A novel Cdk9 inhibitor preferentially targets tumor cells and synergizes with fludarabine.

Elisabeth Walsby,Paul Brennan,Guy Pratt,Chris Pepper,Hao Shao,Abdullah Y Abbas,Shudong Wang,Tracey D Bradshaw,Chris Fegan,Peter M Fischer

doi:10.18632/oncotarget.1568

Abstract

Cdk9 is a key elongation factor for RNA transcription and functions by phosphorylating the C-terminal domain of RNA polymerase II. Here we present direct evidence that cdk9 is important for cancer cell survival and describe the characterization of the potent cdk9 inhibitor CDKI-73 in primary human leukemia cells. CDKI-73 induced caspase-dependent apoptosis that was preceded by dephosphorylation of cdk9 and serine 2 of RNA polymerase II. CDKI-73 was more potent than the pan-cdk inhibitor flavopiridol and showed >200-fold selectivity against primary leukemia cells when compared with normal CD34+ cells. Furthermore, CDKI-73 was equipotent in poor prognostic sub-groups of leukemia patients and showed cytotoxic synergy with the nucleoside analog fludarabine. The Mechanism of synergy was associated with CDKI-73-mediated transcriptional inhibition of MCL1 and XIAP that was maintained when used in combination with fludarabine. Our data present a strong rationale for the development of cdk9 inhibitors such as CDKI-73 as anticancer therapeutics.

Highlights

A number of human cancers, including chronic lymphocytic leukemia (CLL), are associated with the over expression of anti-apoptotic BCL2 family proteins [1,2,3]
Despite the technical challenges associated with genetic modification of primary CLL cells, short-term knockdown of cdk9 was achieved in primary CLL cells to approximately 50% of empty vector controls (Figure 1C) and this resulted in a significant increase in spontaneous apoptosis (Figure 1D)
Cancer cells often appear to demonstrate oncogene addiction for anti-apoptotic proteins in order to maintain their survival advantage and resist apoptosis. One such cancer is chronic lymphocytic leukemia (CLL) in which over expression of BCL2 family proteins is a hallmark of the disease.[3]

Summary

Introduction

A number of human cancers, including chronic lymphocytic leukemia (CLL), are associated with the over expression of anti-apoptotic BCL2 family proteins [1,2,3]. CLL is the most common leukemia in the western world and accounts for almost half of all leukemias in older adults [4]. It is characterized by the progressive accumulation of monoclonal CD5+ B-cells in lymphoid tissues, bone marrow and peripheral blood and by the resistance of neoplastic cells to apoptosis [5]. MCL1, a member of the BCL2 family, is associated with chemoresistance and poor prognosis [7, 8] suggesting that therapeutic strategies targeting this protein may be of particular value in CLL. Given the short half-life of MCL1 [9], one strategy for targeting MCL1 is transient inhibition of transcription

Methods

Results

Conclusion