Enitociclib, a Selective CDK9 Inhibitor, Induces Complete Regression of MYC+ Lymphoma by Downregulation of RNA Polymerase II Mediated Transcription.

Double-hit lymphoma (DH-DLBCL) is an aggressive, and often refractory, type of B-cell non-Hodgkin lymphoma (NHL) characterized by rearrangements in MYC and BCL2. Cyclin-dependent kinase 9 (CDK9) regulates transcriptional elongation and activation of transcription factors, including MYC, making it a potential targeted approach for the treatment of MYC+ lymphomas. Enitociclib is a well-tolerated and clinically active CDK9 inhibitor leading to complete metabolic remissions in 2/7 DH-DLBCL patients treated with once weekly (QW) 30 mg intravenous administration (i.v.). Herein, we investigate the pharmacodynamic (PD) effect of CDK9 inhibition in preclinical models and in blood samples from patients (DH-DLBCL [n=10] and MYC+ NHL [n=5]) treated with 30 mg i.v. QW enitociclib. Enitociclib shows significant regulation of RNA polymerase II Ser2 phosphorylation in a MYC amplified SU-DHL-4 cell line and depletion of MYC and anti-apoptosis protein MCL1 in SU-DHL-4 and MYC overexpressing SU-DHL-10 cell lines in vitro. Tumor growth inhibition reaching 0.5% of control treated SU-DHL-10 xenografts is achieved in vivo and MYC and MCL1 depletion as well as evidence of apoptosis activation after enitociclib treatment is demonstrated. An unbiased analysis of the genes affected by CDK9 inhibition in both cell lines demonstrates that RNA polymerase II and transcription pathways are primarily affected and novel enitociclib targets such as PHF23 and TP53RK are discovered. These findings are recapitulated in blood samples from enitociclib-treated patients; while MYC downregulation is most robust with enitociclib treatment, other CDK9 regulated targets may be MYC independent delivering a transcriptional downregulation via RNA polymerase II.