Tracking CDK1 Dynamics With a Novel Biosensor Mouse and Clinical Cohorts to Predict CDK4/6 Inhibitor Sensitivity in...

High-risk estrogen receptor positive (ER+) early breast cancer (EBC) is managed with endocrine therapies and cyclin-dependent kinase (CDK) 4/6 inhibitors, although no standard molecular markers clinically identify these tumors. CDK1 lies immediately downstream of CDK4/6 but has not been investigated as a prognostic marker for ER+ disease. Here, we integrate real-time imaging of a novel Cdk1 activity biosensor mouse with clinical cohorts to establish CDK1 as a candidate biomarker of progression and therapeutic sensitivity. Using the MMTV-PyMT;MMTV-Cre;Cdk1-FRET biosensor mouse, we visualized Cdk1 upregulation in situ during the transition to invasive ductal carcinoma (IDC). Clinically, we identified that pCDK1^T161 is specifically upregulated in IDC, and that pCDK1^Y15 is prognostic of poor relapse-free survival in stage 1 disease. We further characterized CDK1 activity as part of a dynamic adaptive response to CDK4/6 inhibition: the CDK1/cyclin B1 complex was suppressed, then rebounded within 3 days. In the NeoPalAna trial, non-responders incompletely suppressed CDK1. Palbociclib-resistant cells recovered basal CDK1 in vitro, non-responsive tumors exhibited elevated Cdk1 activity in vivo, and CDK1 was elevated in CDK4/6 inhibitor-resistant patient tumors. Together, our findings position CDK1 as a candidate biomarker for poor prognosis in early disease, and an indicator of adaptive responses and resistance to CDK4/6 inhibition.