Abstract S2-07: cfDNA analysis from BOLERO-2 plasma samples identifies a high rate of ESR1 mutations: Exploratory analysis for prognostic and predictive correlation of mutations reveals different efficacy outcomes of endocrine therapy–based regimens

Background: The Y537S and D538G mutations in ESR1 are recurrent alterations in metastatic breast cancer (MBC) that promote ligand-independent receptor activation and resistance to estrogen deprivation therapy in laboratory models. The clinical prevalence of these mutations is not well established, but has been reported to be ∼10% by tumor sequencing. Cell free DNA (cfDNA) analysis typically reflects tumor-derived genetic alterations and can be used to characterize a population of patients at a common time point. We hypothesized that cfDNA could be used to detect ESR1 mutation at entry and its clinical impact in a large subset from BOLERO-2 that randomized patients with MBC to Exemestane (EXE) or EXE plus Everolimus (EVE). Methods: Patients with ER+/HER2– MBC with prior exposure to at least 1 non-steroidal aromatase inhibitor (NSAI) were enrolled in BOLERO-2. cfDNA was extracted from 560 baseline plasma samples using QIAamp Circulating DNA kit or QIAsymphony DSP Virus/Pathogen kit. Samples were analyzed by droplet digital PCR for Y537S and D538G. Cox-proportional hazards model was used to assess progression free survival (PFS) in patient subgroups defined by each ESR1 mutation, and the prognostic effect of each ESR1 mutation on overall survival (OS). Results: Of 541 evaluable patients (74.7% of study population), 156 (28.8%) had mutation in ESR1 in D538G (21.1%) and/or Y537S (13.3%) with 30 samples having both mutations. Sequencing of 302 archival tumor specimens (244 primary and 57 metastases) from this study only yielded 4 instances of D538G (1.3%) and 1 Y537S (0.3%). In the overall population, both mutations were poor prognostic factors associated with shorter OS (Table 1). PFS results were different for the two mutations. D538G but not Y537S mutation was associated with a shorter PFS with EXE compared to wild type (WT), (hazard ratios, D538G: 1.44 [95%CIs, 1.04-1.99] and Y537S: 0.92 [95%CIs, 0.44-1.93]). The D538G mutant group derived a similar benefit as WT from the addition of EVE to EXE, whereas the Y537S group did not (Table 2). Conclusions: cfDNA analysis identifies a high rate of the Y537S and D538G ESR1 mutations in ER+ NSAI-treated MBC. As the two mutations may only represent 50-60% of all activating ESR1 mutations, the clinical prevalence of ESR1 mutations in ER+ MBC may be much higher than previously reported. Both mutations appear to be associated with a more aggressive disease biology. Interestingly, these two activating mutations appear to have differential effects on EXE and EVE sensitivity, highlighting new areas for research in ER biology. Citation Format: Chandarlapaty S, Sung P, Chen D, He W, Samoila A, You D, Bhatt T, Patel P, Voi M, Gnant M, Hortobagyi G, Baselga J, Moynahan ME. cfDNA analysis from BOLERO-2 plasma samples identifies a high rate of ESR1 mutations: Exploratory analysis for prognostic and predictive correlation of mutations reveals different efficacy outcomes of endocrine therapy–based regimens. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr S2-07.