Bortezomib is ineffective in an orthotopic mouse model of pancreatic adenocarcinoma

The purpose of the present study was to evaluate the potency of the proteasome inhibitor bortezomib +/- gemcitabine in vitro and in vivo in pancreatic carcinoma. It could be shown that bortezomib induced apoptosis and inhibited proliferation of pancreatic carcinoma very efficiently in vitro. In contrast, in an orthotopic pancreatic adenocarcinoma mouse model, gemcitabine treatment inhibited tumor growth, whereas bortezomib promoted it. Bortezomib-treated animals showed significantly higher tumor burden compared with gemcitabine-treated and control animals, although bortezomib was locally active and induced a decrease of proteasome activity, which was most pronounced following the simultaneous administration of gemcitabine. Also, tumor progression was not caused by immunosuppression as a result of proteasome inhibition. Interestingly, anti-CD31 staining of tumors showed that angiogenesis was significantly increased in the tumors of bortezomib-treated mice compared with the tumors of control animals. In addition, bortezomib resulted an increase of pericytes, vascular endothelial growth factor, RGS-5, and hypoxia-inducible factor-1alpha in the tumor. Although this study supports efficacy of bortezomib against pancreatic carcinoma in vitro, it strongly indicates that bortezomib therapy has a significant tumor-promoting effect in vivo by induction of angiogenesis. The data are in accordance with the complete failure of bortezomib in a phase II trial for this indication. Choosing the right schedule of gemcitabine and bortezomib showed some synergistic effects, but the gain might not be big enough to compensate the potentially detrimental effects.