BLU-222, a Potent CDK2 Inhibitor, Synergizes with CDK4/6 Inhibitors to Overcome Drug Resistance in HR–Positive and Triple-Negative Breast Cancer by Inducing p21 and p27

Cyclin-dependent kinase 4/6 inhibitors (CDK4/6is) combined with endocrine therapy are the standard first-line treatment for hormone receptor-positive, HER2−negative (HR+/HER2−) metastatic breast cancer, but resistance develops over time. In triple-negative breast cancer (TNBC), the efficacy of CDK4/6i is unclear. 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 showed significant antitumor effects across 8 models, resulting in durable tumor regression and extended survival. Mechanistically, BLU-222, alone or with palbociclib, upregulated p21 and p27 expression and enhanced p21 binding to CDK2 as well as p21 and p27 binding to CDK4 complexes. CRISPR knockout of p21 or p27 in palbociclib-resistant cells eliminated this synergy. Further, RNA sequencing showed that the combination treatment upregulated senescence and interferon pathways, offering insights into the observed therapeutic synergy.