Abstract PO4-06-12: Distinction of basal-like and triple-negative basal-like breast cancers utilizing a novel comprehensive single-cell liquid biopsy-based test

Abstract Introduction: Breast cancer is a highly heterogeneous disease and is a leading cause of death for women worldwide. Triple-negative breast cancer (TNBC), the most aggressive form of the disease, is clinically defined by the lack of expression of estrogen receptor (ER) and progesterone receptor (PR), as well as lack of over-expression of human epidermal growth factor receptor-2 (HER2). Approximately 70% of all TNBC cases exhibit basal-like cells, characterized by high basal expression of keratins as well as extensive genomic instability with copy number gains and losses across most chromosomes. In this study we show that novel single cell genomic evaluation coupled with immunofluorescence (IF) protein analysis of CTCs can detect TNBC with basal-like genomic features as well as basal-like tumors with expression of ER and HER2. Materials and Methods: Epic Sciences’ DefineMBC™ liquid biopsy test characterizes CTC proteomics and genomics as well as ctDNA for the treatment of metastatic breast cancer. Blood samples from 813 metastatic breast cancer patients, with either newly diagnosed or progressing disease, with historical tissue subtype HR(+)HER2(-), HR(+/-)HER2(+), and TNBC supplied by the referring physician were collected and tested. Up to 5 CTCs were selected and isolated from each sample based on morphology and IF signal. DNA was extracted from individual cells, and whole genome amplified and sequenced at low coverage. Copy number analysis was performed to determine copy number gains and losses across chromosomes, as well as large scale-state transitions (LSTs) and ERBB2 amplification. CTCs characterized with high chromosomal instability (e.g., exhibiting 30 LSTs; 1Q firestorm and 8Q stair step) and high levels of cytokeratin expression (mean fluorescence intensity signal 10K) were categorized as basal-like. Results: By previous metastatic tissue biopsy, 77% were HR(+), 8% were HER2(+), and 12% were TNBC. CTCs categorized in DefineMBC as basal-like subtype (9.8 % of all samples tested) showed to be highly present in patients called as TNBC (79% basal-like). Additionally, 54% of HR(+) and 23% of HER2(+) had basal-like CTCS. Patients characterized with TNBC by metastatic tissue biopsy and by DefineMBC exhibited basal-like cells in 88% of the cases. Conclusion: The results show a novel clinical assay which utilizes CTC protein detection as well as single-cell genomic analysis to identify tumors driven by basal-like cells. Generally, TNBC is classified with negative biomarker expression. We show that CTCs in TNBC display genomically basal-like cells. CTCs from HR(+) and HER2(+) tumors may express basal-like features with preserved ER and HER2 expression. The comprehensive nature of DefineMBC™ utilizing genomic analysis of detected CTCs has provided a new tool to diagnose both basal-like TNBC and basal-like hormone receptor positive and HER2 over-expressing tumors. Citation Format: Alessandra Cunsolo, David Bourdon, Ernest Lam, Giuseppe Di Caro, Nilesh Dharajiya, Timothy Pluard, Lee Schwartzberg. Distinction of basal-like and triple-negative basal-like breast cancers utilizing a novel comprehensive single-cell liquid biopsy-based test 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 PO4-06-12.