Simultaneous Metabolic and Perfusion Imaging Using Hyperpolarized 13C MRI Can Evaluate Early and Dose-Dependent Response to Radiation Therapy in a Prostate Cancer Mouse Model

Abstract

Purpose: To investigate use of a novel imaging approach, hyperpolarized (HP) ¹³C magnetic resonance imaging (MRI) for simultaneous metabolism and perfusion assessment, to evaluate early and dose-dependent response to radiation therapy (RT) in a prostate cancer mouse model.

Methods and materials: Transgenic Adenocarcinoma of Mouse Prostate (TRAMP) mice (n = 18) underwent single-fraction RT (4-14 Gy steep dose across the tumor) and were imaged serially at pre-RT baseline and 1, 4, and 7 days after RT using HP ¹³C MRI with combined [1-¹³C]pyruvate (metabolic active agent) and [¹³C]urea (perfusion agent), coupled with conventional multiparametric ¹H MRI including T2-weighted, dynamic contrast-enhanced, and diffusion-weighted imaging. Tumor tissues were collected 4 and 7 days after RT for biological correlative studies.

Results: We found a significant decrease in HP pyruvate-to-lactate conversion in tumors responding to RT, with concomitant significant increases in HP pyruvate-to-alanine conversion and HP urea signal; the opposite changes were observed in tumors resistant to RT. Moreover, HP lactate change was dependent on radiation dose; tumor regions treated with higher radiation doses (10-14 Gy) exhibited a greater decrease in HP lactate signal than low-dose regions (4-7 Gy) as early as 1 day post-RT, consistent with lactate dehydrogenase enzyme activity and expression data. We also found that HP [¹³C]urea MRI provided assessments of tumor perfusion similar to those provided by ¹H dynamic contrast-enhanced MRI in this animal model. However, apparent diffusion coefficien , a conventional ¹H MRI functional biomarker, did not exhibit statistically significant changes within 7 days after RT.

Conclusion: These results demonstrate the ability of HP ¹³C MRI to monitor radiation-induced physiologic changes in a timely and dose-dependent manner, providing the basic science premise for further clinical investigation and translation.

Figure 1:

A panel of representative MR images of a responding mouse prostate tumor following radiotherapy. Tumor volume and hyperpolarized (HP) [1-¹³C]lactate decreased after radiotherapy, while perfusion increased as shown using HP [¹³C]urea and dynamic contrast enhanced (DCE) MRI data. There was also an increase in the apparent diffusion coefficient (ADC) in this tumor, which is inversely correlated with cellularity. A frequency-calibration phantom for hyperpolarized ¹³C scans can be seen in the upper right corner of the images.

Figure 2:

Changes in normalized hyperpolarized (HP) ¹³C lactate, alanine, and urea following radiotherapy. There is an apparent decrease in lactate (A), an increase in alanine (B) and perfusion as seen in HP [¹³C]urea (C) among volume-decreasing tumors (responders) after radiotherapy. In contrast, there were no apparent changes among non-responders (Day 7 has 3 data points, with overlapping range bars and quantile bars). Multilevel regression analyses revealed a significant decrease in lactate, and significant increases in alanine and urea for responders, and these changes were significantly different from non-responders.

Figure 3:

Intratumoral dose-dependent changes in HP ¹³C pyruvate-to-lactate conversion. There were significant associations between radiation dose and normalized HP lactate (nLac) changes at all time points. Moreover, high-dose regions (10-14 Gy) showed a greater magnitude of decreases in nLac compared to low-dose regions (4-7 Gy), as early as Day 1. High-dose regions also showed significantly decreased nLac from baseline at Day 4 and Day 7, and intermediate-dose regions also showed significantly decreased nLac from baseline at Day 7 (*, P < .05; **, P < .01; ***, P < .001; black *, ** indicate significant difference between high- and low-dose; blue *, *** indicate significant change from baseline; IM = intermediate).

Figure 4:

Dose-dependent changes in immunohistochemical and molecular markers. Tumor regions treated with high radiation dose showed significantly more necrosis/fibrosis (A: arrows - frank necrosis; stars - frank fibrosis) on H&E staining (B) and lower Ki-67 (proliferation) staining (C) compared to their corresponding low-dose regions as well as a separate cohort of untreated TRAMP tumors (20x magnification). Moreover, high-dose tumor regions showed lower lactate dehydrogenase (LDH) activity (E), LDHA expression (F), and MCT3/4 expression (I) compared to low-dose regions, consistent with the decreased hyperpolarized ¹³C lactate signal observed in the high dose regions. Additionally, high-dose regions showed slightly higher anti-caspase-3 (an apoptosis marker) staining (D), higher LDHB expression (G), and lower MCT1 (H) expression compared to low-dose regions or untreated tumors, but these differences were not statistically significant. (U = untreated, H = high, L = low; *, P < .05; **, P < .01; ***, P < .001; ****, p < .001)