Patients with
anaplastic thyroid carcinoma (ATC) typically succumb to their disease months after diagnosis despite aggressive
therapy. A large percentage of ATCs have been shown to harbor the V600E B-Raf point mutation, leading to the constitutive activation of the
mitogen-activated protein kinase pathway. ATC invasion,
metastasis, and angiogenesis are in part dependent on the
gelatinase class of
matrix metalloproteinases (
MMP). The explicit targeting of these two
tumor markers may provide a novel therapeutic strategy for the treatment of ATC. The
MMP-activated
anthrax lethal toxin (LeTx), a novel
recombinant protein toxin combination, shows potent
mitogen-activated protein kinase pathway inhibition in
gelatinase-expressing V600E B-Raf
tumor cells in vitro. However, preliminary in vivo studies showed that the
MMP-activated LeTx also exhibited dramatic antitumor activity against xenografts that did not show significant antiproliferative responses to the LeTx in vitro. Here, we show that the
MMP-activated LeTx inhibits orthotopic ATC xenograft progression in both toxin-sensitive and toxin-resistant ATC cells via reduced endothelial cell recruitment and subsequent
tumor vascularization. This in turn translates to an improved long-term survival that is comparable with that produced by the multikinase inhibitor
sorafenib. Our results also indicate that
therapy with the
MMP-activated LeTx is extremely effective against advanced
tumors with well-established vascular networks. Taken together, these results suggest that the
MMP-activated LeTx-mediated endothelial cell targeting is the primary in vivo antitumor mechanism of this novel toxin. Therefore, the
MMP-activated LeTx could be used not only in the clinical management of V600E B-Raf ATC but potentially in any solid
tumor.