Cell-cycle inhibition and immune microenvironment in breast cancer treated with ribociclib and letrozole or chemotherapy

Abstract In this study, we performed genomic analyses of cell cycle and tumor microenvironment changes during and after ribociclib and letrozole or chemotherapy in the CORALLEEN trial. 106 women with untreated PAM50-defined Luminal B early breast cancers were randomly assigned to receive neoadjuvant ribociclib and letrozole or standard-of-care chemotherapy. Ki67 immunohistochemistry, tumor-infiltrating lymphocytes quantification, and RNA sequencing were obtained from tissue biopsies pre-treatment, on day 14 of treatment, and tumor specimens from surgical resection. Results showed that at surgery, Ki67 and the PAM50 proliferation scores were lower after ribociclib compared to chemotherapy. However, consistent reactivation of tumor cell proliferation from day 14 to surgery was only observed in the ribociclib arm. In tumors with complete cell cycle arrest (CCCA) at surgery, PAM50 proliferation scores were lower in the ribociclib arm compared to chemotherapy (p