CDK2 inhibitor BLU-222 synergizes with CDK4/6 inhibitors in drug resistant breast cancers through p21/p27 induction.

Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) combined with endocrine therapy are the standard first-line treatment for hormone receptor-positive, HER2-negative (HR+/HER2-) metastatic breast cancer, but resistance inevitably develops. In triple-negative breast cancer (TNBC), the efficacy of CDK4/6i remains uncertain. Our study shows that the selective CDK2 inhibitor BLU-222, while effective alone, enhances synergistic activity when combined with CDK4/6i in resistant HR+/HER2- and TNBC models, leading to increased apoptosis and cell cycle arrest. In vivo, combining BLU-222 with palbociclib or ribociclib produced significant antitumor activity across eight resistant models, driving durable tumor regression and prolonged survival. Mechanistically, BLU-222, alone or with palbociclib, upregulated p21 and p27 expression, enhanced p21 binding to CDK2 as well as p21 and p27 binding to CDK4. CRISPR knockout of p21 or p27 in palbociclib-resistant cells eliminated this synergy. Further, RNA sequencing revealed that combination treatment upregulated senescence and interferon pathways, providing mechanistic insight into the observed therapeutic synergy.