Monitoring Early Changes in Tumor Metabolism in Response to Therapy Using Hyperpolarized (13)C MRSI in a Preclinical Model of Glioma

This study shows the use of hyperpolarized (13)C magnetic resonance spectroscopic imaging (MRSI) to assess therapeutic efficacy in a preclinical tumor model. (13)C-labeled pyruvate was used to monitor early changes in tumor metabolism based on the Warburg effect. High-grade malignant tumors exhibit increased glycolytic activity and lactate production to promote proliferation. A rodent glioma model was used to explore altered lactate production after therapy as an early imaging biomarker for therapeutic response. Rodents were surgically implanted with C6 glioma cells and separated into 4 groups, namely, no therapy, radiotherapy, chemotherapy and combined therapy. Animals were imaged serially at 6 different time points with magnetic resonance imaging at 3 T using hyperpolarized [1-(13)C]pyruvate MRSI and conventional (1)H imaging. Using hyperpolarized [1-(13)C]pyruvate MRSI, alterations in tumor metabolism were detected as changes in the conversion of lactate to pyruvate (measured as Lac/Pyr ratio) and compared with the conventional method of detecting therapeutic response using the Response Evaluation Criteria in Solid Tumors. Moreover, each therapy group expressed different characteristic changes in tumor metabolism. The group that received no therapy showed a gradual increase of Lac/Pyr ratio within the tumor. The radiotherapy group showed large variations in tumor Lac/Pyr ratio. The chemo- and combined-therapy groups showed a statistically significant reduction in tumor Lac/Pyr ratio; however, only combined therapy was capable of suppressing tumor growth, which resulted in low endpoint mortality rate. Hyperpolarized (13)C MRSI detected a prompt reduction in Lac/Pyr ratio as early as 2 days post combined chemo- and radiotherapies.