Tumor immune microenvironment of metastatic HR+ HER2- breast cancer treated with CDK4/6 inhibitors.

Abstract

e13059 Background: Adding a CDK4/6 inhibitor (CDKi) to endocrine therapy has meaningfully improves outcomes for patients with metastatic HR+ HER2- breast cancer (mHRBC). In addition to their cytostatic effects on cancer cells, translational work in early-stage HRBC suggested that CDKi have pro-inflammatory effects which could facilitate anti-cancer immune activity. Combination checkpoint inhibitor and CDKi strategies have been studied though toxicities limit further development. CDKi impact on the tumor immune microenvironment (TIME) in mHRBC is not well-defined to date. Methods: We retrospectively identified patient cases with mHRBC treated with a CDKi and available archival samples obtained within 2.5 years of CDKi initiation and 1 year of CDKi discontinuation. FFPE slides from each sample were processed for multiplexed immunohistochemical staining to phenotype immune cell subsets including T cells and functional subsets, dendritic cells, macrophages, and monocytes. Up to ten 1mm 2 regions of interest (ROI) per sample were selected for imaging, optimized for tumor tissue. Individual cells were defined and phenotyped according to a pre-specified hierarchical strategy. Cellular abundance was quantified as immune cell count per mm 2 . Where appropriate due to limitations in sample size, means comparisons were made between groups using the t-test, and unadjusted p values are presented. This study was approved by the OHSU Institutional Review Board. Results: 12 pre-CDKi and 8 post-CDKi samples were collected from 16 unique patients. CDKi resulted in numeric decreases in B cell, CD4+ T cell, CD8+T cell, and monocyte abundance. Decreased macrophage abundance was statistically significant (pre-CDKi mean +/- std dev: 32.37 +/- 30.43 cells/mm 2 ; post-CDKi 12.99 +/- 19.09 cells/mm 2 , p < 0.05) and included CD163+ and CSF1R+ macrophages. Rapid disease progression (rPD) was defined as within 6 months of initiating CDKi, and we observed differences in pre-CDKi immune cell composition between rPD and non-rPD samples. 70% of non-rPD ROI had fewer than 250 immune cells/0.8mm 2 , compared with 22.5% of rPD ROI. rPD samples were characterized by greater abundance of T regulatory cells (rPD 114.81 +/- 70.15 cells/mm 2 ; non-rPD 47.83 +/- 58.61 cells/mm 2 ); and naïve CD8+ T cells (rPD 141.45 +/- 73.51 cells/mm 2 ; non-rPD 56.23 +/- 36.32 cells/mm 2 ). Among our 4 cases with paired pre- and post-CDKi samples, CDKi resulted in an increase in macrophage, Treg and ex-Treg abundance in the rPD case, but decreases in the 3 non-rPD cases. Conclusions: In this dataset of mHRBC samples, CDKi resulted in decreased macrophage abundance rather than increased pro-inflammatory cells. Furthermore, we observed that an immunologically ‘cold’ tumor immune microenvironment was not associated with worse outcome in mHRBC. The TIME of patients with rapid progression during CDKi treatment had higher abundances of immunosuppressive immune cells.