach can facilitate real time monitoring of chemoresistance and therapeutic course correction to minimize the risk of treatment failures.

Citation Format: Darshana Patil, Dadasaheb Akolkar, Sanket Patil, Vishakha Mhase, Sachin Apurwa, Sushant Pawar, Rutuja Bhave, Mahesh Ukade, Samruddhi Raut, Harshal Bodke, Shabista Khan, Raja Dhasarathan, Vineet Datta, Stefan Schuster, Cynthe Sims, Jatinder Bhatia, Chirantan Bose, Ajay Srinivasan, Rajan Datar. Non-invasive evaluation of chemoresistance in breast cancers using circulating tumor associated cells [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS4-36.