INTRODUCTION: Proteasome inhibition provides an attractive approach to cancer 
therapy and may have application in the treatment of breast cancer. However, 
results of recent clinical trials to evaluate the effect of the proteasome 
inhibitor Bortezomib (Velcade, also called PS-341) in metastatic breast cancer 
patients have shown limited activity when used as a single agent. This 
underscores the need to find new and more efficacious proteasome inhibitors. In 
this study, we evaluate the efficacy of the novel proteasome inhibitor BU-32 
(NSC D750499-S) using in vitro and in vivo breast cancer models.
METHODS: We have recently synthesized a novel proteasome inhibitor (BU-32) and 
tested its growth inhibitory effects in different breast cancer cells including 
MCF-7, MDA-MB-231, and SKBR3 by in vitro cytotoxicity and proteasomal inhibition 
assays. The apoptotic potential of BU32 was tested using flow cytometry and 
analyzing cell cycle regulatory proteins. In vivo tumor xenograft studies for 
solid tumor as well as tumor metastasis were conducted using MDA-MB-231-GFP 
cells.
RESULTS: We report for the first time that BU-32 exhibits strong cytotoxicity in 
a panel of cell lines: MDA-MB-231 (IC50 = 5.8 nM), SKBR3 (IC50 = 5.7 nM) and 
MCF-7 cells (IC50 = 5.8 nM). It downregulates a wide array of angiogenic marker 
genes and upregulates apoptotic markers, including Bid and Bax. Incubation of 
MDA-MB-231 cells with BU-32 results in the accumulation of cell cycle inhibitor 
proteins p21 and p27 and stabilization of the tumor suppressor protein p53. 
Studies in in vivo solid tumor and metastasis models show significant effect 
with a 0.06 mg/kg dose of BU-32 and marked reduction in tumor burden in the 
skeleton.
CONCLUSIONS: We have shown that BU-32 is effective in cultured breast cancer 
cells and in breast cancer xenografts. The results suggest its potential benefit 
in breast cancer treatment.