Abstract PO5-04-05: Clinical validation of the concordance performance of ERBB2 status by single-cells genomics ERBB2 (HER2) amplification Assay in Circulating Tumor Cells (CTCs) from patients with matched HER2 status from metastatic tissue biopsies

Background Liquid biopsies are a non-invasive diagnostic approach for detecting CTCs that may provide clinically actionable information for treatment decisions for metastatic breast cancer (MBC) patients when a conventional biopsy is infeasible. Blood-based liquid biopsy has the potential advantage that it allows broad sampling of metastatic tumor clones because it is capable of and likely to obtain cellular and genomic contributions from more than one metastatic site. Here we report the concordance performance of HER2 positivity by single-cell CTC genomics ERBB2 (HER2) amplification in patients with matched HER2 status from metastatic tissue biopsies. Methods A prospective Clinical Experience Program for CTC ERBB2 (HER2) Assay across 25 different United States-based cancer centers, was used to recruit 128 MBC patients with documented standard-of-care metastatic tissue biopsies performed within the last 5 years (Days from tissue biopsy to liquid biopsy, Median: 19.5; interquartile range: 40.75) CTCs from blood collection at each respective clinical site were isolated from plasma, nucleated cells were plated, & slides were bio-banked for Immunofluorescent staining & subsequent imaging in a CAP CLIA lab. CTCs were identified using Epic Sciences digital imaging & machine learning algorithms. Individual CTCs were sequenced for genomic quantification of chromosomal instability (LSTs) and ERBB2 (HER2) amplification. Results Within the clinical validation cohort of 128 MBC patients, 30% (38/128) were HER2-positive by standard pathologic criteria (IHC 3+ or IHC2+/ISH+) in metastatic tissue biopsies. Among the 64 out of 128 cases with reportable results by the CTC ERBB2 (HER2) amplification on chromosomally unstable CTCs, 31% (20/64) of patients had ERBB2 amplification. Concordance between CTC single cell genomics and available tissue results for HER2 positivity showed a sensitivity of 69% and specificity of 78%. We then addressed the effects of two known biological confounders to the use of tissue biopsy for HER2 status determination: 1) tissue biopsies performed on bone metastases, 2) tumor evolution driven by treatment pressure over time leading to potential HER2 conversion and tumor heterogeneity. Thus, we performed subgroup analysis including only validation set patients with tissue biopsies performed on non-bone tissues, which showed slightly improved concordance to the tissue comparator with a Sensitivity of 86%, and Specificity of 75%. Next, to exclude the possibility of tumor evolution we selected only patients with the HER2 status comparator performed on non-bone tissue biopsies contemporaneous (Days from tissue biopsy to liquid biopsy, Median: 14; interquartile range: 22) to the blood draw for the CTC ERBB2 (HER2) amplification assay. This showed dramatically improved concordance performance with a Sensitivity of 100%, and a Specificity of 75%. Conclusions Blood and tissue are two biologically distinct sample types where variations in HER2 and other biomarker results might be expected due to analytic, temporal and clinical differences. These data indicate that determination of HER2 status by single cell genomic ERBB2 (HER2) amplification assay can aid in the clinical assessment of ERBB2 status in patients with metastatic breast cancer (MBC) for whom tissue biopsy for HER2 status is not available, or infeasible. Additionally, these results provide evidence of the contribution of site differences (bone) of metastatic biopsies and also tumor evolution to discordance between the CTC ERBB2 (HER2) Assay and tumor biopsy. Citation Format: Giuseppe Di Caro, Ernest Lam, Megan Slade, Shuguang Huang, Rick Wenstrup, Lee Schwartzberg. Clinical validation of the concordance performance of ERBB2 status by single-cells genomics ERBB2 (HER2) amplification Assay in Circulating Tumor Cells (CTCs) from patients with matched HER2 status from metastatic tissue biopsies [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 PO5-04-05.