Abstract Background: AR plays a pivotal role in a subset of triple-negative BC, particularly the luminal androgen receptor (LAR) subtype, which is known to be resistant to chemotherapy. Notably, enzalutamide, an AR inhibitor, improved the pathological complete response (pCR) rate in ER+/HER2- BC in a randomized clinical trial, although the underlying biological mechanisms remain unclear. Early-phase studies have implicated preliminary clinical benefit when enzalutamide was combined with ICIs in LAR-TNBC. Building on these clinical findings, our study investigated how AR expression affects the tumor microenvironment (TME) and modulates the efficacy of ICIs in ER+/HER2- BC. Methods: A total of 4070 BC patients from five cohorts with transcriptome of bulk tumors (I-SPY2 GSE173839, TCGA, METABRIC, SCAN-B GSE96058, GSE271080NCT02955395) and two cohorts with Single-cell transcriptome (GSE246613, GSE167036) were analyzed. High expression of AR was defined as the top two-thirds of expression levels for each cohort. LAR-TNBC was defined as the top two-thirds of AR-expressing tumors within the TNBC/Basal subtype. Results: ER+/HER2- BC exhibited substantially lower levels of genomic instability compared with LAR-TNBC in TCGA. This low genomic instability is consistent with the immunologically “cold” phenotype characteristic of ER+/HER2- disease.Among ER+/HER2- BC, AR expression was consistently and negatively associated with number of Tumor-Infiltrating Lymphocytes (TILs) consistently in the TCGA, METABRIC, and SCAN-B cohorts (all p 0.05). Further, AR expression was associated with reduced infiltration of CD8+ T cells, dendritic cells (DCs), M1 macrophages in the METABRIC and SCAN-B cohorts (all p 0.05). Moreover, high AR expression was associated with lower pCR rates after neoadjuvant ICI treatment in ER+/HER2- BC in the I-SPY2 trial (p=0.002). Single-cell transcriptomic analyses revealed that AR was expressed not only in epithelial cells but also in various stromal and immune cell populations, including fibroblasts, T cells, and myeloid cells. Notably, AR expression in epithelial and endothelial cells were inversely correlated with ICI response. Analyzing the NCT02955395 trial cohort, we found that enzalutamide treatment significantly increased the infiltration of CD8+ T cells, DCs, and M1 macrophages in ER+/HER2- BC, similar to that observed in AR-low tumors (all p 0.05). Conclusion: We found that AR expression was inversely associated with immune and myeloid cell infiltration as well as ICI response in ER+/HER2- BC. Given that AR suppression with enzalutamide induced a similar immune microenvironmental profile, combining ICIs with this treatment may provide clinical benefit in ER+/HER2- BC. Citation Format: Jun Arima, Kohei Chida, Rongrong Wu, Takafumi Shima, Toru Kuramoto, Hiroki Hamamoto, Kohei Taniguchi, Nao Kawaguchi, Ryo Tanaka, Yoshiro Imai, Kosei Kimura, Mitsuhiko Iwamoto, Kenichi Hakamada, Takashi Ishikawa, Sang-Woong Lee, Kazuaki Takabe. Androgen receptor expression negatively correlates with immune infiltration and response to immune checkpoint inhibitors in ER-positive/HER2-negative breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2684.
Arima et al. (Fri,) studied this question.