ESR1 mutations in circulating tumor DNA (ctDNA) are associated with CTCs and increased hormone receptors in metastatic tumor tissues of patients with metastatic breast cancer (MBC).

1057 Background: The monitoring of ctDNA and circulating tumor cells (CTCs) in patients with MBC predicts metastasis and prognosis. We previously reported that HER2 and ESR1 alterations in ctDNA were associated with predicted metastasis in MBC (2019 ASCO#1036). Furthermore, ctDNA can be used to evaluate tumor heterogeneity (2020 ASCO #1028). Here, we report that baseline ctDNA ESR1 mutation is a key point associated with tumor tissue characteristics and CTCs, which may help to elucidate disease resistance in MBC. Methods: This study included 288 hormone receptors positive MBC patients who received systemic treatment under an IRB-approved clinical trial (NU16B06) at NU Lurie Cancer Center (2016-2021). Baseline plasma ctDNA was analyzed by Guardant360 NGS for ESR1 mutations. CTC enumerations were performed by using 7.5ml blood in CELLTRACKS (Menarini). Estrogen receptor (ER), progesterone receptor (PR), HER2 and Ki67 in each patient’s biopsy tumor tissue before surgery, surgical tumor tissue and metastatic tumor tissue were evaluated by NU PathCore. Kruskal-Wallis was used for statistics. Results: Of the 288 patients, ESR1 mutations were found in 18 hotspots from 38 patients (ESR1Mut, 13.19%) and there were 250 patients without any mutation (ESR1WT, 86.81%). Median of Total CTCs (/7.5ml) and HER2+ CTCs (/7.5ml) were significantly increased in ESR1Mut group compared to ESR1WT group, total CTCs were 8.0 vs 1.0 (P=0.006) and HER2+ CTCs were 1.5 vs 0 (P=0.014), respectively. There were significant differences on hormone receptors expression (positive cells %) in tumor tissues between ESR1Mut group and ESR1WT group: 1) ESR1Mut group has significant higher expression in ER and PR in biopsy tumor tissues. The mean of ER in ESR1+group was 90.48% vs 48.18% in ESR1WT group (p<0.001). The mean of PR in ESR1Mut group was 40.86% vs 25.60% in ESR1WT group (p<0.001). Meanwhile, there is not significant difference on HER2 expression in ESR1Mut group compared ESR1WTgroup; 2) In the surgical tumor tissues, the mean ER was 97.64% in ESR1Mut group which was significantly higher than 47.90% in ESR1WT group (p<0.001), while the PR was 45.33% and 24.32%, respectively; 3) In metastatic tumor tissues, the mean of ER in ESR1Mut group was 84.75% vs 34.89% in ESR1WTgroup (p<0.001) and the mean of PR in ESR1Mut group was 32.25% vs 9.33% in ESR1WT group (p<0.001). Furthermore, median of Ki67 in ESR1Mut group is 28.33% which was significantly higher than 18.75% in ESR1WT group (p<0.01). Conclusions: Baseline ctDNA ESR1 mutations not only had higher total CTCs and HER2+ CTCs but also significantly correlated with high hormone receptors and proliferation in tumor metastatic tumor tissues. The synergy of ctDNA ESR1 mutation and tissue pathological characteristics expands the early predictive role of ctDNA monitoring metastatic prognosis for clinical decision-making.