Abstract P5-11-05: Concordance of genomic alterations with targeted sequencing of circulating tumor DNA (ctDNA) and circulating tumor cells (CTC) in endocrine-resistant metastatic breast cancer

Background: Liquid biopsy has emerged as a valuable strategy for analyzing resistance biomarkers in advanced breast cancer. Differences in genomic alterations between CTC and cfDNA has been reported before and may reflect differences in the cell compartments of origin. Our aim was to determine the mutational concordance between CTC and cfDNA in luminal metastatic breast cancer (MBC) with different levels of endocrine resistance. Methods: We evaluated the mutational spectrum in matched cfDNA and CTC samples of 38 patients with luminal (hormone receptors [HR] positive, HER2 negative) MBC. Using QIAmp Circulating Nucleic Acid Kit, cfDNA was isolated from plasma samples (5 mL), and from CTC samples obtained from plasma (7.5 mL). CTC were isolated by Epcam-based immune-magnetic enrichment and confirmed with AdnaTest Breast Cancer Detect (Epcam, Muc1, HER2). Sequencing of both cfDNA and CTC was performed using the Oncoming Breast cfDNA Assay (150 hotspots, 10 genes). Limit of detection range was 0.01-0.15%, with 92.1% cases equal or less than 0.1%. Concordance between mutational profile of CTC and cfDNA was analyzed and correlated with endocrine resistance and patient characteristics. Results: Forty-seven patients with HR+ HER2- MBC were included, yielding enough cfDNA for analysis in 38 cases (80%), with isolation of CTC in 8 cases (16.7%). Rate of mutation detection in CTC was 87.5% (7/8), with TP53 mutations in all cases and no cases of ESR1 mutations; two cases of ERBB3 and PIK3CA mutations were found only in CTC, without a correlate in cfDNA. In cfDNA, genetic alterations were observed in 63.4% of cases, with 70.8% of TP53 mutations, 37.5% of PI3KCA mutations and 25% ESR1 mutations (2 D538G, 3 Y537S, 2 V392I). Average number of mutations was 2.28 in CTC and 1.96 in cfDNA, with a range of 1-4 in both cases. Concordance between cfDNA and CTC was low, with only 20% of CTC mutations simultaneously detected in cfDNA. CTC were only detected in cases with primary or secondary endocrine resistance. The percentage of patients with genetic alterations in cfDNA varied according to the degree of endocrine resistance (no resistance, 42.8%; secondary resistance, 65.2%; primary resistance, 75%), although the difference was not statistically significant. An association was found between the rate of detection of mutations in cfDNA and the response status (progression disease: 90.9%; stable disease: 57.1%; objective response: 33.3%) (p=0.04). Conclusions: The low rate of CTC detection in HR positive MBC may limit its utility for guiding therapeutic decisions. However, the information provided by CTC sequencing does not completely overlap with the data provided by cfDNA, and may be complementary to cfDNA in endocrine-resistant MBC. Citation Format: Ayala de la Pena F, Navarro Manzano E, Fernandez Perez MP, Garcia Martinez E, de la Morena Barrio P, Ivars Rubio A, Gonzalez Billalabeitia E, Fernandez Sanchez A, Teruel Montoya R, Marin Zafra G. Concordance of genomic alterations with targeted sequencing of circulating tumor DNA (ctDNA) and circulating tumor cells (CTC) in endocrine-resistant metastatic breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-11-05.