Hyperpolarized 13C lactate, pyruvate, and alanine: noninvasive biomarkers for prostate cancer detection and grading

Abstract

An extraordinary new technique using hyperpolarized (13)C-labeled pyruvate and taking advantage of increased glycolysis in cancer has the potential to improve the way magnetic resonance imaging is used for detection and characterization of prostate cancer. The aim of this study was to quantify, for the first time, differences in hyperpolarized [1-(13)C] pyruvate and its metabolic products between the various histologic grades of prostate cancer using the transgenic adenocarcinoma of mouse prostate (TRAMP) model. Fast spectroscopic imaging techniques were used to image lactate, alanine, and total hyperpolarized carbon (THC = lactate + pyruvate + alanine) from the entire abdomen of normal mice and TRAMP mice with low- and high-grade prostate tumors in 14 s. Within 1 week, the mice were dissected and the tumors were histologically analyzed. Hyperpolarized lactate SNR levels significantly increased (P < 0.05) with cancer development and progression (41 +/- 11, 74 +/- 17, and 154 +/- 24 in normal prostates, low-grade primary tumors, and high-grade primary tumors, respectively) and had a correlation coefficient of 0.95 with the histologic grade. In addition, there was minimal overlap in the lactate levels between the three groups with only one of the seven normal prostates overlapping with the low-grade primary tumors. The amount of THC, a possible measure of substrate uptake, and hyperpolarized alanine also increased with tumor grade but showed more overlap between the groups. In summary, elevated hyperpolarized lactate and potentially THC and alanine are noninvasive biomarkers of prostate cancer presence and histologic grade that could be used in future three-dimensional (13)C spectroscopic imaging studies of prostate cancer patients.

Figure 1

A, diagram of the [1-¹³C] pyruvate and the metabolic pathways relevant to this study. The hyperpolarized ¹³C spectra (B) and peak height plots (C) show the time course for the hyperpolarized [1-¹³C] pyruvate and its metabolic products following the injection of 350 μL of hyperpolarized pyruvate. The pyruvate was injected at a constant rate from 0 to 12 s. The MR spectra were acquired every 3 s from a 28-wk-old TRAMP mouse with a high-grade primary tumor using a 5° flip angle and a 10-mm-thick slice. The peak height plot was corrected for the amount of magnetization used to record the previous n spectra by dividing each peak height by cosⁿ(5°). The hyperpolarized pyruvate quickly reached a maximum at 24 s before being converted to lactate and alanine. Based on this time course, the subsequent MRSI data were recorded between 35 and 49 s, a time when the hyperpolarized lactate signal was roughly constant. Glut., glutamate; α-KG, α-ketoglutarate; ALT, alanine transaminase.

Figure 2

Axial T₂-weighted ¹H image depicting the primary tumor and lymph node metastasis from a TRAMP mouse with a high-grade primary tumor (A) and the overlay of an interpolated hyperpolarized ¹³C lactate image following the injection of 350 μL of hyperpolarized [1-¹³C] pyruvate (B). After spatially zero filling and voxel shifting the ¹³C spectra to maximize the amount of tumor in the voxels, a subset of the spectral grid was selected (C) and displayed (D). The three-dimensional MRSI was acquired with a nominal voxel resolution of 135 mm³ and zero filled to a resolution of 17 mm³. The spectra show substantially elevated lactate in the high-grade primary tumor compared with the low-grade tumor shown in Fig. 3. In addition, the metabolite signal is significantly lower in the necrotic regions of the primary tumor. Lac, lactate; Ala, alanine; Pyr, pyruvate.

Figure 3

Axial T₂-weighted ¹H image depicting the primary tumor from a TRAMP mouse with a low-grade primary tumor (A) and the overlay of an interpolated hyperpolarized ¹³C lactate image following the injection of 350 μL of hyperpolarized [1-¹³C] pyruvate (B). The numerical range of the color map in the lactate image was reduced to half the range used in Fig. 2 to allow visualization of the lower hyperpolarized lactate levels. After spatially zero filling and voxel shifting the ¹³C spectra to maximize the amount of tumor in the voxels, one voxel from the primary tumor was selected (C) and displayed (D). The three-dimensional MRSI was acquired with a nominal voxel resolution of 135 mm³ (dashed white box) and zero filled to a resolution of 17 mm³ (solid white box). The spectrum shows prominent signals from lactate and pyruvate and smaller signals from alanine and pyruvate hydrate.

Figure 4

A to D, representative H&E-stained sections and hyperpolarized ¹³C spectra for one case from each of the histologically defined groups. The histology slides were processed using 5-μm-thick sections and photographed with × 40 magnification. The hyperpolarized ¹³C spectra represent voxels taken from MRSI data sets and normalized to correct for differences in polarization and receiver sensitivity. The normalized spectra illustrate the strong correlation that exists between the amount of hyperpolarized ¹³C lactate and the progression of the disease from the normal prostates to the low-grade primary tumors and the high-grade primary tumors.

Figure 5

Numerical comparisons of hyperpolarized ¹³C metabolites with the pathologically defined groups. Bar plot summarizes the peak area-to-noise ratios of the ¹³C-labeled lactate, pyruvate, and alanine for the four histologically defined groups (A). Columns, average; bars, SD. The lactate peak area SNR values were statistically different for all four groups, except that low-grade tumors were not different from lymph node metastases (P < 0.05). Although the alanine SNR values were not quite statistically different, they exhibited similar trends to the lactate values as shown by the lactate versus alanine plot (B). The box plot of the individual values for the lactate SNR (C) shows that there was almost no overlap between the normal prostates, low-grade tumors, and high-grade tumors. The plot of lactate versus THC (D) suggests that much of the within-group scatter of the lactate SNR may be related to variability in the amount of hyperpolarized carbon delivered to the tissue. All the data presented in these plots were obtained from five normal mice, four TRAMP mice with low-grade primary tumors, and three TRAMP mice with high-grade primary tumors.