Dynamic contrast-enhanced CEST MRI using a low molecular weight dextran

Abstract

Natural and synthetic sugars have great potential for developing highly biocompatible and translatable chemical exchange saturation transfer (CEST) MRI contrast agents. In this study, we aimed to develop the smallest clinically available form of dextran, Dex1 (molecular weight, MW ~ 1 kDa), as a new CEST agent. We first characterized the CEST properties of Dex1 in vitro at 11.7 T and showed that the Dex1 had a detectable CEST signal at ~1.2 ppm, attributed to hydroxyl protons. In vivo CEST MRI studies were then carried out on C57BL6 mice bearing orthotopic GL261 brain tumors (n = 5) using a Bruker BioSpec 11.7 T MRI scanner. Both steady-state full Z-spectral images and single offset (1.2 ppm) dynamic dextran-enhanced (DDE) images were acquired before and after the intravenous injection of Dex1 (2 g/kg). The steady-state Z-spectral analysis showed a significantly higher CEST contrast enhancement in the tumor than in contralateral brain (∆MTR_asym^{1.2 ppm} = 0.010 ± 0.006 versus 0.002 ± 0.008, P = 0.0069) at 20 min after the injection of Dex1. Pharmacokinetic analyses of DDE were performed using the area under the curve (AUC) in the first 10 min after Dex1 injection, revealing a significantly higher uptake of Dex1 in the tumor than in brain tissue for tumor-bearing mice (AUC[0-10 min] = 21.9 ± 4.2 versus 5.3 ± 6.4%·min, P = 0.0294). In contrast, no Dex1 uptake was foundling in the brains of non-tumor-bearing mice (AUC[0-10 min] = -1.59 ± 2.43%·min). Importantly, the CEST MRI findings were consistent with the measurements obtained using DCE MRI and fluorescence microscopy, demonstrating the potential of Dex1 as a highly translatable CEST MRI contrast agent for assessing tumor hemodynamics.

Keywords: CEST; MRI; brain tumor; dextran; permeability.

Figure 1.. Timeline of CEST and DCE MRI acquisitions.

Two 7-min steady state full Z-spectra acquisitions were performed before and at approximately 10 minutes after the injection of 2 g/kg Dex1 in saline solution (infusion time=1 min). Dynamic CEST MRI was started at 2 min before the injection to 18 min after the injection at a temporal resolution of 10 s. Two WASSR scans (~ 33 sec) were acquired to assess the B₀ inhomogeneities before and after each CEST acquisition. Dynamic contrast-enhanced (DCE) MRI was acquired using a T_1w single-slice FLASH sequence (TR/TE=18/3 ms) from 1 min before the injection to 12 min after the i.v. injection of ProHance® (0.05 mL bolus over ~3 s).

Figure 2.. In vitro CEST characterization of Dex1.

a) The chemical structure of dex1 (average m = 6). B) NMR spectra of 100 mM dex1 in D₂O and H₂O (pH 6.5, 20 °C). C) Z-spectra, and D) MTR_asym plots of 20 mM Dex1 (20 mg/mL) and PBS at pH 6.0–8.0. E) Comparison of CEST signal of 1.8 mg/mL Dex1 (concentration per glucosyl unit ~ 22 mM) and 20 mM glucose at pH =7.4. F) Plot of 1-S^1.2ppm/S₀ at pH = 7.4 vs. concentration of Dex1 (in glucose units), showing a good linear correlation. All studies were performed using 3-second long CW pulse (B₁ =1.8 μT) at 37°C unless otherwise noted.

Figure 3.. DexCEST (B₁=1.8 μT/3 sec) contrast at 3T.

A) MTR_asym parametric maps of Dex1 (20 mM) at a function of pH (37 °C). Note that a D-glucose sample (20 mg/mL, pH 7.4) was included as control. B) Z-spectra and C) MTR_asym of 10 mM Dex1 at pH ranging from 6.0 to 8.0. D) pH dependency of DexCEST contrast at 3T as compared to that at 11.7T.

Figure 4.. Dex1-enhanced CEST MRI in the GL261 brain tumor model.

A) From left to right: T2w, CEST images, and Gd-enhanced image of a representative mouse. B-C) Mean ROI Z-spectra and MTR_asym plots before and after dextran1 injection in the tumor (B) and contralateral brain (C). D-E) Comparison of the pre- and post-injection CEST contrast in the tumor (D) and brain (E) of 5 mice. F) Comparison of dextran1-induced contrast enhancement in the tumor and brain in 5 mice. G) Immunofluorescence image showing the high uptake of Dex1 in the tumor, in which blue is nuclei (stained with DAPI) and red is dextran (labeled with Cy5.5). Right lower area: tumor; left upper area: normal brain. *: P<0.05, **: P<0.01, N.S.= not significant (Student’s t test, two tailed and paired, n=5).

Figure 5.. DDE CEST MRI in the GL261 brain tumor model.

A) T₂w image showing the three ROIs for tumor (T), brain (B) and ventricle (V). B-D) Dynamic changes of the mean CEST contrast, as quantified by ΔS^1.2ppm/S₀ (t)%= [S^1.2ppm/S₀ (t=0)- S^1.2ppm/S₀ (t)]x100% (B). C) AUC map between 0 to 10 min after the injection of Dex1. D) Comparison of the mean AUC values in the tumor and brain. E-F) Dynamic Gd concentration change after the injection of 0.1 mmol/kg ProHance (~ 0.05 mL of 0.5 M solution). F) DCE AUC (0–10 min) map. G) Comparison of the mean AUC (0–10 min) in the tumor and brain. *: P<0.05 (Student’s t test, two tailed and paired, n=4).

Figure 6.. DDE MRI in the normal brain.

A) T₂w image of a representative mouse B) AUC map between 0 to 10 min after the injection of Dex1.C) Comparison of the mean AUC (0–10 min) in non-tumor bearing mice with that of tumor in tumor-bearing mice.