CEST MRI monitoring of tumor response to vascular disrupting therapy using high molecular weight dextrans

Purpose: Vascular disrupting therapy of cancer has become a promising approach not only to regress tumor growth directly but also to boost the delivery of chemotherapeutics in the tumor. An imaging approach to monitor the changes in tumor vascular permeability, therefore, has important applications for monitoring of vascular disrupting therapies. Methods: Mice bearing CT26 subcutaneous colon tumors were injected intravenously with 150 kD dextran (Dex150, diameter, d~ 20 nm, 375 mg/kg), tumor necrosis factor-alpha (TNF-α; 1 µg per mouse), or both (n = 3 in each group). The Z-spectra were acquired before and 2 h after the injection, and the chemical exchange saturation transfer (CEST) signals in the tumors as quantified by asymmetric magnetization transfer ratio (MTR_asym) at 1 ppm were compared. Results: The results showed a significantly stronger CEST contrast enhancement at 1 ppm (∆MTR_asym = 0.042 ± 0.002) in the TNF-α-treated tumors than those by Dex150 alone (∆MTR_asym = 0.000 ± 0.005, P = 0.0229) or TNF-α alone (∆MTR_asym = 0.002 ± 0.004, P = 0.0264), indicating that the TNF-α treatment strongly augmented the tumor uptake of 150 kD dextran. The MRI findings were verified by fluorescence imaging and immunofluorescence microscopy. Conclusions: High molecular weight dextrans can be used as safe and sensitive CEST MRI contrast agents for monitoring tumor response to vascular disrupting therapy and, potentially, for developing dextran-based theranostic drug delivery systems.