CDK2 Inhibition Exerts RB-Independent Antitumor Activity in CDK4/6 Inhibitor–Resistant HR+/HER2− Breast Cancer

Abstract Intrinsic and acquired resistance to CDK4/6 inhibitors (CDK4/6i) is a critical challenge in hormone receptor–positive (HR+), human epidermal growth factor receptor 2–negative (HER2−) metastatic breast cancer (MBC). CDK2 inhibitors (CDK2i) show promise in this context, with several currently under early-phase clinical evaluation. In this study, by analyzing tumor and liquid biopsies from patients treated with the selective CDK2i PF-07104091, we observed that PF-07104091 monotherapy achieved disease stabilization in a cohort of patients with CDK4/6i-resistant HR+/HER2− MBC. Notably, responses were observed irrespective of retinoblastoma (RB) expression and phosphorylation, whereas nonprogressive disease appeared more frequently among patients whose tumors retained wild-type TP53. Studies in established and patient-derived cell lines further substantiated that the growth-suppressive effects of CDK2i were independent of RB status in CDK4/6i-resistant models. CDK2i activity reduced the DNA replication rate, increased DNA damage, and inhibited mitotic entry, with growth inhibition dependent on p53 expression. These findings indicate that CDK dependency shifts from CDK4/6 toward CDK2 as cells transition from a CDK4/6i-sensitive to a CDK4/6i-resistant state. The distinct mechanisms of CDK2i and CDK4/6i support enhanced synergistic activity in HR+/HER2− MBC with acquired resistance to CDK4/6i therapy. Significance: Integrated analysis of CDK2 inhibitor-treated patients and experimental models reveals RB1-independent and p53-dependent mechanisms of CDK2 inhibition in breast cancer, supporting biomarker-guided therapeutic strategies and rational combination approaches.