ALBERT

Description: Abstract 1789 Active agents for the treatment of CLL are available. However, CLL eventually progresses to refractory disease. Defects in apoptosis, cytogenetic abnormalities and alterations in non-malignant cells of the micro-environment are likely to be the major causes of therapeutic resistance. Therefore, novel agents are required to abrogate apoptosis blocks and to alter the micro-environment of CLL. One approach targets cyclin-dependent kinases (Cdk). Specifically, Cdk9 promotes initiation and elongation steps in transcription maintaining expression levels of anti-apoptotic proteins with inherently rapid turnover rates. Inhibitors of cyclin-dependent kinases (Cdks) have been previously shown to exert cytotoxic effects on CLL cells by inhibiting Cdk7 and Cdk9. However, inhibitors used in these studies suffer from lack of selectivity, making unambiguous interpretation of results difficult, and leading to unspecific toxicity. Here, we studied the novel Cdk inhibitor LDC-9-A, which exhibits potent and selective inhibitory activity against Cdk9. The in vitro cytotoxicity of LDC-9-A was evaluated after 24 and 48 hours in CLL cases (n=8) and the CLL derived prolymphocytic cell line MEC-1. After a culture period of 48 hours LD50 values ranged between 0.1 and 1.9 μM for primary CLL cells and MEC-1 cells, respectively. Western blot analysis of protein extracts from MEC-1 cells showed that LDC-9-A down-modulates the anti-apoptotic proteins Bcl-2 and Mcl-1 in a dose- and time-dependent manner. In order to study the in vivo anti-CLL activity of LDC-9-A 9–12 month-old transgenic TCL1 mice (n=4) were treated for two consecutive days with an orally bioavailable LDC-9-A formulation (50 mg/kg body weight). Control groups consisted of TCL1 mice receiving vehicle only (n=3) and wild-type control littermates (n=4) receiving LDC-9-A. Numbers of peripheral blood leukemic cells (CD19+CD5+CD3−) and non-malignant T-cells (CD19−CD5+CD3+) were longitudinally evaluated by flow cytometry. Remarkably, LDC-9-A treatment of TCL1 transgenic mice led to a decrease of blood leukemic cells to 6.3±2.0% (mean±SEM) of baseline levels (p