5.36 Effect of Valproic Acid on Fludarabine Activity in Chronic Lymphocytic Leukemia

Abstract

Valproic acid (VPA) is a first-generation anti-epileptic drug with histone deacetylase (HDAC) inhibition activity and has been shown to inhibit proliferation and induce apoptosis in various leukemic cells, including chronic lymphocytic leukemia (CLL). We show that the combination of VPA and fludarabine (VF) are synergistic in vitro against leukemic cell lines (BJAB, I-83 and NALM-6), as well as primary CLL cells. The VF combination induced cleavage of caspase-8, caspase-9, BID and caspase-3, implicating both the intrinsic and extrinsic pathways of apoptosis. This synergistic interaction was also associated with enhanced hyper-acetylation of histones and induction of DNA damage, as compared to VPA and fludarabine alone. The VF combination also induced greater degradation of the inhibitor of B(IB) protein, suggesting a greater liberation of the Nuclear FactorB (NFB) complex. This liberation of NFB appeared to be necessary for the synergy, as inhibiting this step by the addition of Bay-11-7082 was sufficient to suppress the VF synergy. Interestingly, despite the greater reduction in IBprotein levels, the VF combination reduced the levels of the NFB target proteins that inhibit apoptosis, including X-linked inhibitor of apoptosis protein (XIAP). Having established the effects of VPA in vitro, a phase II clinical trial was initiated at CancerCare Manitoba to determine the activity of VPA in CLL, either when used alone or in combination with fludarabine. Five patients, aged 70-79 years, who had received at least 1 prior therapy with fludarabine have to date been examined. Three out of 5 patients were fludarabine-resistant, as defined by a lack of response to fludarabine or relapse 6 months after completion of treatment with fludarabine. No responses were seen after 28 days using VPA alone. However, in 4 patients who continued on VPA with fludarabine, 3 patients showed a 50% fall in lymphocyte/lymph node size after receiving 5 cycles of the combination. In conclusion, VPA enhances the cytotoxic activity of fludarabine in CLL and other lymphoid cells in a synergistic manner, triggering both the intrinsic and extrinsic apoptotic pathways. The VPA-fludarabine combination appears to be also effective in vivo in cases of relapsed/refractory CLL patients and VPA may serve as an effective adjunct for other drugs in this disease.