Preclinical activity of the JAK1/2 inhibitor ruxolitinib on malignant plasma cell growth and survival.

Abstract

e18565 Background: In multiple myeloma, cytokines in the tumor environment, in particular interleukin-6 (IL-6), support the growth and survival of malignant plasma cells. Binding of IL-6 to its receptor leads to gp130 dimerization and activation of JAKs and STAT3. Ruxolitinib (INC424)is the first small molecule JAK inhibitor approved for the treatment of patients with myelofibrosis. The aim of our study was to evaluate the effects of ruxolitinib on malignant plasma cells. Methods: Cell growth was studied in seven myeloma cell lines including the IL-6 dependent INA-6. Ruxolitinib was tested at 0.0625 µmol/L to 8 µmol/L. Proliferation of plasma cell enriched patient samples was measured by [3]H-thymidine uptake, apoptosis by flow cytometry upon annexin V/7-AAD staining. Levels of STAT3 and ERK1/2 phosphorylation were determined by Westernblot analysis. IC50 concentrations and combination index were calculated with CalcuSyn. IL-6 levels were determined by ELISA. Results: A significant inhibition of plasma cell growth with ruxolitinib was achieved in IL-6 dependent INA-6 cells (IC50 0.22 µmol/L). Complete growth inhibition at 1 µmol/L was seen in the absence and presence of bone marrow stromal cells. Stromal cell viability and IL-6 production were not affected. The number of apoptotic INA-6 cells upon treatment with ruxolitinib at 1 µmol/L increased 3.6- and 7.2-fold (after 48 and 72 hours, respectively), consistent with the reduction of IL-6 induced STAT3 phosphorylation. A similar strong inhibitory activity of ruxolitinib (IC50 0.16 µmol/L) was observed in tumor cells of a patient with plasma cell leukemia proliferating in response to IL-6. In contrast, none of the myeloma cell lines that grow autonomously were sensitive, pointing to the kinase specificity of the drug. Using INA-6 as a model, combinations with other signaling inhibitors revealed additive to synergistic effects with PI3K, mToR and IGF-1R inhibitors. Conclusions: In multiple myeloma, ruxolitinib has a strong cytotoxic activity against malignant plasma cells that require IL-6 for growth and survival. This warrants further clinical testing but also points to the need of identifying molecular markers to predict benefit from JAK inhibitor treatment.