Dual NAMPT and BTK Targeting Leads to Synergistic Killing of Waldenström Macroglobulinemia Cells Regardless of MYD88 and CXCR4 Somatic Mutation Status.

Purpose: Nicotinamide phosphoribosyltransferase (Nampt) regulates intracellular NAD+ pool and is highly expressed in a number of malignancies. FK866, a selective inhibitor of Nampt, depletes intracellular NAD+ levels, thereby blocking cellular metabolism and triggering sensitization to other drugs and cell death. Here we characterized the anti-tumor effects of Nampt inhibition in Waldenstrӧm Macroglobulinemia (WM). Experimental Design: We investigated Nampt role in MW cells using both mRNA and protein expression analyses. We have also used loss-of-function approaches to investigate the growth and survival effects of Nampt on MW cells and further tested the anti-MW activity of dual Nampt and BTK inhibition in vitro and in vivo. Results: We found that WM cells exhibit high levels of Nampt compared with normal B cells. Loss of function studies suggested a potential oncogenic role of Nampt in WM cells, and BTK-inhibitor ibrutinib and FK866 resulted in a significant and synergistic anti-WM cell death, regardless of MYD88 and CXCR4 mutational status. Cell death was associated with: 1) activation of caspase-3, PARP and down-regulation of Mcl-1; 2) enhanced intracellular ATP and NAD+ depletion; 3) inhibition of NF-kappa B signaling; and 4) inhibition of multiple pro-survival signaling pathways. In a murine xenograft WM model, low-dose combination FK866 and Ibrutinib is well tolerated, significantly inhibits tumor growth, and prolongs host survival. Conclusions: our results show intracellular NAD+ level as crucial for proliferation and survival of WM cells, and provides the mechanistic preclinical rationale for targeting Nampt, either alone or with Ibrutinib, in WM.