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

Abstract

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.

Keywords: CEST; MRI; dextran; permeability; vascular disrupting therapy.

FIGURE 1

Illustration of using dexCEST MRI to monitor the tumor responses to antivascular therapies. A, Schematic of the effect of antivascular therapies such as TNF-α on the extravasation of dextran molecules. The picture on the left shows a “normal” vessel around tumor cells, consisting of endothelial cells but lacking pericyte coverage. While it has greater permeability than healthy vessels, molecules larger than the pore size cannot easily pass. Upon TNF-α treatment, as shown in the picture on right, tumor endothelial cells are selectively damaged by TNF-α, resulting in an enormous augmentation of vessel permeability and strong extravasation of large molecules in the tumor. B, Chemical structure of Dex150. C, CEST characteristics of Dex150 in PBS as shown by the Z spectrum and MTR_asym plot of 3.6 mg/mL mM Dex150 (20 mM per glucose unit or 24 μM per dextran molecule, in 10 mM PBS, pH = 7.3). CEST MRI was performed using a 4-s-long CW radiofrequency pulse (B₁ = 3.6 μT) at 37°C

FIGURE 2

DexCEST contrast enhancement in CT26 tumors before and 2 h after the administration of Dex150. A, From top to bottom, T2w anatomical images with tumors indicated by the yellow arrows (top), dexCEST parametric maps (middle), and overlaid images showing the dexCEST signal within the tumors (bottom) of the representative mice before and 2 h after the injection of Dex150. B, Corresponding MTR_asym plots before and after injection. C, Mean pre- and post-dexCEST contrast in the tumors (n = 3)

FIGURE 3

DexCEST contrast enhancement in CT26 tumors before and 2 h after the administration of Dex150 and TNF-α. A, From top to bottom, T2w anatomical images with tumors indicated by the yellow arrows (top), dexCEST parametric maps (middle), and overlaid images showing the dexCEST signal within the tumors (bottom) of the representative mice before and 2 h after the injection of Dex150 and TNF-α. B, Corresponding MTR_asym plots before and after injection. C, Mean pre- and post-dexCEST contrast in the tumors (n = 3)

FIGURE 4

DexCEST contrast enhancement in CT26 tumors before and 2 h after the administration of TNF-α. A, From top to bottom, T2w anatomical images with tumors indicated by the yellow arrows (top), dexCEST parametric maps (middle), and overlaid images showing the dexCEST signal within the tumors (bottom) of the representative mice before and 2 h after the injection of Dex150 and TNF-α. B, Corresponding MTR_asym plots before and after injection. C, Mean pre- and post-dexCEST contrast in the tumors (n = 3)

FIGURE 5

Comparison of CEST signal changes in mice injected with Dex150, TNF-α, and the combination of Dex150 and TNF-α (n = 3 each group)

FIGURE 6

Fluorescence validation. A, Fluorescence imaging of Dex150-FITC in the tumor. The top panel shows the images of 2 representative mice, with and without TNF-α treatment, respectively, at 2 h after Dex150-FITC injection; the middle panel is the corresponding ex vivo images of excised tumors; and the bottom panel is the bar plot of the average fluorescence intensities in the tumors receiving saline, Dex150 and Dex150+TNF-α, respectively. B, Fluorescence microscopic images of DAPI (top), CD31 (middle) and Dex150 (bottom), respectively