Single-Cell Dynamics Determines Response to CDK4/6 Inhibition in Triple-Negative Breast Cancer

Abstract

<b>Purpose:</b> Triple-negative breast cancer (TNBC) is a heterogeneous subgroup of breast cancer that is associated with a poor prognosis. We evaluated the activity of CDK4/6 inhibitors across the TNBC subtypes and investigated mechanisms of sensitivity.<b>Experimental Design:</b> A panel of cell lines representative of TNBC was tested for <i>in vitro</i> and <i>in vivo</i> sensitivity to CDK4/6 inhibition. A fluorescent CDK2 activity reporter was used for single-cell analysis in conjunction with time-lapse imaging.<b>Results:</b> The luminal androgen receptor (LAR) subtype of TNBC was highly sensitive to CDK4/6 inhibition both <i>in vitro</i> (<i>P</i> < 0.001 LAR vs. basal-like) and <i>in vivo</i> in MDA-MB-453 LAR cell line xenografts. Single-cell analysis of CDK2 activity demonstrated differences in cell-cycle dynamics between LAR and basal-like cells. Palbociclib-sensitive LAR cells exit mitosis with low levels of CDK2 activity, into a quiescent state that requires CDK4/6 activity for cell-cycle reentry. Palbociclib-resistant basal-like cells exit mitosis directly into a proliferative state, with high levels of CDK2 activity, bypassing the restriction point and the requirement for CDK4/6 activity. High CDK2 activity after mitosis is driven by temporal deregulation of cyclin E1 expression. CDK4/6 inhibitors were synergistic with PI3 kinase inhibitors in <i>PIK3CA</i>-mutant TNBC cell lines, extending CDK4/6 inhibitor sensitivity to additional TNBC subtypes.<b>Conclusions:</b> Cell-cycle dynamics determine the response to CDK4/6 inhibition in TNBC. CDK4/6 inhibitors, alone and in combination, are a novel therapeutic strategy for specific subgroups of TNBC. <i>Clin Cancer Res; 23(18); 5561-72. ©2017 AACR</i>.