T-cell prolymphocytic leukemia (T-PLL) is a rare leukemia with an aggressive disease course, characterized by chemotherapy resistance and a median overall survival of less than two years. Apart from defects in DNA damage response genes like ATM and genes of JAK/STAT signaling, overexpression of MYC, often caused by gain of chromosomal material, is frequently observed in T-PLL. Intravenous application of the anti-CD52 antibody Alemtuzumab has been demonstrated to improve progression-free survival and is currently considered the standard of care. In younger patients remissions can be consolidated by allogeneic stem cell transplantation, which results in long-term remission in a small subgroup of patients. The prognosis in the majority of patients relapsing from their disease is dismal and thus novel treatment options are urgently needed. Cyclin-dependent kinase 9 (CDK9), a serine/ threonine kinase which is ubiquitously expressed, is the catalytic unit of the positive transcription elongation factor (P-TEFb). In the presence of cyclin T, p-TEFb stimulates transcription elongation by phosphorylation of the carboxy-terminal domain (CTD) of RNA polymerase II (RNA Pol II). CDK9 inhibition by LDC526, a specific CDK9 inhibitor, leads to reduced phosphorylation of CTD-Ser2, Ser5 and Ser7 of RNA Pol II and subsequently to the downregulation of MYC and MCL-1.


CD3+CD4+ tumor cells from cryopreserved peripheral blood samples of 11 T-PLL patients were enriched to a purity of >95% by fluorescence-activated cell sorting (FACS) or magnetic activated cell separation (MACS). RNA and DNA were isolated using the RNeasy Mini Kit® or the QIAamp DNA blood midi kit® (Qiagen, Hilden, Germany), respectively. To determine IC50 values, we performed cell viability assays for T-PLL cells of 11 patients treated with LDC526 and a panel of reference compounds compared to DMSO controls (CellTiter-Glo Luminescent Cell Viability Assay; Promega, Fitchburg, WI, USA). Gene expression profiling was performed in primary T-PLL cells treated with the CDK9 inhibitor LDC526 in a concentration of 10µM compared to DMSO (n=3) on Clariom S Pico Assays (Affymetrix, Santa Clara, CA, USA) and analyzed by gene set enrichment analysis (GSEA) and GeneTrail analysis. Gene expression measurement was performed with quantitative real-time PCR (qPCR) for MYC (Taqman, Thermo Fisher Scientific, Waltham, MA, USA). Protein expression was analyzed for pCTD Ser2, Ser5 and Ser7, MYC, p-STAT3 and MCL-1 by Western Blot (n=4) after LDC526-treatment in three different concentrations (10, 30 and 100 µM).


LDC526 inhibited T-PLL cell survival in vitro in a dose-dependent manner. The median IC50 was 1.7 µM (range: 0.9 -3.5) which compared favourably to a panel of reference compounds including fludarabine, clofarabine, methotrexate, cytarabine and ruxolitinib (median IC50 values 6.4, 6.2, 29.9, 8.3, and 29.2, respectively). Similar experiments with BAY1143572, a structurally closely related CDK9 inhibitor, which is currently investigated in phase I studies of acute myeloid leukemia, revealed an even lower median IC50 of 0.9 µM (range: 0.5 - 1.7). Of note, the purine analogues fludarabine and clofarabine were only active in some of the patients tested, whereas both CDK9 inhibitors showed activity in all samples analyzed.

Gene expression profiles of LDC526 treated T-PLL cells compared to DMSO controls revealed an enrichment of genes in gene sets associated with interferon and TNFA signatures, MYC targets as well as JAK/STAT genes, which were subsequently confirmed by qPCR experiments. At the protein level LDC526 dose-dependently reduced phosphorylation of CTD-Ser2, Ser7 and to a lesser extent of Ser5 compared to DMSO controls resulting in efficient down-regulation of MYC, MCL-1 and p-STAT3.


Our experiments identify CDK9 as a novel therapeutic target in T-PLL which can be efficiently inhibited by LDC526 and BAY1143572 at clinically meaningful concentrations. CDK9 inhibition resulted in both reduced T-PLL cell survival and down-regulation of MYC and JAK/STAT signaling, which have a key role in the molecular pathogenesis of T-PLL.


Dürig: Lead Discovery Center: Research Funding.

Author notes


Asterisk with author names denotes non-ASH members.

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