Inhibition of the proteasome has emerged as a clinically effective anticancer 
therapeutic approach in recent years. Bortezomib (Velcade®) showed extremely 
high potency against a wide range of cancer cell lines. Ixazomib 
(MLN9708-MLN2238), the second-generation proteasome inhibitor, selectivity and 
potency were similar to that of bortezomib, is currently being investigated in 
phase I studies. It shows superior antitumor activity in hematologic malignancy, 
especially multiple myelomas. In this study, for the first time, we evaluated 
and compared the antiproliferative and apoptotic effects of the novel proteasome 
inhibitor MLN2238 (the active form of MLN9708) with bortezomib using in vitro 
chronic myeloid leukemia. Cytotoxic and apoptotic effects of MLN2238 and 
bortezomib were determined by trypan blue dye exclusion assays, WST-1 cell 
proliferation assay, increased AnnexinV-PI binding capacity, changes in 
caspase-3 activity and loss of mitochondrial membrane potential (JC-1). 
Associated with proteasome pathway NFκB1 and c-myc mRNA expression levels were 
examined by the qRT-PCR method. We observed that cytotoxic and apoptotic effects 
on K562 cells were started at 5 μm of MLN2238 and 1 μm of bortezomib after 24 
and 48 h. Also, MLN2238 and bortezomib downregulated NFκB1 and c-myc mRNA 
expression at 24 h. Our result revealed that MLN22238 and bortezomib had 
significant cytotoxic and apoptotic effects on K562 cells. Here, we first 
demonstrate in vitro data that support the development of MLN2238, by direct 
comparison with bortezomib on K562 cells.