Valproic Acid Induces Apoptosis in Chronic Lymphocytic Leukemia Cells through Activation of the Death Receptor Pathway and Potentiates TRAIL Response.

Background: The anti-leukemic in vitro activity of valproic acid (VPA), a commonly used antiepileptic agent, was tested on lymphocytes derived from 40 patients with chronic lymphocytic leukemia (CLL) (Binet stage A=34, B=3, C=3). These patients had not been previously treated or remained untreated for the previous 6 months. Combined analysis of ZAP-70, CD38 and IgV_H mutational status was performed for each patient.

Methods: Mononuclear cells were incubated with VPA at 1, 5 and 10 mM for 24 hours. Cell viability was assessed by trypan blue exclusion assay, apoptosis by annexin V/propidium iodide(PI) labelling and PI staining after cell permeabilisation. Caspase activation was studied by flow cytometry analysis after cell treatment with selective caspase inhibitors.

Results: Exposure of CLL cells to VPA resulted in dose-dependent cytotoxicity and apoptosis in all CLL patients tested. VPA-treatment induced apoptotic changes in CLL cells including phosphatidylserine (PS) externalisation and DNA fragmentation. The mean apoptotic rate was similar between IgV_H mutated and unmutated patients or ZAP-70+/ZAP-70- cases. VPA induced apoptosis by the extrinsic pathway involving engagement of the caspase-8 dependent cascade. Although CLL cells are commonly resistant to death receptor-induced apoptosis, VPA increased significantly the sensitivity of leukemic cells to TRAIL (tumor necrosis factor α-related apoptosis-inducing ligand). In addition, VPA overcomed the prosurvival effects of bone marrow stromal cells.

Conclusions: These data indicate that VPA, at the pharmacological concentration of 1 mM, is a potent inducer of apoptosis in CLL and should be further explored as a single agent. Also the combination of VPA and TRAIL may be a promising approach in the treatment of CLL.