Molecular imaging of tumors by chemical exchange saturation transfer MRI of glucose analogs

Abstract

Early detection of the cancerous process would benefit greatly from imaging at the cellular and molecular level. Increased glucose demand has been recognized as one of the hallmarks of cancerous cells (the "Warburg effect"), hence glucose and its analogs are commonly used for cancer imaging. One example is FDG-PET technique, that led to the use of chemical exchange saturation transfer (CEST) MRI of glucose ("glucoCEST") for tumor imaging. This technique combines high-resolution MRI obtained by conventional imaging with simultaneous molecular information obtained from the exploitation of agents with exchangeable protons from amine, amide or hydroxyl residues with the water signal. In the case of glucoCEST, these agents are based on glucose or its analogs. Recently, preclinical glucoCEST studies demonstrated the ability to increase the sensitivity of MRI to the level of metabolic activity, enabling identification of tumor staging, biologic potential, treatment planning, therapy response and local recurrence, in addition to guiding target biopsy for clinically suspected cancer. However, natural glucose limits this method because of its rapid conversion to lactic acid, leading to reduced CEST effect and short signal duration. For that reason, a variety of glucose analogs have been tested as alternatives to the original glucoCEST. This review discusses the merits of these analogs, including new data on glucose analogs heretofore not reported in the literature. This summarized preclinical data may help strengthen the translation of CEST MRI of glucose analogs into the clinic, improving cancer imaging to enable early intervention without the need for invasive techniques. The data should also broaden our knowledge of fundamental biological processes.

Keywords: Cancer; MRI; chemical exchange saturation transfer (CEST); contrast agent; diagnosis; molecular imaging.

Figure 1

CEST MRI kinetic measurements in D1-DMBA-3 breast tumors at different times following IV injections of 1.0 g/kg of 2DG/FDG and 1.5 g/kg of D-Glc (20). CEST, chemical exchange saturation transfer; 2DG, 2-deoxy-D-glucose; FDG, 2-fluoro-2-deoxy-D-glucose; D-Glc, D-glucose.

Figure 2

Bar graph showing % of MTR_asym of 20 mM D-Glc solution (10% D₂O) at several PBS concentrations at the typical frequency offsets of the hydroxyl peaks (A) 1.3 ppm, (B) 2.1 ppm and (C) 2.88 ppm from the water peak (T=37 °C, 11.7T). (The peak at 0.66 ppm that belongs to the hydroxyl peak at the 6-position of D-Glc was not included in the MTR calculations because of its proximity to the water signal.) ¹H NMR spectra of the hydroxyl protons of 0.1 M D-Glc solution (T=4 °C, pH=5.4, 11.7T). Spectra were recorded on a fresh sample (D) and several hours after the sample preparation (E). MTR_asym, magnetization transfer asymmetry.

Figure 3

Comparison of in vitro chemical exchange saturation transfer (CEST) MRI of glucose (“glucoCEST”) NMR signals of several glucose analogs. Solutions of 20 mM were measured at T=37 °C, saturation pulse duration of 3 s, and attenuation of 2.4 µT, at the 11.7T (Rivlin M, 2018, unpublished results). MTR_asym, magnetization transfer asymmetry.

Figure 4

¹H-decoupled ³¹P NMR spectra of metabolites extracted from brains of mice bearing 4T1 tumors. (A) and (B) are extracts from control brains (without treatment). (C) and (D) are extracts from brains of mice administrated [6-¹³C] 3-O-Methyl-D-glucose (3OMG) [1.0 g/kg, per-os (PO)]. The peaks were referenced to GPC (0.49 ppm). Each spectrum represents a single specific brain of a mouse. The peaks were assigned according to previously published data: GPC, glycerphosphocholine; GPE, glycerphosphoethanolamine; Pi, inorganic phosphate. As no metabolic phosphorylated products were obtained, no peaks were observed around 5.1 ppm. ¹H-decoupled ¹³C NMR spectra of metabolites extracted from brains of mice bearing 4T1 tumors. (E) is an extract from the control brain (without treatment) and (F) is an extract from the brain of mice administrated [6-¹³C] 3OMG (1.0 g/kg, PO). Each spectrum corresponds to overnight data accumulation and represents a single specific brain of a mouse. The resonance of [6-¹³C] 3OMG is shown at 63.3 ppm in spectra (F). The peaks were referenced to DSS (0 ppm). As no metabolic phosphorylated products were obtained, no peaks were observed around 66 ppm (Rivlin M, 2018, unpublished results). More details regarding the extracts procedure can be found at Rivlin et al. (29).

Figure 5

3-O-Methyl-D-glucose (3OMG) chemical exchange saturation transfer (CEST) vs. CEST MRI of glucose (glucoCEST) kinetics measurements in 4T1 breast tumor model in the same animal. (A) An anatomical T₂-weighted image before D-glucose administration. (B,C) CEST map before (B) and ~60 min after (C) Per os (PO) treatment with D-Glc, 1.5 g/kg (at frequency offset of 1.2 ppm, B₁=2.4 µT). No remarkable CEST contrast was obtained in the tumor vis-a-vis the baseline. (D) An anatomical T₂-weighted image before 3OMG administration. (E,F) CEST map before (E) and ~60 min after (F) PO treatment with 3OMG, 0.7 g/kg (at frequency offset of 1.2 ppm, B₁=2.4 µT). Approximately 4% CEST was obtained in the tumor with reference to the baseline. Green arrows point to the tumor, the red arrow to the urinary bladder. (G) The time series of the % CEST achieved in 4T1 tumor following treatment with D-Glc (1.5 g/kg) vs. 3OMG (0.7 g/kg). Figure taken with permission from (29).

Figure 6

3-O-Methyl-D-glucose (3OMG) chemical exchange saturation transfer (CEST) MRI of malignant brain tumor. (A) Anatomical image of the mouse brain. (B) The area under the curve image calculated for the last three minutes of the CEST scan (using a single CW magnetization transfer prepulse of strength 1.5 µT and duration 2 s), a period of 8–11 min post injection of 3OMG (3 g/kg, 1.9M, 200 mL). Figure taken with permission from (59).

Figure 7

3-O-Methyl-D-glucose (3OMG) chemical exchange saturation transfer (CEST) MRI and ¹⁸FDG PET/CT images from five tumors of a murine model (4T1 cells). (A) A coronal view of an anatomical T₂-weighted MR images (7T field) before 3OMG administration showing the tumor (green arrow) and the urinary bladder (red arrow). (B) % CEST images 60 min after PO administration with 3OMG, 1.0 g/kg (at a frequency offset of 1.2 ppm, B₁=2.4 µT). A significant CEST contrast was obtained in the tumor and the urinary bladder as well as areas suspected to be metastases. (C) ¹⁸FDG PET/CT coronal view obtained 60 min after IV injection of ¹⁸FDG showing accumulation mainly in the tumor (green arrow) and urinary bladder (red arrow). (D) Correlation between 3OMG % CEST contrast and % ID/mL value in the five tumors from a murine model ± SD. The CEST and PET/CT measurements were performed 8 and 10 days after implantation of the tumors, respectively (29).

Figure 8

In vivo 6DG CEST MRI measurements in a 4T1 tumor (7T field). (A) A T₂-RARE anatomical image before administration of the agent; (B) MTR_asym image at 1.2 ppm following treatment with 6DG (2.0 g/kg, IP), overlaid onto the T₂ anatomical image; (C) The MTR_asym plots for 4T1 tumor before (red curve) and after treatment (green curve) with 6DG. In the inset: full Z spectra for 4T1 tumor at 2 time periods following administration of 6DG. Total estimated time of scan was 80 min for CW pulses of 2 s duration and attenuation of 2.5 µT (106 Hz) at 106 frequencies offsets that were divided into two series and sampled alternately and simultaneously. The mean intensities in the selected ROI in the tumor were used for the MTR_asym plot. B₀ inhomogeneity corrections were made (Rivlin M, 2018, unpublished results).

Figure 9

¹H-decoupled ³¹P NMR spectra of extracts from 4T1 tumors (A) untreated and (B) and (C) treated with 2.6 g/kg (PO) and 2.0 g/kg (IV) of 2OMG, respectively. The peaks were assigned according to previously published data (79): GPC, glycerophosphocholine; GPE, glycerophosphoethanolamine; Pi, inorganic phosphate. The spectra were calibrated according to GPC (0.49 ppm). Rivlin M, 2018, unpublished results.

Figure 10

In vivo 2OMG CEST MRI measurements in a 4T1 tumor (7T field). (A) A T₂-RARE anatomical image before administration of the agent; (B) MTR_asym image at 1.0 ppm following treatment with 2OMG (3 g/kg, IP), overlaid onto the T₂ anatomical image; (C) The MTR_asym plots for 4T₁ tumor before (red curve) and after treatment (green curve) with 2OMG. In the inset: full Z spectra for 4T₁ tumor at two-time periods following administration of 2OMG; total estimated time of scan was 80 min (same CEST protocol as described in Figure 8 caption). Rivlin M, 2018, unpublished results.