– AZ5576 (AZ-5576) is a potent, selective CDK9 inhibitor with an IC 50 <5 nM, decreases phosphorlyation of Ser2-RNAPII in cells with an IC 50 of 96 nM.AZ5576 downregulated Mcl-1 and MYC protein abundance across multiple tested DLBCL cell lines and in primary DLBCL cells.MYC sensitizes DLBCL cells to AZ5576, genetic downregulation of MYC in U-2932 and OCI-LY19 cells resulted in diminished susceptibility to AZ5576.AZ5576 dose-dependently downregulated MYC transcription of a luciferase reporter containing multiple canonical MYC-MAX-binding sites, an effect more pronounced compared with multi-CDK inhibitor dinaciclib.AZ5576 restricts growth of xenografted DLBCL tumors in vivo.