The Novel JAK2 Inhibitor NVP-BSK805 Has Cytotoxic Activity on Malignant Plasma Cells

Abstract 2993

In multiple myeloma (MM) and plasma cell leukemia, activation of the JAK/STAT pathway is induced by interleukin (IL)-6, which is produced and secreted into the tumor microenvironment primarily by stromal cells. Upon binding of IL-6 to its specific alpha-chain receptor, dimerization of the gp130 signaling subunits leads to activation of associated JAK kinases and STAT transcription factors. In particular, STAT3 has been shown to be essential for myeloma cell growth and survival. NVP-BSK805 (Novartis) is a novel substituted quinoxaline JAK2 inhibitor tool compound which displays more than 20-fold selectivity for JAK2 over the other JAK family members and more than 100-fold selectivity over a panel of additional kinases (Baffert et al., Mol Cancer Ther 9:1945, 2010). The study presented here aims at growth inhibitory effects of NVP-BSK805 in malignant plasma cells. NVP-BSK805 inhibited the growth of six human myeloma cell lines displaying dose-dependent activity with IC50 concentrations between 2.6 μ mol/L and 6.8 μ mol/L. Among the cell lines, IL-6 dependent INA-6 cells were most sensitive to the inhibitory effects of the compound: both IL-6 and bone marrow stromal cell induced proliferation as measured by [³H]-thymidine uptake was completely inhibited at 4 μ mol/L and IC50 concentrations were less than 1 μ mol/L. Viability of the stromal cells was not significantly affected. NVP-BSK805 concentrations as low as 0.5 μ mol/L were sufficient to yield a marked reduction of IL-6 induced STAT3 phosphorylation and complete abrogation at 2 μ mol/L, thereby blocking essential survival signals. Accordingly, treatment of INA-6 cells with NVP-BSK805 for 48 hours led to significant apoptosis starting at 2 μ mol/L with a 30% increase in annexin V-positive cell numbers compared to DMSO controls. Importantly, NVP-BSK805 showed potent cytotoxic activity on plasma cell-enriched primary tumor samples from patients with extramedullary plasma cell disease that are highly responsive to IL-6: in 3 out of 4 tumor samples the IC50 concentrations were between 0.5 μ mol/L and 0.6 μ mol/L. These studies are extended to combinations of NVP-BSK805 with PI3K, mToR, MAPK, HDAC, and IGF-1R inhibitors in order to optimize targeted therapy strategies facing different pathway alterations in individual myeloma patients. In INA-6 cells, synergistic activity was found combining NVP-BSK805 with rapamycin and the MEK1 inhibitor U0126. Preclinical in vivo studies are ongoing. Our results with NVP-BSK805 substantiate the use of JAK inhibitors as a therapeutic strategy for patients with MM. Since results from studies with pan-JAK inhibitors such as pyridone 6 indicate involvement of additional JAK kinases, the choice of the optimal compound will depend on its JAK family selectivity and the biology of JAK signaling in MM.