Cytotoxic Effect of Bendamustine Combined with Kinase Inhibitors in Hematologic Cell Lines

Abstract 4912

Bendamustine, a kind of alkylating agent, has demonstrated a favorable anti-tumor activity both alone and in combination with rituximab in low grade B cell lymphoma. Because the kinase cascades, including B-cell receptor signaling, are necessary for the proliferation and survival of the lymphoma cells, the kinase inhibitors have been developed as molecular target agents of malignant lymphomas. We investigated the cytotoxic effect of bendamustine, both as a single agent and in combination with these new kinase inhibitors in human lymphoid cell lines. We used three B-cell lines, BALL-1 (acute lymphoblastic leukemia), SKLY16 (B-cell lymphoma) and DHL4 (diffuse large B cell lymphoma), and T-cell line THP-6 in this study. These cells were treated with bendamustine alone or combined with enzastaurin (a PKC-β inhibitor), CAL-101 (a p110δ PI3K inhibitor), PCI-32765 (a Btk inhibitor) or R406 (a Syk inhibitor) for 48 hours. Drug-induced cytotoxicity was evaluated using the MTT assay. Cell viability in each experiment was normalized using untreated controls. The treatment with bendamustine alone decreased cell viability in a dose-dependent manner on these three B-cell lines. In contrast, T-cell line THP-6 showed the resistance to bendamusitine. For the combination study, cells were exposed to bendamustine (10μM) and one of these kinase inhibitors at various concentrations simultaneously for 48 hours. CAL-101 and PCI-32765 did not enhance the cytotoxic effect on all of B-cell lines. In BALL-1 and SKLY16 cells, the combination with enzastaurin or R406 resulted in a higher cytotoxic activity than that induced by bendamustine alone. In DHL4 cells, the treatment combined with R406 inhibited cell growth effectively, but not with enzastaurin. To evaluate whether these drug combinations are synergistic, a combination index (CI) was calculated and normalized isobolograms were constructed from non-constant ratio drug combinations using Calcusyn software. The CI values were less than 0. 7 in SKLY16 cells treated with bendamustine and enzastaurin, indicating that these produced synergistic cytotoxic effects in cell line-dependent manner. Our results show that enzastaurin might potentiate the cytotoxic activity of bendamustine in vitro and be a good candidate for the combination with bendamustine.