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.