Abstract 4570: ESR1-mutant circulating tumor DNA is associated with plasma interleukin-2 (IL-2) leading to circulating tumor cell proliferation in metastatic breast cancer

Abstract Introduction: Immune suppression is one of the most important causes of metastasis in metastatic breast cancer (MBC). Monitoring circulating tumor DNA (ctDNA) and circulating tumor cells (CTCs) in patients with MBC is predictive of a worse prognosis. We previously reported that ctDNA ESR1 alterations are associated with disease resistance in MBC (2022-ASCO #1057). Here, we sought to examine if ctDNA ESR1 mutations correlated with plasma IL-2 level, a key cytokine in the MBC immune microenvironment, leading to changes in CTCs levels following systemic treatment. Methods: Whole blood samples (7.5ml/each) were collected from 50 ER+ (12 HER+ and 38 HER2-) patients with MBC before (base line, BL) and 3 months (follow-up evaluation, FE) after systemic therapy at Northwestern Memorial Hospital (IRB-NU16B06). Plasma ctDNA was analyzed using the Guardant360 NGS-based assay and CTC enumeration was performed in the FDA-approved CELLTRACKS System (Menarini). The CTCs were classified as CK+/DAPI+/CD25-. ELISA (Fisher) for IL-2 was analyzed by using patients’ plasma. Kruskal-Wallis test was used for statistics. Results: Of the 50 patients, CTCs were found in 26 patients at BL level and in 24 patients at the follow-up FE level, respectively. ESR1 mutations were identified in 12 hotspots from 10 patients (ESR1Mut, 20%), while 40 patients had no mutations (ESR1WT, 80%). The median number of total CTCs (/7.5 ml) was significantly higher in the ESR1Mut group compared to the ESR1WT group, CTCs were 23.2 vs. 10.3 (P=0.006) at the BL level and 27.8 vs. 14.7 (P=0.014) at the FE level, respectively. Meanwhile, the corresponding IL-2 levels were significantly lower in the ESR1Mut group compared to the ESR1WT group, with levels of 71.9 pg/ml vs. 81.3 pg/ml (P<0.01) at the BL level and 68.3 pg/ml vs. 77.1pg/ml (P<0.01) at the FE level. In the ESR1Mut group after treatment, patients with decreased IL-2 levels (median decrease of 17.38 pg/ml) showed an average increase of 9.4 CTCs, while those with increased IL-2 levels (median increase of 19.8 pg/ml) showed an average decrease of 3.3 CTCs. The results indicate that ESR1 mutations correlated with lower IL-2 levels and higher CTC counts both before and after treatment, and changes in IL-2 levels were inversely correlated with changes in CTC counts. Conclusions: ctDNA ESR1 mutations were associated not only with lower plasma IL-2 levels but also with significantly higher CTC counts, even after systemic treatment. Our new findings demonstrate that, in addition to contributing to endocrine therapy resistance, ESR1 mutations may also suppress plasma IL-2 and weaken the immune environment in MBC, leading to an increase in CTCs escaping immune surveillance and promoting metastasis. The synergy between ctDNA ESR1 mutations and IL-2 expands the predictive role of ctDNA monitoring in assessing metastatic prognosis for clinical decision-making. Citation Format: Qiang Zhang, Justin W. Zhang, Jianhua Jiao, Andrew A. Davis, Lorenzo Gerratana, Youbin Zhang, Xinkun Wang, Paolo D'Amico, Weijun Qin, Lisa Flaum, Akhil Chawla, Janice Lu, Leonidas C. Platanias, William J. Gradishar. ESR1-mutant circulating tumor DNA is associated with plasma interleukin-2 (IL-2) leading to circulating tumor cell proliferation in metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4570.