Combination of bendamustine and entinostat synergistically inhibits proliferation of multiple myeloma cells via induction of apoptosis and DNA damage response.

Bendamustine, a hybrid molecule of purine analog and alkylator, induces cell death by activation of apoptosis, DNA damage response, and mitotic catastrophe. Entinostat, a selective class I inhibitor of histone deacetylase (HDAC), exerts anti-tumor activity in various cancer types, including multiple myeloma (MM). We sought to determine the combinatorial effects of bendamustine and entinostat on MM cells. Cell growth assays showed that bendamustine or entinostat inhibited proliferation in a dose-dependent manner, and their combinations synergistically induced growth inhibition in all MM cells tested. An apoptotic-ELISA and western blot assays on PARP cleavage and caspase-8 and caspase-3 revealed that bendamustine in combination with entinostat exhibited a much more potent activity than either agent alone to promote the MM cells undergoing apoptosis in a dose-dependent manner. Flow cytometric analysis found that entinostat exhibited distinct effects on cell cycle progression in different lines and bendamustine mainly arrested the cells at S phase, whereas their combinations dramatically blocked the S cells entering G2/M phase. Furthermore, studies on DNA damage response indicated that phospho-histone H2A.X (P-H2A.X), a hall marker of DNA double strand break, along with phosphorylated CHK2 (P-CHK2) was significantly enhanced by the combinations of bendamustine and entinostat as compared to either agent alone. These molecular changes were correlated with the increases in mitotic catastrophe. Collectively, our data demonstrate that bendamustine in combination with entinostat exhibit potent anti-proliferative/anti-survival activity in MM cells via induction of apoptosis and DNA damage response. Regimens consisting of bendamustine and/or entinostat may represent novel therapeutic strategies against MM.