Tc-99m-Labeled Tricarbonyl His-CNA35 as an Imaging Agent for the Detection of Tumor Vasculature

Given the importance of angiogenesis for a tumor's survival and growth, several therapeutic strategies rely on the selective inhibition of angiogenesis and the destruction of existing tumor vasculature. These strategies raise the need for a noninvasive tool to evaluate tumor vasculature. We describe the radiosynthesis and evaluation of an imaging tracer that specifically binds tumor subendothelial collagen and thereby images tumor vasculature. Methods: Tc-99m-tricarbonyl was prepared and labeled with His-collagen-binding adhesion protein 35 (CNA35). After in vitro specificity testing, in vivo biodistribution and dosimetric studies were performed in healthy nude mice via planar imaging. Tc-99m-(CO)(3) His-CNA35 was evaluated for in vivo imaging of tumor vasculature in a HT29 colorectal carcinoma xenograft. Results: The labeling procedure yielded a compound with 95%-99% radiochemical purity and good in vitro stability. An in vitro binding test confirmed specificity and functionality. Tc-99m-(CO)(3) His-CNA35 rapidly cleared from the blood and pre-dominantly accumulated in the kidneys and liver. The effective dose for a proposed single injection of 500 MBq of Tc-99m-(CO)(3) His-CNA35 is 3.70 mSv per organ or 2.01 mSv/g of tissue. Tumors were successfully visualized, and uptake correlated with ex vivo immunohistochemical staining of tumor vasculature. Conclusion: Tc-99m-(CO)(3) His-CNA35 may be a useful radioligand for the in vivo detection of tumor vasculature through subendothelial collagen binding. A noninvasive method of imaging tumor vasculature that could provide a reliable assessment of tumor vasculature would allow evaluation of the effectiveness of commonly used antiangiogenic therapies and determination of their optimal dosing and scheduling.