Abstract 1955: Investigating anti-tumor immunity with CDK4/6 inhibition in triple negative breast cancer
Authors: Guo Qiuchen , Spasic Milos , Goreczny Gregory , Maynard Adam , McAllister Sandra 
Published: July 2021

Abstract
The purpose of our study is to evaluate the immunological effects of CDK4/6 inhibitors in pre-clinical models of triple negative breast cancer (TNBC). CDK4/6 inhibitors, including abemaciclib, are currently approved to treat patients with metastatic hormone receptor-positive (HR+), Her2-negative breast cancer. We previously reported in our HR+ model that CDK4/6 inhibitors promote anti-tumor immunity by inducing tumor cell antigen presentation via activating tumor endogenous retroviral elements, which induce type III interferon production and MHC-I upregulation (Goel, DeCristo, et al., Nature, 2017). We also uncovered important anti-tumor effects on the immune system: CDK4/6 inhibitors decrease T regulatory cell proliferation without affecting cytotoxic CD8 T cell numbers. TNBC has been considered a poor candidate for CDK4/6 inhibitor therapy, as tumors often lose retinoblastoma (Rb) protein expression/function, which is critical for CDK4/6 inhibitor-induced cell cycle arrest. However, Rb mutation or loss occurs in only ~20% of TNBC cases and we found Rb expressing murine and human TNBC cell lines decreased proliferation in vitro in response to abemaciclib. In our preclinical model of TNBC, abemaciclib suppressed tumor growth Consistent with previous findings, TNBC tumor cells upregulated MHC-I upon abemaciclib treatment, suggesting increased antigen presentation. Tumor cell-surface PD-L1 was also increased with abemaciclib both in vitro and in vivo, as assessed by flow cytometry and RT-qPCR. These results are encouraging, given that αPD-L1 therapy (Atezolizumab) in combination with chemotherapy (nab-paclitaxel) has recently been approved as standard care for metastatic TNBC and the IMpassion130 trial reported enhanced progression-free and overall survival in patients with PD-L1+ tumors. However, treatment with Palbociclib did not increase tumor PD-L1 level, suggesting the potential functional variances between different CDK4/6 inhibitors. Our data therefore suggest the potential efficacy of abemaciclib in combination with αPD-L1 in the treatment of Rb+ TNBC. We also found that abemaciclib increased CD8+ and CD4+ T cells and decreased PD1+ CD8 and CD4 cells in the spleen. Furthermore, total numbers of naïve CD8+ and CD4+ T cells increased with abemaciclib treatment, suggesting a favorable anti-tumor systemic immunological effect. We recently performed RNAseq on abemaciclib-treated TNBC and while we do not have the results at this time, they will be analyzed and prepared for presentation at the meeting. Deeper analysis of the mechanisms involved in regulation of PD-L1 expression and enhancement of naïve T cells should enable us to evaluate combination therapies using CDK4/6 inhibitors for this particularly deadly breast cancer subtype.
Citation Format: Qiuchen Guo, Milos Spasic, Gregory Goreczny, Adam Maynard, Sandra McAllister. Investigating anti-tumor immunity with CDK4/6 inhibition in triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1955.