Association of HER2 alterations and ESR1 mutations in cell-free DNA (cfDNA) with circulating tumor cells (CTCs), multiple metastasis, and prognosis in stage III/IV breast cancer (BCa)

1036 Background: The monitoring of CTCs and cfDNA in metastatic BCa showed ability to predict treatment resistance and survival. Here we report a highly significant correlation between HER2 alterations and ESR1 mutations of cfDNA with CTCs and prognosis of BCa, which may help to predict disease recurrence and treatment benefit. Methods: A total of 85 blood samples (7.5ml/each) were collected from 85 patients with stage III/IV BCa who received treatments at Northwestern RHLCCC. CTC enumeration was performed in FDA approved CELLTRACKS ANALYZERII System (Menarini). Plasma cfDNA was analyzed using Guardant360 NGS-based assay including a 73-gene panel for genomic alterations or mutations. We previously reported cut-off of 5.7% (2018 ASCO) was used to dichotomize the prognostic value of cfDNA percentage. Kruskal-Wallis test was used for statistics. Results: Of the 85 whole samples analyzed, there were 72 samples and 67 samples without ESR1 mutations (ESR1-) and HER2 alterations ( HER2-) respectively, and there are 13 samples and 18 samples that had ESR1 mutations ( ESR1+) and HER2 alterations ( HER2+, 10 amplified, 7 mutated, 1 for both) respectively. CTC positive (≥5) were detected in 13/57 ESR1-HER2- samples (Group 1) and 5/15 ESR1- HER2+ samples (Group 2), 7 /10 ESR1+ HER2- samples (Group 3), 3/3 ESR1+ HER2+ samples (Group 4). The median CTCs number/sample in Group 3 (15 CTCs) and Group 4 (12 CTCs) were significantly higher than Group 1 (0 CTC) and Group 2 (2 CTCs) (P = 0.0020). There were a significant higher average metastasis sites in Group 3 (3 sites) and Group 4 (3 sites) in compared to Group 1 (2 sites) and Group 2 (1 site) (P = 0.0035). Furthermore, patients in Group 4 ( ESR1+ HER2+) has the worst prognosis in compared to other groups (P = 0.0151) on overall survival. Conclusions: Both ESR1 mutations and HER2 alterations in cfDNA contribute to CTCs detection and disease metastasis sites independently, when ESR1 mutations plays a major role. The synergy of ESR1 mutation and HER2 alteration expands the predictive role of liquid biopsy tests monitoring the metastatic prognosis and endocrine resistance for clinical decision-making.