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Evaluation of the Effects of Endocrine Therapy and Abemaciclib on Host and Tumor Immune Cell Repertoire/Function in Advanced ER+/HER2- Breast Cancer

To characterize the advanced and metastatic breast tumor immune landscape and the impact of CDK4/6 inhibition on its composition and function, we used scRNA-seq to analyze tumor biopsies pre- and post- treatment with ET and the CDK4/6i, abemaciclib. Patients with ER+/HER2- advanced or metastatic breast cancer starting first- or second-line CDK4/6i with ET were enrolled in the “Impact of Endocrine Therapy and Abemaciclib on Host and Tumor Immune Cell Repertoire/Function in Advanced ER+/HER2-Breast Cancer” (NCT04352777) trial. For each patient, the same lesion was biopsied prior to treatment (T1 = baseline, pretreatment) and following four weeks of continuous abemaciclib and ET (T2 = 4 weeks post-treatment). A total of 26 pre- and post-treatment tumor biopsies from diverse sites (recurrent breast and bone, liver, and lymph node metastases) from 13 patients who had varying responses to treatment were analyzed. Following tissue dissociation, positive selection of CD45+ cells was used to enrich for tumor-infiltrating immune cells. We find that genes associated with interferon response are downregulated in T cell populations and genes associated with antigen presentation were upregulated in macrophages and dendritic cells following treatment. Raw single-cell sequencing data from the 10 patients that consented to data sharing will be made available through dbGaP.