Abstract PO1-16-02: Correlation of Circulating Tumor Cells (CTC) with Clinical Characteristics, Pathological Factors, and Treatment Response in Patients with Metastatic Breast Cancer (MBC)

Abstract Background: Circulating Tumor Cells (CTC) can be isolated in 40-80% of patients with metastatic breast cancer (MBC). High levels of CTC, defined as ≥5/7.5 ml of blood are associated with shorter progression-free survival and overall survival. CTC can be tested for clinically relevant biomarkers, such as ER, and HER2 status, and have the potential to guide therapy as well as help with disease monitoring. We investigated CTC detection rate, its correlation with clinical characteristics, pathological characteristics, and response to treatment in patients with any type of metastatic breast cancer who progressed on at least one line of therapy. Methods: Eligible patients with MBC were enrolled in the IMAGE-II study (Individualized Molecular Analyses Guide Efforts in Breast Cancer) NCT02965755, in which we obtained archival tissue as part of the standard of care, and prospectively collected serial blood samples for CTC analysis. We compared CTC levels and biomarker status with clinical factors, tissue-based pathology, and molecular biomarkers. We assessed whether changes in CTC count correlated with treatment response. Blood samples were collected at baseline (Day 1), after 1-2 weeks of therapy, after 8-12 weeks, and at subsequent restaging (every 8-12 weeks). Medical records were reviewed every 3 months for ongoing treatment response, and death. Samples were processed at the Biocept CLIA-certified Laboratory. CTC were enumerated by the presence of CD45- and DAPI+ cells. Further CTC characterization was performed by antibody staining of specific protein biomarkers (ER, HER2). Fisher’s exact test was used to evaluate the association between baseline CTC 5 and ≥5/7.5ml with patient characteristics. McNemar’s test was used to assess discordance between tissue-based and CTC-defined markers. Results: Between 1/26/2018 and 12/31/22 baseline samples were collected from 70 women with a median age of 56 (36-82) who were enrolled at four study sites. CTC was detected in 59 (84%) of participants. Baseline CTC counts were 5/7.5 ml in 37/59 (63%) and ≥5/7.5 ml in 22/59 (37%) of participants. Differences between breast cancer subtype and CTC-receptor status were observed (Table 1). Among the 41 patients who were ER-positive, 31 (76%) were CTC ER-negative(p 0.001). Among 29 patients who were ER-negative, 4 (14%) were CTC ER-Positive (P 0.001). Among the 7 patients who were HER2 positive, 2(29%) were HER2 positive on CTC. Among 63 patients who were HER2 negative, 7(11%) had HER2 expression on their CTC. The discordance in the classification of HER2 wasn't statistically significant (p=0.77). Elevated CTC at baseline was more frequently detected in younger participants ( 50 years old) (55% vs 27%; P=0.22), in Black women compared to White (60 % vs 29 %, P=0.27), and in participants with visceral vs non-visceral metastasis (52% vs 28 %, P=0.40). Patients with CTC ≥5 vs 5/7.5mL at baseline had a shorter duration of anti-HER2-based therapy (61.5 vs 836 days, p=0.04). There were no statistically significant differences among participants who received chemotherapy agents (316.5 vs 365.5 days, P=0.88), endocrine therapy (376 vs 490 days, P=0.89), or overall therapy (272 vs 390 days, P=0.21). Conclusions: We observed significant differences in the expression of ER between tumor tissue and CTC, which can be partly due to tumor evolution over time. Additionally, participants who are young, Black, and those who have visceral metastasis may have higher CTC counts. Higher CTC counts were associated with a shorter duration of anti-HER2 therapy. Although it didn’t meet statistical significance, a similar trend was observed in patients who received chemotherapy and endocrine therapy. Table 1: MBC Tissue-Based Subtype and CTC-receptor status at baseline Citation Format: Asnakech Bayable, Tingchang Wang, Amanda Blackford, Jessica Tao, Jenna Canzoniero, Seoho Lee, Faith Too, Barbara Blouw, Mary Wilkinson, Rima Couzi, Antonio Wolff, Christie Hiton, Ben Park, Vered Stearns, Cesar Santa-Maria. Correlation of Circulating Tumor Cells (CTC) with Clinical Characteristics, Pathological Factors, and Treatment Response in Patients with Metastatic Breast Cancer (MBC) 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 PO1-16-02.