Imaging Glioblastoma Metabolism by Using Hyperpolarized [1-13C]Pyruvate Demonstrates Heterogeneity in Lactate Labeling: A Proof of Principle Study

Abstract

Purpose To evaluate glioblastoma (GBM) metabolism by using hyperpolarized carbon 13 (¹³C) MRI to monitor the exchange of the hyperpolarized ¹³C label between injected [1-¹³C]pyruvate and tumor lactate and bicarbonate. Materials and Methods In this prospective study, seven treatment-naive patients (age [mean ± SD], 60 years ± 11; five men) with GBM were imaged at 3 T by using a dual-tuned ¹³C-hydrogen 1 head coil. Hyperpolarized [1-¹³C]pyruvate was injected, and signal was acquired by using a dynamic MRI spiral sequence. Metabolism was assessed within the tumor, in the normal-appearing brain parenchyma (NABP), and in healthy volunteers by using paired or unpaired t tests and a Wilcoxon signed rank test. The Spearman ρ correlation coefficient was used to correlate metabolite labeling with lactate dehydrogenase A (LDH-A) expression and some immunohistochemical markers. The Benjamini-Hochberg procedure was used to correct for multiple comparisons. Results The bicarbonate-to-pyruvate (BP) ratio was lower in the tumor than in the contralateral NABP (P < .01). The tumor lactate-to-pyruvate (LP) ratio was not different from that in the NABP (P = .38). The LP and BP ratios in the NABP were higher than those observed previously in healthy volunteers (P < .05). Tumor lactate and bicarbonate signal intensities were strongly correlated with the pyruvate signal intensity (ρ = 0.92, P < .001, and ρ = 0.66, P < .001, respectively), and the LP ratio was weakly correlated with LDH-A expression in biopsy samples (ρ = 0.43, P = .04). Conclusion Hyperpolarized ¹³C MRI demonstrated variation in lactate labeling in GBM, both within and between tumors. In contrast, bicarbonate labeling was consistently lower in tumors than in the surrounding NABP. Keywords: Hyperpolarized ¹³C MRI, Glioblastoma, Metabolism, Cancer, MRI, Neuro-oncology Supplemental material is available for this article. Published under a CC BY 4.0 license.

Keywords: Cancer; Glioblastoma; Hyperpolarized 13C MRI; MRI; Metabolism; Neuro-oncology.

Graphical abstract

Figure 1:

Hyperpolarized ¹³C MR images from all seven patients. (A) Grayscale axial contrast-enhanced ¹H three-dimensional (3D) T1-weighted fast spoiled gradient-echo (FSPGR) images through the center of the lesion for each patient and the corresponding unenhanced images overlaid with the (B) pyruvate, (C) lactate, and (D) bicarbonate color maps summed over the time course.

Figure 2:

Histograms of the (A) lactate-to-pyruvate and (B) bicarbonate-to-pyruvate ratios in each voxel from the section through the center of the lesion for each patient (n = 7) with an overlying polynomial fit; glioblastoma (GBM) data are shown in blue, and the normal-appearing brain parenchyma (NABP) data are shown in red.

Figure 3:

Average labeled metabolite distribution for the entire patient cohort (n = 7). Histograms show the (A) average lactate-to-pyruvate and bicarbonate-to-pyruvate ratios and (B) normalized signal intensities for pyruvate, lactate, and bicarbonate with an overlying polynomial fit. Normalization was performed relative to the normal-appearing brain parenchyma (NABP). Ratios for glioblastoma (GBM) are shown in blue, and ratios for NABP are shown in red.

Figure 4:

Dependence of metabolite signal ratios (lactate-to-pyruvate and bicarbonate-to-pyruvate ratios) on (A) tumor volume, (B) volume of enhancing tissue, and (C) percentage of nonenhancing tumor core. Each point represents an individual participant. The lesion volume and the volume of enhancing tissue are expressed in centimeters cubed; the nonenhancing core is expressed as a percentage of the entire lesion volume. The R² values, representing the goodness of each fit, and the corresponding P values for each regression are given. The level of significance was set at .05.

Figure 5:

Relationship between lactate dehydrogenase A (LDH-A) expression and labeling of lactate and pyruvate following injection of hyperpolarized [1-¹³C]pyruvate. Scatterplots show the relationship between LDH-A expression and the lactate-to-pyruvate and bicarbonate-to-pyruvate ratios. Each point represents a tissue sample. The R² values, representing the goodness of fit, and P values for each regression are shown.

Figure 6:

(A–C) Proton images, hyperpolarized ¹³C MR images, and immunohistochemical (IHC) data from participant 7 (74-year-old man with glioblastoma). (A) Grayscale axial three-dimensional (3D) T2-weighted (T₂W), fluid-attenuated inversion recovery (FLAIR), and gadolinium-based contrast agent (GBCA)–enhanced 3D T1-weighted (T₁W) fast spoiled gradient-echo images through the center of the lesion. There is a lesion within the right anterior temporal lobe demonstrating T2-weighted and FLAIR hyperintensity involving the right insula and external capsule and reaching the lentiform nucleus. (B) The corresponding pyruvate and lactate maps summed over the entire time course and the lactate-to-pyruvate (LP) ratio map are shown in color superimposed on the T1-weighted images before contrast enhancement. The metabolic maps reveal heterogeneity, with higher pyruvate and lactate being shown in the medial aspect of the lesion; the LP ratio was particularly higher in the posterior part of insula. (C) Representative IHC imaging, shown with a 20× magnification, from the target region of interest highlighted on the ¹H and ¹³C MR images (blue circle) stained for ki-67, monocarboxylate transporter 1 (MCT1), and carbonic anhydrase IX (CAIX). Details on IHC analysis are provided in Appendix E1 (supplement); in brief, the antibodies used for staining were: M7240 for ki-67, HPA003324 for MCT1, and NCL-L-CAIX for CAIX. Histopathologic findings demonstrated a homogeneous high-grade tumor with MIB-1 staining of approximately 8%, high MCT-1 staining, and no significant staining for CAIX.