CEST‐MRI for glioma pH quantification in mouse model: Validation by immunohistochemistry

Abstract

In glioma, the acidification of the extracellular tumor microenvironment drives proliferation, angiogenesis, immunosuppression, invasion and chemoresistance. Therefore, quantification of glioma extracellular pH (pHe) is of crucial importance. This study is focused on the application of the YbHPDO3A (ytterbium 1,4,7-triscarboxymethyl-1,4,7,10-tetraazacyclododecane) probe for in vivo glioma pHe quantification using chemical exchange saturation transfer (CEST)-MRI and its correlation with tumor metabolism assessed by immunohistochemistry. The U87 glioma mouse model was used (n = 18) and MRI performed at 4.7 T. CEST-MRI of YbHPDO3A solutions at different pH values showed two resolved CEST spectra at 71 ppm and 99 ppm, both sensitive to pH variations, allowing therefore calculation of the ratiometric curve for in vivo pH quantification. In vivo MRI sequences consisted of T for tumor localization, T * to assess YbHPDO3A biodistribution by exploiting its magnetic susceptibility effect and CEST for glioma pHe mapping. T * images show that YbHPDO3A extravasates in tumor in regions with damaged blood-brain barrier. The pHe is calculated only in these regions. Hematoxylin/eosin histology and Ki-67, CA-IX (carbonic anhydrase 9) and NHE-1 immunohistochemical staining were performed; their expression rates were compared with the in vivo pHe values. On the basis of the cell proliferation marker Ki-67, two groups were defined: one group with a lower mitotic index (MI% < 20% = mean value) and a mean pHe value of 7.00 (low-proliferation/high-pH group) and the other with MI% > 20% and an acidic pHe of 6.6 (high-proliferation/low-pH group). CA-IX and NHE-1 were over-expressed in the high-proliferation/low-pH group (CA-IX, 92 ± 7% versus 30 ± 13%; NHE-1, 84 ± 8% versus 35 ± 11%), indicating an acidic/hypoxic microenvironment. These immunohistochemical results are consistent with our pHe mapping (Pearson correlation coefficient > 0.70) and provide evidence for the feasibility of the CEST-MRI method with the YbHPDO3A probe for glioma pHe quantification at 4.7 T. Importantly, the YbHPDO3A probe has similar chemical and biological properties to the clinically approved MRI contrast agent GdHPDO3A. This makes the method promising for a clinical translation.

Tumor pH and Protein Concentration Contribute to the Signal of Amide Proton Transfer Magnetic Resonance Imaging

“…Alternative endogenous CEST MRI methods such as AACID (34) have been used in this way to investigate the tumour pH change associated with administration of dichloroacetate (35), topiramate (36) and lonidamine (37) in preclinical experiments. Additionally, exogenous diaCEST (38–41) and paraCEST (42,43) contrast agents have been developed that successfully measure tumour extracellular pH, though these methods are limited by contrast agent availability to the tumour, which may be limited in areas of particularly poor perfusion (44) or in the brain where the blood-brain barrier may be partially intact in the early stages of tumour growth.…”

Use of FCC-NMRD relaxometry for early detection and characterization of ex-vivo murine breast cancer

“…Cross-linked albumin was obtained by using a well-established procedure 27,28 . Briefly, BSA (Bovine Serum Albumin, Sigma Aldrich) was dissolved in water (150 mg/mL) and the solution placed in a heated bath at 80 °C for 10 minutes.…”

Imaging tumor acidosis: a survey of the available techniques for mapping in vivo tumor pH

“…Yb 3+ -HPDO3A showed a pH sensitivity in the range 6.5-8.0 at a field strength of 7 T and demonstrated in vivo tumor pHe maps in a murine melanoma model [96]. In a recent study using Yb 3+ -HPDO3A, it was shown that tumor pHe mapping can also provide information about tumor aggressiveness in a rat glioma model [97]. The advantage of PARACEST agents endowed with highly shifted resonances is further witnessed in the case of Tm 3+ -HPDO3A pH-sensitive agent, which allows in vivo tumor pH mapping at lower magnetic fields (1 T), due to the far-shifted hydroxyl resonances of the two diastereoisomers (71 and 185 ppm, respectively) [98].…”