Bortezomib is well-known for inducing cell death in cancer cells, specifically 
through the mechanism of proteasome inhibition. Thiostrepton, a thiazole 
antibiotic, has also been described for its proteasome inhibitory action, 
although differing slightly to bortezomib in the proteasomal site to which it is 
active. Previously we had shown the synergic effect of bortezomib and 
thiostrepton in breast cancer cells in vitro, where sub-apoptotic concentrations 
of both proteasome inhibitors resulted in synergic increase in cell death when 
combined as a treatment. Here, we administered such a combination to MDA-MB-231 
xenograft tumors in vivo, and found that the effect of complementary proteasome 
inhibitors reduced tumor growth rates more efficiently than compared with when 
administered alone. Increased induction of apoptotic activity in tumors was 
found be associated with the growth inhibitory activity of combination 
treatment. Further examination additionally revealed that combination-treated 
tumors exhibited reduced proteasome activity, compared with non-treated and 
single drug-treated tumors. These data suggest that this drug combination may be 
useful as a therapy for solid tumors.