Analysis of the anti-proliferative and the pro-apoptotic efficacy of Syk inhibition in multiple myeloma.

Background: Multiple myeloma (MM) is a clonal B cell malignancy characterized by proliferation of malignant plasma cells in the bone marrow. Despite high-dose melphalan therapy with autologous stem cell transplantation (ASCT) and the introduction of immunomodulatory drugs like bortezomib or lenalidomide, that have been associated with improved survival, MM is still incurable and new treatment options are needed. In B cell malignancies such as chronic lymphocytic leukaemia (CLL) or diffuse large B cell lymphoma (DLBCL), Syk (spleen tyrosine kinase) inhibitors have shown promising in vitro and first clinical results. In our study, we analyzed the potential of Syk as a target in MM.

Methods: The MM cell lines AMO-1, U266 and RPMI8226 and primary MM cells were treated with the Syk inhibitors BAY61-3606, R406 or Piceatannol and proliferation, migration and apoptosis induction were analyzed. Effects on involved intracellular signaling cascades were determined by Western blotting. Furthermore, we analyzed synergistic and additive effects of Syk inhibitors in combination with established anti-myeloma drugs and experimental inhibitors (e.g. PI-3-Kinase inhibitor NVP-BEZ235).

Results: Incubation of MM cell lines as well as primary MM cells with Syk inhibitors resulted in a reduced proliferation and stromal cell-derived factor-1 alpha (SDF-1 alpha) induced migration that was accompanied by a concentration dependent inhibition of the MAP-Kinase, characterized by reduced phosphorylation of ERK an p38 molecules, and NF-kappaB signalling pathways. Furthermore, Syk inhibition induced apoptosis in MM cells in a dose-dependent manner, characterized by reduced expression of pro-caspase 3, increased PARP-1 cleavage and enhanced release of cytochrome c. In addition combined treatment of MM cells with Syk inhibitors and NVP-BEZ235 (dual PI3-kinase/mTOR inhibitor) or MAPK inhibitors (PD98059, SP600125, U0126, SB203580) resulted in increased apoptotic activity of the drugs.

Conclusions: Our results show that Syk inhibition might represent a promising new treatment option in MM with an increased efficacy when combined with MAP kinase inhibitors. Furthermore, our study strongly underlines the potency of Syk inhibitors as a potential therapeutic treatment option for MM patients.