Bireociclib (XZP-3287) is a novel potent, selective cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor with IC50 of 2.7/16 nM respectively. Bireociclib (XZP-3287) exhibits a significantly stronger selective inhibitory effect on CDK4 than on CDK6, but with additional inhibitory activity against CDK2 and CDK9. Bireociclib (XZP-3287) reduced the phosphorylation level of the Rb protein at Ser780, thereby impeding tumor cell division and inducing cell cycle arrest at the G0/G1 phase. Bireociclib (XZP-3287) resulted in significant tumor regression in a ZR-75-1-R tamoxifen-resistant breast cancer model.
