Abstract C248: Targeting the tumor microenvironment: Vascular-specific damage by tirapazamine influenced by hypoxia and vascular stability
Authors: Baker Jennifer H. E. , Kyle Alastair H. , Flannagan Erin , Methot Stephen , Balbirnie Andrew , Minchinton Andrew I. 
Published: June 2012

Abstract
Background: Tirapazamine (TPZ; SR 4233; 3-amino-1,2,4-benzotriazine-1,4-di-N-oxide) is a Phase II/III bioreductive anti-cancer agent with greater toxicity to hypoxic vs oxygenated cells in vitro. We have previously reported that TPZ is able to mediate dose-dependent, irreversible central vascular dysfunction in vivo, leaving a hallmark viable rim of surviving peripheral vessels as seen with other vascular targeting agents (VTAs).
Purpose: To explore how this agent, which is a bioreductive cytotoxin in vitro, is able to mediate catastrophic damage to tumor endothelium, likely the most oxygenated cell population in solid tumors, we have investigated the role of the tumor microenvironment including tumor hypoxia and vascular function.
Methods: Vascular dysfunction may be qualitatively and quantitatively assessed using multiplex immunohistochemistry-based staining of frozen tumor sections for vasculature, perfusion, hypoxia, apoptosis and proliferation. Additional features of the microenvironment such as vascular permeability, architecture and support cells were also assessed using this tumor-mapping approach.
Modification of tumor hypoxia was achieved via induction of mild anemia or low oxygen content gas breathing of tumor-bearing mice, and vascular function was increased by over-expression of VEGF in tumor models or systemic administration of nitric oxide (NO), or decreased via inhibition of nitric oxide synthase (NOS).
Results: DCE-MRI studies correlated with tumor mapping data have shown that greater pre-treatment perfusion in HCT116 colorectal xenografts predicts for decreased sensitivity to the anti-vascular effects of TPZ. In contrast to observations using DCE-MRI, increasing tumor perfusion and vascular function caused an increase in tumor vessel sensitivity to TPZ, increasing both the magnitude and frequency of response in tumors. Both NOS inhibition and excess NO availability were able to potentiate anti-cancer effects of TPZ through enhancement of its vascular damage.
Increasing tumor hypoxia by decreasing blood oxygenation was also found to potentiate the anti-vascular effects of TPZ, with otherwise resistant HT29 colorectal xenograft tumors showing a strong vascular dysfunction effect.
Vascular architecture and tumor microenvironmental features were mapped in sensitive HCT116 and resistant HT29 colorectal xenograft models, with vascular maturity, permeability, microregional location of proliferating cells and hypoxia all identified as distinct in the two models.
Summary: This work demonstrates that the vascular targeting effects of TPZ are potentiated by both hypoxia and vascular destabilization, and emphasizes the importance of specifically investigating the activity of anti-cancer agents in the context of the tumor microenvironment in vivo. This work was funded by a grant from CIHR & Jennifer Baker is a recipient of a graduate studentship from Michael Smith Foundation for Health Research.
Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C248.