Prognosis and tumor-infiltrating lymphocytes (TILs) in estrogen receptor (ER)-low metastatic breast cancer (MBC): Analysis from a large multi-institutional cohort and the TONIC phase II trial with nivolumab.

Abstract

1100 Background: Although 1% as ER cutoff to discriminate ER+/HER2- from triple-negative (TN) BC serves the purpose of maximizing access to endocrine therapy, it may be suboptimally informative in terms of prognosis and immune checkpoint inhibitor (ICI) benefit. In the early setting, we reported that ER-low BC (ER 1-9%/HER2-) is immunologically and prognostically similar to TN (ER 0%) [Massa, 2023]. Here we focus on MBC and aim to assess prognosis of ER-low BC, including the conversion from ER+(≥10%)/HER2- to ER-low at relapse. We also compared ER-low and TN in terms of outcome after ICI, and TILs. Methods: We included two cohorts of MBC patients (pts): a multicentric retrospective cohort and pts with ER<10%/HER2- MBC enrolled in the TONIC trial testing nivolumab +/- induction therapy. All pts had ER and HER2 status assessed on metastatic tumor lesion; in the retrospective cohort ER and HER2 status on primary tumor was also available. Survival endpoints in the retrospective cohort: overall survival (OS, from BC diagnosis), post-relapse survival (PRS, from BC relapse). Exploratory endpoints from the TONIC trial: clinical benefit rate (CBR), OS and PFS (both from randomization). TILs were assessed on MBC samples (both cohorts) and on matched primary tumor (retrospective cohort). Results: We included 1112 pts in the retrospective cohort: the prevalence of ER-low BC was 3.6% in primary tumor and 5.9% in MBC lesions. In the TONIC trial, 15/110 pts had ER-low MBC. In the retrospective cohort, ER-low and TN MBC showed similar outcome, and significantly unfavorable vs ER+/HER2- MBC (Table). Among pts with ER+/HER2- primary BC, 6.7% converted to ER-low and 14.7% to TN at relapse. ER+/HER2- BC pts converting to ER-low had similar outcome than those converted to TN, and significantly poorer than pts with stable ER+/HER2- (Table). In the TONIC trial, ER-low MBC pts, compared to TN, showed similar response to ICI (CBR: 20.0% vs 22.1%, p=1), similar PFS and numerically worse OS (Table). TIL levels on MBC samples were similar between ER-low and TN both in the retrospective (median: 6% vs 5%, p=0.83) and the TONIC cohorts (median: 5.5% vs 5.0%, p=0.56) and were higher than ER+/HER2- (median: 3%). Conclusions: We confirmed that ER-low and TN MBC show similar poor outcome and the shift from ER+/HER2- to ER-low has a comparable unfavorable prognostic impact compared to conversion to TN. Our results support the hypothesis that ER-low and TN are immunologically akin and similarly prone to respond to ICI. [Table: see text]