The Novel HSP90 Inhibitor IPI-504 Synergizes with Bortezomib in Mantle Cell Lymphoma Cells by Targeting Both NF-kappaB Signaling and Unfolded Protein Response and Leading to Increased Mitochondrial Apoptosis.

Mantle cell lymphoma (MCL) is an aggressive B lymphoid neoplasm with a mature B-cell phenotype and genetically characterized by the t(11;14)(q13;q32) leading to cyclin D1 overexpression with the consequent deregulation of cell cycle at the G1-S checkpoint. MCL cells present a constitutive activation of the NF-kappaB pathway which leads to the overexpression of several anti-apoptotic regulators. Recently, MCL cells have been shown to express high levels of the chaperone heat shock protein of 90 kDa (HSP90) and to respond well to the ansamycin derivative 17-AAG, an HSP90 inhibitor. We have analyzed the sensitivity to the novel, highly soluble, 17-AAG derivative IPI-504 (Infinity Pharmaceuticals) on a panel of eleven human MCL cell lines and primary cells from MCL patients, which differ in their p53-dependent pathway status, growth characteristics and sensitivity to cytotoxic drugs. We observed that IPI-504 heterogeneously exerted cytostatic effect among MCL samples, with IC₅₀ ranging from 0.06 to 15.4 μM, irrespective of the mutational status of the client protein p53 and/or expression levels of other client proteins such as cyclin D1 and BCL-2. IPI-504 activity involved the downregulation of the client proteins IKKb and phospho-AKT, and consequent inhibition of NF-kappaB signaling. In the most sensitive samples, these events led to the induction of cell death characterized by loss of mitochondrial membrane potential, activation of caspases and phophatidylserine exposure. IPI-504 cytotoxic activity was increased by cotreatment with pharmacological inhibitors of IKK and AKT. Noteworthy, IPI-504 showed remarkable synergistic interaction with the proteasome inhibitor bortezomib, reaching combination indexes (CIs) between 0.53 and 1.094. Efficiency of this latest combination was found to be correlated with the ratio between HSP90 and the co-chaperone HSP70 (P<0.05), whose levels were increased following treatment with both IPI-504 and bortezomib, thus limiting the cytotoxicity. Accordingly, pharmacological inhibition of HSP70 sensitized MCL cells primarily resistant to IPI-504. We compared the modulation of some components of the physiologic unfolded protein response (UPR) among samples harboring different sensitivity to IPI-504 and bortezomib. We observed, in the most sensitive cells, a complete inhibition of the spliced isoform of XBP-1 and phospho-eIF2a after a long time exposure to the combination, a fact that was associated to increased upregulation of the proapoptotic BH3-only protein NOXA and induction of apoptosis. Considering that both bortezomib and IPI-504 have already demonstrated selective cytotoxicity against malignant B cells, combining these two drugs may represent an attractive model for the design of a new and rational combination therapy for MCL.