Background: Waldenström’s Macroglobulinemia (WM) is an incurable lymphoplasmacytic lymphoma with limited options of therapy. Histone deacetylase (HDAC) inhibitors represent promising new treatment strategy in B cell malignancies. We therefore investigated the in vitro effect of the novel hydroxamic acid derivative HDAC inhibitor LBH589 in WM.

Methods: WM cell lines (BCWM1 and WSU-WM) and IgM secreting low-grade lymphoma cell lines (MEC1, RL) were used. Bone marrow primary CD19+ cells and bone marrow stromal cells (BMSC) were obtained from patients with WM after informed consent. Cytotoxicity and DNA synthesis were measured by MTS assay and thymidine uptake assay. Cell signaling and apoptotic pathways were determined by Western Blot and immunofluorescence.

Results: LBH589 induced a significant decrease of proliferation and triggered cytotoxicity in all cell lines tested and primary CD19+ WM cells (IC50 of 20–40nM), even in the presence of BMSC, IL-6 and IGF-1, which induce resistance to conventional therapies. Importantly, LBH589 did not induce cytotoxicity in healthy donor peripheral blood mononuclear cells. LBH589 induced both intrinsic and extrinsic apoptotic pathways, with caspase-9, caspase-8, caspase-3, and PARP cleavage in a dose-dependent manner. We also demonstrated significant upregulation of the proapoptotic transcription factor p53 and down-regulation of the anti-apoptotic proteins BclxL, Mcl-1 and c-myc. We then demonstrated that LBH589 induced apoptosis in WM cells in a caspase-independent manner through induction of autophagy, as shown by upregulation of LC3B and Rab7 expression. We further determined the mechanism of action of LBH589 in WM, investigating the effect of LBH589 on histone acetylation and NF-kB pathways. We found that LBH589 induced a dose-dependent increase in histone H3-H4 acetylation; and inhibited both canonical and non-canonical pathways of NF-κB, as shown by western blot and immunofluorescence. In addition, LBH589 augmented rituximab, fludarabine, bortezomib, and perifosine-induced cyotoxicity in WM cells.

Conclusion: LBH589 has significant antitumor activity in WM in vitro, providing the framework for clinical trials evaluating LBH589 as a new therapeutic agent in patients with WM.

Disclosures: No relevant conflicts of interest to declare.

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