Abstract A44: Clinical significance of ESR1 mutations by cell-free DNA in recurrent/metastatic breast cancer

Purpose: We aimed to develop a droplet digital Polymerase Chain Reaction (ddPCR)-based method for the sensitive detection of estrogen receptor (ER) α (ESR1) mutations of tumor tissues and the corresponding cell-free DNA (cfDNA) in recurrent/ metastatic breast cancer. Experimental Design: We investigated a total of 325 tumor specimens (270 primary breast cancer specimens and 55 ER-positive recurrent/metastatic tumor specimens, in which 33 recurrent/ metastatic tumor specimens had the corresponding plasma). We investigated the quantification of rare ESR1 mutations, four representative types, Y537S, Y537N, Y537C, and D538G in 33 extracted genomic DNA (gDNA) and its corresponding cfDNA using ddPCR system that simultaneously performed thousands of PCRs on a nanoliter scale. Results: A total of 33 (60 %) of 55 recurrent/ metastatic breast cancer specimens had the corresponding plasma which were evaluated for ESR1 mutations' analysis for cfDNA. We verified the correlation of each ESR1 mutation between cfDNA and tissue gDNA. Only ESR1 Y537C was correlated between cfDNA and tissue gDNA (r = 0.56, P = 0.0006). ESR1 Y537S in tissue gDNA were tend to be correlated Y537N in cfDNA (r = 0.35, P = 0.043). CEA (Carcinoembryonic antigen), which is one of the representative breast cancer tumor markers, was correlated with ESR1 Y537S in cfDNA (r = 0.74, P < 0.0001) and tend to be correlated with Y537S in tissue gDNA (r = 0.36, P = 0.033) and D538G in cfDNA (r = 0.54, P = 0.0011). We also evaluated the correlation between the presence of ESR1 mutations and best overall response (BOR) to post-endocrine therapy. BOR to post-endocrine therapy was 25 stable disease (SD) and 8 progressive disease (PD) in 33 patients. Using each selected cut-off, a total of 4 (50%) of 8 PD patients had ESR1 cfDNA mutations as follow: each patient had Y537S, Y537N, D538G, and double mutations (Y537S and D538G). The presence of ESR1 cfDNA mutations was much higher in PD group, compared with SD group (P = 0.0073) Conclusions: We demonstrated the sensitive detection and accurate quantification of low frequency ESR1 mutations in 33 tumor tissues and the corresponding cell-free DNA (cfDNA) using ddPCR assay in recurrent/ metastatic breast cancer. Our results highlight that ESR1 mutations may be of clinical relevance for metastatic breast cancer patient. Citation Format: Takashi Takeshita, Yutaka Yamamoto, Mutsuko Yamamoto-Ibusuki, Toko Inao, Aiko Sueta, Saori Fujiwara, Hirotaka Iwase. Clinical significance of ESR1 mutations by cell-free DNA in recurrent/metastatic breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A44.