The CDK9 inhibitor, AZD4573, currently in clinical trials for hematological malignancies, acted synergistically with palbociclib in these ER+ in vitro 2D and 3D models. Additionally, in two independent endocrine- and palbociclib-resistance patient-derived xenografts, treatment with AZD4573 in combination with palbociclib and fulvestrant resulted in tumor regression.